Young-Gil Jung,Wonseok Yng,Yong Joo Kim,She.K.Kim,Young-Ok Yoon,Hyunkyu Lim,*,Do Hyng Kim

aAdvanced Materials and Process R&D Department,Korea Institute of Industrial Technology,156,Gaetbeol-ro,Incheon 21999,Republic of Korea

b Department of Metallurgical Engineering,Yonsei University,50,Yonsei-ro,Seoul,Seodaemun-gu 03722,Republic of Korea

Abstract In this study,the effects of Ca addition on the microstructure,thermal properties,and mechanical properties of a Mg-6.0Zn-1.2Y-0.7Zr(ZWK611)alloy at room temperature and 150°C were investigated.With an increase in the Ca content,the ignition resistance of the ZWK611 alloy improved and the grains became fine.The as-cast ZWK611 alloy consisted mainly of the dendriticα-Mg matrix and I-phase(Mg3Zn6Y)at the grain boundaries.On the other hand,the τ-(Ca2Mg6Zn3)and I-phases were formed in the Ca-added ZWK611 alloy.The fraction of the τ-phase increased with an increase in the Ca content.After the solid-solution treatment,these phases remained partially at the grain boundaries of the Ca-added ZWK611 alloys and an additional W-phase(Mg3Zn3Y)was observed.The phases remaining at the grain boundaries restricted the grain growth through the grain boundary pinning effect.The aging treatment resulted in the formation of MgZn’precipitates in the α-Mg matrix of the alloys.These precipitates were more uniformly distributed in the Ca-added alloys than in the alloys without Ca.Thus,the heat treatment-induced precipitation improved the tensile and creep properties of the T6-treated alloys.The T6-treated ZWK611+0.7Ca alloy exhibited the best mechanical properties at room temperature and 150°C among all the tested alloys.? 2021 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:Mg-Zn-Y alloys;Heat-treatment;Precipitate;Mechanical properties;Thermal analysis.

1.Introduction

The commercial cast Mg alloys are mainly classifie into Mg-Al-,Mg-Zn-,and Mg-RE(rare-earth element)-based alloys.Mg-Al-based alloys are AZ and AM series alloys such as AZ91D and AM60 alloys.These alloys offer an excellent combination of mechanical properties,corrosion resistance,and die-castability.However,they have lower creep resistance above 125°C due to the poor thermal stability ofβ-Mg17Al12,which makes them not suitable for applications at elevated temperatures[3].In case of Mg-RE-based alloys such as WE43 and EV31 alloys,these alloys show good mechanical properties at room temperature and significan creep resistance,but they are still restricted due to a cost disadvantage because of the high content of RE elements such as Nd and Gd,restricting the automotive application[4].Among Mg-Zn-based alloys,Mg-Zn-RE-Zr alloys offer excellent mechanical properties and creep resistance at room and high with small amounts of RE[5].Especially,the addition of Y to Mg-Zn-based alloys results in the formation of several secondary phases,which significantl affect the properties of the alloys[6,7].In general,depending on the Zn/Y ratio,three types of ternary phases are formed in the Mg-Zn-Y system:The I-,W-,and long-period stacking ordered(LPSO)phases.Among the these phases,the I-phase with a unique crystal structure(icosahedral quasicrystal structure)can induce significan strengthening effect and improves the ductility of Mg-Zn-Y alloys because of its high hardness,strong bonding,and low interfacial coefficien by coherent interfaces with theα-Mg matrix[8].Additionally,it was reported that the I-phase was more effective in the mechanical properties at room temperature with the higher ductility when compared with the W-phase[6].Thus,a relatively large amount of Wphase is not rather effective in the mechanical properties at room temperature.On the other hand,the W-phase shows good thermal stability at elevated temperatures and high elastic modulus and higher hardness than theα-Mg matrix and LPSO phases[9,10].Furthermore,the LPSO phase in the Mg alloys has been considered as the greatest strengthening secondary phase,which provides the strengthening effect by the formation of kink bands[11].In general,it is reported that this phase is easier to be formed when the Y content is higher than Zn content.However,it is limited to use a high amount of RE in Mg alloys due to the high cost as the commercial application[9-11].Moreover,the addition of Ca to Mg-Zn-based alloys results in the formation of theτ-phase and Mg2Ca depending on the Zn/Ca ratio.Ca is an effective grain refine for Mg alloys[12].Furthermore,the addition of Ca significantl improves the ignition resistance of Mg alloys by forming a dense oxide fil on the surface[13].

As mentioned above,in previous studies,the effects of various alloying elements such as Y and Ca etc.,on Mg-Zn-based alloys have been reported[5-13].However,these studies have been mostly focused on the development of wrought Mg alloys than the cast Mg alloys.Besides,so far,there have been only few studies on the effects of Ca addition to Mg-Zn-Y alloys with I-phase for the cast Mg alloy.S.Naghdali et al.had reported on the solidificatio behavior and microstructure evolution of as-cast Ca-added Mg-5.0Zn-1.0Y alloy with I-phase,but their mechanical properties at room and elevated temperature were not discussed at all[14].In addition,for the application as the cast Mg alloy in the future,heat-treatment is essential for improving the mechanical properties and microstructural stability of as-cast alloys.The solid-solution aging treatment(T6-treatment)in particular,has been found to be useful for improving the mechanical properties of Mg-Znbased cast alloys.This is because the T6-treatment induces the formation of fin precipitates,which obstruct the movement of dislocations in the alloy[15].

For this reason,to improve the ignition resistance and mechanical properties of cast Mg alloy,as the fift minor alloying element,Ca was selected.Therefore,it is obvious that there is a need to investigate the effects of Ca addition on microstructure and mechanical properties of the as-cast and heat-treated Mg-Zn-Y-Zr alloys with I-phase.Consequently,in this study,we developed a Ca-added Mg-Zn-Y-Zr alloy for the application of cast Mg alloy in EV parts and subjected it to heat-treatment to achieve excellent ignition resistance and mechanical properties at room and elevated temperatures.The effects of the addition of Ca on the microstructures and mechanical properties of the Mg-6.0Zn-1.2Y-0.7Zr(ZWK611)alloy were investigated.The secondary phases formed by the addition of Ca were identified The effects of Ca addition on the grain size and the distribution of the precipitates formed in theα-Mg matrix after the T6-treatment were also investigated.The correlation between the mechanical properties and microstructure of the alloy was investigated.

2.Experimental procedure

2.1.Materials preparation

The nominal and measured(by inductively coupled plasma atomic emission spectroscopy)chemical compositions of the Mg-6.0Zn-1.2Y-0.7Zr-xCa(x=0,0.3,0.5,and 0.7wt.%)alloys are listed in Table 1.The alloys were fabricated by melting commercial pure metals(Mg,Zn)and master alloys(Mg-3.8Ca,Mg-25Y,and Mg-33Zr)in a steel crucible under the protection of a mixed gas atmosphere of SF6and CO2.The melt was held at 740°C for 1h and was transferred to a permanent steel mold(40mm x 90mm x 150mm)at 700°C.

Table 1Chemical compositions of the ZWK611-xCa(x=0,0.3,0.5,and 0.7wt.%)alloys.

Table 2EDS analysis results for the SEM images shown in Fig.6(a)-(d).

Fig.1.Hardness variation curves of the tested alloys under the different aging time at 180 °C.

2.2.Heat-treatment

To optimize the heat-treatment conditions,the specimens were solution-treated at 380°C for 8h followed by quenching in hot water at 80°C to prevent the hot-cracking after solution heat treatment.After the solid-solution treatment,the alloys were isothermally aged at 180°C for different durations(3-48h)to achieve the peak aged condition,i.e.to determine the aging time corresponding to the peak hardness.As shown in Fig.1,the hardness of each aged specimen was measured using a Rockwell hardness tester(Daekyung tech Co.Ltd.,Republic of Korea)under 60 kgf of the F-scale.The T6-treatment(380°C/24h+180°C/12h)of the as-cast ingots was carried out on the basis of the hardness test results.

2.3.Thermal analysis

The ignition temperature of the alloys was measured using a differential thermal analyzer(DTA,TA instrument SDT Q600)over the temperature range of 100-1300°C at a heating rate of 10°C/min under a dry air atmosphere.In addition,after the T6-treatment,the variation in the phase transformation(dissolution and precipitation)energy was analyzed using differential scanning calorimetry(DSC,Perkin Elmer)under continuous heating from 100 to 510°C at the heating rate of 20°C/min in an Ar gas atmosphere.

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2.4.Microstructures and phase analysis

The microstructural analyses of the alloys were carried out using optical microscopy(OM)(Nikon MA200),scanning electron microscopy(SEM)(FEI QUANTA 200F),and energy dispersive X-ray spectroscopy(EDS).For the preparation of the OM and SEM samples,a solution of 2 vol% nitric acid+ethyl alcohol was used as an etchant.In addition,for preparing the grain size measurement samples,a solution of 80mL ethanol,4.8g picric acid,10mL acetic acid,and 10mL distilled water was used as an etchant.The grain size of the alloys was measured using the ASTM E112 standard test method.The samples for transmission electron microscopy(TEM)(JEOL 2001 FX)were prepared by ion milling(Gatan,Model 600).To identify the secondary phases in the alloys,high-resolution X-ray diffraction(HR-XRD)(Rigaku Smart-Lab)with Cu Kαradiation was used.The diffraction patterns were recorded over the 2θrange of 20-80° at a scanning speed of 2 °/min.

Fig.2.The ignition temperature results of the tested alloys.

2.5.Mechanical properties

Dog-bone-type specimens with ASTM B557-84(gauge length:30mm,diameter:6.0mm)standard were prepared for carrying out the tensile tests of the alloys at room temperature and 150°C.Room temperature elongation was measured using the extensometer(Epsilon Tech.corp.),and the hightemperature elongation was calculated by measuring the actual elongated length after the test.The tensile tests were carried out at a cross-head speed of 1.0mm/min.Tensile creep specimens with a gauge length of 15mm,thickness of 3mm,and width of 7mm were machined and the creep test was carried out at 150°C/100MPa.

3.Results

3.1.Ignition temperature

The ignition temperature results of the alloys are shown in Fig.2.From the exothermic peaks associated with the onset of ignition,the ignition temperatures of the ZWK611+xCa alloys withx=0,0.3,0.5,and 0.7wt.% were calculated to be 714.50,887.48,960.44,and 983.25°C,respectively.The ignition temperature of the alloys increased and gradually saturated with an increase in the Ca content.This indicates that the addition of Ca improved the ignition resistance of the ZWK611 alloy.It has been reported that the addition of Ca significantl improves the ignition resistance of Mg alloys[16,17].According to Ha et al.[18],the addition of Ca effectively decreases the oxidation rate of molten Mg alloys through the formation of a thin and dense CaO-MgO fil on the surface,thus retarding the oxidation and ignition of the alloy.

Fig.3.Optical images of the ZWK611+xCa(x=0,0.3,0.5,and 0.7wt.%)alloys:(a-d)as-cast and(e-h)T6-treated.

Fig.4.Optical images to compare the grain sizes of the ZWK611+xCa(x=0,0.3,0.5,and 0.7wt.%)alloys:(a-d)as-cast and(e-h)T6-treated.

3.2.Microstructure

The optical micrographs of the as-cast and T6-treated ZWK611+xCa(x=0,0.3,0.5,and 0.7wt.%)alloys are shown in Fig.3(a-h).It can be observed from Fig.3(a-d)that the microstructures of the as-cast ZWK611+xCa(x=0,0.3,0.5,and 0.7wt.%)alloys consisted of dendritic primaryα-Mg and interdendritic eutectic phases.In addition,net-like secondary phases were formed along the grain boundaries ofα-Mg with an increase in the Ca content.On the other hand,in the case of the T6-treated alloys,the volume fraction of the secondary phases reduced through the solid-solution treatment process and discontinuous net-like microstructures were observed,as shown in Fig.3(e-h).Furthermore,a large fraction of the secondary phases remained at the grain boundaries even after the solid-solution treatment.The microstructures etched by the picric etchant were observed to investigate the effect of Ca addition on the grain size of the alloys(Fig.4(ah)).The average grain size results of the alloys are shown in Fig.5.The value of average grain size for as-cast alloys are 36.61±1.46μm,26.65±1.50μm,21.64±1.03μm,and 18.92±0.40μm,respectively.Also,the value of average grain size for T6-treated alloys are 46.41±1.53μm,32.07±2.17μm,24.15±1.60μm,and 20.14±0.78μm,respectively.The grain size of the tested alloys decreased with an increase in the Ca content.The grains in the T6-treated alloys were larger than those in the as-cast alloys.It is wellknown that Ca is an effective grain refine for Mg alloys,and the grain size of Mg alloys decreases significantl with the addition of up to 1.0wt.% Ca[14,19,20].In addition,the degree of grain growth gradually decreased with an increase in the Ca content and a saturated curve was obtained,as shown in Fig.5.

Fig.5.Average grain sizes of the tested alloys measured by ASTM E112 standard test method.

The SEM images of the tested alloys are shown in Fig.6,and the corresponding SEM-EDS results are listed in Table 2.As can be observed from Fig.6(a)and(b),the as-cast alloys showed two types of secondary phases(bright and gray phases).The bright phases(marked as“a”and“b”in Fig.6(a)and 6(b))in the as-cast ZWK611 and ZWK611+0.7Ca alloys were composed of Mg,Zn,and Y.A small amount of Ca was detected in the case of the ZWK611+0.7Ca alloy.The compositions of these phases were similar to that of the I-phase.This phase showed eutectic pocket morphology[21].The gray phase(marked as“c”in Fig.6(b))in the as-cast ZWK611+0.7Ca alloy was composed of Mg,Zn,and Ca.This composition corresponds to theτ-phase.The morphology of this phase was slightly different from that of the I-phase.The SEM images of the T6-treated ZWK611 and ZWK611+0.7Ca alloys are shown in Fig.6(c)and 6(d),respectively.The T6-treated ZWK611 alloys also showed the Mg-Zn-Y phases(marked as“a”and“b”in Fig.6(c)).Most of these phases dissolved after the solid-solution treatment.However,some of the Mg-Zn-Y phases remained at the grain boundaries.The compositions of these phases were similar to that of the I-phase.On the other hand,in the case of the T6-treated ZWK611+0.7Ca alloy,the Mg-Zn-Y and Mg-Zn-Ca phases(marked as“c”and“d”in Fig.6(d),respectively)with compositions similar to those of the W-andτ-phases,respectively,remained at the grain boundaries.The EDS mapping analysis of the T6-treated alloys was carried out determine the amounts of the elements dissolved in theirα-Mg matrix(Fig.7).The EDS mapping results showed that theα-Mg matrix of both the T6-treated ZWK611 and ZWK611+0.7Ca alloys consisted of Zn and Y.The Zn and Y contents of the ZWK611+0.7Ca alloy were higher than those of the ZWK611 alloy.Ca was also detected in theα-Mg matrix of the ZWK611+0.7Ca alloy.

3.3.XRD analysis

Fig.8 shows the HR-XRD patterns of the as-cast and T6-treated ZWK611+xCa(x=0,0.3,0.5,and 0.7wt.%)alloys.All the as-cast alloys showed the diffraction peaks characteristic of theα-Mg and I-phases.Weak W-phase diffraction peaks were also observed.The as-cast ZWK611 alloys with Ca also showed the diffraction peaks characteristic of theτ-phase.On the other hand,the T6-treated alloys with Ca showed only the diffraction peaks corresponding to the W-andτ-phases.The diffraction peaks corresponding to the I-phase were not observed in the XRD patterns of these alloys.Fig.8(c)shows the magnifie XRD patterns of the as-cast and T6-treated alloys over the 2θrange of 37.5-42.5° showing the variation in the diffraction peaks of the alloys after the T6-treatment.The most intense peak of the I-phase is observed at 2θ=38.4°.It has been reported that the MgZn phase in Mg-Zn-based alloys mainly precipitates during long-term aging at 200°C and its crystallographic structure is similar to that of MgZn2,whose diffraction peaks are observed at 2θ=40-42°[22,23].Therefore,in this study,the diffraction peaks corresponding to the MgZn2phase of the T6-treated alloys were observed at 2θ=39.5-41.5°.The I-phase peak intensity of the ZWK611 alloys decreased slightly after the T6-treatment.The I-phase diffraction peaks of the Ca-added ZWK611 alloys flattene after the T6-treatment.

3.4.Mechanical properties

Fig.6.SEM micrographs of ZWK611 and ZWK611+0.7 alloys:(a,b)as-cast and(c,d)T6-treated specimens;(e)and(f)are high magnificatio image of area‘A’and‘B’,respectively.

Fig.7.EDS elemental mapping results for the SEM images shown in Fig.6(c)and(d):(b and e)Zn,(c and f)Y,and(g)Ca.

Fig.8.HR-XRD patterns for(a)as-cast and(b)T6-treated specimens of the tested alloys,(c)The intensity variation at 37.5-42.5 ° range after the T6-treatment.Note that the most intense peak of I phase is at 38.4 ° and MgZn2 diffraction peaks is mainly detected at 40-42 ° range[18,19];■a-Mg●I-phase(Mg3Zn6Y)▼W-phase(Mg3Zn3Y2),◆τ-phase(Ca2Mg6Zn3),and▲MgZn2(MgZn’precipitate).

The tensile test results of the as-cast and T6-treated ZWK611+xCa(x=0,0.3,0.5,and 0.7wt.%)alloys are listed in Table 3.The tensile properties for the tested alloys at room temperature and 150°C are shown in Fig.9.As shown in Fig.9(b),in the case of the as-cast alloys,the tensile yield strength at room temperature increased with the addition of Ca because of the formation of theτ-phase and an increase in its volume fraction and the grain refinemen effect.The elongation of the as-cast alloys increased slightly with the addition of 0.3wt.%Ca.However,the elongation decreased with a further increase in the Ca content.The T6-treatment improved the tensile yield strength of the tested alloys at room temperature by about 15-20%.The T6-treated ZWK611+0.7Ca alloy exhibited the highest tensile yield strength among all the T6-treated alloys.The T6-treated ZWK611+0.7Ca alloy also showed the highest strength at 150°C among all the tested alloys,as shown in Fig.9(d).

Table 3Tensile-test results for the tested alloys at room temperature and 150°C.

Fig.9.Stress-strain curves and mechanical properties of the as-cast and T6-treated ZWK611+xCa(x=0,0.3,0.5,and 0.7wt.%)alloys at room temperature((a)and(b))and 150°C((c)and(d)).

Fig.10 shows the creep curves of the tested alloys under the applied stress of 100MPa at 150°C.The creep strains and secondary creep rate of the tested alloys are listed in Table 4.The creep strains of the T6-treated ZWK611,ZWK611+0.3Ca,ZWK611+0.5Ca,and ZWK611+0.7Ca alloys were 0.712%,0.302%,0.274%,and 0.215%,respectively.Also,their secondary creep rate are 1.007×10?8/s,4.944×10?9/s,5.739×10?9/s,and 4.547×10?9/s,respectively.The creep properties of the alloys at 150°C exhibited a trend similar to that shown by the tensile properties at 150°C.The creep properties of the Ca-added ZWK611 alloys were superior to those of the ZWK611 alloys.The T6-treated ZWK611+0.7Ca alloy exhibited the lowest creep strain among all the tested alloys.

Fig.10.Creep test curves of the tested alloys at 150°C and 100MPa.

Table 4Creep strain and secondary creep rate at 150°C and 100MPa.

4.Discussion

4.1.Investigation of microstructures

In this study,the effects of Ca addition on the microstructure and mechanical properties of Mg-Zn-Y-Zr alloys were investigated.The OM,SEM,and XRD analyses revealed that the microstructure of the as-cast ZWK611 alloy consisted mainly of theα-Mg and I-phases with a small amount of the W-phase(Mg3Zn3Y2).Although the I-phase in Mg-Zn-Y alloys is commonly formed by a eutectic reaction(L→α-Mg+I-phase),the W-phase is directly formed by the melt through a eutectic reaction during the initial solidificatio stage.This phase then transforms into the I-phase by the end of the solidificatio process via a peritectic reaction(L+Wphase→α-Mg+I-phase)[24,25].Therefore,in this study,the W-phase did not fully transform into the I-phase,and some of the W-phase remained in the alloys after the solidifica tion process.The Ca-added as-cast ZWK611 alloys showed an additional Mg-Zn-Ca phase,which was identifie to be theτ-phase.Theτ-phase volume fraction of the alloys increased with an increase in the Ca content to form a continuous grain boundary network.In addition,the Mg2Ca phase was not detected in this study.It has been reported that the Ca-containing phases formed by the addition of Ca in Mg-Zn-based alloys are related to their Zn/Ca atomic ratios[26].The Mg2Ca andτ-phases are formed when the Zn/Ca atomic ratio is less than 1.2,while only theτ-phase is formed at the Zn/Ca atomic ratios higher than 1.2[27,28].In this study,the Zn/Ca atomic ratios of the tested alloys were 5.22-12.37.Thus,the formation of theτ-phase in the Ca-added ZWK611 alloys in this study is consistent with the results reported previously.

Fig.3(e-h)show the microstructures of the T6-treated alloys.The metallographic observation revealed that unlike the case of the as-cast alloys(Fig.3(a-d)),the original interdendritic secondary phases in the T6-treated alloys dissolved in the matrix after the solid-solution treatment.In the case of the T6-treated ZWK611 alloy,most of the I-phases dissolved into the matrix after the solid-solution treatment,while some of these phase remained partially at the grain boundaries,as shown in Figs.3(e)and 6(c).However,in the case of the Caadded T6-treated ZWK611+xCa alloys,a relatively large amount of the secondary phase remained at the grain boundaries compared to the T6-treated ZWK611 alloy despite the solid-solution treatment.The volume fraction of this phase increased with an increase in the Ca content,as shown in Fig.3(f-h).The secondary phases left at the grain boundaries of the Ca-added ZWK611 alloys after the T6-treatment consisted of the Mg-Zn-Y and Mg-Zn-Ca phases,as revealed by the SEM and EDS analyses.The Mg-Zn-Ca phase was identifie to be theτ-phase(marked“d”in Fig.6(d)).Meanwhile,the Zn/Y atomic ratio of the Mg-Zn-Y phase was 2.4(marked“c”in Fig.6(d)).Although the Zn/Y atomic ratio of the Mg-Zn-Y phase was intermediate to those of the I-and W-phases,its chemical composition was more similar to that of the Wphase.Furthermore,Fig.11(a)and(b)showed a TEM image and the corresponding selected-area diffraction(SAD)pattern for W-andτ-phases in T6-treated ZWK611 alloy.The SAD patterns exhibited[110]zone of the face-centered cubic structure(Fmm,a=0.683nm)of the W-phase and[101]zone of hexagonal structure(P63/mmc,a=0.9725nm,c=1.0148nm)of theτ-phase[30,31].From these results,it can be inferred that the Ca addition affected the I-phase volume fraction of the alloys and the W-phase fraction of the Ca-added ZWK611 alloys was relatively larger than that of the ZWK611 alloys.Besides,the W-phase exhibited higher thermal stability than I-andτ-phases(melting point of W-,I-,andτ-phases:About 520,450,and 400°C,respectively)[29,30,32].Thus,in the case of the T6-treated Ca-added ZWK611 alloys,not only theτ-phase but also a moderate amount of the W-phase also remained at the grain boundaries after the solid-solution treatment,as shown in Fig.11.Consequently,the presence of the W-phases at the grain boundaries contributed to the suppression of grain growth through the grain boundary pinning effect during the heat-treatment,as shown in Figs.4 and 5.Also,although the fraction of the formed W-phases during the solidificatio process cannot be accurately determined,their fraction would be not changed after the T6-treatment(solid-solution temperature:380°C)due to the thermally stable than I-andτ-phases.Besides,in a previous study[30],for the Mg-Zn-Y with I-phase,in case of undergoing a long time heat-treatment at above 420°C,it was reported that the I-phase transformed into the W-phase and thus,the solidsolution treatment effect may be rather reduced.Therefore,a moderate amount of W-phase for the Mg-Zn-Y with I-phase is improved on the mechanical properties at elevated temperature,but a relatively large amount of W-phase could be a rather negative effect on the mechanical properties at room temperature because it was not more effective than the Iphase as mentioned above[6].Fig.6(e)and(f)(labeled as“A”and“B”in Fig.6(c)and(d),respectively)showed the magnifie image ofα-Mg.The long-rod-like and globularlike particles could be clearly observed in theα-Mg matrix of the T6-treated ZWK611 alloys as shown in Fig.6(f)when compared with Fig.6(e).Also,as shown in Fig.12,the TEM and EDS analysis results showed that these particles consisted of Zn and Zr.According to previous studies[29,33],it was reported that these were identifie to be Zr-containing particles related to Zn-Zr particles.Therefore,it can be concluded that with an increase in the amount of dissolved Zn in theα-Mg matrix tiny globular-and long-rod-shaped Zr-containing particles precipitated in theα-Mg matrix.These results are consistent with the EDS elemental mapping results(Fig.7).

Fig.11.TEM images obtained using STEM mode and corresponding SAD patterns of secondary phase in the T6-treated ZWK611+0.7Ca alloy:(a)W-phase(Mg3Zn3Y2)and(b)τ-phase(Ca2Mg6Zn3).

Fig.12.The TEM image of T6-treated ZWK611+0.7 alloy obtained using STEM mode:EDS analysis results on the TEM image.

4.2.Thermal analysis

Phase transformation analysis was carried out using DSC to examine the variations in the I-andτ-phases of the alloys after the T6-treatment.The DSC curves of the tested alloys are shown in Fig.13.As mentioned above,it has been reported that the I-phase is unstable at about 420°C and shows a melting temperature of about 450°C[30].On the other hand,the melting temperature of theτ-phase is about 400°C[32].The as-cast ZWK611 alloy showed an endothermic peak at 458.7°C corresponding to the melting temperature of the I-phase.The Ca-added ZWK611 alloys on the other hand,showed an endothermic peak at 395°C(Fig.13(a))corresponding to the melting temperature of theτ-phases.These alloys exhibited two-step peaks.This is because theτ-phases preferentially dissolved and simultaneously diffused around the I-phase because of the formation of theτ-phases around the I-phase with a continuous grain boundary network.Thus,in the Ca-added ZWK611 alloys,the dissolution of the I-phase occurred at lower temperatures than that in the ZWK611 alloy.The endothermic peak area of the T6-treated alloys was smaller than that of the as-cast alloy.In addition,the T6-treated alloys showed broad exothermic precipitation peaks at about 210-310°C.It is well-known that the Mg-Zn binary eutectic temperature is about 340°C.Thus,the precipitated MgZn2phase slowly decomposed in the matrix with an increase in temperature[34].Fig.14 shows the enthalpies(ΔH)of I-andτ-phases calculated from the endothermic peaks of the alloys using the Gauss function.The calculated enthalpies(ΔH)are listed in Table 5.With the addition of Ca,the enthalpy(ΔH(As,I-phase))of the I-phase in the as-cast ZWK611 alloy decreased,while the enthalpy(ΔH(As,τ-phase))of theτ-phase increased.The amounts of the I-andτ-phases dissolved in theα-Mg matrix could be calculated by measuring the variation in the enthalpy of the alloys(ΔH(As-T6))after the solid-solution treatment.In the case of the Ca-added ZWK611 alloys,the I-phases were almost entirely dissolved in theα-Mg matrix,whereas the amount of the dissolvedτ-phases decreased gradually with an increase in the Ca content.This is because the dissolution of the Iphases in theα-Mg matrix was accelerated by the formation of a continuous network at the grain boundaries.It can also be attributed to the difference in the solubility limits of Y and Ca(3.40 and 0.82 at.%,respectively)[35].Therefore,with an increase in the Ca content,theτ-phase showed smaller atomic diffusion than the I-phase.Thus,a driving force(such as thermal energy and time)was required for the dissolution of theτ-phases.

Fig.13.DSC curves of the(a)as-cast and(b)T6-treated ZWK611+xCa(x=0,0.3,0.5,and 0.7wt.%)alloys.

Fig.14.Enthalpy(ΔH)results of the I-and τ-phases,as obtained from the endothermic peaks in the DSC curves of the alloys.

Table 5Enthalpy(ΔH)results of the I-and τ-phases.

4.3.Correlation between the mechanical properties and microstructure

The formation of theτ-phase improves the mechanical properties of Mg-Zn-based alloys[36].The tensile yield strength at room temperature of the as-cast alloys increased with the formation of theτ-phase,volume fraction increased with an increase in the Ca content,as shown in Fig 9(b).However,the elongation of the as-cast alloys firs increased with the addition of 0.3wt.% Ca and then decreased with a further increase in the Ca content.It has been reported that a high stress concentration at theτ-phase/matrix interface can trigger the formation of voids,leading to the nucleation of cracks because of the weak atomic bonding with theα-Mg matrix[37].Therefore,the largeτ-phase volume fraction of the as-cast Ca-added Mg-Zn-Y-Zr alloys deteriorated their ultimate tensile strength and elongation at room temperature.

Fig.15.TEM images taken along the directions parallel to[0001]Mg(a and c,multi-beam images)and[010]Mg(b and d,diffracting vector indicated)for the T6-treated ZWK611 and ZWK611+0.7Ca alloys;(e-h)are the magnifie images of(a-d),respectively.

According to the previous studies[15,38-40],it was reported that the precipitates in Mg-Zn-based alloys have been identifie to be MgZn’.Also,the chemical composition and structure of these precipitates are similar to those of MgZn2.Likewise,it can be confirme that the diffraction peaks corresponding to the MgZn’precipitate in the T6-treated alloys were observed by the result of XRD analysis in this study(Fig.8).Furthermore,TEM analysis was carried out at the zone axis of parallel and perpendicular planes to the basal of theα-Mg matrix plane to understand the difference of distribution for the MgZn’precipitates in non-Ca added and Caadded alloys.Fig.15 shows the TEM images of the precipitation occurring in theα-Mg matrix of the alloys.In general,Mg-Zn-based alloys exhibit two types of precipitates(β’1andβ’2)during the aging treatment.Theβ’1precipitates exhibit rod-like morphology and lie in the(110)Mgplanes with the[0001]Mggrowth direction.Theβ’2precipitates exhibit discshaped morphology and lie parallel to the(0001)Mgplanes.In addition,theβ’1precipitates grow up to a few micrometers,while theβ’2precipitates can grow only up to a few hundred nanometers[38].However,in this study,the size of the observed MgZn’precipitates was less than 100nm,which is smaller than the sizes reported previously.The addition of rare earth elements can decrease the age hardening rate of Mg-Zn-based alloys and decelerate their over-aging by suppressing the nucleation of precipitates[39].The distribution of precipitates was more homogeneous in the Caadded ZWK611 alloys than in the ZWK611 alloy.The addition of trace amounts of Ca to the Mg-4Zn alloy can significantly enhance its age-hardening response by the refinemen and homogeneous distribution of precipitates[40].However,the mechanism underlying this observation needs further investigation.Therefore,the Ca addition increased the tensile yield strength of the T6-treated alloys at room temperature by precipitation hardening and grain refinement In addition,not only theτ-phase but also the relatively thermally stable Wphase remained at the grain boundaries of the alloys after the T6-treatment.The volume fractions of these phases increased with an increase in the Ca content.This effectively inhibited the grain boundary migration and sliding,thus improving the creep resistance of the alloy.

Overall,the present work investigated the effects of Ca addition on microstructure and mechanical properties of ascast and heat-treated Mg-Zn-Y-Zr alloy with the I-phase.For the as-cast alloy series,it was confirme thatτ-phase around the I-phase was formed by the Ca addition to ZWK611 alloy and also,according to the partial change of the volume fraction of I-phase,a relatively large amount of the W-phases existed in Ca-added ZWK611 alloys.The volume fraction of these phases with a continuous grain boundary network was increased and the grain of the tested alloys refine with the increase of Ca content.Also,these phases partially remained at grain boundaries despite the solid-solution treatment.For the heat-treated alloy series,with the solid-solution treatment,theτ-phases had preferentially dissolved in theα-Mg matrix and then,simultaneously diffused around the I-phase because of the formation of a continuous grain boundary network.Eventually,the dissolution of the I-phases was accelerated by the addition of Ca and a relatively large amount of Zn atom in theα-Mg matrix of Ca-added ZWK611 alloys existed more than that of ZWK611 alloy after the solid-solution treatment.Thus,with the aging treatment,the precipitation hardening effects of Ca-added ZWK611 alloys had significantl enhanced than that of ZWK611 alloy without Ca.

In summary,when it combined with experimental and discussed results in this study,Fig.16 shows the schematic illustration of microstructure for the present studied alloys.Thus,by the enhanced MgZn’precipitation hardening effect in theα-Mg matrix together with the grain boundary pinning effect by the remainingτ-phase and a moderate amount of thermally stable W-phase at the grain boundaries,the mechanical properties of T6-treated Ca-added ZWK611 alloys at room temperature and 150°C were more improved than those of T6-treated ZWK611 alloy.Therefore,in this study,it could be confirme that the Ca addition to heat-treated Mg-Zn-Y-Zr alloy with the I-phase acts as a significan role in the mechanical properties at room temperature and 150°C.Furthermore,the results of the present study would be an important background on the new application of Mg alloy in the EV parts and lead to a positive effect on the development of the cast Mg alloy in the future.

Fig.16.The schematic illustration of microstructure for the present studied alloys.

5.Conclusion

We investigated the effects of heat-treatment on the microstructure and mechanical properties of Ca-added Mg-Zn-Y-Zr alloys.The main conclusions are as follows:

(1)The ignition temperature of the Ca-added ZWK611 alloys increased with an increase in the Ca content,indicating that the ignition resistance of the Mg-Zn-Y-Zr alloy improved with the addition of Ca.

(2)With the addition of Ca,theτ-phase was formed in the ZWK611 alloy.The volume fraction of theτ-phase increased with an increase in the Ca content.In addition,the volume fraction of the I-phase changed owing to the formation of theτ-phase.A relatively large amount of the W-phase was formed in the Ca-added ZWK611 alloys and it remained at the grain boundaries even after the T6-treatment.

(3)The MgZn’precipitates formed in the ZWK611 alloy were smaller than those observed in Mg-Zn binary alloys.With the addition of Ca to the ZWK611 alloys,not only the grain boundary pinning effect by the remainingτ-and W-phase at grain boundaries but also the homogeneous distribution of precipitates by the heattreatment act as an important role on the mechanical properties of the tested alloys at room temperature and 150°C.

(4)The T6-treated ZWK611+0.7Ca alloy showed the highest tensile yield strength at room temperature owing to the precipitation hardening and grain refinemen effects.Also,a moderate amount of thermally stable Wphase remaining at the grain boundaries after the T6-treatment significantl contributed to the suppression of grain growth and the improvement in the mechanical properties of the alloy at 150°C.

Declaration of Competing Interest

The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.

Acknowledgments

This study was carried out with the support of the Korea Institute of Industrial Technology as"Enterprise demand-based production technology commercialization project(KITECH JG-20-0003)".