Jingfeng Song, Jing Chen, Xioming Xiong, Xiodong Peng, Dolun Chen,Fusheng Pn

a National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, China

b Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, Ontario M5B 2K3, Canada

Abstract More than 4000 papers in the fiel of Mg and Mg alloys were published and indexed in Web of Science (WoS) Core Collection database in 2021.The bibliometric analyses indicate that the microstructure, mechanical properties, and corrosion of Mg alloys still are the main research focus.Mg ion batteries and hydrogen storage Mg materials have attracted much attention.Significan contributions to the research and development of magnesium alloys were made by Chongqing University, Shanghai Jiaotong University, and Chinese Academy of Sciences in China, Helmholtz Zentrum Hereon in Germany, Ohio State University in the United States, the University of Queensland in Australia,Kumanto University in Japan, and Seoul National University in Korea, University of Tehran in Iran, etc..This review is aimed to summarize the progress in the development of structural and functional Mg and Mg alloys in 2021.Based on the issues and challenges identifie here,some future research directions are suggested.

? 2022 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Peer review under responsibility of Chongqing University

Keywords: Magnesium alloys; Cast magnesium alloys; Wrought magnesium alloys; Bio-magnesium alloys; Mg based energy storage materials; Processing technologies; Corrosion and protection.

1.Introduction

In September 2020, China proposed the ‘carbon neutrality'and ‘emission peak' strategies, which have attracted worldwide attention.Extensive application of magnesium (Mg) and magnesium alloys is one of the best solutions to achieve the goal.Mg alloys are the lightest structural materials, which have a great potential for weight-saving and CO2emission reduction.Besides, Mg alloys have high specifi strength and stiffness, superior damping performance, good biocompatibility, large hydrogen storage capacity, and high theoretical specifi capacity for battery, etc.[1-6].Hence, magnesium and its alloys have been applied in the fiel of aerospace, automotive, 3C (computers, communications, and consumer electronics) etc.in the world.In addition, the application of Mg and Mg alloy in biomedical and energy sectors has recently attracted increasing attention.However, a lot of difficultie still need to be overcome to expand the further applications of magnesium alloys[7-12].The relatively low strength,poor plasticity, and inferior corrosion resistance of magnesium alloys impede the structural applications, while the problems on the fast degradation rate of Mg alloys and narrow hydrogen charging and discharging window need to be solved in functional materials to broaden the future application of Mg alloys [13-20].

In the past year of 2021, more than 4000 papers in the fiel of Mg and Mg alloys were published and indexed in the authoritative database of “Web of Science Core Collection”.Based on such a literature search, the research trends and hotspots of magnesium alloys were analyzed.The presentwork aims to review the important advances of magnesium and its alloys worldwide in 2021, in order to boost the multifaceted scientifi research of magnesium alloys and promote the development and application of magnesium alloys globally.

Fig.1.Published Mg-related papers in the past 20 years in the Web of Science (WoS) Core Collection database.

2.Overview of Mg research in 2021

2.1.Overall status of Mg research

The published Mg-related papers in 2021 were searched in the Web of Science (WoS) Core Collection database on February 10, 2022.Fig.1 presents a simple search results in the past 20 years using ‘Magnesium or Mg alloy' as the topic(blue comlumn).To reveal more precisely the publications on Mg and Mg alloys, a more sophisticate retrieval strategy is applied.Briefl , ‘Mg alloy', ‘Magnesium hydrogen', ‘Magnesium battery', ‘Magnesium biodegradable', ‘Magnesium corrosion', ‘Magnesium mechanical' were used as topics with a certain rule in the WoS Core Collection database.After the duplicates are automatically eliminated, the results are shown with red comlumn in Fig.1.It is seen from both the simple and refine searches that the number of publications gradually increases from 2002 to 2020 and the number of publications in 2021 is slightly lower than that of 2020.The slight decrease in 2021 would mainly be due to the fact that some publications in 2021 have not yet been indexed in WoS by February 10, 2022.It is expected that the total number of publications in 2021 would be higher than that of 2020 in the WoS database.The gradual increase in publications illustrates that the research on Mg and Mg alloys is a hot spot in the fiel of materials science and engineering in the past 20 years and has attracted more and more attention.

Fig.2.Statistical analysis of the distribution of countries with at least 5 Mg papers published in 2021: (a) Paper percentage in different countries and regions; (b) network visualization among different countries.

With the refine or more precise retrieval, 4060 papers on Mg and Mg alloys in total were collected by February 10,2022.Statistical analysis was conducted via the VOSviewer software.The distributions of countries and regions and organizations that published Mg papers were analyzed on the basis of the above-mentioned literature.88 countries and regions in total have published Mg and Mg alloys in 2021.Fig.2 shows a statistical analysis result of the distribution of countries and regions with at least 5 Mg publications in 2021.Fig.2(a)shows a distribution of countries and regions, where Chinaremains the country that publishes the most Mg papers with a contribution of 40.38%, followed successively by India, the USA, Germany, and Japan, etc.Fig.2(b) shows the network visualization among different countries and regions.The size of circle represents the number of published papers while the width of the link lines among different countries and regions indicates the collaboration intensity.About 23.81% of Mg papers were published based on international collaborations in 2021.Intensive international collaborations are seen among China, the USA, Germany, Australia, England and India.

Fig.3.Statistical analysis of organizations publishing at least 15 Mg papers in 2021: (a) top 20 organizations; (b) network visualization among different organizations.

Fig.3 shows the statistical analysis result of organizations that published at least 15 Mg papers in 2021.The top 20 organizations are shown in Fig.3(a).Chongqing University has published 163 Mg papers, remains the top spot in 2021 like 2020, followed by Chinese Academy of Sciences, Shanghai Jiaotong University, Northeastern University, and Harbin Institute of Technology.Helmholtz Zentrum Hereon and Max Plank Society from Germany, University of Tehran from Iran,and University of Queensland from Australia are also positioned in the top 20 spots in 2021.Fig.3(b) shows the network visualization among different organizations.Similarly,the circle area or size represents the number of published papers, while the width of the link lines among different organizations indicates the collaboration activities.65.76% of Mg papers are published based on collaborations, which suggests that collaborations among different organizations can significantl accelerate Mg research.

2.2.Statistics and analysis of journals publishing Mg papers

According to the number of magnesium papers published in 2021, the top 20 journals are listed in Table 1.Journal ofMagnesium and Alloyspublishes the most papers, followed byMaterials Science and Engineering AandMaterials.The network visualization among different journals that published at least 10 Mg papers in 2021 is shown in Fig.4.Similarly,the wider links between two journals reveal more citations between them.The results indicate that the correlations among theJournal of Magnesium and Alloys, Materials Science and Engineering A, and Journal of Alloys and Compoundsare quite close.

Table 1Top 20 journals with Mg papers worldwide in 2021.

Fig.4.Network visualization among different journals published at least 10 Mg papers in 2021.

Fig.5.Statistical analysis of county (region) distribution of Mg papers published in the Journal of Magnesium and Alloys in 2021.

Journal of Magnesium and Alloys(JMA) published 161 papers in 2021, with an increase of 45% from 2020.The impact factor (IF) of JMA increases rapidly from 7.115 in 2019 to 10.088 in 2020, ranking No.1 among the 80 journals in Metallurgy & Metallurgical Engineering category (JCR Q1).Fig.5 shows the statistical analysis result of country (region)distribution of Mg papers published in the JMA in 2021.The number of collaborations among different institutions is 108,accounting for 67%.More than half of the articles in the JMA involved institutional collaborations, which is also similar to the whole trend.The statistical results confir that the JMA enjoys collaborative academic achievements.

2.3.Research hotspots in 2021 based on bibliometric analysis

The top 150 keywords by relevance, based on the Mg and Mg alloy articles published in 2021 are shown in Fig.6.The larger size of the circle reflect the more times of keywordsused.For instance, ‘Microstructure', ‘Mechanical properties',and ‘Corrosion' are the top three keywords, which implies that the microstructure, mechanical properties and corrosion of magnesium alloys continue to be the research hot areas.

Fig.6.Network visualization among different keywords in Mg-related papers.

In addition, the similar color of circles indicates a high relevance of the keywords.There are mainly four colors in Fig.6, i.e., red for ‘microstructure' and ‘mechanical properties' group, green for ‘corrosion' group, purple for ‘hydrogen storage' group, and blue for ‘magnesium battery'.The largest group is the red ‘microstructure' and ‘mechanical properties',suggests that the mechanical properties and microstructures of structural Mg alloys as well as their processing technologies in 2021 still attracted much attention in the R&D of Mg and Mg alloys.Research in the second largest green group of‘corrosion' is very near to the yellow ‘bio-Mg' group, indicating that bio-Mg alloys have attracted increasing attention and are closely related to the corrosion properties.

Interestingly, in each color group, the interconnected keywords are identifie as the sub-hot spots in each hot field as listed in Table 2.From the listed keywords, the sub-hot spots are revealed.

Table 2Keywords in each research group.

Table 3Mechanical properties of RE-containing cast magnesium alloys.

In short, the bibliometric analysis of keywords is capable of showing both the hot field and the hot spots in each hot field which could be used to guide the research directions and topics.Based on the bibliometric analysis results, the re-search field on Mg and Mg alloys could be generally grouped into four main categories: (1) traditional structural cast and wrought Mg alloys that focused mainly on microstructure and mechanical properties, (2) functional materials including Mg battery, hydrogen storage Mg materials, and bio-Mg materials, (3) processing technologies of Mg and Mg alloys, and(4) corrosion and protection of Mg and Mg alloys.

3.Structural Mg alloys

Light weighting of structural materials is one of the most efficien ways to save energy and reduce CO2emission,which has become a main strategy of most countries in the world.For instance, as stated in the ‘Energy-saving and new energy vehicle technology roadmap' released by China, the weight of vehicles should reduce 10% in 2020, 20% in 2025, and 35% in 2035, compared to that in 2015.The application of Mg structural materials provides a potential solution since the density of Mg alloys is the lowest among the metallic structural materials.Hence, investigation on structual Mg alloys is still a hot spot.

3.1.Cast Mg alloys

3.1.1.Rare-earth containing high strength cast Mg alloys

The mechanical properties of RE-containing cast Mg alloys developed in 2021 with an ultimate tensile strength (UTS)above 250 MPa are listed in Table 3 [21-28].

Guohua Wu's group [21]investigated the role of Gd on the mechanical properties of Mg-3Nd-4.5Gd-0.2Zn-0.5Zr alloy under permanent mold casting.The peak-aged Mg-3Nd-4.5Gd-0.2Zn-0.5Zr alloy exhibited a relatively good combination of strength and ductility (YS = 200 MPa,UTS = 343 MPa, (elongation) EL = 5.4%) due to the dense distribution ofβ′′phases with a higher aspect ratio.They also developed a permanent cast alloy Mg-9Gd-1Yb-0.5Zn-0.2Zr with a high YS of 229 MPa in the peak-aged condition[25].The high YS of this peak-aged alloy is attributed to the dense co-precipitation of thinγ′′plates and prismaticβ′andβ1precipitates.

In addition,some high pressure die cast(HPDC)Mg alloys were developed.Suming Zhu et al.[24]found that an addition of 0.3 wt.% Mn to AE44 (Mg-4Al-4RE) alloy resulted in the precipitation of nanoscale Al-Mn particles.The UTS, YS and EL of the T5 treated HPDC AE44-0.3Mn alloy were obtained to be 284 MPa, 192 MPa and 11.4%, respectively, which exhibited a better strength-ductility combination than most diecast Mg alloys and A380 alloy.Bai et al.[26]developed a novel HPDC WZA631 (Mg-6Y-3Zn-1Al) alloy showing YS,UTS, and EL of 175 MPa, 281 MPa, and 9.8%, respectively.The strength was enhanced due to the solute atoms Y and Zn into the matrix inhibiting the dynamic recovery by migrating dislocations, and the formed LPSO phase hindering dislocation movement along with increased dislocation storage.Furthermore, the coherent interface of LPSO phase with Mg matrix weakens the occurrence of interfacial cracking and twinning, thus promoting the ductility.

3.1.2.Rare earth-free high strength cast Mg alloys

Some newly developed RE-free cast Mg alloys were reported in 2021, with the mechanical properties of these alloys listed in Table 4.Cui et al.[29]found that Mg-6Al-4Zn-1.2Sn alloy exhibited a high UTS of 240 MPa, YS of 136 MPa and EL of 14%.The high UTS and EL were due to the low amount of shrinkage and the high YS was attributed to the decreased grain size.

3.1.3.Heat-resistant cast magnesium alloys

Mg alloy castings used in heat-resistant aerospace components have recently attracted a lot of attention.The mechanical properties of some newly-developed heat-resistant alloys are listed in Table 5.

Shouxun Ji's group [32]developed a new HPDC Mg-REAl alloy with superior mechanical properties at elevated temperatures.The YS of the alloy was 94 ± 1.8 MPa at 300 °C,which was 42% and 20% higher than that of AE44 and Mg-4 wt.% RE (La, Ce, Nd) at 300°C.The UTS of the Mg-RE-Al alloy was the highest among the three alloys at 150°C, 250°C and 300°C.The strengthening of Mg-RE-Al was attributed to the enhanced affinit of Nd and Gd to Al atoms.Jung et al.[33]reported that a T6-treated Mg-6.0Zn-1.2Y-0.7Zr-0.7Ca(ZWK611+0.7Ca)alloy showed the highest strength at 150 °C (the YS was 143 ± 2.6 MPa, the UTS was 179 ± 10.7 MPa and the EL was 8.2 ± 0.9%) and the lowest creep strain (0.22%) at 150°C/100 MPa among all the tested alloys.The addition of Ca led to the formation of uniformly distributedτ-(Ca2Mg6Zn3) phase and I-phase.Yang et al.[34]revealed that Mg-8Al-1.0Gd-1.0Nd (T6 heat treatment) exhibited an excellent tensile creep resistance which was superior to AZ91 alloy.The steady-state creep rate and creep strain of Mg-8Al-1.0Gd-1.0Nd at 150°C/60 MPa were 1.03 × 10-8s-1and 0.31%, respectively.

3.2.Wrought Mg alloys

3.2.1.Traditional commercial wrought Mg alloys

In 2021, many researchers used special processing technology to control the microstructure in order to enhance themechanical properties of commercial wrought magnesium alloys [35-37].Table 6 summarizes the mechanical properties of traditional commercial wrought magnesium alloys.

Table 4Mechanical properties of some RE-free cast Mg alloys.

Table 5Mechanical properties of some newly-developed heat-resistant Mg alloys.

Table 6Mechanical properties of traditional commercial wrought Mg alloys at room temperature reported in 2021.

Shan et al.[38]developed a technology of combining equal channel angular pressing (ECAP) with electropulsing treatment (EPT) to enhance the mechanical property of commercial AZ61 magnesium alloy.‘ECAP+EPT' technology can optimize the microstructure and largely improve the YS,UTS, EL of AZ61 alloy to 330 MPa, 448 MPa, and 15%,respectively.The AZ80 prepared by Zhang et al.[40]by multi-directional forging exhibited excellent mechanical performance.The UTS reached 402 MPa with the EL above 17%.In addition, Zou et al.[42]found that ZK60 alloy rods after radial forging at 300 °C showed a high tensile strength of 341 MPa and a high elongation of 27.1%.

3.2.2.High strength wrought Mg alloys

Table 7 summarizes the mechanical properties of ultra-high strength rare-earth based wrought magnesium alloys developed in 2021.

Adding rare earth elements is one of the effective methods to enhance the mechanical properties of wrought magnesium alloys [54-57].Ma et al.[43]found that with the addition of 0.5 wt.% La, the YS of Mg-9Gd-3Y-0.5La-0.5Zr can reach 480 MPa and the EL is close to 6%.The addition of La leads to more precipitates and results in fine grains.Su et al.[44]optimized Gd and Y content and obtained high mechanical properties.The extruded Mg-1.75Gd-0.75Y-0.5Zn-Mn (at.%, Mg-14.5Gd-2.3Y-1.1Zn-0.3Mn (wt.%)) alloy exhibited a high UTS of 520 MPa.The alloy with Y/Gd atomic ratio of 0.4 has high peak hardness and mechanical properties.Furthermore, the addition of Ag [49]and Sm [58]can also improve the mechanical properties of magnesium alloys.

The high strength of materials is often at the expense of plasticity, which is a well-known dilemma in materials science and engineering.Some researchers have developed materials with both high strength and good ductility.Zhen et al.[46]improved the comprehensive mechanical properties of Mg-9.5Gd-4Y-2.2Zn-0.5Zr (wt.%) alloy through an alternating aging process,so that its YS reaches 425 MPa and its UTS reaches 493 MPa, and the elongation (EL) is 11.2%.Furthermore, Tong et al.[47]obtained Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr(wt.%) alloy with ultra-high yield strength (417 MPa) and high plasticity (12.9%) by multi-directional forging (MDF)and aging treatment.Li et al.[48]achieved a simultaneous increase in the strength and plasticity of the extruded Mg-13Gd alloy through aging treatment.The YS of 400 MPa and EL of 15% are mainly attributed to the formation of high-density nano-sizedβ' precipitates and a certain proportion of precipitation-free areas in the structure dominated by dynamic recrystallization and fin grains.S.Kamado's group[50]developed a Mg-8.0Gd-4.0Y-1.0Mn-0.4Sc alloy with an UTS of 425 MPa and an EL of 10.6%.

Rare earth elements can effectively improve the mechanical properties of wrought magnesium alloys, but the cost is generally very high.Some researchers have also developed wrought magnesium alloys with low-cost and high performance, which are listed in Table 8.

Table 7Mechanical properties of high strength rare-earth wrought Mg alloys at room temperature developed in 2021.

Table 8Mechanical properties of high strength and low-cost wrought Mg alloys at room temperature reported in 2021.

Hucheng Pan's group[59]developed an ultra-high strength wrought Mg-1Ca-1Al-0.3Zn-0.4Mn (wt.%) alloy.The YS,UTS, and EL of this alloy through conventional extrusion reach 435 MPa, 449 MPa, and 4.2%, respectively.This is mainly because the nano-precipitates are formed, i.e., a large number of nanoscale second phase precipitates can be observed.Dingfei Zhang's group [60]found that during the aging treatment, the addition of Sn refine the precipitates and increases the density of the precipitates.The YS, UTS,and EL of the peak-aged Mg-6Zn-1Mn-2Sn-0.5Ca alloy can reach 379 MPa, 407 MPa and 7.5%, respectively.Yan et al.[62]prepared Mg-5Zn-xSr alloy (x= 0, 0.2, 0.6, 1.0 wt.%)by high strain rate rolling.The rolled Mg-5Zn-0.6Sr alloy has the best UTS of 359 MPa and EL of 20%.

3.2.3.High plasticity wrought Mg alloys

Magnesium alloy with hexagonal close-packed(HCP)crystal structure does not have sufficien independent slip systems and thus exhibits poor ductility.Recently, there have been many reports on microalloying [64,65]or heat treatment to obtain high-plasticity magnesium alloys [66-69].Table 9 summarizes the mechanical properties of typical high plasticity magnesium alloys developed in 2021.

Bin Jiang' group [70]found that extruded Mg-4Gd-0.5Zr-0.5Nd alloy showed a plasticity of 42.3%, Mg-4Gd-0.5Zr-1.5Nd alloy has excellent strength of 224 MPa and plasticity of 39.3%.Mg-4Zn-1Gd alloy developed by K.S.Shin's group [71]exhibited good comprehensive mechanical properties with a UTS of 336 MPa and an EL of 33.9%.Due to the low content of RE,this Mg-4Zn-1Gd alloy shows good industrialization potential.Xiaoqin Zeng's group [73]developed a RE free Mg-1.8Zn-0.2Ca alloy exhibited high elongation of～30% and strength of 265 MPa.The Mg-1Gd-0.5Zn-0.3Ce alloy developed by Bin Jiang's group [72]and the Mg-2Zn-0.3Ca-0.2Ce-0.1Mn (ZXEM2000) alloy developed by Zhao et al.[75]have high plasticity of about 30%.In addition, The high-plasticity Mg-Zn-Ca-Mn alloys were developed through adding Ca and Mn elements into Mg-Zn alloy by Xianhua Chen's group [74].It was found that Mg-4Zn-0.3Ca-0.2Mn alloy exhibited the highest EL of 30%, Mg-4Zn-0.3Ca-0.7Mn alloy has excellent strength of 289 MPa and plasticity of 26%.

More slip systems can be activated in Mg alloys at high temperature than at room temperature.Thus, some Mg alloys with superplasticity were reported in 2021.Table 10 summarizes the developed wrought Mg alloys with superplasticity at high temperatures in 2021.Sun et al.[83]reported the highest plasticity of 782% of peak aged Mg-10Gd-3Y-1.5Zn-1Zr(wt.%) alloy at 450 °C at a strain rate of 5 × 10-3s-1.The precipitation of the new 14H LPSO phase and Mg24Y5delays the separation of grain boundaries and improves the bearing capacity of strain accumulation,which accounts for the superplasticity.Malik Abdul et al.[84]studied the superplasticity of fine-graine extruded ZK61 alloy.At a tensile strain rate of 1 × 10-3s-1and a temperature of 673 K, an elongation at break (FE) of 400% can be obtained.Liu et al.[85]studied the effect of the isothermal repeated upsetting and extrusion process (RUE) on the superplasticity of ZK60 magnesium alloy.The optimized superplasticity is 142% at 653 K.Chen et al.[86]found that the two-phase Mg-Li alloy exhibits superplasticity of 307% at 623 K.

3.2.4.Superlight wrought Mg alloys

Ultra-light wrought magnesium alloys are mainly magnesium-lithium alloys with a density of 1.4-1.65 g/cm3.Table 11 summarizes the mechanical properties of the ultra-light wrought magnesium-lithium alloy developed in 2021.

Table 9The mechanical properties of typical high plasticity wrought Mg alloys at room temperature reported in 2021.

Table 10The mechanical properties of wrought Mg alloys with superplasticity at high temperatures reported in 2021.

Table 11Mechanical properties of the superlight wrought Mg-Li alloys developed in 2021.

Michael Ferry's group[87]prepared a body-centered cubic Mg-14Li-7Al alloy with high specifi strength of ～350 kN m kg-1by cast +quenching.They proposed a spinodal decomposition strengthening mechnism for ultralightweight Mg alloy with specifi yield strengths surpassing almost every other engineering alloy.The compelling morphological, chemical,structural, and thermodynamic evidence for the spinodal decomposition were provided.Yang et al.[88]used rotary die forging to introduce a large number of twins and stacking faults into the coarse grains.The bulk Mg-4Li-3Al-3 Zn alloy with a UTS of 409 MPa has been successfully prepared.Cao et al.[89]developed a new Mg-2.76Li-3Al-2.6Zn-0.39Y alloy by multi-directional forging (MDF) and rolling process,which also has excellent mechanical properties.Ruizhi Wu's group[90]prepared Mg-14Li-xSn alloys and reported that the strength and ductility of Mg-14Li-6Sn alloy after 50% cold rolling have been improved.

3.2.5.Laminated composite sheets of wrought Mg alloys

In order to overcome some shortcomings of magnesium alloys, such as low mechanical properties, low formability at room temperature and corrosion resistance, laminated composites of magnesium alloy have been developed greatly in recent years.Table 12 presents the mechanical properties of laminated composites of Mg alloys reported in 2021.

Table 12Mechanical properties of the mechanical properties of laminated composites of Mg alloys reported in 2021.

Wang et al.[91]fabricated Ti6Al4V/AA6061/AZ31 laminated composites by hot rolling.It was found that the UTS and YS of the Ti/Al/Mg laminated composites increased,and the elongation decreased with increasing rolling reduction.The highest UTS of 476 MPa was achieved when the rolling reduction was 50%.When a rolling temperature of 350°C was employed, comprehensive tensile properties(UTS = 384 MPa, elongation = 15.5%) of AZ31/5052 laminated composites were achieved[93].AZX611[94]and ZK60[95]were also used to prepare laminated composites and good mechanical properties were obtained.In addition, laminated composite sheets of the same magnesium alloy were also developed to improve the mechanical properties.For example,Wei et al.[97]prepared four-layer Mg-14Li-3Al-2Gd sheets by accumulative roll bonding (ARB).With increasing ARB paths, both the strength and interfacial bonding strength increased, and the elongation decreased constantly.Bai et al.[99]investigated the atomic diffusion at the interface during the extrusion bonding of pure magnesium and Mg-Al-Zn-RE alloy.The results confirme that atomic diffusion indeed occurred across the interface during extrusion.The gradients inelement concentration, local stresses, and hydrostatic pressure were considered to be the necessary conditions for extensive atomic diffusion to occur.

3.3.Microstrutures

Guohua Wu's group [100]successfully refine the grain size of GW83 from 832μm to 229μm by applying ZrCl4instead of Zr.They verifie the feasibility of ZrCl4alloying at low temperature for grain refinemen of sand-cast GW83 alloy.Shu-Qing Yang et al.[101]found that the adding of MgAl2O4+ Ca has better refinin effect on Mg-3 wt.%Al alloy than adding MgAl2O4only.The grain size of Mg-3 wt.%Al was hugely refine from 660 ± 55μm to 178 ± 12μm by adding of 2 wt.% MgAl2O4and 0.2 wt.% Ca, due to Ca significantl promotes Mg adsorption on MgAl2O4(110) surface.

Yuansheng Yang's group [102,103]investigated the microstructure and microsegregation evolution of Mg-6Al-4Zn-1.2Sn (wt.%) cast magnesium alloy with sub-rapidly solidification which provides essential information for the application of Mg alloy in the thin-wall castings.It is found that the microsegregation of Al, Zn, and Sn in the alloy decreases with the increase in cooling rate.

HAADF-STEM was extentively used to reveal the precipitation behavior of Mg alloys.Guohua Wu's group [21]revealed that the matrix precipitation of Mg-3Nd-4.5Gd-0.2Zn-0.5Zr was dense distribution ofβ′′and fewβ′.The EDS results indicated that Gd incorporated intoβ"precipitates and is likely to be substituting for Nd atoms, leading to the strongly enhanced precipitation kinetics and greatly augmented volume fraction of p" phase.Therefore, the dense distribution ofβ′′phases with higher aspect radio lead to substantial enhancements of alloy strength.In the case of peak aged Mg-9Gd-1Yb-0.5Zn-0.2Zr alloy, there was a specifi mutually perpendicular dense distribution between the fine-scal basalγ′′and prismaticβ′andβ1phases [104].The dense co-precipitation of thinγ′′plates and prismaticβ′andβ1precipitates with different contrasts lead to high YS of Mg-9Gd-1Yb-0.5Zn-0.2Zr alloy.Mingzhe Bian's group [105]revealed that the heterogeneously nucleated AgMg4at the grain boundaries resulted in microvoid formation in the general high misorientation angle grain boundaries.Therefore, the ductility in precipitation hardenable Mg-12Ag-0.1Ca alloy decreased significantl.

To sum up, much progress has been made in cast and wrought Mg alloys in 2021 compare to that in 2020 [106].In the case of high strength RE containing cast Mg alloys,the UTS, YS, EL of Mg-Nd-Gd-Zn-Zr alloy developed in 2021 was reported as 343 MPa, 200 MPa, and 5.4%.The mechanical properties are comparable to those of Mg-6Gd-3Y-0.5Zr alloy reported in 2020 with an UTS of 340 MPa and EL of 6.2% [106].However, as reviewed by Song et al.[107], Chongqing University has reported a permanent mold cast Mg-10Gd-2Y-1Zn-0.5Zr alloy which exhibited an UTS of 351 MPa and an EL of 10.2%.Due to such a high content of RE, the high cost of Mg-10Gd-2Y-1Zn-0.5Zr alloy limits its wide application to some extent.In the case of HPDC Mg alloys, the UTS of Mg-4Al-4RE-0.3Mn alloy developed in 2021 exceeded 280 MPa, and the elongation maintained at 11%[24,26].The UTS and EL of HPDC Mg-4Al-3La-0.3Mn-2Gd alloy developed in 2020 were reported as 284 MPa and 14% [106], which is slightly higher due to the higher RE content.

In the case of commercial wrought Mg alloys, excellent mechanical properties (UTS= 448 MPa, EL=15%) of the commercial AZ61 alloys achieved by ‘ECAP+EPT' technology were reported in 2021.It is of great importance to commercialize such technology for the industrialization of Mg and Mg alloys.In the case of ultra-high strength RE containing wrought Mg alloys in 2021, the highest UTS was 520 MPa obtained for extruded and aged Mg-14.5Gd-2.3Y-1.1Zn-0.3Mn alloy.However, it was reported that the highest UTS of wrought Mg-8Gd-3Y-0.4Zr alloy developed in 2020 was 710 MPa, which was prepared by rotary swaging technology and aging.In the case of low cost wrought Mg alloys in 2021, the highest UTS reaches 449 MPa for a Mg-1Ca-1Al-0.3Zn-0.4Mn alloy, the EL is 4.2%.In 2020, the same research group reported a similar low-cost Mg-1.0Ca-1.0Al-0.2Zn-0.1Mn alloy having an UTS of 425 MPa and an EL of 11%.Thus, the enhanced UTS is at the expense of EL.In addition, a new spinodal decomposition strengthening mechanism was reported in ultralight Mg-14Li-7Al alloy in 2021.

In the future, low-cost and high performance cast and wrought Mg alloys with high industrilization potential are still in demand.

4.Functional Mg materials

In addition to the high specifi strength and stiffness of structural Mg alloys, Mg and Mg alloys also exhibited good biocompatibility, large hydrogen storage capacity, and high theoretical specifi capacity for battery, damping properties,fast dissolve properties, electromagnetic shielding properties.Consequently, Mg and Mg alloys have great potential in energetic and biomedical applications.However, the fast degradability of bio-Mg alloys presents the biggest challenge for the biomedical application.The massive application of Mg based hydrogen materials was mainly restricted by the high thermodynamic stability and poor kinetics of hydrogen absorption and desorption.In the case of rechargeable Mg batteries(RMBs), although Mg has ultrahigh theoretical capacity and high safety, RMBs still fall short of their potential meritdue to the lack of practical electrolytes and cathodes [108].

4.1.Bio-magnesium alloys

Mg alloy as a type of metallic biomaterials possesses the advantage of degradability, but its degradation process produces various complex changes in the material and the organism [109,110].The development of new bio-Mg alloys has consistently been a research hotspot.Investigation on bio-Mg alloys mainly focuses on improving the biodegradation or biological responses via alloying and surface coating.

Alloying elements, such as Zn, Ca, Cu, Mn, and Zr, with good biocompatibility are the primary considerations in the composition design of Mg alloys [111-114].Thein-vitroresults show that Mg-5Sn-4In alloy has a minimum corrosion rate of 0.36 mm/y [115].Bulk amorphous materials developed in the Mg-Zn-Ca system also show good mechanical properties, with strengths reaching 700 MPa [116].Rare earth elements Y, Ce, Pr, Gd, Dy, Yb, Sm, and Eu were evaluated before being used as biomedical Mg alloys [117].Sc appears to be suitable from a biological perspective.Given the paucity of data, Tb and Ho may have some potential, the applicability of Tm, Lu, and Er remains unclear.Further investigations are needed to determine the physiological effect of each rare earth element.

Surface modification including coating, chemical transformation, and surface treatment, is applied to control the biodegradation of bio-Mg alloys.For example,MAO/CS composite coatings have been generated on the surface of Mg-1.75Zn-0.56Ca alloys to protect the matrix and improve biocompatibility in physiological environments [118].Biocompatible Ti has also been coated on the Mg surface via the physical vapor deposition (PVD) technique to improve the corrosion resistance.nontoxic, non-allergic,βTi-29Nb-13Ta-4.6Zr (TNTZ) has been deposited on pure Mg and AZ31 Mg alloys [119],in-vitrocorrosion tests show that theEcorrof the coated sample is about 400 mV, being higher than that of the uncoated Mg sample.

In terms of biological responses, only a few studies have examined the metabolism of typical alloying elements in living organisms, whereas numerous studies have been conducted on the implantation of Mg alloys in animals.For example, Willumeit-R?mer Regine's group [120]evaluated Mg-10Gd alloy in implantation and conducted in-vivo fluores cence molecular tomography to measure the initial increase in the fluorescenc signal at the implantation site.The results revealed cellular stress at the implantation site, and the signal did not diminish until day 42.The Mg-10Gd alloy affected the connective tissue on the implant side.

In addition to the regular improvement of biodegradation of bio-Mg alloys,the new Resoloy rare earth Mg-Dy alloy for bioresorbable vascular implant is developed [121]as an alternative to WE43 scaffolds developed by Biotronik, Inc., which received CE approval in 2016.In-vitrodegradation tests have shown that the combination of MgF2passivation and PLLA surface coating on the Resoloy Mg backbone results in slower degradation and longer support times (40+ days), compared with WE43 alloy.This findin sheds light on the industrialization of bio-Mg alloys for the bioresorbable vascular implant application.Hess et al.[122]revealed the anti-tumor mechanism of extracellular Mg2+, Mg2+enhances T cell effect or function by raising LFA-1 outside-in signaling, thereby increasing the efficien y of T cells immunotherapies.

4.2.Mg battery

4.2.1.Mg-air battery

Magnesium has been considered as a good anode material for metal-air batteries because of its high specifi capacity(2200 mAh), high discharge voltage (3.03 V), relatively lowcost and abundant global reserves.In 2021, the anode materials of magnesium air battery mainly include the following magnesium alloys, such as AZ31, Mg-Li, Mg-Bi, Mg-Ca-Zn,Mg-Gd-Zn alloys, etc.The average discharge voltage and anode efficien y of these alloys are listed in Table 13.Among these alloys, Mg-0.7Sn-1.4Y alloy shows the highest anode efficien y of 53.1% at a current of 10 mA·cm-2.

Table 13Average discharge voltage and anode efficien y of Mg alloys for Mg-air battery.

The discharge performance of magnesium-air battery is mainly improved by microalloying and processing modifica tion.In Wu's work [123], the discharge characteristics of twoα-Mg-based lithium alloys(LAZ131 and LAZ531)were studied.It was observed that grain boundaries are easier to be corroded than grains, thus accelerating the corrosion process of alloy materials.Jiang et al.[127]adopted a powder metallurgy technique to prepare a modifie AZ31 anode with extremely fin grains (667 ± 291 nm).Compared with the coarse AZ31 alloy with a grain size of 473 ± 154μm, the modifie AZ31 alloy shows significantl higher activity and higher capacity as the anode material of magnesium-air battery during discharge.In addition,Chen et al.[124]found that the discharge voltage and anode efficien y of magnesium-air battery with extruded Mg-xBi (x= 0.5, 1.0, 2.0 wt.%) as anode decrease with increasing bismuth alloying element underconstant current discharge.The magnesium-air battery shows good anode discharge activity and stable discharge process when the anode material is Mg-0.5Bi due to the thin and loose fil of discharge products.

In terms of cathode material of magnesium-air battery,Dong et al.[128]synthesized copper disulfid nanosheets by one-step hydrothermal method.The half-wave potential of CuMnO2was achieved at 0.63 V, along with a limiting diffusion current density of 5.60 mA·cm-2at 1600 rpm in 0.1 M KOH electrolyte.Besides, the half-wave potential shows only a small change of 22 mV after 5000 CV cycles,suggesting the outstanding stability and durability.Liu et al.[129]found that MnO2/reduced graphene oxide (rGO) layered hybrids have a high BET surface area for synergistic effects on the introduction of rGO and the morphological transformation of anchor MnO2.

In the case of electrolyte, Leong et al.proved that the bielectrolyte acid salts can significantl improve the voltage and power performance of magnesium air batteries [130], which provides spread of electrochemical windows and a decrease in passivation, doubling the peak power density and having an open voltage of 46%.In Ye's work, Mg-air batteries (pure Mg as an anode material) with ultrahigh average specifi capacity of 2190 mAhg-1and high energy density of 2282 Wh kg-1had been achieved by designing a dual-layer gel electrolyte.Organic gel protects magnesium anode from corrosion, and chloride ions in hydrogel are helpful to produce unique needle-like discharge products, instead of the dense passivation layer commonly reported [131].

4.2.2.Rechargeable Mg batteries

Rechargeable Mg batteries (RMBs) are considered as highly promising candidates for next-generation large-scale energy storage systems, owing to the ultrahigh theoretical capacity (3833 mAh cm-3or 2205 mAh g-1) of Mg having the abundant resource, and its high safety (no obvious dendrite morphology during Mg plating process) [132].However,although tremendous breakthroughs (including free-chlorine electrolyte designs, and the development of high-capacity conversion-type cathodes, availably artificia solid electrolyte interphase (SEI) on Mg metal (Fig.7)) were obtained two decades ago, RMBs still fall short of their potential merit[108].The main bottlenecks still originated from a lack of practical electrolytes and cathodes that would enable high energy density and power density [133].

(1) Anode of rechargeable Mg batteries

The Mg metal is currently an ideal anode for RMBs, owing to the high theoretical mass and volume capacities, and low redox potential(-2.37 V vs.H+/H).But the formation of passive layers on the surface of the Mg anode in most conventional electrolytes (e.g., Mg(ClO4)2, Mg(PF6)2or Mg(TFSI)2salts in carbonate or ether solvents) restricts the reversible plating/stripping of Mg2+.Thus, some effective strategies,such as designing the protective layer on Mg metal, developing the Mg alloy anodes, or searching the insertion-type or conversion-type metal compound anodes, have been proposed to promote the development of RMBs anodes [134].

Zhao et al.designed a bismuth (Bi)-based artificia protective layer on Mg metal by the displacement reaction between Mg and BiCl3in solution [135].The Mg-Mg symmetric cell assembled by the modifie Mg anode exhibited a relatively lower overpotential of ～0.6 V and maintained cycling stability over 4000 h in Mg(TFSI)2/DME electrolyte.Zhang et al.prepared an artificia interlayer made of amorphous MgCl2@polymer on Mg metal surface byin-situchemical reaction of metallic Mg with H3PO4and SiCl4in sequence[136], the corresponding symmetric Mg cell exhibited a low overpotential of ～0.25 V and stable cycles over 700 h in Mg(TFSI)2/DME electrolyte.Dou et al.constructed a “MgF2-rich” SEI on Mg metal surface with high electronic insulation though the electrolyte modificatio [137].The symmetric cell exhibited superior cycling performances of over 1150 h with low polarization.In addition, introducing the sodium cations[138]or Mg(BH4)2[139]in Mg-based electrolytes could also decrease the interface impedance and reduce the passive layer.

The deposition behavior of Mg is still controversial.Most researchers demonstrated that there was no Mg dendrite formation during Mg plating/stripping process in most conventional electrolytes.Recently, Kwak et al.observed that the spherical Mg seeds covered the substrate in APC electrolytes at the relatively low rate of 2 mA cm-2[140].When current density was increased to 10 mA cm-2, needle-like dendritic growth could be observed.Moreover, they introduced Au magnesiophilic sites in the substrate, which effectively suppressed the Mg dendritic growth.Bae et al.designed an amorphous MgO-wrapped Zn-skeleton as a unique current collector for anode-free Mg battery [141].This MgO/Zn interlayer behaves as a mixed ionic-electronic conductor, ren-dering uniform Mg nanoparticles upon electroplating.It also improved the charge transfer kinetics and lowered the cell impedance.Song et al.proposed vertically aligned N- and Odoped carbon nanofibe arrays on carbon cloth(VNCA@C)as current collector for Mg battery [142].The evenly nanoarray was favorable to homogenize the surface current density, and the microchannels built in this 3D host promoted the uniform nucleation of Mg.Under a high current density of 10 mA cm-2, the carbon host delivered a reduced nucleation overpotential of 429 mV and an elongated Mg plating/stripping cycle life (110 cycles).

Fig.7.Current design strategies for rechargeable Mg-based batteries [108].

The alloy-type anodes usually avoid the passivation reaction from the conventional electrolytes, but the huge volume expansion/shrinkage during Mg2+alloying and de-alloying leads to a serious capacity fading.Song et al.focused on liquid gallium (Ga) electrodes owing to the self-healing properties [143].In order to improve the poor wettability of liquid Ga on stainless steel (SS) mesh, they constructed a CuGa2layer on the surface of SS mesh and the wettability of liquid Ga was significantl enhanced.The liquid Ga electrode also exhibited good compatibility with Mg(TFSI)2.

(2) Cathode of rechargeable Mg batteries

For cathode, the high-polarization of Mg2+leads to strong electrostatic interactions between Mg2+and anion lattices,further resulting in serious voltage polarization and low magnesiation degree [144].In 2021, some high-performance sulfid or selenide cathode materials were reported, as listed in Table 14.Among these materials, the reversible specifi capacity of defect-rich Cu7.2S4nanotubes could reach 314 mAh g-1at 0.1 A g-1[145].Meanwhile,it also exhibited excellent rate capability of 91.7 mAh g-1at 1.0 A g-1and cycling stability over 1600 cycles with capacity decay of 0.0109% per cycle at 1.0 A g-1.Shen et al.reported a high-performance FeS2cathode using a copper current collector [146].They found that the formation of copper nanowires during discharge could activate the redox reactions of FeS2to realize the fourelectron transfer, thus delivering a significantl enhanced reversible capacity of 679 mAh g-1at 50 mAh g-1.

Table 14Summary of the electrochemical performance of cathode materials for RMBs reported in 2021.

Table 15Information on nanostructured systems.

Table 16Information on catalyst doping systems.

The high-voltage RMBs cathode is of great interest but rarely reported.Fluorophosphates with the inserted magnesium ion channels and high discharge voltages (＞3 V) have been studied to some extent [155].Rubio et al.[154]found that Na3V(PO4)2F2cathode could perform the reversible multi-electron storage through the V4+/V3+and V5+/V4+redox couples for RMBs, thus the specifi capacity of 136 mAh g-1could be obtained.

(3) Electrolyte for rechargeable Mg batteries

The chloride-free magnesium battery electrolyte with wide electrochemical stable windows, high ionic conductivity, and good compatibility with electrode materials are significan for developing high-performance RMBs.

Sun et al.introduced an effective additive, reduced perylene diimide-ethylene diamine (rPDI), in simple Mg(TFSI)2/DME electrolyte [156].A high power den-sity (2.0 mW cm-2) and a stable cycle life (＞200 cycles)in an Mg-organic full cell could be achieved.Horia et al.used tetrabutylammonium borohydride as a moisture scavenger to control the moisture content in chloride-free Mg(HMDS)2/DME electrolyte [157].This electrolyte demonstrated a low polarization voltage of 0.25 V over 1000 cycles,an average coulombic efficien y of 98.3% over 150 cycles in asymmetrical cell and no obvious corrosive reaction to current collectors.

Fig.8.(a) The molecular structure and denticity of the methoxyethyl-amine chelants.The overpotentials at the 10th cycle of Mg||Mg cells at (b) 1.5 mA cm-2 for 1.5 mAh cm-2 and (c) 0.1 mA cm-2 for 0.1 mAh cm-2.(d) CEs of Mg//SS cells cycled at 0.1 mA cm-2.(e) Long-term cycling of the Mg//SS cell.(f)Scheme of solvated Mg2+ and electron transfer in Mg0.15MnO2 host.(g) Charge-discharge curve of Mg//Mg0.15MnO2 cell at 0.5 C.(h) Cycling performance and CE of Mg//Mg0.15MnO2 at 0.5 C [158].

Very interestingly, Hou et al.found a family of methoxyethyl-amine chelants (Fig.8a) that could tune solvation structure of simple Mg(TFSI)2/DME electrolyte, improving the interfacial charge transfer kinetics and suppressing the passivation reaction [158].This electrolyte demonstrated low polarization voltage of ～0.1 V(Fig.8b,c)and a high coulombic efficien y of over 99.5% (Fig.8d, e).Moreover, this electrolyte system provided an excellent compatibility for layered Mg0.15MnO2cathode, enabling a fast diffusion kinetics of the solvated Mg2+in the interlamination (Fig.8f).Thus, a high average discharge voltage of ～2.3 V and a reversible capacity of 190 mAh g-1after 200 cycles in Mg//Mg0.15MnO2cell could be achieved (Fig.8g, h).

The high-voltage boron-based electrolytes also exhibit large potential in RMBs due to high oxidation stability, high ionic conductivity, and weak corrosion to current collectors [159].But the complex synthesis process and expensive raw materials hinder their widespread application.Ren et al.reported a magnesium tetra(trifluoroethanoloxy)borat(Mg[B(Otfe)4]2) electrolye by two methods of microcrystal redissolution andin-situreaction [160].First, a stoichiomet-ric reaction between Mg(Bu)2and BH3·THF was performed with 2,2,2-trifluoroethano (HOtfe) under ambient conditions.Then, the Otfe- from Mg(Otfe)2was subsequently separated and transferred to B(Otfe)3via the transmetalation reaction,generating Mg-B complex product separated by solvent vacuum extraction.This electrolyte exhibited an average coulombic efficien y of ～99% and low overpotential of 0.2 V and an oxidation stability over 3 V vs.Mg2+/Mg at SS.

(4) Mg-S batteries

Magnesium-sulfur batteries with high mass/volumetric energy density, great safety and low cost have been considered as one of most potential RMBs [161].Nowadays, these issues including insulating nature of elemental sulfur, sluggish redox kinetics between S and Mg2+, severe volume changes,low conductivity and low utilization of polysulfides etc., still extremely limit the electrochemical performance [162].The results reported to date are still far from the expected performance indicators.

In order to improve the redox kinetics, Xu et al.introduced the Ag current collector in S cathode [163].The composited S cathode delivered an improved specifi capacity of ～1200 mAh g-1and a long cycling life of 100 cycles.Zhao et al.designed a Co3S4@MXene heterostructure sulfur host to improve the electrochemical redox kinetics [164].The Co3S4@MXene-S cathode could display a high sulfur utilization with a specifi capacity of 1220 mAh g-1and retain a capacity of 528 mAh g-1after 100 cycles.

The sulfur conversion reaction is also associated with the properties of electrolyte solvent, such as permittivity, viscosity and chemical stability.Zou et al.found that the Mg-S reactions in the commonly used DME-based electrolyte was“solid-solid” reactions due to the low polysulfid solubility of MgSxin DME, thus leading to sluggish kinetics and poor reversibility[165].The polysulfid solubility could be increased by using high-donor-number solvents (e.g., dimethyl sulfoxide, dimethyl formamide), which increases the discharge capacity from 660 to ～1500 mAh g-1and decreases the sulfur overpotential from＞600 to ～200 mV at 0.1 C.This work provided critical insights into the electrolyte design for Mg-S batteries.In addition, Mg-S batteries suffer from serious self-discharge behavior.Ford and Schaefer et al.revealed that the decreased self-discharge capacity could reach to 32%of the firs cycle discharge capacity in 0.5 M MgFPB/DEG electrolyte [166].In response, preventing contacts of S8and polysulfid with the Mg anode will prevent the firs step of self-discharge.The electrolyte modification the optimization of sulfur host and artificia Mg SEI design may be effective to improve this issue.

In 2021, the practical matching between the available cathode, anode and electrolyte, and the manufacture of largecapacity soft-package RMBs are still absent.Chongqing University successfully produced the soft-package RMBs as a result of extensive fundamental studies on high-capacity sulfide/oxid cathodes, magnesium alloy anodes and low-cost electrolytes.It is expected to facilitate the commercialization of RMBs and the revolution of energy storage market.

4.3.Hydrogen storage Mg materials

Among many metal hydrides, Mg-based hydrogen storage materials have attracted wide attention due to the high theoretical hydrogen storage density of 7.6 wt.% and bulk hydrogen storage density of 110 kg/L.However, due to the high thermodynamic stability and poor kinetics of hydrogen absorption and desorption, it is difficul to apply Mg-based hydrogen storage materials on a large scale commercially.To this end,researchers around the world continue to investigate on alloying, nanostructuring, mixing with additives and changing reaction path and so on.

The first-principl calculations are commonly used to predict the structure and properties of materials and explore the mechanism of hydrogen storage.Ding et al.[167]found that Mg14Ti4H36system shows the best hydrogen storage performance in complex indexes such as substitution energy, desorption temperature and bulk modulus.Magnesium hydrides can obtain excellent hydrogen storage performance through the substitution of transition metal elements and adjustment of substitution concentration and configuration which provide a new idea for the performance improvement of Mg-based hydrogen storage materials.

Alloying, as the most effective method to improve the absorption/desorption performance of Mg-based hydrogen storage materials, is usually optimized by adjusting the internal structure of the alloy.As a common means of alloying modification element substitution has been widely concerned by researchers.For example, Zhang et al.[168]studied the hydrogen storage performance of Mg98Ni2-xLaxalloy (x= 0,0.33, 0.67 and 1) in which lanthanum replaced part of nickel.He et al.[169]investigated the hydrogen storage properties of La10-xRexMg80Ni10(x= 0 or 3; RE = Sm or Ce) alloys, which was prepared by vacuum induction casting with elements Ce and Sm replacing part of lanthanum.It has been proved that the thermodynamics and kinetics of hydrogen storage in Mg-based hydrogen storage alloys have been greatly improved by the element substitution.

In recent years, some studies have shown that the existence of LPSO phase in Mg-based hydrogen storage alloys will significantl affect the hydrogen storage properties.Nano-sized rare earth hydrides (REHx) generated byin-situdecomposition of LPSO phase can be evenly distributed at the grain boundary of Mg matrix, thus effectively reducing the catalytic dead zone and improving the catalytic effi ciency.In addition, these nanosized REHx hydrides can also be fi ed on the grain boundaries of Mg nanocrystals to inhibit the growth of Mg/MgH2grains, thus improving the stability of the nanophase structure.Mao et al.[170]studied the hydrogen storage performance and catalytic mechanism of Mg98.5Gd0.5Y0.5Zn0.5by adding Gd to Mg98.5Y1Zn0.5alloy.Thein-situgenerated RE(Gd/Y)Hx(x=2,3)nano-hydride by the LPSO phase decomposition can not only promote the hydrogen absorption/desorption of Mg matrix asin-situcat-alysts, but also inhibit the growth of Mg/ MgH2grains via pinning effect during hydrogenation and dehydrogenation.

In addition, besides the conventional casting method to obtain Mg-based hydrogen storage alloy, researchers have also developed other special preparation methods, such as DC arc plasma method [171], mechanical alloying [172], and melt spinning method [173]and so on.The alloys obtained by these special preparation processes not only solve the problem that cannot be prepared by conventional casting methods,but also have special structure, which opens a new fiel of study on the hydrogen storage properties of Mg-based hydrogen storage alloys.

Nanostructuring and catalyst doping, as the most rapid and effective methods to improve the dynamic properties of Mgbased hydrogen storage materials, have also attracted the attention of researchers all over the world.Tables 15 and 16 list some relevant information of nanostructures and catalyst doping systems, respectively, reported in recent years.Studies have shown that nanostructures can have profound effects on the hydrogen storage properties of Mg-based hydrogen storage materials by increasing the specifi surface area,enriching grain boundaries/defects, and shortening hydrogen transport paths.Xin Zhang's team[174]successfully prepared ultrafine sized MgH2nanoparticles with a major size of 4-5 nm for the firs time through a novel metathesis process of liquid-solid phase driven by ultrasound, and achieved 6.7 wt.% reversible storage of hydrogen at 30°C.

In recent years, a family of two-dimensional layered MXenes materials has shown great potential in catalysis, sensors,conversion, energy storage, gas absorption and electronic devices due to their layered structure, relatively large surface area, significan chemical durability and high electrical conductivity.Therefore, MXenes materials came into the attention of researchers of Mg-based hydrogen storage materials.They mixed the MXenes materials into MgH2by ball milling, and greatly improved the hydrogen absorption and desorption performance of MgH2.For example, Zhihui Tian et al.[64]mixed 5 wt.% Ti3C2into MgH2, which not only led to a significan reduction in activation energy of MgH2(ΔH = 57.5 kJ/mol), but also achieved hydrogen desorption of 4.8 wt.% within 10 min at 300°C.Based on this result, Hou et al.[178]successfully realized Ni particles loaded on Ti3C2by hydrothermal method, which further improved the hydrogen absorption/desorption kinetics of MgH2.The MgH2+5 wt.% Ni/Ti3C2-WE can desorb 6.25 wt.% hydrogen within 600 s at 275°C and absorb 4.55 wt.%hydrogen within 1200 s at 100°C.Obviously, the full contact between Ni nanoparticles and Ti3C2matrix and the rich electronic interaction brought by the interface can significantl improve the ab/desorption performance of MgH2.In addition, the electron transfer and the unique structure of Ni/Ti3C2-WE in multivalent Ti (Ti0, Ti2+, Ti3+, Ti4+) are also the key factors for superior catalytic activity of Ni/Ti3C2-WE.

In addition to the application of Mg-based materials in solid hydrogen storage, they can also be used in the fiel of hydrolytic hydrogen production.Pang et al.[182]investigated the effect of monovalent alkali metal cations in seawater on hydrogen production behavior of (Mg10Ni)10Ce alloy.The results show that the (Mg10Ni)10Ce alloy has the shortest hydrolysis induction time within 20 min in KCl solution at 25°C, and the highest H2yield is 173.4 mL g-1.The results of this study lay a solid foundation for the development of green portable hydrogen generators and the popularization of clean hydrogen energy and emission reduction.

Based on the thermodynamics and kinetics of de/hydriding reactions, catalyst addition is an effective method to enhance the properties of Mg-based hydrogen storage materials [5].Liu et al.[183]synthesized smaller and uniform graphenesupported TiO2nanoparticles (TiO2@rGO) via solvothermal method and found that the MgH2-70TiO2@rGO composite exhibited superior kinetic properties.The composite can desorb 6.0 wt.% hydrogen within 6 min at 300 °C and absorb 5.9 wt.% hydrogen within 2 min at 200 °C.Besides, Mg is also a vital substitution element for the property improvement of La-Y-Ni hydrogen storage alloys.For example, the LaY1.25Mg0.75Ni9alloy exhibited good overall electrochemical properties with the maximum discharge capacity of 308.4 mAh/g and capacity retention of 69.0% after 100 cycles,which were 10.7% and 47.1% higher than those of the original LaY2Ni9alloy, respectively [184].

To achieve the practical application of hydrogen, the metal hydride tank (MHT) is a widely used device for the safe storage and delivery of hydrogen.However, due to the highly complex hydrogen reaction and transport, the design of a MHT with desired properties is difficult Lin et al.[185]developed a numerical model that can accurately simulate the absorption and desorption processes in a double-layered annulus MHT and optimized the parameters of the MHT with desired properties.This physical model based and numerical simulation guided strategy is of general value for the design of MHT in hydrogen-related applications.

Chongqing University has investigated Mg-based hydrogen storage materials,and successfully discovered novel Mg-Ni-Y and Mg-Ni-Nd systems, which show high hydrogen storage capacity, low operating temperature, and robust recyclability.Moreover, Chongqing University also established an integrated hydrogen storage system, including light-weight tanks,solar-power-based hydrolysis systems and fuel-cell, demonstrating their capability for potential applications.

4.4.High modulus and high damping Mg alloys

It is difficul for Mg alloy to maintain high elastic modulus and high damping capacity simultaneously, which hinders the further development and application of magnesium alloys.Wang et al.[186]prepared Mg-8Li-4Y-2Er-2Zn-0.6Zr alloy with high damping capacity, high elastic modulus (E)through heat treatment and cold rolling.With the presence of 18R LPSO and later formed twins and kinks, the damping capacity increases from 0.01 to 0.02.In addition, the modulus of elasticity of the cold-rolled alloy reaches 48.9 GPa.

Xianhua Chen's group develops magnesium alloys with both high modulus and high strength.They found that the addition of Ge element to the matrix Mg-Gd-Ag-Mn alloy could improve the elastic modulus and strength of the alloy at the same time [187].Among the alloys containing Ge,Mg-10Gd-1.5Ag-0.2Mn-3.5Ge alloy exhibited the best comprehensive mechanical properties, with E, UTS and EL of 51 GPa, 423 MPa and 10%, respectively.In addition, they designed a new as-cast Mg-Y-Zn-Al-Li alloy with a high elastic modulus of 52.9 GPa [188].

4.5.Dissoluble magnesium alloys

In recent years, dissoluble magnesium alloys have broad application prospects because of their strength to bear high pressure and controllable degradation rate.However, it is key to enhance the degradation rate and simultaneously maintain good mechanical properties.Many researchers have found that Ni has a great influenc on the mechanical properties and degradation rate of soluble magnesium alloys, the developed Mg alloys in 2021 are summarized in Table 17.

Table 17Compressive and degradation properties of dissoluble Mg alloys developed in 2021.

Liu et al.[189]added Ni to hollow glass microsphere/Mg alloy composites.The results show that adding Ni leads to the formation of Al3Ni2intermetallic, refine the matrix and reduces the function of corrosion barrier of theβ-phase.As a result, the UCS increases to 393 MPa, and the average degradation rate dramatically increases to 95.6 mg cm-2h-1at 93°C.Wang et al.[190]developed a high strength Mg-10 Gd-3Y-0.3Zr-0.8Ni alloy with a moderate degradation rate.The UCS is as high as 597 MPa and the degradation rate is 24 mg cm-2h-1at 93°C.Wang et al.[193]rationally designed Mg-Y-Ni dissoluble magnesium alloys with a combination of high strength and rapid degradation with the help of the special structure of LPSO phase.This suggested that the Ni-containing LPSO structure could not only improve the mechanical properties, but also accelerate the corrosion of magnesium alloys.In addition, Wang et al.[194]investigated the effect of compositions, contents and morphologies of Nicontaining LPSO phase on the mechanical and degradation properties and their corrosion mechanisms.The higher potential difference between the Ni-containing LPSO and the magnesium matrix promotes the occurrence of galvanic corrosion and enhances the degradation rate.Thus, the degradation rate is accelerated with increasing Ni-containing LPSO content.However, the continuous distribution of LPSO phase can act as a corrosion barrier to prevent corrosion expansion and reduce the degradation rate.Moreover, Ma et al.[191]proposed novel Mg-7.2Y-2.8Ni alloys containing rodshaped LPSO phase.The average degradation rate increases by 27% and the UCS increases to 456 MPa in an annealed state.

4.6.Electromagnetic shielding Mg alloys

Current studies on the electromagnetic shielding performance of Mg alloys have become increasingly extensive and thorough.Some new shielding Mg alloy materials, such as low-RE content Mg-Sn series alloys, RE-free Mg-Li series alloys and Mg-based composite materials, have been developed.

Xianhua Chen's group has successfully developed Mg-Sn-Zn alloys with minor amounts of Ca and Ce, which exhibit superior shielding effectiveness (SE) and high strength [195].When the Sn content is 3 wt.%, the Mg-Sn-Zn-Ca-Ce alloy possesses SE value of 91-114 dB at 30-1500 MHz, which is mainly attributed to the regular arrangement of Mg2Sn precipitates.Zhang et al.have studied the SE and corrosion resistance of Mg alloys fabricated by electro-pulsing[196].The SE of the alloys (～87 dB) at a high frequency(1500 MHz) increases by 109.4%.The enhanced SE is attributed to the dissolution and refinemen of precipitates.At present the shielding properties of Mg-based composites have become a research hotspot.Wang et al.have found a novel Mg-Li-Zn-Gd/MWCNTs composite with high SE and mechanical strength (X-band) [68].The EMI SE of the ARB5(fi e accumulative roll bonding cycles) sheet in the X-band frequency range is increased up to 77-96 dB.

To sum up, a new Resoloy rare earth Mg-Dy alloy with excellent mechanical properties and biocorrosion behavior as an alternative to WE43 scaffolds for bioresorbable vascular implant is developed, which helps promote the industrializa-tion of bio-Mg alloys for bioresorbable vascular implant application.A simple low-cost Mg(TFSI)2/DME electrolyte in Mg//Mg0.15MnO2cell with a high average discharge voltage of ～2.3 V and a reversible capacity of 190 mAh g-1after 200 cycles is developed.In addition, Chongqing University successfully produced the soft-package RMBs by virtue of extensive fundamental studies on high-capacity sulfide/oxid cathodes, magnesium alloy anodes and low-cost electrolytes.An ultrafine-size MgH2nanoparticles with a major size of 4-5 nm led to 6.7 wt.% reversible storage of hydrogen at 30 °C.Chongqing University also established an integrated hydrogen storage system, including light-weight tanks, solarpower-based hydrolysis systems and fuel-cell, demonstrating their capability for potential applications.

In the future, controlling the fast degradability of bio-Mg alloys remains to be a hot topic.In the case of rechargeable Mg batteries (RMBs), low-cost Mg battery pack with high battery performance is still under development, and the industrialization of RMB requires more attention.Further improvement in the kinetics of hydrogen absorption and desorption of Mg based hydrogen materials is very important.The repeatability of high hydrogen storage performance needs to be improved, massive production of Mg based hydrogen materials as well as the integrated storage devices should be further developed.

5.Processing technologies

As stated in Section 2.3, the mechanical properties and microstructure of Mg alloys have attracted much attention in both 2020 and 2021, indicating the significanc of such research areas.Processing technologies play a vital role in the mechanical properties and microstructural development.Thus,cast technology, plastic processing technology, as well as the newly emerged additive manufacturing technology were developed in 2021.In addition, China produces over 80% primary Mg in the world.The development of technology to produce ultra-high purity Mg is of great importance.Furthermore, welding and joining of Mg alloy parts are essential for their actual application.

5.1.Preparation technology of high-purity Mg

The purity of magnesium significantl affects the performance of the alloys, such as the biocompatibility, corrosion resistance and mechanical properties.Therefore, many researchers devoted to produce high purity magnesium, much progress was made.Several innovative processes were developed to produce magnesium with super high purity of 99.999 wt.%.

Lee et al.[197]developed a novel electrolytic process to produce high-purity (99.999 wt.%) magnesium.The electrolysis of MgO was conducted using liquid tin (Sn) cathode and carbon (C) anode.The Mg-Sn alloys were obtained with a current efficien y of 86.6% at 1053 K.The high-purity Mg(99.999 wt.%) was produced from the Mg-Sn alloy by vacuum distillation at 1200 - 1300 K for a duration of 5 - 10 h.Liang et al.[198]developed one-step vacuum distillation technology to produce high purity magnesium.They designed a horizontal fractional condensing vacuum (HFCV) furnace to purify primary magnesium ingot, as shown in Fig.9.Magnesium with purity higher than 99.999 wt.%(5 N)was collected in high temperature region and magnesium with purity higher than 99.99 wt.% (4 N) was collected in the low temperature region in one step.Yuan Tian et al.[199]investigated the distribution and evaporation principles of impurities in distilled magnesium metal using a low vacuum (8 × 104Pa)distillation purification The results show that the optimum preparation temperature for high-purity Mg (99.99 wt.%) is 750 °C.

5.2.Cast technology for high strength Mg alloys

In addition to the traditional permanent mold cast and sand cast, optimizing and developing the casting process is one of the important measures to obtain high-strength magnesium alloy products [200-202].

The traditional fabrication of seamless Mg-Gd-Y-Zn-Zr alloy ring parts needs to undergo complex large deformation processing, which is easy to cause cracking and the decrease of YS due to excessive strain.Ma et al.[203]prepared the ring-shaped Mg-8.5Gd-4Y-1Zn-0.4Zr (wt.%) alloy via centrifugal casting and ring-rolling process, as shown in Fig.10.It is the firs time to use centrifugal casting and ring rolling process to prepare rings for Mg-RE-Zn alloys.The outer diameter and thickness of the centrifugal cast ring are 380 mm and 21 mm, respectively.After an accumulative rolling reduction of 80% and peak aging, the UTS of this ring-rolled alloy is further enhanced, reaching 511 MPa, and the EL reaches 5.3%.

In the case of the as-cast Mg alloys, the cooling rate plays a key part in controlling the microstructure.Shaokang Guan'sgroup[204]prepared the sub-rapidly solidifie Mg-Zn-Y-Nd alloy samples with diameters of 2-8 mm by step-copper mold casting.This result suggests that the sub-rapid solidificatio can significantl enhance the solid solubility limit of alloying elements, refin grains, and alter the type of precipitation phases.The sub-rapid solidificatio would be a promising technique to improve the mechanical properties of Mg alloys, which can be used as biodegradable mini-implants.

Fig.9.The saturated vapor pressure of each element in magnesium ingot at different temperatures [198].

Fig.10.Schematic diagram of centrifugal casting and ring rolling process [203].

Yu et al.[205]prepared the vacuum die casting AZ91D alloy and studied the correlation of 3D defect-band morphologies and mechanical properties of the alloy.It is proved that the porosity became lower and the defect band became much less distinct with the introduction of vacuum.The average volume of porosity decreased from 8.1 × 10-6mm3to 4.2 × 10-6mm3.The UTS and EL of vacuum-4 m/s castings continuously increased and reached 253 MPa and 7.6%,respectively.

5.3.Additive manufacturing technology of Mg alloys

Additive manufacturing (AM) technology has become significantl helpful in developing complex geometries and personalized products with a high customization freedom by computational modeling.In the past year, AM of magnesium has gained attention to overcome some issues in the processes and techniques in biomedical materials and structural materials [206-209].

5.3.1.Additive manufacturing technology of biomedical Mg alloys

Wang et al.[210]fabricated three different types of porous Mg-Nd-Zn-Zr alloy scaffolds by selective laser melting (SLM) process.The fabricated biomimetic, diamond,sheet-based gyroid scaffolds exhibit different pore structures with a unit cell size of 2 mm, 1.5 mm, 2.2 mm, respectively.By regulating the strut thickness, all scaffolds result in the same porosity of 75% and average pore size of 800μm.Pou-álvarez et al.[211]prepared open-porous scaffolds of WE43 Mg alloy with a body-center cubic cell pattern by laser powder bed fusion with different strut diameters.The scaffolds show excellent mechanical properties and thein-vitrobiocompatibility tests suggest that PEO treatment is necessary to ensure cell proliferation.

Long et al.[212]reported the 3D-printed innovative multifunctional PLGA/Mg porous scaffolds for comprehensive postsurgical management of clinical treatment of Osteosarcoma (OS).The scaffolds have well interconnected macropores with a pore size around 415 ± 24μm, and micropores ranging from 5 to 50μm are also distributed on the strut surface of the scaffold frameworks.Dong et al.[213]developed 3D-printed customized Mg/PCL composite scaffolds with enhanced osteogenesis and biomineralization.The Mg/PCL scaffolds display a pore size of 480 ± 25μm, a fibe diameter of 300 ± 25μm, and a scaffold porosity of about 66%, which provide a compatible condition for tissue regen-eration and bone healing.Marco Boi et al.[214]fabricated 3D PCL scaffolds reinforced with a novel Mg-doped bioactive glass (Mg-BG) and displayed good mechanical properties and biological reactivity.For the 50/50 composite, a fibe-fibe distance of 293 ± 14 μm and a fibe diameter of 331 ± 14 μm were obtained.

5.3.2.Additive manufacturing technology of structural Mg alloys

Salehi et al.[215]presented a detailed comparative analysis of the consolidation of as-printed Mg-5.06Zn-0.15Zr alloy parts by conventional (CT) and microwave (MW) sintering.This work enlightens the great potential of MW sintering as an attractive alternative to reduce energy consumption and shorten the lead time of the binder jetting technology.Julmi et al.[216]developed the high-quality WE43 components by laser powder bed fusion process (PBF-LB).The microstructure of the PBF-LB metal parts consisted of much more refine grains compared to as-cast parts.

Klein et al.[217]studied the microstructure and mechanical properties of a wire-arc additive manufactured AZ61A alloy.The mechanical properties lie in-between those of typical cast and wrought alloys, demonstrating the feasibility of processing Mg alloys by wire-arc additive manufacturing.Wang et al.[218]fabricated AZ31 magnesium alloy thin-wall component with 50 layers by the modifie cold metal transferwire-arc additive manufacturing (CMT-WAAM).Compared with the as-forged and as-cast AZ31 alloy, the thin-wall component exhibits an UTS of 226 MPa and a high EL of 28.3%in the deposition direction.Liming Peng's group [219]investigated the influenc of friction stir processing (FSP) and aging heat treatment on the microstructure and mechanical properties of selective laser melted (SLMed) Mg-Gd-Zr alloy.The results show that FSP can lead to porosity reduction from 0.78 to 0.015% as well as grain refinement Wen et al.[220]reported a formation mechanism of hot cracking in laser powder bed fusion (LPBF) prepared ZK60 magnesium alloy.The hot cracking was attributed to a wide solidificatio temperature range, a low eutectic temperature as well as a high thermal stress.The serious solidificatio and liquation cracking hugely deteriorated the relative density of LPBF samples.

The 3DP (three dimensional printing) of Mg alloy is an additive manufacturing process in which bonded magnesium alloy powder by binder and then densifie by sintering.This new manufacture process can rapidly produce parts with complex structures and shape.In the future this manufacture process is likely to replace some parts of the casting process.Compared with other additive manufacturing processes, the 3DP additive manufacturing process has high safety and low cost.Although this process has been relatively mature in other alloy systems, the sintering densificatio is still one of the difficultie in Mg alloys.

There are only 2 reports on 3DP Mg alloy in 2021,as listed in Table 18.Jingfeng Wang's group [221]developed a new sintering process which used high viscosity liquid Mg alloy to enhance the density and network-like of MgO framework structure to keep the original shape.The UCS of AZ91D Mg alloy prepared by the 3DP process can reach to 354 MPa.This means that the properties of 3DP Mg alloy can basically reach the properties of its as-cast alloy.Moreover, they introduced the two-step sintering process and combined it with the network-like of MgO framework structure [222], leading to an increased compressive strength to 393 MPa while the sintering time was significantl reduced.This proves that when the performance of 3DP magnesium alloy is close to that of as-cast magnesium alloy, and its production cost could be lower.

Table 18Sintering process and properties of the Mg alloys prepared by 3DP in 2021.

5.4.Plastic processing technologies of Mg alloys

5.4.1.Extrusion technology

In 2021, extrusion processes are modifie to refin the microstructure and further improve the properties of magnesium alloys.

Zhou et al.[223]developed a new differential thermal equal channel angular extrusion process (DTECAP) for Mg-Sn-Zn-Zr (TZK) alloy.The TZK alloy exhibited an excellent balance of strength and ductility.Interestingly, Wang et al.[224]designated a constrained groove extrusion (CGP) process to fabricate Mg alloy sheets with improved texture, as shown in Fig.11.A double-peak basal texture with the basal pole tilted from ND to RD appeared in the traditional CGP route, while an inclination of the basal pole from ND to TD was observed in cross-CGP routes.Dingfei Zhang's group[225]developed semi-solid extrusion(SSE)process to prepare AZ31 magnesium alloy sheets with uniform microstructure,fin grain size, weakened texture and improved mechanical properties.

5.4.2.Rolling technology

Huiyuan Wang's group [226]developed sub-rapid solidificatio (SRS) followed by hard plate rolling (HPR) to prepare Mg-6Zn-0.2Ca alloy sheet.Compared with the conventional solidificatio and combined with the HPR, it was observed that the fracture elongation increased from ～17% to～23% without sacrificin the tensile strength (～290 MPa).This is mainly due to the refine Ca2Mg6Zn3eutectic phase which can effectively alleviate or avoid crack initiation.Wu et al.[227]prepared LA51/LA141 alloy composite plate with bimodal-grained microstructure by cumulative roll bonding(ARB).Due to the synergistic effect of bimodal grain microstructure and the activation of non-basal slip, the Mg-Li alloy composite sheet produced by ARB method presents a longer softening stage and thus maintains a high plasticity.

5.4.3.Forging technology

Forging processing has a wide application prospect because large samples can be readily deformed with conventional tools.Huang et al.[228]studied strain-controlled multipass multi-directional forging of Mg-Gd-Y-Zn-Ag-Zr alloy with dimensions of 460 mm × 260 mm × 160 mm.Zhimin Zhang's group [229]optimized the multi-directional forging(MDF) process of pre-solutionized Mg-13Gd-4Y-2Zn-0.5Zr alloy.Many plate-like Mg5RE phases are present in 4 passesin the MDFed sample.Chen et al.[230]successfully prepared high-strength Mg-Li alloy by cold rotary swaging process.The results show that nano-grains are formed in the central region of the alloy bar after fi e passes of swaging.

Fig.11.Schematic illustration of CGP process [224].

Fig.12.Schematics of the step-ladder radial forging experiment (a), experimental equipment (b), and macroscopic morphology of the step-ladder specimen after three passes (c), where RD, PD, and TD represent the radial, processing, and tangential directions in this study, respectively [36].

Zou et al.[36]utilized radial forging(RF)to prepare ZK60 magnesium alloy bar with step-shaft, as shown in Fig.12.The results show that by increasing RF passes, the grains in different radial sections are gradually refine to form a bimodal microstructure composed of coarse (～14.1μm) and fin (～2.3μm) grains.Besides, with increasing RF passes,the initial micro-sizedβ-phase is gradually broken and dissolved into the matrix, and finall evolving to form a larger area of nano-sized Zn2Zr spherical particles, which are uniformly distributed in the grain interior.Excellent mechanical properties including higher tensile strength and ductility were obtained after three RFed passes.

5.4.4.Friction stir processing technology

Friction stir processing (FSP) is also regarded as an effective technology to improve the microstructure and enhance themechanical properties.FSP is a type of severe plastic deformation technology with high potential to be commercialized.

Fig.13.Illustrations of (a) the FSE procedure, (b) the hollow ram and (c) the vertical section and sampling position.Here, ED and RD refers to the extrusion direction and the radial direction of the fabricated Mg-RE rods, respectively [234].

Xu et al.[231]modifie the microstructure and mechanical properties of die-casting AZ91D by low-temperature FSP.They found that the microstructure was greatly refine by friction stirring due to continuous dynamic recrystallization,twin induced dynamic recrystallization and particle stimulated nucleation recrystallization.Luo et al.[232]improved the stretch formability of AZ61 alloy sheet by multi-pass friction stir processing (M-FSP) to an Erichsen value of 3.7 mm.This is mainly due to the grain refinemen produced by MFSP and the existence of extension twins to accommodate the deformation during Erichsen cupping test.Banglong Fu et al.[233]proposed a novel differential rotation refil friction stir spot welding(DR-refil FSSW)to improve the mechanical properties of cast Mg alloy welds.A bimodal microstructure with weakened texture compared to conventional refil FSSW was obtained, and the deformation incompatibility between the stir zone (SZ) and thermal mechanically affected zone(TMAZ) was avoided, leading to a significan increase in the tensile lap shear strength (TLSS) of the welded cast alloy.The welds have 50% higher TLSS than that of standard refil FSSW welds.

Li et al.[234]proposed a friction stir extrusion to fabricate rare-earth magnesium alloy rods with high strength and ductility, as shown in Fig.13.In their study, the refine grains and uniform dispersion of the second phase were obtained under the coupled thermo-mechanical effect, resulting in the yield strength, ultimate strength and compression strain of the alloy increasing by 42.5%, 63.6% and 35.5%, respectively.

5.5.Welding and joining technology of Mg alloys

Welding and joining technology of magnesium alloys is a key to their engineering applications.The welding process optimization of magnesium alloys and the welding of dissimilar materials were developed in the past year.

5.5.1.Welding process

The optimization and development of welding methods and processes aim to improve the welding manufacturing effi ciency, enhance product quality, and reduce the welding energy consumption and cost.

Chen et al.[235]found that the pulse current makes the microstructure transition from the SZ to the heat-affected zone(HAZ) smoother and improves the UTS and EL of EFSW joints of AZ31B Mg alloy.When the pulse current is 400A,the UTS are 286 MPa, reaching 96.4% of the base material and EL is 12.1%.Xu et al.introduced high frequency ultrasonic vibration (UV) to the arc welding of MB3 Mg alloy to reduce defects, improve weld microstructure and joint strength.The UTS of the optimized butt joint by UV treatment reached 239 MPa, which is equivalent to 93% of base metal [236].Chang et al.[237]optimized the parameters of TIG welding with an alternating cusp-shaped magnetic fiel of AZ91 alloy.With an excitation current of 2 A and an excitation frequency of 80 Hz, the arc was compressed, and thus obtain a smooth weld with the largest depth-to-width ratio and fine-graine microstructure.

5.5.2.Welding of dissimilar materials

Joining of magnesium alloys with other materials such as Al alloys, steel, polymer or even different Mg alloys is quite important for the application of Mg alloy components.Thus,research was carried out on the dissimilar welding.

Fu et al.[238]utilized a novel refil friction stir spot welding (refil FSSW) to weld AZ31 Mg alloy and galvanized DP600 steel.The defect-free welds with high strength were successfully obtained in a wide window of parameters.Dileep Singh et al.[239]prepared the magnesium-steel welds by impact welding, and revealed that Fe-rich particles getting deep into the Mg matrix is conducive to the dissimilar metal joining.Liu et al.[240]studied laser welding of AZ31B magnesium alloy and DP590 dual-phase steel with concave groove joint.The mechanical property of magnesium/steel joints is improved due to the meshing effect of the concave groove and the metallurgical effect of adding Sn powders.In addition, the resistance rivet welding (RRW) was used to join Mg alloy to steel,and a metallurgical-mechanical hybrid joint was formed among the semi-tubular rivet [241].

Welding of Mg and Al were extensively investigated in the past year.Wu et al.[242,243]found that ultrasonic vibration could reduce the thickness of intermetallic compounds in the whole weld and improve the tensile strength of each part of the Al/Mg alloy welds by FSW.Acarer et al.[244]prepared Mg-Al composite by explosive welding and found that the UTS of the AZ31-Al5005 composite is 178 MPa, which is lower than that of the individual components owing to the formation of cracks.Galvanic corrosion was also observed in the joints.Kumar et al.[245]eliminated intermetallic compounds via Ni interlayer during friction stir welding of dissimilar Mg/Al alloys.The Ni can drastically cease the atomic diffusion of Mg and Al across their interfaces, eliminate typical intermetallic compounds Al3Mg2and Al12Mg17, leading to the formation of Al3Ni and Mg2Ni compounds.In addition,Paidar et al.[246]used the modifie friction stir clinchingbrazing (MFSC-B) and probe-less friction stir spot brazing(PFSSB) processes to weld AA2024-T3 and AZ31 Mg alloys.The tensile shear failure load in the MFSC-B joint (4369 N)is improved due to improved material fl w and intense dislocation density.

The newly developed friction stir-arc welding method was applied by Jiang et al.[247]to prepare dissimilar AM60/AZ31 joints.After the TIG arc heating, the UTS and YS of the two joints were similar, and the EL of the FSWARC joint was improved from 9.0 to 14.1%, which is attributed to the increase of Schmid factor for the basal slip and extension twinning caused by the weakening of the texture, the growth of the grain size and the annihilation of the residual dislocations in the NZ-middle.

Wang et al.[248]obtained AZ31-carbon fibe reinforced polymer (CFRP) joint by friction stir interlocking technique(FSI).Wang et al.[249]provided a new method for the control of welding quality in the process of riveting-welding hybrid bonding for magnesium and CFRP.The weld joints obtained by the reverse model operation have a favorable appearance, high tensile shear load and favorable plasticity.

To sum up, the centrifugal casting and the sub-rapid solidificatio by step-copper mold casting for high-strength Mg alloys were developed in the past year.Both bio-Mg alloys and structural Mg alloys with complex pore structure were successfully produced.However, the number of reports on 3DP Mg alloys is still far below the traditional additive manufacturing process like SLM.In addition, the commercialization of AM to prepare Mg alloy parts is still in its infancy.Current public reports mainly focused on AM of Mg alloy test coupons instead of real components.Several plastic processing technologies were reported to either improve the mechanical properties or weaken the textures.However, the commercialization and cost control of the plastic processing technologies remain a big challenge.In the aspect of joining and welding, publications are mainly seen on friction stir welding, which are used to weld Mg alloy parts with relatively simple geometry.However, argon arc welding of Mg alloys was seldom reported, although argon arc welding was extensively used in industries to repair some casting defects of large and complex Mg alloy parts.

In the future, the vacuum die casting might become the development trend for high strength Mg alloys.The 3DP Mg alloy still needs more attention and development.Special attention should be paid to additive manufacturing of large and complex Mg alloy parts with low cost and high safety.Plastic processing technologies of Mg alloys with low cost and industrialization potential are still in need.More attention is suggested to be paid to the welding of Mg alloy castings with complex morphology, since Mg alloy castings occupy over 80% market of Mg alloy parts.

6.Corrosion and protection of Mg alloy

Due to the extremely low electrode potential of Mg(-2.363 V), Mg alloys are believed to have a poor corrosion resistance and tend to have galvanic corrosion when joining with other metals.Researchers mainly attempt to understand the corrosion mechanism and explore possible ways to improve corrosion resistance of magnesium alloys.Recently,environmental corrosion of Mg alloys has been investigated to reveal the practical corrosion behavior in various service conditions, which is vital for industrialization of Mg alloys.In addition, bio-magnesium alloys have attracted increasing attention, the bio-corrosion of Mg alloys is also summarized.

6.1.Corrosion behavior of Mg alloys

6.1.1.Environmental corrosion of Mg alloys

The corrosion behavior of magnesium alloys in atmospheric environment is very different from that in laboratory accelerated tests.The atmospheric gasses, temperature, humidity and ultra-violet radiation have a significan effect on the corrosion behavior of magnesium alloys.As reviewed in the past, corrosion data of AZ31 in marine, haze and desert environments were well documented [106,107].

Jiang et al.[127]studied the corrosion behavior of Mg-Nd alloys in the South China Sea environment, and observed that NaCl deposited on the surface of Mg-Nd alloys after exposed in the marine environment, then the NaCl dissolved in high humidity environment.The electrochemical reaction between iron and magnesium alloys occurred severely and rapidly.The corrosion process can be divided into 4 steps: (1) adsorption of water vapor, (2) dissolution of NaCl and corrosive gas in water film (3) the electrochemical reaction on the surface of specimens, and (4) surface drying-wetting alternating action.Song et al.[97]analyzed the corrosion behavior of AM60 magnesium alloy in Shenyang industrial atmospheric environment.The corrosion pitting and sediment were formed on the surface after exposure for 1 month.When the exposure time increased to 6 months, the corrosion sites greatly increased, and obvious microcracks were observed.The corrosion products of AM60 after exposure for 6 months were MgO, Mg(OH)2, MgSO4and MgCO3.Liu et al.[92]developed a superamphiphobic coating to improve the environmental corrosion resistance of magnesium alloys.The coating can remain superamphiphobic after ultraviolet irradiation(λ= 254 nm, 672 h), abrasion (50 cycles,1.0 kPa), sandimpact (≥10 cycles), strong acid/alkaline solution, organic solvents immersion (n-hexane, ethylene glycol, ≥48 h), high temperature (200 °C, 72 h) and acidic industrial atmosphere(Ph = 4.6, 40 d).

6.1.2.Corrosion mechanism of Mg alloys

Second phase has a complex influenc on the corrosion behavior of magnesium alloy matrix.Liu et al.[250]found that the small dispersed LPSO phase of MgY3.6Zn1.2Zr0.16 alloy accelerated the micro-galvanic corrosion between the second phase and magnesium matrix and increased the hydrogen evolution rate.On the contrary, the large dominant LPSO phase of MgY2.8Zn1.9Zr0.16 alloy could provide a barrier for the magnesium matrix during corrosion and block the corrosion expansion.Zhang et al.[251]reported the Al-Si eutectic of Mg-4Li-6(Al-Si) alloy deteriorated the corrosion resistance of Mg-4Li alloy.The poor corrosion resistance was attributed to the increased micro-galvanic corrosion of the precipitates and the texture change from a basal texture to non-basal texture.

The mechanism of Mn on inhibiting magnesium alloy corrosion caused by Fe was clarifie by Yang et al.[252].The element of Fe in Mg-6Mn alloy was incorporated into Mn to form Mn (Fe) phase.With the corrosion time increased,a dense Mn-rich corrosion product fil was formed on the partial surface, which can isolate the second phase and corrosive solution.Then the corrosion rate of Mg-6Mn alloy was decreased.

Merson et al.[253]studied the influenc of corrosion products on stress corrosion cracking of Mg-Zn-Zr alloy in NaClbased solutions.The embrittlement of Mg-Zn-Zr alloy after pre-exposure was related to the thickness, weight and hiding power of corrosion products.The more corrosion products deposited on the surface, the more serious the embrittlement of the specimen, which was attributed to the large volume of hydrogen in the product layer.Therefore, the removal of corrosion products from the surface of specimen pre-exposed to corrosion solution can inhibit the stress corrosion cracking of magnesium alloys.

Jia et al.[6]proposed a corrosion mechanism of AZ80 magnesium alloy with phosphate conversion coating after impaction.The corrosion occurred firs at indentation areas because of the cracks and damage on the coating.With increasing immersion time to 24 h, the coating in the indentation area dropped and fell off.The coating without damage disappeared completely after immersion for 48 h.

Mg alloys have great potential to be used as biomedical materials, thus the bio-corrosion behavior in the simulated body flui (SBF) and Dulbecco's modifie eagle's medium(DMEM), etc.were extensively studied.

Sun et al.[254]studied the corrosion behavior of ZK60 magnesium alloy in SBF with different pH values.A lot of pitting occurred in the solution with a pH value of 5.2.The corrosion mechanism changed from filifor corrosion to uniform corrosion with the pH value of SBF increased from 7.4 to 9.0.Chen et al.[255]analyzed the protection capability of biodegradation product layer on Mg-1 Zn alloy.In SBF, the corrosion resistance of Mg-1 Zn decreased during dynamic strain, due to the damage of the corrosion product Mg(OH)2layer.In SBF-containing protein, the protein was spontaneously adsorbed to the degradation product layer.A stable and fl xible composite anticorrosive fil was formed,resulting in the improvement of corrosion resistance of magnesium alloy.

Norbert Hort's group [256]studied the degradation behavior of Mg-Nd alloys with different amounts of intermetallics in DMEM and 10% fetal bovine serum (FBS) solution.The intermetallic Mg41Nd5particles in the Mg-Nd alloy affected the degradation of alloy in two opposite ways.The positive effect was related to the compact and protective corrosion product layer, the negative effect was attributed the microgalvanic corrosion between Mg41Nd5and matrix.In the early stage of degradation, the negative effect was predominant,while in the later stage of degradation, both negative and positive effects were present and jointly determined the corrosion performance.Azzeddine et al.[257]compared the corrosion behavior of binary magnesium-rare earth alloys of Mg-1.44Nd, Mg-0.3Ce, Mg-0.41Dy and Mg-0.63Gd in 2 mL DMEM with 10% FBS solution.The results show that the existence of sulfur can improve the corrosion resistance of Mg-RE alloys.

6.2.Corrosion resistance improvement of Mg alloys

Magnesium alloys are easily corroded because of the high reactivity, and the loose and porous self-corrosion products on the surface, which cannot effectively protect the matrix.Therefore, many researchers have been working on the development of new methods, materials and technologies that can delay the corrosion of magnesium alloys.

6.2.1.High-efficienc corrosion inhibitors of Mg alloys

Addition of corrosion inhibitor is one of the most effective methods to keep the Mg alloys away from the corrosion, which has the advantage of low-cost and easy operation.In 2021, corrosion inhibitors including 1-n-butyl-2-decylpyrazole bistrifluoromethanesulfon imide ([BDePz][NTf2]) [258], 1-n-butyl-2-octylpyrazole bis(trifluoromet ylsulfonyl)amide ([BOPz][NTf2]) [254],commercial corrosion inhibitors (LPS3 [259], LPS2 [259],AMLGuard [259]and Ardrox 3961 [259]), tri(bis(2-ethylhexyl)phosphate) (Ce(DEHP)3) [260], Na2MoO4[261],etc., were applied and the corrosion inhibition efficiencie are summarized in Table 19.

Table 19Corrosion inhibition efficien y of Mg alloys with different corrosion inhibitors in 2021.

Table 20Corrosion rates of Mg alloys in different conditions reported in 2021.

Wang et al.[262]investigated the synergistic corrosion inhibition of organic corrosion inhibitor L-Phenylalanine (LPhe) and inorganic corrosion inhibitor (Zn(NO3)2) of AZ31B alloy in a 0.05 wt.%NaCl solution,and found that the highest inhibitory efficien y is 93.2%, which is much better than the single L-Phe-or Zn(NO3)2.The dense fil containing Zn2+-L-Phe-complex on the surface of magnesium alloy suppresses the corrosion of the alloy.Gao et al.[258]reported that the maximum inhibition efficien y of [BDePz][NTf2]for AZ91Din the 0.05 wt.% NaCl solution was 90.4%, which was attributed to the coverage of corrosion inhibitor to reduce the exposure area of magnesium alloy, where the insoluble small molecules reacted with corrosion products along with the hydrophobic effect of corrosion inhibitor.Aqueous molybdate provided effective corrosion inhibition of WE43 magnesium alloy in a 0.05 M NaCl solution [261].When the concentration at and above 100 mM, the competitive adsorption of molybdate anions on the surface with the formation of a Mo(VI)-Mo(V) mixed-valence protective layer suppresses further corrosion attack.

6.2.2.Microalloying for Mg alloys

Alloying is an important way to improve the corrosion properties of magnesium alloys.In 2021, a large number of studies have been carried out to improve the corrosion resistance of Mg alloys by alloying, such as Ca [263], Mn[264-266], Y [97,267,268], Gd [269,270], Sr [271,272], Sm[273], and other alloying elements [20].The corrosion rates are summarized in Table 20.

Adding an appropriate amount of calcium can improve the corrosion resistance of magnesium alloy.The corrosion rate of AZ31-0.5Ca was ～0.72 mm·y-1, about 7.4 times lower than AZ31 alloy [263].Chen et al.[97]improved the corrosion resistance of AZ63 alloy in 3.5 wt.% NaCl solution by adding Y and explored the mechanism as follows: (1)improving the content of Al2O3in the oxide layer; (2) facilitating the intermetallic phase transformation from continuous Mg17Al12to discrete Al2Y phase and efficientl alleviating the micro-galvanic corrosion; (3) forming relatively continuous and dense layers.Dargusch et al.[271]found that the addition of Sr introduced a Sr-containing intermetallic phase(Mg8Al4Sr) into the AE42 alloy, increasing the volume fraction and continuity of the intermetallic network.The corrosion rate of the Sr-containing alloy was ～0.56 mm·y-1, which is comparable to that of high-purity Mg.In addition, Sung Soo Park et al.reduced the corrosion property of hot-rolled Mg-8Sn-1Al-1 Zn alloy by Smmicroalloying [273]from 19.0 to 2.7 mm·y-1.

Furthermore, alloying is also utilized to improve the hightemperature oxidation resistance of magnesium alloys.Le et al.[268]investigated the effect of Y addition on hightemperature oxidation behavior and product of AZ80 alloy.The alloying element Y facilitated the transformation of loose flocculen shape oxide layer (MgO) on AZ80 surface into dense granular oxide layer (MgCO3), which can prevent alloy matrix from further oxidation.High content of Gd is conducive to the formation of multi-layer film composed of Gd2O3-ZrO2-MgO-cubic phases on the surface of the alloy from the inner to outside,and significantl increases the hightemperature oxidation resistance[269].Yuan et al.[270]studied the effects of co-alloying of Ca and Gd on the high temperature oxidation of Mg alloys.They concluded that with an addition of ～0.5 wt.% Ca, Mg-3.5Gd-0.5Ca alloy is with the fines microstructure, lowest roughness and good high temperature oxidation resistance.

6.2.3.Surface treatment for Mg alloys

To suppress the corrosion of Mg alloys,many surface treatment procedures and coatings have been proposed to enhancethe corrosion resistance of Mg alloys, including surface alloying[274,275],high pulsed electron beam treatments(HCPEB)[276], laser surface melting (LSM) [277], superhydrophobic surface [5,278,279], organic coatings [280-282], nickel coating [283,284], conversion fil [285-288], etc.The corrosion rate of magnesium alloys with different surface treatments in 3.5 wt.% NaCl is summarized in Fig.14, and the bio corrosion rate is listed in Table 20.Among the developed coatings, the PEO-Seal and Ni-P composite coating on AZ91D alloy exhibited an excellent corrosion resistance of 0.001304 ± 0.000110μA·cm-2in 3.5 wt.% NaCl.

Table 21Corrosion rate of magnesium alloys in different surface treatment conditions reported in 2021.

Fig.14.Corrosion rate of magnesium alloys with different surface treatment conditions in 3.5 wt.% NaCl reported in 2021.

Surface alloying via thermal diffusion is a cost-effective technique to produce a Surface alloying via thermal diffusion is a cost-effective technique to produce a corrosionresistant layer for Mg alloys.The aluminizing is the most promising approach to enhance the corrosion resistance.Song et al.[274]prepared an Al-alloyed layer through thermal diffusion on Mg surface with the passive current density of 19.4 ± 9.7μA/cm2, which acted as an effective barrier to protect the substrate Mg from scratching and corrosion attack.In addition, Hou et al.[275]constructed the double-layer protective fil on Mg-In alloy.The formation of In-rich layer and compact Mg(OH)2fil retarded the dissolution, and greatly improved the corrosion resistance of the alloy (Table 21).

Table 22Newly published ISO standards on magnesium and magnesium alloys in 2021.

Surface modificatio is another effective way to improve the corrosion resistance of magnesium alloys.Zhang et al.[276]modifie surface of Mg-4Sm-2Al-0.5Mn alloy with HCPEB, and found that corrosion current density (Icorr) of the alloy treated by 15 pulses was the lowest (1.48 × 10-6A·cm-2).Besides, the effects of LSM on the microstructure and surface topography evolution in AZ31B magnesium alloy were explored by Dahotre et al.[277], and the corrosion rate of LSM AZ31B Mg samples (0.5 mm/year) was significantl lower than the untreated AZ31B (8 mm·y-1).Compared with the traditional method, chemical methods have gradually become a research hotspot.Wang et al.[278]prepared superhydrophobic surfaces with tunable water adhesion on rolled Mg-3Al-1 Zn (AZ31) and Mg-9Al-1 Zn (AZ91) alloy sheets through simple chemical etching and surface modification which is beneficia for durable anti-corrosion ability.Besides, preparing super-hydrophobic coating by homemade spraying suspension composed of the mixture of poly(methyl methacrylate) (PMMA), acetone, and stearic acid (SA)-modifie ZnO nanoparticles was also a simple method [5].Cui et al.[279]improved the performance of microarc oxidation/diamond-like carbon coating (MAO/DLC) by polydimethylsiloxane (PDMS) on AZ31B Mg alloy, with espe-cially high superhydrophobicity (CA = 110.5°) and corrosion resistance (icorr= 9.93 × 10-3μA cm-2).

As one of the simplest and most economical methods to improve the corrosion resistance of magnesium, coatings including organic coating (chitosan-based [281]), inorganic coating (electrophoretic deposition fil [282], Ni coating [283,284]) and conversion coating (phosphate conversion coating (PCC) [285,288], trivalent chromium conversion coating (TCC) [291], layered double hydroxide coating(LDH)[286,290,292-296],plasma electrolytic oxidation coating(PEO)[297-300])etc.,have made some progress in 2021.Yuansheng Yang's group [288]developed a PCC free of fluo rine,chromium and nitrite,which showed perfect morphology and corrosion resistance.Li et al.[294]investigated the corrosion resistance of micro-arc oxidized AZ91D magnesium alloy by hydrothermal treatment, leading a decrease of corrosion rate by three orders of magnitude (3.54 × 10-4mm/y).In addition, Wu et al.conducted an in-depth study on LDHs grownin-situon the surface of rolled AZ31 [301]and Mg-Ca alloys [302], the LDH conversion fil greatly improved the corrosion resistance of the magnesium alloys.The rare earth Ce [293], graphene oxide [303], corrosion inhibitor[304]and MXene [287]were successfully doped with LDHs,which can effectively protect the magnesium substrate from corrosion.

To meet the strict demand for corrosion resistance, the research also focuses on composite coatings.Zhou et al.[284]prepared a Ni-P composite coating on Mg alloys,and the coating with sealing treatment had excellent corrosion resistance (0.001304μA·cm-2).Besides, Gnedenkov et al.[289]prepared the composite calcium-phosphate coating on MA8 Mg alloy, and the corrosion current density is 7.6 × 10-10A cm-2, which decreased by four orders of magnitude.In addition, Liang Wu et al.have been devoted to endow LDHs coatings with self-healing properties in order to further improve the corrosion resistance of composite coatings[305].The yttrium-doped Mg-Al LDHs fil was prepared on a magnesium alloy by a hydrothermal method.The ternary Mg-Al-Y LDHs coating was emphatically constructed, which can trap chloride ions into the interlayer in a corrosive environment, thereby triggering the stable precipitation of Y element, and had a certain self-healing ability [301].

In brief,the atmospheric temperature,gasses,humidity and ultra-violet radiation have a significan influenc on the corrosion behavior of magnesium alloys.Magnesium alloys show unique corrosion behavior in South China Sea, Shenyang industrial atmosphere and high temperature environments.In addition, the second phase, intermetallic, pH values of corrosive solution and corrosion product layer have a great effect on the corrosion behavior of magnesium alloys.Corrosion resistance of magnesium alloys can be significantl improved by adding corrosion inhibitors and alloying elements or performing surface treatment.As one of the simple and efficien methods,coating has a remarkable protective effect on magnesium alloy surface.Among them, superhydrophobic coating,self-healing coating and composite coating are widely concerned by researchers, and are expected to promote the wide applications of magnesium alloys.

In the future, more studies and analyses on the corrosion behavior of magnesium alloys in different atmosphere environments are still needed.Special attention should be paid to the corrosion mechanisms of magnesium alloys.The corrosion protection technique via coatings should be further developed towards functionalization and commercialization.

7.Other progresses

7.1.Standards

In 2021, three standards are newly published which are all proposed by China, as listed in Table 22.ISO 23,694:2021,ISO 23,700:2021, and ISO 8287:2021 focused on extruded rods, bars and tubes, the rolled plates and sheets, and primary magnesium, respectively.All the three ISO standards were equally settled as British standards, which are BS ISO 23,694:2021, BS ISO 23,700:2021, BS ISO 8287:2021.The standard settlement indicates that the quality of ISO standards on magnesium is highly recognized by the Great Britain.It is anticipated that the establishment of these important international standards will pave the way for the further development and applications of magnesium and magnesium alloys in a variety of industrial sectors.

In 2021, all the fi e standards proposed by China were still under development and have been proceeded from CD(committee draft) stage to DIS (draft of International Standard), as listed in Table 23.ISO/CD 4155, ISO/WD 4177,ISO/CD 4181, ISO/CD 4188 and ISO/CD 4189 were proposed to measure the content of Ni, Cr, Sr, As, and Na with inductively coupled plasma atomic emission spectrometry, respectively.However, due to the current COVID situation, the interlaboratory tests of these elements in other countries were quite challenging.

Table 23ISO standards on magnesium and magnesium alloys under development in 2021.

Table 24Distribution of granted patents on magnesium and magnesium alloys in different countries and regions in 2021.

7.2.Patents

The granted Mg-related patents in 2021 were searched in SooPAT with “Mg or magnesium alloy” as keyword.Aftermanually eliminating the irrelevant data, 707 granted patents in total were found.

According to the number of Mg-related granted patents published in 2021, the top 10 countries and regions of the granted patens are listed in Table 24.China granted the most patents on Mg and Mg alloys, followed by Japan, the United States, South Korea and Europe.In addition, many papers on magnesium and magnesium alloys have been published in these countries, indicating that they value the academic and industrial development of magnesium and magnesium alloys.

7.3.International awards

In 2021, both International Mg Society (IMS) and International Magnesium (IMA) Association issued awards on magnesium and magnesium alloys.Both awards are of great importance to the R&D and application of magnesium and magnesium alloys.

IMS issued the 2021 International Mg Science and Technology Award (Annual Award).Eight types of awards were conferred to 27 winners who have made significan contributions to the magnesium R&D and application.These awards covered many aspects including magnesium smelting, Mgbased structural materials, Mg-based functional materials and so on.The award effectively promoted the discussion and communication between scientists, young researchers, and students, stimulated the interaction and cooperation between universities, research institutes and enterprises, boosted the development and application of magnesium and magnesium alloy science and technology, and provided a long-term platform for international communication and cooperation.

IMA issued fi e types of awards, which are Automotive Cast Product/Process, Commercial Cast Product/Process,Process, Wrought Product, and Environmental Responsibility.The award effectively boosted the application and industrial development of magnesium and magnesium alloys.

7.4.Other industrial progress

Magnesium and magnesium alloy industry have attracted much attention in 2021.Many mega enterprises are very interested in joining in the Mg industry,indicating that Mg and Mg alloys have great application potential.Baosteel Metal Co.,Ltd.plans to invest 18.2 billion Yuan to promote the R&D and industrilization of Mg and Mg alloys.In additon,Baosteel and Chongqing University co-established a ‘Joint Research and Development Center for Advanced Magnesium Technology', with the aim to promote the high-quality development of China's magnesium industry.Guangdong National Technology Co., Ltd plans to invest 500 million Yuan in 5 years in total to the ‘Development and Application of Solid-State Magnesium-Based Hydrogen Storage Materials and Technology'as well as R&D of magnesium-ion-based batteries.Western Magnesium Corporation in the United States plans to invest 1 billion dollors to build a primary Mg production base,which aims to produce 100,000 ton primary Mg.In 2021, the location of the production base is chosen as Harrison in the Ohio State.

8.Summary and outlook

The publications on magnesium and magnesium alloys continue to increase in the past 20 years, indicating that research and development of magnesium alloys have attracted more and more attention.According to the bibliometric analysis, microstructures, mechanical properties, and corrosion of magnesium alloys are the main research focus.The emerging research hot spots involve mainly energy storage magnesium materials, such as Mg ion batteries, hydrogen storage Mg materials.

In the structural Mg alloys,some new progress are made in cast and wrought Mg alloys.In the case of cast Mg alloys, a Mg-3Nd-4.5Gd-0.2Zn-0.5Zr alloy with an UTS of 343 MPa,and an EL of 5.4% was developed.In the case of wrought alloys, an UTS of 448 MPa, and EL of 15% can be achieved in commercial AZ61 alloy via ‘ECAP+EPT' technology.It is of great importance to commercialize such a technology for the industrialization of Mg and Mg alloys.Furthermore,a new spinodal decomposition strengthening mechanism was reported in ultralight Mg-Li-Al alloys.

Functional magnesium materials such as bio-magnsium alloys and energy storage Mg materials have made some new progress.In the case of bio-magnesium alloys, a new Resoloy rare earth Mg-Dy alloy for biodegradable cardiovascular stents application is developed, which is a good alternative to CE approved WE43 alloy.In the case of Mg batteries, a family of methoxyethyl-amine chelants has been developed, which could tune solvation structure of simple Mg(TFSI)2/DME electrolyte, improving the interfacial charge transfer kinetics and suppressing the passivation reaction.Additionally, rechargeable magnesium soft-pack cells were successfully assembled.In the case of hydrogen strorage materials, ultrafin MgH2nanoparticles were successfully synthesized, which reached 6.7 wt.% reversible storage of hydrogen at 30°C.A lot of catalysts, such as TiFe, Ti3C2, Ni/ Ti3C2and so on, were found to improve the thermodynamic and kinetic properties of MgH2.

Although much progress has been made on the research and development of magnesium alloys, there are still some challenges limiting the massive application of magnesium and magnesium alloys.In the case of Mg materials development,mechanical properties of structual magnesium alloys at both room temperature and elevated temperature, and the industrialization of Mg batteries, hydrogen storage Mg materials need to be improved.In the aspact of processing technologies,the microstructual evolution and heat treatment of vacuum die castings, the cost and safety of additive manufacturing of large complex Mg alloy parts, plastic processing technologies with low cost and industrialization potential,and welding of castings with complex morphology requires extentive invesitigation.In the case of corrosion and protection of Mg alloys, the high corrosion-resistant magnesium alloys and effective surface treatment of magnesium alloys should be further developed to meet the application requirements on most occasions, the environmental corrosion behavior of Mg alloys needs more attention.

Acknowledgments

The authors would like to thank the experts worldwide(including Dr.Liang Wu, Dr.Jia She, Dr.Jianbo Li, and Dr.Shuangshuang Tan, etc.) who have contributed to this review by providing important information and commenting on the preparation.The financia support from the Guangdong Major Project of Basic and Applied Basic Research(2020B0301030006), National Natural Science Foundation of China (NSFC) (No.52071036), Key Research and Development Program of Zhejiang Province (No.2021C01086), and the Fundamental Research Funds for the Central Universities Project (Nos.2021CDJCGJ009, SKLMT-ZZKT-2021M11) is also gratefully acknowledged.