Tinshuo Zho ,Yobo Hu,b,? ,Cho Zhng ,Bing He ,Tinxu Zheng ,Aito Tng,b ,Fusheng Pn,b,?

a College of Materials Science and Engineering,Chongqing University,Chongqing 400044,China

b National Engineering Research Center for Magnesium Alloys,Chongqing 400044,China

Abstract In general,different extrusion conditions will affect the microstructure of magnesium alloys and further determine the mechanical properties.The effects of extrusion parameters and heat treatment processes such as extrusion speed,pre-forging,annealing time,extrusion ratio and cooling rate on the microstructure,texture evolution and tensile properties of Mg-2Gd-0.3Zr alloys were investigated in this study.Compared with the as-cast alloy,the extrusion process significantly refine the grains and exhibit the rare earth texture.With the increase of extrusion speed and annealing time,the growth of recrystallized grains is accelerated,leading to the increase of elongation.Large pre-forging deformation achieves to refin the grains by promoting recrystallization nucleation,resulting in increased strength of Mg-2Gd-0.3Zr alloy.Decreasing the extrusion ratio or increasing the cooling rate will introduce coarse un-DRXed grains,which transformed the texture into the basal texture.In particular,the effect of rapid cooling on refining the recrystallized grains is also obvious.Different extrusion conditions influence the mechanical properties of the Mg-2Gd-0.3Zr alloy through the grain size,proportion of non-recrystallization region and texture type.

Keywords: Mg-2Gd-0.3Zr alloy;Extrusion conditions;Microstructure;Texture;Mechanical properties.

1.Introduction

Magnesium alloys are regarded as ideal structural materials in the engineering field [1,2].It is generally believed that magnesium alloys have broad application prospects in the aerospace,automotive and digital products owing to their low density [3-7].However,its hexagonal close-packed (hcp)lattice structure lacks sufficient number of active slip systems,resulting in poor formability of magnesium alloy at room temperature [6,8,9].Therefore,the development of high ductility magnesium alloys has been strongly desired in manufacturing industry.

One of the common strategies to improve ductility is to introduce alloying elements into pure magnesium [10-12].In recent years,research on Mg-RE alloys has aroused great interest.It is reported that the addition of low content rare earth elements such as Gd,Y,Dy and Nd can not only effectively refin the grains but also change the texture type,thereby improving the ductility of magnesium alloy [13-15].Liu et al.studied the microstructure and mechanical properties of Mg-RE binary alloys,and found that the elongation of Mg-2.0Gd (wt.%) and Mg-0.2Ce (wt.%) alloys can reach more than 30% at room temperature after extrusion [16,17].Using in-situ high energy X-ray diffraction techniques,Xu revealed that solid solution Gd atoms activate prismatic slip in the early stages of compression deformation [18].Investigations on the Mg-Zn-Gd alloys have indicated that segregation of Gd at grain boundaries affects the recrystallizationbehavior and promotes the formation of rare earth texture[19].Generally,the coarse as-cast grains of magnesium alloys also limit the improvement of ductility [20].Since the lattice constant of Zr (a=0.323mm,c=0.514mm) is very close to Mg (a=0.321mm,c=0.521mm),it can provide nucleation sites for the solidification of magnesium melt.This effectively refine the initial as-cast grains of the alloy,so Zr is considered an ideal nucleating agent.In the past few years,our laboratory has successfully prepared a series of high plastic wrought Mg-Gd-Zr alloys [21,22].

It is well known that hot extrusion process is one of the important methods to enhance mechanical properties of magnesium alloys,which can eliminate casting defects [23-25].Different extrusion parameters will affect the microstructure of the alloy.In order to obtain good mechanical properties,it is necessary to select the appropriate extrusion conditions.The combination of slow extrusion speed and relatively low temperature can inhibit the growth of recrystallized grains,thus obtaining higher strength [26].Tong pointed out that different extrusion ratios determine the volume fraction of un-DRXed microstructure and achieve the transformation to strong basal texture [27].Studies have shown that the pre-deformation before extrusion contributes to the recrystallization behavior and finally obtains ultra-fin grain [28].However,there is a lack of literature to report the effects of extrusion parameters on Mg-2Gd-0.3Zr alloy.The aim of this work is to further improve the mechanical properties of Mg-2Gd-0.3Zr alloy by optimizing the extrusion conditions.Moreover,the influence of extrusion parameters and heat treatment processes on microstructure,texture evolution and mechanical properties were systematically studied.

2.Experimental procedures

The Mg-2Gd-0.3Zr alloy ingots used in this experiment were prepared by semi-continuous casting.The actual chemical composition was determined using X-ray fluores cence spectrometry (XRF-1800) and the result was listed in Table 1.

Table 1Chemical composition of studied alloys.

This work adjusted the microstructure of the as-extruded Mg-2Gd-0.3Zr alloy by changing extrusion parameters and heat treatment processes,such as die-exit speed,pre-forging deformation,annealing time,cooling rate,and extrusion ratio.The detailed process parameters are described below and summarized in Table 2.

Table 2Extrusion conditions of Mg-2Gd-0.3Zr alloys.

Table 3Tensile mechanical properties of as-extruded Mg-2Gd-0.3Zr alloys at different die-exit speeds.

(1) Conventional extrusion:Before extrusion,the cast ingot with diameter 80mm and height 50mm was preheated at the extrusion temperature for 1h.Then,the alloy was directly extruded with extrusion ratio of 27 at a die-exit speed of 5m/min under the 420 °C.

(2) Different extrusion speeds:Before extrusion,the cast ingot with diameter 80mm and height 50mm was preheated at the extrusion temperature for 1h.Then,the alloy was directly extruded with extrusion ratio of 27 at the 420 °C,and the die-exit speed used were 1m/min and 20m/min,respectively.

(3) Different pre-forging deformation:The pre-forging deformation of the sample is controlled by changing the shape of the cast ingot.In order to achieve the pre-forging deformation of 7% and 63%,two specification of ingots were selected,with the corresponding sizes ofΦ80×40mm and 45×45×100mm respectively.Except that the extrusion temperature is adjusted to 430 °C,other extrusion parameters are consistent with conventional extrusion processes.

(4) Different annealing time:The extruded bars obtained by conventional extrusion process were annealed at 420 °C,with annealing time of 90min,180min and 270min,respectively.

(5) Different extrusion ratio:Before extrusion,the cast ingot with diameter 80mm and height 50mm was preheated at the extrusion temperature for 1h.Besides the conventional extrusion process,the extrusion ratio of 8 was also selected.In order to obtain fine grains in the microstructure,extrusion was performed at 350 °C.

(6) Different cooling rate:The extruded bars obtained by conventional extrusion process were immediately water quenched,and the cooling rate was measured to be about 200 °C/s.

Fig.1.Optical microstructure of Mg-2Gd-0.3Zr alloy (a) as-cast,(b) as-extruded.

After grinding on SiC grit paper,the samples were etched with a solution of 2.0g picric acid,4mL acetic acid and 28mL anhydrous ethanol.Microstructure of as-cast and extruded Mg-2Gd-0.3Zr alloys were observed by using optical microscopy(OM,ZEISS Axiovert 40MAT),scanning electron microscope (SEM,JEOL JSM-7800F) and.X-ray diffraction(XRD,D/max-1200V) analysis was performed to determine phase and macro-texture.The electro-polishing samples were characterized by electron backscatter diffraction(EBSD)technique,and the obtained data were imported into the channel 5 software for analysis.Transmission electron microscope(TEM,FEI Talos F200S) was carried out at 200kV to detect the second phase.Tensile specimens with a gauge length of 35mm and a diameter of 5mm were machined by extruded bar along ED.Tensile tests were conducted on a CMT-5105 material testing machine using an initial strain rate of 2.0 ×10-3s-1at room temperature.

3.Results and discussions

3.1.Microstructure and properties of Mg-2Gd-0.3Zr alloys for conventional extrusion

The optical microstructures of as-cast and as-extruded Mg-2Gd-0.3Zr alloys are clearly shown in Fig.1.It can be seen that the microstructure of as-cast alloy consists of equiaxed grains without dendritic.Compared with other alloys,the ascast grain size of Mg-2Gd-0.3Zr alloy is very small,with an average grain size of only 26μm.This phenomenon is due to the fact that Zr acts as the nucleus of heterogeneous nucleation forα-Mg during solidification In addition,the molten alloy was casted into a water-cooled mould,so direct-chill casting also plays a key role in the fine grain size.After extrusion,the grain was refine obviously.In particular,the microstructure of the sample exhibits the characteristics of double-sized grains,which are composed of large-sized normal grains and fine grain bands distributed along the extrusion direction.

Fig.2.X-ray diffraction patterns of as-cast and as-extruded Mg-2Gd-0.3Zr alloys.

X-ray diffraction patterns of as-cast and as-extruded Mg-2Gd-0.3Zr alloys are depicted in Fig.2.It is apparent that both samples contain theα-Mg phase.There is a weak diffraction peak in the as-extruded alloy at 22.3°,which corresponds to the characteristic diffraction peak of Mg5Gd[29,30].However,it is difficult to observe the diffraction peak of Mg5Gd in the as-cast alloy.This is because semicontinuous casting mostly uses water cooling treatment.During solidification the cooling rate is too fast that the second phase cannot be precipitated from theα-Mg.Compared with the commercial AZ31 extruded alloy,the (0002) plane of the extruded Mg-2Gd-0.3Zr alloy has lower peak intensity [31].This implies that the alloy does not form the traditional fiber texture during the extrusion process.

In order to further determine the phases contained in the alloy,the samples are characterized by SEM and TEM.One can see that in Fig.3 there are very few nano-scale cubic phase and spherical particle phase in as-extruded Mg-2Gd-0.3Zr alloy.EDS analysis reveal that the cubic phase is the metastableβ”-Mg3Gd compounds and the spherical particle phase is theα-Zr,where the size of Mg3Gd is about 500nm and the radius of particle Zr is close to 300nm [30,32].From the above observations,the second phase present in the alloy is not only small in number but also nanometer in size.On the one hand,these second phases cannot promote grain nucleation by PSN mechanism during the recrystallization process,and on the other hand,they cannot effectively pin the dislocation movement during extrusion.Therefore,it can be considered that these second phase will not affect the properties of the alloy in this study.

Fig.3.SEM,TEM and EDS results of as-extruded Mg-2Gd-0.3Zr alloy.

Fig.4.(0001) pole figure and inverse pole figure of as-extruded Mg-2Gd-0.3Zr alloy.

Fig.4 indicates the macro-texture of the as-extruded Mg-2Gd-0.3Zr alloy taken from the cross section.The results show that the texture of as-extruded Mg-2Gd-0.3Zr alloy is obviously different from that of AZ31,where the basal texture is significantly weakened.It has the maximum texture intensity of only 2.5.In other words,this means that no traditional fiber texture is formed in the Mg-2Gd-0.3Zr alloy.It can be seen from the (0001) pole figure that the c-axis of most of the grains deviates from the ED to the outside by about 45°.In addition,the inverse pole figure presents that the texture component of the alloy is mostly＜122＞// ED,showing the typical rare earth texture.This is similar to the texture type of Mg-Gd alloy studied by Stanford [33].The addition of rare earth weakens the texture of magnesium alloys.This special texture is formed due to the segregation of rare earth atoms at the grain boundaries during extrusion [34].

3.2.Influence of the die-exit speed on microstructure and properties of Mg-2Gd-0.3Zr alloys

Fig.5 presents the EBSD maps and (0001) pole figure of Mg-2Gd-0.3Zr alloys extruded at different die-exit speeds.Fully dynamical recrystallization (DRXed) occurs in all extruded samples,and there is no un-DRXed structure distributed along the ED.At low extrusion speed (1m/min),the grain size distribution becomes more uniform,and there are fewer fine grain bands in the microstructure,compared with the microstructure at the extrusion speed of 20m/min.According to statistics,the average grain size is about 3.1μm.By increasing the extrusion speed to 20m/min,the grain size increases with an average grain size of 3.7μm.Compared with slow extrusion,there are more fine grain bands in alloys using fast extrusion.There are obvious differences between the grain size in the fine grain zone and the normal grain size.Previous studies have shown that Zener-Hollomon (Z)parameter can be used to qualitatively evaluate DRXed grain size (dDRX),which expressed by the following formula [35-37].

where A is a constant,and n is the power law exponent.

In addition,the relationship between strain rate () and Zener-Hollomon (Z) parameter can be define as [38]:

where Q is the diffusion activation energy,and R is the gas constant,and T is the temperature,and the average strain rate() during the plastic deformation process can be calculated by the following equation [39,40].

Fig.5.EBSD maps and (0001) pole figure of as-extruded Mg-2Gd-0.3Zr alloys at different die-exit speeds:(a) 1m/min (b) 20m/min.

where DBis the diameter of the cast ingot,and DEis the diameter of the extrusion bar,and VRis the die-exit speeds,and ER is the extrusion ratio.Combined with Eqs.(1),(2) and(3),it can be concluded that the DRXed grain size gradually decreases with the increase of extrusion speed.However,the experimental results show that the coarser grains will be formed at the faster extrusion speed,which is contradictory to the conclusion obtained by the above equation.Obviously there must be other more important factors that determine the grain size.This abnormal phenomenon is due to the fact that severe plastic deformation and friction generate a large amount of heat during rapid extrusion.The rapid increase of temperature in the extrusion barrel leads to the coarsening of DRXed grain size.The different extrusion speed not only affects the grain size but also changes the texture of the alloys.It can be observed that the Mg-2Gd-0.3Zr alloy exhibits a weaker texture at slower extrusion speeds.The orientation distribution of most grains is relatively random.As the extrusion speed increased,the texture distribution became more concentrated.Meanwhile,the texture components tend to tilt towards＜20＞and＜010＞.Lu pointed out that with the increase of extrusion speed,basal-oriented grains have a growth advantage over other oriented grains [41].

Fig.6.Tensile stress-strain curves of as-extruded Mg-2Gd-0.3Zr alloys at different die-exit speeds.

Fig.7.Distribution of Schmid factor for the (a,c) basal slip and (b,d) {102} tensile twining at different die-exit speeds:(a,b) 1m/min (c,d) 20m/min.

The tensile stress-strain curves of as-extruded Mg-2Gd-0.3Zr alloys at different die-exit speeds are illustrated in Fig.6.The values of mechanical properties are given in Table 3.In both conditions,the Mg-2Gd-0.3Zr alloys exhibit excellent ductility,which have an elongation of more than 50%.For slow extrusion,the yield strength,tensile strength,and elongation of the alloy are 187MPa,223MPa,and 52%,respectively.The slower extrusion exhibits a significantly higher yield strength.Although the average grain size difference is only 0.6μm,the size difference of normal grains is significant.For slow-extruded samples,the average grain size of normal grains is 4.9μm,while for fast-extruded samples,the average grain size is 7.0μm.In addition,the proportion of fine grain bands in slow-extruded samples is 7.6%,while the proportion in fast-extruded samples is 4.5%.It is believed that the number of fine grain bands also plays an important role in the mechanical properties of the alloys.When the extrusion speed is increased to 20m/min,the yield strength and tensile strength of the alloy are reduced to 105MPa and 182MPa,respectively,but the elongation is greatly increased to 62%.It is generally considered that basal slip and tensile twinning are the main factors affecting the ductility of magnesium alloys [42,43].Fig.7 presents the distribution of Schmid factor for basal slip and {102} tensile twining along the ED at different extrution speeds.Usually,different texture types will affect the initiation of the slip system.For faster extrusion,the average Schmid factors value of the basal slip is only 0.33,which weakens the contribution of basal slip to ductility.However,it is worth noting that the value of the average Schmid factors for {102} tensile twinning reaches 0.42 at 20m/min extrusion.Twins can coordinate plastic deformation by adjusting grain orientation.Among them,the maximum grain size of the fast-extruded samples is 26.1μm,while the maximum grain size of the slow-extruded samples is 14.9μm.There is no doubt that twins are highly dependent on grain size,and the increase of grain size can effectively reduce the CRSS required to produce twins [44].Therefore,for the rapidly extruded Mg-2Gd-0.3Zr alloy,a large number of tensile twins are activated in the grains during stretched along the ED,which improves the ductility of the alloy at room temperature.

Table 4Tensile mechanical properties of as-extruded Mg-2Gd-0.3Zr alloys in different pre-forging.

3.3.Influence of the pre-forging on microstructure and properties of Mg-2 Gd-0.3Zr alloys

Fig.8 shows EBSD maps,(0001) pole figure and grain size distributions of as-extruded Mg-2Gd-0.3Zr alloys in different pre-forging.It is found that the alloys prepared by two kinds of pre-forging deformation have undergone fully recrystallization.As shown in Fig.8(a),after pre-forging 7%,the alloy has a slightly larger grain size,with an average size of 3.4μm.The presence of fine grain bands is uniformly distributed in the microstructure.When the amount of deformation for pre-forging is 63%,the grains are obviously refined By measurement,the average grain size of the alloy is 2.3μm.In addition,with the increase of strain,the proportion of fine grains (grain size＜1μm) gradually increases,and the fraction of fine grains in the two samples are 34%and 60%,respectively.It becomes difficult to distinguish normal grains from grains in the fine grain bands.Both samples exhibit typical rare earth textures.In alloy with pre-forging 7%,the basal poles of most grains are tilted outward along ND.However,the texture distribution becomes more random as the strain increases.And the peak intensity is reduced from 7.25 to 3.02.It is believed that the pre-forging process accelerates the DRX behavior of the alloy as the main reason for the weakening of the texture.

Fig.8.EBSD maps,(0001) pole figure and grain size distributions of as-extruded Mg-2Gd-0.3Zr alloys in different pre-forging:(a) 7% (b) 63%.

The tensile stress-strain curves of the as-extruded Mg-2Gd-0.3Zr alloys in different pre-forging are shown in Fig.9.Corresponding mechanical properties are listed in Table 4.Further comparison reveal that the alloy after pre-forging 63% has higher strength,in which the yield strength,tensile strength and elongation are 180MPa,223MPa and 47%.This improvement in strength is due to the effect of fine-graine strengthening.Pre-forging process can not only effectively break the coarse as-cast grains but also effectively refin the recrystallized grains.In order to further understand the mechanism of grain refinement in the pre-forging process,Peng observed the microstructure of the sample after the forging stage[28].It is found that a large fraction of twins induced in the as-cast microstructure when small deformation occurs at the forging stage.Higher residual strain energy is stored near the twin boundaries,which can be released through the nucleation of DRXed grains.Therefore,twin recrystallization mechanism plays a dominant role at small strains [45].However,under large strains,fin grains form a necklace-like structure along the original grain boundaries.This is due to the high density dislocation produced by the pre-forging process,which is evolved into dislocation entanglement.Therefore,a large number of nucleation core are provided for the continuous dynamic recrystallization (CDRX).Meanwhile,fin grains can generate more random orientations,resulting in weakened texture.Apparently,the strength of the Mg-2Gd-0.3Zr alloy prepared by pre-forging deformation 7% is relatively low.The yield strength and tensile strength of the alloy are only 120MPa and 188MPa respectively,but it has excellent ductility with elongation exceeds 60%.

Fig.9.Tensile stress-strain curves of as-extruded Mg-2Gd-0.3Zr alloys in different pre-forging.

3.4.Influence of the annealing time on microstructure and properties of Mg-2 Gd-0.3Zr alloys

Fig.10 presents the EBSD maps of as-extruded Mg-2Gd-0.3Zr alloys annealed at 420 °C for different times.In the three samples,the alloys are composed entirely of fully recrystallized microstructure.It is worth noting that the grains of the alloy are gradually coarsened with the prolonging of annealing time.This is because there are very few second phases in the alloy,which cannot effectively hinder the grain growth.Therefore,the average grain size of the alloy after annealing for 90min,180min and 270min is 6.2μm,7.6μm and 8.5μm,respectively.In particular,fin grained bands are still present in the alloy after annealing for 270min.This is mainly related to the growth rate of the grains.With the extension of annealing time,the texture morphology had not significantly changed.This indicates that all the grains grown at the same rate,did not show preferential growth phenomenon.In addition,the tensile stress-strain curves of the as-extruded Mg-2Gd-0.3Zr alloys at 420 °C for different times are shown in Fig.11.The measured mechanical properties values are summarized in Table 5.It can be found that the annealing treatment deteriorates the strength of the alloys.All samples have the same tensile strength,which are close to 162MPa.With the extension of annealing time,the yield strength of the alloys decreased slightly,and the strength for 90min,180min and 270min is 80MPa,72MPa and 64MPa,respectively.It can be concluded from the Hall-Petch relationship that grain growth is the reason for the gradual decline in yield strength.Moreover,the ductility of the alloy can be improved by annealing process,and its elongation can reach 68%,71% and 75%,respectively.

Table 6Tensile mechanical properties of as-extruded Mg-2Gd-0.3Zr alloys for different extrusion ratio.

Fig.11.Tensile stress-strain curves of as-extruded Mg-2Gd-0.3Zr alloys annealed at 420 °C for different times.

Table 5Tensile mechanical properties of as-extruded Mg-2Gd-0.3Zr alloys annealed at 420 °C for different times.

Fig.10.EBSD maps and inverse pole figure of as-extruded Mg-2Gd-0.3Zr alloys annealed at 420 °C for different times:(a) 90min (b) 180min (c) 270min.

Fig.12.EBSD maps,inverse pole figure and misorientation angle distributions of as-extruded Mg-2Gd-0.3Zr alloys for different extrusion ratio:(a) 27 (b) 8.

3.5.Influence of the extrusion ratio on microstructure and properties of Mg-2 Gd-0.3Zr alloys

Fig.12 shows the EBSD maps,inverse pole figure and misorientation angle distributions of as-extruded Mg-2Gd-0.3Zr alloys prepared with different extrusion ratios.It can be observed that both samples consist of two different sizes of grains.When the extrusion ratio is 27,the alloy has undergone fully recrystallization with an average grain size of 2.7μm.Meanwhile,the bimodal microstructure of the alloy is composed of large grains and fine grain bands.As the extrusion ratio decreases to 8,the microstructure contains not only recrystallized grains but also un-DRXed grains elongated along the ED.The average size of the recrystallized grains is about 2.3μm.It can be judged that the fraction of un-DRXed grains will rise with the decrease of extrusion ratio.This is because the energy provided by the smaller extrusion strain is not sufficient to drive the recrystallization process.In addition,the extrusion temperature also affects the recrystallization behavior of the alloy.With the decrease of extrusion temperature,the speed of grain boundary migration slows down,which not only refine the recrystallized grains but also increases the volume fraction of un-DRXed grains.It can be found that there are a large number of high-angle grain boundaries (HAGBs) in the sample with the extrusion ratio of 27.This indicates that the larger extrusion strain makes the dislocation have a high migration rate,and then the low-angle grain boundaries (LAGBs) gradually change to HAGBs.New recrystallization grains are formed by migration and coalescence of sub-grain boundary [46].It is well known that a large number of unconverted LAGBs remain within the un-DXed grains,so that the LAGBs accounts for a high proportion in the alloy with an extrusion ratio of 8.Furthermore,the texture type with extrusion ratio of 27 belongs to rare earth texture,and the orientation distribution of most grains is random.However,as the extrusion ratio decreases,the texture exhibits typical fiber texture characteristics with [010]axis parallel to the ED,and the maximum intensity increases from 2.20 to 4.46.This is mainly caused by the rising of the un-DRXed grains fraction.

Fig.13.Tensile stress-strain curves of as-extruded Mg-2Gd-0.3Zr alloys for different extrusion ratio.

Fig.13 exhibit the tensile stress-strain curves of asextruded Mg-2Gd-0.3Zr alloys with various extrusion ratio,and the values of relevant mechanical properties are summarized in Table 6.For Mg-2Gd-0.3Zr alloy with an extrusion ratio of 27,the yield strength,tensile strength and elongation are 140MPa,197MPa and 51%,respectively.The rare earth texture in the alloy formed during extrusion is the main reason for the high ductility.Meanwhile,most of the grains can easily initiate basal slip.As the extrusion ratio decreases,the strength of the alloy is significantly improved,and its yield strength and tensile strength can reach 193MPa and 227MPa respectively.Compared with pre-forging 63%,a large number of un-DRXed grains are introduced into the microstructure of the alloy with an extrusion ratio of 8,which results in the transformation of the rare earth texture to the stronger basal texture.These grains in the hard orientation increase the stress required to start the slip system.Therefore,the improvement in strength is attributed to the combination of texture strengthening and fine-graine strengthening.However,the reduction of the extrusion ratio will deteriorate the ductility of the Mg-2Gd-0.3Zr alloy to some extent.One of them is that the defects in the original as-cast microstructure cannot be completely eliminated by using smaller extrusion strain.Moreover,it is easy for cracks to form inside the coarse un-DRXed grains and further propagation leads to material failure.

Table 7Tensile mechanical properties of as-extruded Mg-2Gd-0.3Zr alloy with the cooling rate of 200 °C/s.

3.6.Influence of the cooling rate on microstructure and properties of Mg-2 Gd-0.3Zr alloys

Fig.14 shows the EBSD maps,inverse pole figure and corresponding misorientation angle distributions of asextruded Mg-2Gd-0.3Zr alloy obtained with water cooling.It is known from the measurement that the cooling rate of the alloy subjected to water cooling treatment can reach 200°C/s.There is obvious grain refinement in which the average size of the recrystallized grain is about 1.5μm.In addition,the microstructure of the alloy consists of coarse un-DRXed grains elongated along the ED,which occupy an area fraction of 23.5%.With the increase of cooling rate,the texture exhibits a strong＜010＞basal texture indicating that the basal plane of most grains is parallel to the ED.Meanwhile,the maximum texture intensity increases to 4.17.For sample with the cooling rate of 200°C/s,the fraction of LAGBs increased significantly .In particular,the LAGBs are mainly observed inside the un-DRXed grains.The tendency of dislocation entanglement to form dislocation cells is weakened.These large numbers of LAGBs mean that many dislocation slips have not reached an equilibrium state,thus the recrystallization process is suppressed [47-49].

Fig.14.EBSD maps,inverse pole figure and misorientation angle distributions of as-extruded Mg-2Gd-0.3Zr alloy with the cooling rate of 200 °C/s.

Fig.15.Tensile stress-strain curves of as-extruded Mg-2Gd-0.3Zr alloy with the cooling rate of 200 °C/s.

The tensile stress-strain curves of as-extruded Mg-2Gd-0.3Zr alloy at water cooling is shown in the Fig.15,and the values of tensile properties are summarized in Table 7.As the cooling rate increases to 200°C/s,the EL of the alloy decreases to 42%.But the strength is obviously improved,with the TYS and UTS reaching 253MPa and 260 MPa,respectively.The water quenching maintained the microstructure after hot extrusion,so there will be more un-DRXed grains.Compared with other extrusion processes,water cooling treatment can obtain recrystallized grains with relatively small size.It is well known that the lattice distortion at the grain boundary is serious and the orientation of grains on both sides is different,which makes the dislocations blocked at the grain boundary.In general,the dislocation plugging near the grain boundaries of fine grains causes less stress concentration [50].Therefore,the plastic deformation of fine grains must be caused only by applying large stress.Furthermore,the fraction of non-recrystallization region is about 24%.During stretching along ED,the hard orientation characteristics of the un-DRXed grains make the basal slip and tensile twins unable to start.Since the recrystallization behavior is suppressed,the strain energy stored during the extrusion process cannot be completely released,which will also increase the strength of the alloy [30].It can be concluded that the joint action of fine grain strengthening and texture strengthening makes the strength greatly increased.

Fig.16 depicts the relationship between tensile yield strength and elongation of various as-extruded magnesium alloys,which includes Mg-Zn-RE system alloys and commercial wrought magnesium alloys [17,19,51-64].Beyond that,the experimental data obtained in this work are also included.It can be observed that the strength of Mg-Zn-RE system alloys is relatively high,and its yield strength can reach up to nearly 500MPa.Unfortunately,maintaining high strength comes at the expense of the ductility of the alloy,which has a maximum elongation of less than 5%.The deterioration of ductility means that the processing of magnesium alloys at room temperature becomes difficult which will limit the wide application of magnesium alloys.In addition,the yield strength and elongation of commercial magnesium alloys are close to the medium level,indicating that their comprehensive mechanical properties are stable.It is worth noting that Mg-2Gd-0.3Zr alloys prepared by different extrusion processes exhibit superior mechanical properties.Compared with Mg-Zn-RE system alloys,the Mg-2Gd-0.3Zr alloys have ultrahigh ductility,and its elongation can reach 75%.Moreover,the Mg-2Gd-0.3Zr alloys still maintains the medium strength than the commercial wrought magnesium alloys,with the highest strength exceeding 250MPa.It can be found from the Fig.16 that the comprehensive mechanical properties of the Mg-2Gd-0.3Zr alloys are located above the C-shaped curve.Therefore,this result indicates that as-extruded Mg-2Gd-0.3Zr alloys can provide excellent strength-ductility balance,which provides the possibility for further expanding the application of low-cost magnesium alloys.

Fig.16.Relationship between tensile yield strength and elongation of various as-extruded magnesium alloys.

5.Conclusions

In this study,the effects of different extrusion parameters and heat treatment processes on the microstructure,texture and mechanical properties of Mg-2Gd-0.3Zr alloy were systematically discussed.The main conclusions can be drawn as follows:

(1) The as-cast Mg-2Gd-0.3Zr alloy consists of equiaxed grains with an average size of only 26μm due to the role of Zr as the nucleus of heterogeneous nucleation.Conventional extrusion significantly refine the as-cast grains and fine grain bands are uniformly distributed in the microstructure.

(2) Rapid extrusion heats up the extrusion barrel and results in grain coarsening.Similarly,prolonging the annealing time will also accelerate the grain growth.This makes it easier to activate {102} tensile twins,therefore improves the ductility of the Mg-2Gd-0.3Zr alloy.After annealing for 270min,the elongation of the alloy reached 75%.

(3) Both the small extrusion ratio and the fast cooling process can inhibit the recrystallization behavior and generate many un-DRXed grains.The evolution of the texture from the rare earth texture to the strong basal texture means that it is difficult to initiate basal slip.At the cooling rate of 200 °C/s,the yield strength of the Mg-2Gd-0.3Zr alloy exceeds 250MPa.The great increase in strength is attributed to the combined effect of fine grain strengthening and texture strengthening.

Data availability statement

The authors declare that the work described was original research that has not been published previously,and not under consideration for publication elsewhere.

Acknowledgements

The authors are very grateful for the support from the National Key Research and Development Program of China(No.2016YFB0301102),the Postgraduate Education Fund of Chongqing University (No.201704020).