M.G.Jing,C.Xu,H.Yn,T.Nkt,Z.W.Chen,C.S.Lo,R.S.Chen,S.Kmo,E.H.Hn

a College of Mechatronics and Control Engineering,Shenzhen University,3688 Nanhai Ave,Shenzhen 518060,China

b The Group of Magnesium Alloys and Their Applications,Institute of Metal Research,Chinese Academy of Sciences,62 Wencui Road,Shenyang 110016,China

cSchool of Materials Science and Engineering,Harbin Institute of Technology,Harbin 150001,China

d Department of Mechanical Engineering,Nagaoka University of Technology,Nagaoka 940-2188,Japan

Abstract The static recrystallization and associated texture evolution were investigated in an extruded Mg-Zn-Gd alloy with bimodal microstructure based on a quasi-in-situ electron back-scatter diffraction(EBSD)method.The typical rare earth(RE)texture formed during annealing,evolving from the bimodal microstructure with[100]basal fibe texture that consisted of fin recrystallized(RXed)grains and coarse unrecrystallized(unRXed)grains elongated along the extrusion direction.In both RXed and unRXed regions,the RXed nucleation produced randomized orientations without preferred selection and the RXed grains with RE texture orientation had more intensive growth ability than those with basal fibe orientation,thereby leading to the preferred selection of RE texture orientation during grain growth.The relationships between stored strain energy,solute drag,grain growth and texture evolution are discussed in detail.This study provided direct evidence of the RE texture evolution in an extruded Mg-RE alloy,which assists in understanding the formation mechanisms for RE texture during extrusion and better developing wrought Mg alloys with improved formability.? 2020 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 alloy;Rare earth texture;Extrusion;Annealing;Recrystallization.

1.Introduction

Commercial Mg alloys usually develop a strong basal texture during thermomechanical processing with consequent poor formability and strong yield asymmetry,which limits their wide applications.The trace addition of rare earth(RE)elements in Mg alloys has been proven as an effective way to modify the basal texture either during thermomechanical processing or on subsequent annealing,contributing to the significan improvement in ductility[1-4].Typically,RE texture was firstl evidenced in extruded Mg alloys with a new<111>component parallel to the extrusion direction(ED)[1,5-8].However,the mechanisms for the formation of such RE texture in extruded Mg alloys is still the subject of much debate.

Various hypotheses have been proposed to clarify the formation of RE texture,such as dynamic recrystallization(DRX)during hot deformation[9-11],preferred nucleation of oriented grains[1,12-17]and preferred growth of specifi orientation[7,18-20].Recently,solute drag effect has drawn great attraction,as RE solutes tend to segregate at grain boundaries(GBs)or dislocations[10,19,21,22].Suffi cient solute drag is likely to alter the DRX mechanism[9,10]and subsequent grain growth[19,23],which modifie the fi nal extrusion texture.Hadorn et al.[10]evidenced a signifi cant segregation of Gd atoms at GBs in accompany with RE texture in extruded Mg-Gd alloy and hypothesized that the formation of such RE texture was ascribed to the continuous DRX mechanism that requires less mobile GBs due to the Gd segregation.Our recent study[24]unveiled experimentally that continuous DRX contributed to the weakening of[100]basal fibe texture and facilitated the formation of[20]basal fibe instead of RE texture component,and further proved that the preferred growth of recrystallized(RXed)grains with RE texture orientation is the key reason for the formation of RE texture in dilute Mg-RE alloys during extrusion.Previous studies[7,18,20]have also confirme the preferred selection of RE texture orientation in extruded Mg-RE alloys during static annealing.Despite these efforts,it is still unclear how the competitive selection of various orientations evolves during RXed nucleation and grain growth and certainly more sound evidence is needed to clarify such issue.

Recently,Guan et al.[14,15]utilized aquasi-in-situelectron back-scatter diffraction(EBSD)method to monitor the texture evolution in cold-rolled Mg-RE alloy during annealing and found that the RXed grains originating from double twins and shear bands made great contribution to the texture weakening.It is noteworthy that the severe cold rolling is possible to introduce high density of double twins and shear bands,which provide sufficien nucleation sites for recrystallization to alter the fina texture.However,this is not the case for extrusion.Despite the observed double twins and shear bands during extrusion,their effect on the texture formation is supposed to be quite limited due to low volume fraction[1,25,26].Uniquely,the Mg alloys exhibited a typical bimodal microstructure at the die-exit during extrusion,which consisted of fin equiaxial RXed grains and coarse unrecrystallized(unRXed)grains elongated along the ED[24,27-29].

In the current study,therefore,we investigated the recrystallization and associated texture evolution during annealing in an extruded Mg-Zn-Gd alloy with typical bimodal microstructure based on thequasi-in-situEBSD method aiming to provide direct and sound evidence of the preferred selection of RE texture orientation at the stage of nucleation and grain growth.This study will offer insight into the mechanisms for the formation of RE texture during extrusion,thereby better developing new wrought Mg-RE alloys with high performance.

2.Experimental procedures

Alloy ingot with a composition of Mg-1.58Zn-0.52Gd(wt.%)was prepared by high purity 99.9% Mg,99.9% Zn,99.95% Gd by resistance melting under the protection of a gas mixture of SF6(1 Vol.%)and CO2(99 Vol.%).Billet of 43mm in diameter and 35mm in height was machined from the ingot and homogenized at 480 °C for 12h,and finall quenched into hot water.Then the homogenized billet was extruded using indirect extrusion method at 350 °C with a ram speed of 1mm/s and an extrusion ratio of 20 to obtain a typical bimodal microstructure with[100]basal fibe texture as provided in our previous studies[29,30].

The extruded sample was ground and polished using emery papers,Al2O3(0.3μm)suspension and OP-S suspension(0.04μm sized SiO2particles).EBSD was conducted using a JEOL JSM-7000F field-emissio scanning electron microscope(FE-SEM)equipped with an EDAX-TSL EBSD system operating at 15kV.The EBSD data were analyzed by OIM Analysis software and particularly the grains that belong to[100]basal fibe,[20]basal fibe and RE texture components were partitioned in discrete inverse pole figur using Tolerance Mode highlighting with tolerance angle of 15°for further analysis.Fiducial marks were made on the surface of the extruded sample after OP-S polishing,such that the area scanned for EBSD was precisely relocated following each annealing at 350 °C for 5,15,30,60,240min to permit re-scanning of the same area.Thequasi-in-situEBSD collected data from a large area of 0.4mm×1.2mm containing 5000～20,000 grains with a step size of 0.7μm.After each EBSD scan,the samples were sealed in a small aluminum foil and annealed in a tube furnace with fl wing argon for protection from oxidation.After annealing,the samples were quenched into cold water and slightly polished using OP-S suspension(0.04μm sized SiO2particles)to remove any oxidized layer to guarantee the next EBSD scan.The thickness reduction after such slight polishing was less than 1μm,measured by a micrometer.

3.Results

Fig.1 shows thequasi-in-situEBSD results of microstructure evolution in the extruded alloy during annealing at 350°C for various time intervals.The EBSD inverse pole figur maps in the same area(Fig.1a-e)allowed us to track the origin of the RXed grains and exactly follow their subsequent grain growth.It is evident that the typical bimodal microstructure of the extruded alloy,which consisted of fin RXed grains and coarse unRXed grains elongated along the ED as presented in our previous study[29],evolved with increasing RXed fraction and grain size during annealing.After 5min annealing,the RXed grains started to grow and consume the adjacent grains in both unRXed and RXed regions.When annealing from 5min to 240min,the RXed grain number decreased from 19,908 to 5020 and the RXed area fraction increased from 0.418 to 0.837(Fig.1f).Besides,the RXed grains became larger from 5.6μm to 27.7μm during annealing and meanwhile the corresponding RXed grain size distributions became wider(Fig.1g).These results indicate the occurrence of RXed grain growth during annealing.Here,the RXed area fraction was determined by an Image-Pro-Plus 6.0 software based on the examined inverse pole figur maps[26].

Fig.2 shows the texture evolution in the extruded alloy during annealing.As provided in our previous studies[29,30],the extruded alloy exhibited a strong[100]basal fibe texture,i.e.,the basal planes and[100]directions of the crystallites parallel to the ED,with a weak[20]basal fibe component and the maximum texture intensity was 13.2.Such extruded texture stayed stable upon annealing for 5min.After 15min annealing,the texture intensity decreased and a new RE texture component emerged as indicated by red arrows.With further annealing,the newly formed RE texture component became gradually strengthened at the expense of[100]basal fibe component and after 240min annealing RE texture component was clearly observed at the position between[24]//ED and[22]//ED,which belong to the range of the orientation deviating approximately 40～60°from[0001]//ED.This is consistent with the previously reported RE textures in Mg-Gd alloys after extrusion[20,31,32].Such texture evolution is indicative of the preferred selection of RE texture orientation in the extruded alloy during annealing.

Fig.1.Quasi-in-situ EBSD results showing the microstructure evolution in the extruded alloy during annealing at 350 °C for various time intervals:(a-e)inverse pole figur maps,(f)RXed grain number and area fraction as a function of annealing time,and(g)corresponding RXed grain size distributions.

Fig.2.Inverse pole figure of the extruded alloy during annealing at various time intervals.

To specificall demonstrate the evolution of various grain orientations during annealing,the RXed grains with typical[100]basal fibe,[20]basal fibe and RE texture orientations were extracted and separately analyzed,and the results are presented in Fig.3 and Fig.4.The inverse pole figur maps(Fig.3)clearly show the evolution of RXed grains with each orientation in both unRXed and RXed regions.Interestingly,the[100]basal fibe component became weakened gradually with intensity from 14.3 to 8.8,while the RE texture became strengthened with nearly stable[20]basal fibe component(Fig.3).This exhibits good agreement with the overall texture evolution observed in Fig.2.To further confir the above results from a statistical perspective,the grain number,number fraction and area fraction of the RXed grains with[100]basal fibe,[20]basal fibe and RE texture orientations as a function of annealing time are provided in Fig.4.The[100]basal fibe orientation contained the most RXed grains,followed successively by RE texture and[20]basal fibe orientations and the RXed grain number of each orientation decreased gradually from 5min to 240min,suggesting the dominant effect of RXed grain growth during annealing(Fig.4a).The number and area fraction of RXed grains with each orientation based on the whole grain number and area examined in the same EBSD area is shown in Fig.4b and c.The RXed grains with RE texture orientation exhibited an evident rising tendency in both number and area fraction,while the RXed number and area fraction of[100]and[20]basal fibe orientations stayed nearly constant during annealing.Hence,the preferred selection of RE texture orientation can be further confirme to occur during annealing,which contributes to the gradual strengthening of RE texture component.

Fig.3.Quasi-in-situ EBSD inverse pole figur maps and corresponding inverse pole figure referring to the ED of the highlighted RXed grains with[100]basal fibe,[20]basal fibe and RE texture orientations during annealing.

To provide more intuitionistic evidence for the undergoing recrystallization process accompanied with the RXed nucleation and grain growth in the extruded alloy with bimodal microstructure,typical unRXed region R1 and RXed region R2 were extracted from Fig.1a-e and analyzed in detail as presented in Fig.5 and Fig.6,respectively.In the unRXed region R1(Fig.5),we labelled the typical RXed grains A-M and highlighted their crystallographic orientations in the inverse pole figure The green and yellow backgrounds in Fig.5f indicate the basal fibe and RE texture orientations,respectively.As shown in Fig.5a-e,fresh RXed grains were observed to form and the RXed grains exhibited an obvious growth tendency during annealing.Here,we focused on the following aspects to clarify the orientation evolution during annealing.i)RXed nucleation.The grains A,B and G formed newly after 15min annealing and the grain N was observed to form after 60min annealing.From the inverse pole figur(Fig.5f),the grains A and N belonged to the basal fibe orientation,while the grains B and G showed RE texture orientation clustering at[24]//ED.It is supposed that the RXed nucleation did not show any preferred selection of grain orientations.ii)Grain growth.The grains with RE texture orientation such as B,E and K displayed overwhelming growth advantage over other grains.Especially the grain K grew from～14.9μm to～65.2μm after annealing from 5min to 240min,which largely consumed the unRXed grain and connected with the grain E.Comparatively,the grains like A,C,D,F and H,etc.,close to the basal fibe orientation grew relatively slowly.It is noted that the size of grain H increased from～16.4μm to～38.8μm after annealing from 5min to 240min and swallowed the adjacent grains L and M with similar orientations,but exhibited much less growth ability than the grains E and K.

Fig.4.Grain number,number fraction and area fraction of the RXed grains with[100]basal fibe,[20]basal fibe and RE texture orientations as a function of annealing time.

In the RXed region R2(Fig.6),we labelled the typical RXed grains using A-M and 1-8,and highlighted their crystallographic orientations in the inverse pole figure After 5min annealing,many RXed grains were observed with size of～1 μm and they showed randomized orientations as can be confirme from the gray points in the inverse pole figure This further suggests that the RXed nucleation tended to produce a wide spectrum of orientations without any preferred selection.With further annealing,the RXed grain number decreased a lot due to the occurrence of grain growth and coalescence.It is observed that the RXed grains such as A-J grew continuously and survived during 240min annealing.Among these grains,the grains A-J,F-H and J belonged to the RE texture orientation,while the grains E and I showed the basal fibe orientation,which confirm the preferred growth of RXed grains with RE texture orientation in the RXed region.In the following,we further tracked some typical RXed grains with RE texture orientation to present their nucleation and subsequent grain growth in detail.i)RXed grain F.After 60min annealing,the grain F was observed to nucleate with size of～0.5μm at triple junctions of adjacent grains.Upon annealing,the grain F grew to a size of～10μm and the RXed grains 1-8 disappeared at the same time.It is evident that the grain F nearly occupied the original location of grains 1-8,while the other surrounding grains such as B-D and H exhibited slight grain growth.This indicates that the grain F played the dominant role in the consumption of the grains 1-8,but it should be noted that the grains like B-D and H may serve as the auxiliary candidates that participated in such consumption.The striking feature here is that the grain F and other possible candidates all belonged to the RE texture component and most of these swallowed grains possessed the basal fibe orientation except the grain 6 as indicated by blue points in Fig.6f.Similar cases were also observed with respect to the grains D and J as indicated by red arrows in Fig.6a-b.ii)RXed grain H.After 5min annealing,the grains H and M with RE texture orientation were observed in the relatively large grain K with basal texture orientation.The grain H exhibited a strong growth tendency with further annealing and the grain K was observed to be consumed mainly by the grain H after 30min annealing.

The above results point to the fact that the formation of RE texture component was mainly attributed to the preferred selection of RE texture orientation in the extruded alloy with bimodal microstructure during annealing involving the RXed nucleation and grain growth in both unRXed and RXed regions,i.e.,the RXed nucleation was prone to produce randomized orientations without preferred selection and the RXed grains with RE texture orientation were evidenced to have more intensive grain growth and survival ability than those with basal fibe orientation.This well explains the evolution and formation of RE texture during annealing as shown in Fig.2.

Fig.5.Quasi-in-situ EBSD results presenting the RXed nucleation and grain growth during annealing in typical unRXed region R1 selected in Fig.1a-e:(a-e)inverse pole figur maps and(f)inverse pole figur showing the crystallographic orientations of highlighted RXed grains.The green and yellow backgrounds in(f)indicates the basal fibe and RE texture orientations,respectively.

Fig.6.Quasi-in-situ EBSD results presenting the RXed nucleation and grain growth during annealing in typical RXed region R2 selected in Fig.1a-e:(a-e)inverse pole figur maps and(f)inverse pole figur showing the crystallographic orientations of highlighted RXed grains.The light gray points in(f)indicate the orientations of the whole grains examined in(a),and the green and yellow backgrounds indicate the basal fibe and RE texture orientations,respectively.

4.Discussion

Second phase particles[33,34],deformation twins[12,14,35]and shear bands[1,15,36]have been claimed as the primary nucleation sites for recrystallization in deformed Mg alloys,which are commonly associated with appreciable texture weakening.In this study,no sound evidence of particle-stimulated nucleation(PSN)of recrystallization was observed in the extruded Mg-Zn-Gd alloy[24,29,37],which made its contribution to the texture negligible.Recently,Guan et al.[14,15]found that double twins and shear bands acted as the main nucleation sites for recrystallization in cold-rolled Mg-RE alloy and produced a diversity of randomized orientations that remained constant during grain growth,thereby contributing to texture weakening.Different from the case of cold rolling,the volume fraction of double twins and shear bands was quite low during extrusion[1,25,26],which is supposed to have a limited effect on the fina texture formation.Besides,it is noted that high density of double twins and shear bands were observed to form in cold-rolled RE-free Mg alloys,such as AZ31 alloy[38],but with typical basal texture after recrystallization.Hence,the effectiveness of these preferential nucleation sites for recrystallization on the formation of RE texture is still debatable.

Fig.7.Average KAM values of different grains as a function of annealing time:(a)the whole,unRXed and RXed grains and(b)the RXed grains with[100]basal fibe,[20]basal fibe and RE texture orientations.

In this study,thequasi-in-situEBSD results provided the direct evidence of preferred selection of RE texture orientation during grain growth in the extruded Mg-Zn-Gd alloy with bimodal microstructure.Previous studies[7,18,20]have revealed the texture transition from[100]basal fibe to RE texture component in extruded Mg-RE alloy during annealing.Wu et al.[7]proposed that the driving force for the preferred grain growth is the difference in stored strain energy between the grains with RE texture orientation and those with[100]basal fibe orientation.To determine the evolution of stored strain energy in the bimodal microstructure during annealing,the kernel average misorientation(KAM)values,which generally signify the stored strain energy(i.e.dislocation density)within materials[7,39,40],of different grains including the whole,unRXed and RXed grains as well as the RXed grains with typical[100]basal fibe,[20]basal fibe and RE texture orientations were analyzed as presented in Fig.7.The average KAM values of the whole,unRXed and RXed grains decreased gradually after 5min annealing and then increased abnormally after 60min annealing(Fig.7a).Also,the similar tendency was observed in the RXed grains with different orientations(Fig.7b).Such abnormal increase of KAM value was most likely due to the accumulated surface strain introduced by the repeated sample polishing.To be more exact,the evolution of KAM value during annealing from 5min to 60min was only considered here and the KAM maps corresponding to the unRXed region R1 and RXed region R2 are presented in Fig.8.In the R1,the RXed grains with low KAM value were prone to form in the unRXed grain with high KAM value and specificall the RXed grains A,B,G and N were observed to nucleate at the interface between the RXed and unRXed grains.This suggests that the previously formed RXed GBs acted as main nucleation sites for recrystallization in the unRXed region.These RXed grains grew effectively with the consumption of the neighboring un-RXed grain during annealing,which is favorable for the reduction and homogenization of strain energy distributing in the bimodal microstructure(Fig.7a).In the R2,the RXed grains showed almost the same level of KAM value except the grain K,but still with the occurrence of preferred growth of RXed grains with RE texture orientation.Thus,the heterogeneous distribution of stored strain energy in the bimodal microstructure provided a critical driving force for accelerating the RXed nucleation and subsequent grain growth in the unRXed region,but can not elucidate the preferred selection of RE texture orientation in both unRXed and RXed regions.Other mechanism is likely to operate at the same time.

For the extruded AZ31 alloy,the RXed grains with[20]basal fibe orientation possessed an overwhelming growth advantage over those with other orientations during annealing,leading to the formation of[20]basal fibe texture[41].A similar tendency was also observed in pure Mg[42]and Mg-Zn binary alloy[43].As for dilute Mg-RE or Mg-Ca alloys,it has been suggested that solute drag effect plays important roles in texture formation due to the strong interaction between solutes and GBs or dislocations[10,19,21,22],which is likely to alter the selection of RXed grain orientation during recrystallization and grain growth[14,19,43,44].Basu et al.[19]proposed that enhanced solute drag by Gd segregation at GBs amplifie the growth advantage of off-basal orientation over basal orientations during grain growth in rolled Mg-Gd alloy.Besides,Zeng et al.[43]reported the evolution and formation of weak basal texture in dilute Mg-Zn-Ca alloy during annealing usingquasi-in-situEBSD method and provided direct evidence of co-segregation of Ca and Zn at GBs using high-angle annular dark-fiel scanning transmission electron microscopy(HAADF-TEM).They further hypothesized that Zn and Ca solutes were prone to co-segregate strongly at high-energy GBs of RXed grains with[20]basal fibe orientation,effectively reducing the mobility of such GBs by solute drag effect and thus leading to the weakening of basal texture[43].The molecular dynamic simulation also supported that the segregation of RE element homogenized the GB energy and mobility distributions among GB types,which largely reduced the preferred selection of basal fibe orientation that is highly active in pure Mg and in turn provided a growth advantage of RXed grains with off-basal or RE texture orientations[45].Most recently,Guan et al.[44]established the relationship between GB misorientation and solute segregation regarding the recrystallization mechanism for the firs time in dilute Mg-Zn-Ca alloy by coupling HAADF-STEM and EBSD techniques and found that solute segregation occurred along a wide range of GBs instead of only along special boundary types in the observed local area.However,it is still difficul to conclude whether the solute segregation occurs uniformly or only at some specifi type of GBs from a statistical perspective,which needs further clarificatio in the future.What becomes clear is that the solute drag with the help of precipitates was effective to restrict the GB mobility of RXed grains with basal fibe orientation[44],which would otherwise grow favorably in conventional REfree Mg alloys.Therefore,it is reasonable to believe that the preferred selection of RE texture orientation in this case was most likely due to the RE solutes segregating preferentially at high-energy GBs of RXed grains with basal fibe orientation,which effectively restricted the mobility of such GBs and thus reduced the survival ability of these basal oriented grains during grain growth.

Fig.8.Quasi-in-situ EBSD KAM maps presenting the evolution of local misorientation within grains during annealing from 5min to 60min in the typical unRXed region R1 and RXed region R2 selected in Fig.1a-e.

It is noted that the solute segregation is closely related to the temperature,since the velocity of GBs becomes high enough to break free of any RE solutes atmosphere that develops at high temperatures[21].This indicates the significanc of controlling the extrusion parameters that were associated with actual extrusion temperature for obtaining the RE texture in Mg-RE alloys during extrusion.Besides,it is necessary to provide sufficien recrystallization process via adjusting the extrusion process to transform the bimodal microstructure into fully RXed microstructure and thus guarantee the occurrence of grain growth with consequent preferred selection of RE texture orientation.These finding will assist in understanding the mechanisms responsible for the formation of RE texture in Mg-RE alloys during extrusion and offer theoretical guidance for optimizing the extrusion process to modify the microstructure and texture,thus better developing new wrought Mg-RE alloys with improved formability and ductility.

5.Conclusions

In summary,the static recrystallization and texture evolution were investigated in an extruded Mg-Zn-Gd alloy with bimodal microstructure using thequasi-in-situEBSD method.The typical bimodal microstructure that consisted of fin RXed grains and coarse unRXed grains evolved with the RXed nucleation and grain growth during annealing and it is found that RE texture component became strengthened at the expense of[100]basal fibe component.In both RXed and unRXed regions,the RXed nucleation produced randomized orientations without preferred selection and the RXed grains with RE texture orientation were evidenced to have more intensive grain growth and survival ability than those with basal fibe orientation,consequently presenting the preferred selection of RE texture orientation.The stored strain energy served as a critical driving force for accelerating the RXed nucleation and grain growth in the unRXed region rather than facilitating the preferred selection of RE texture orientation.The RE solutes was supposed to segregate preferentially at high-energy GBs of RXed grains with basal fibe orientation,which restricted the mobility of such GBs and in turn led to the preferred growth of RXed grains with RE texture orientation.This study offers insight into the mechanisms for the formation of RE texture during extrusion and further provides a pathway for developing new wrought Mg-RE alloys with high performance.

Acknowledgments

The authors gratefully acknowledge the financia supports from the National Natural Science Foundation of China(NSFC,No.52005340 and 51601193),State Key Program of National Natural Science of China(No.51531002),National Key Research and Development Program of China(No.2016YFB0301104),National Basic Research Program of China(973 Program,No.2013CB632202),Guangdong Basic and Applied Basic Research Foundation(No.2019A1515110541)and Shenzhen Bureau of Industry and Information Technology(No.ZDYBH201900000008).