Anisotropy in dynamic recrystallization behavior of AZ31 magnesium alloy

S.M.Ftemi,S.Kheyrdi,H.Pul

aDepartment of Metallurgy and Materials Engineering,Shahid Rajaee Teacher Training University,136-16785 Tehran,Iran

b Institute of Metallurgy and Materials Science,Polish Academy of Sciences(PAS),25 Reymonta Street,30-059 Krakow,Poland

Abstract Deformation behavior as well as microstructural evolutions of a rolled AZ31 magnesium alloy with and without pre-existing extension twins were studied using compression tests which performed along different orientations at a temperature range of 25-350 °C.The results implied that the initial texture not only influenc the evolution of fl w stress,but also change the size and fraction of recrystallized grains.In contrast to samples parallel to rolling and transverse directions,compression along normal direction resulted in a respectful softening at 150 °C.The largest size and fraction of new grains at 250 °C were recorded after deformation along rolling direction,while the maximum fl w softening was observed during deformation along normal direction.The anisotropy in microstructural evolutions was still retained at 350 °C.Pre-existing twins could reduce the anisotropy of material in respect of fl w stress as well as DRX progression,where TD sample showed the lowest DRX fraction at 250 °C.Quaternion misorientation data obtained from EBSD analysis of pretwinned material implied that initial texture could not significantl influenc fina texture.A different misorientation distribution was realized after deformation of pretwinned material along ND and RD directions.? 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:Slip;Recrystallization;Grain size;Growth;Twin.

1.Introduction

There is currently growing interest in developing new applications for magnesium alloys in the aerospace and automotive industry sectors considering their desirable properties such as low density and high specifi strength[1].However,their hexagonal symmetry leads to workability limitations in magnesium alloy,which restrict their potential applications.The workability of magnesium is influence by different deformation mechanisms including basal and nonbasal slip as well as contraction and extension twinning[2].The ease of slip on basal plane of magnesium and related operating dislocation mechanisms were well-known to promote sharp textures in wrought alloy[3].Nevertheless,the predominant mechanisms will depend strongly on the loading mode and direction with respect to the c-axes of the grains[4].Accordingly,a textured material exhibited anisotropic mechanical behavior and as well as microstructural evolution along its different direction during deformation.The latter limits,in turn,the formability of material during secondary industrial processing[5].The anisotropic and asymmetric behavior of textured magnesium alloys at room temperatures have been the subject of different studies[6-8].

Processing of wrought magnesium alloys usually perform at high temperatures.With increasing temperature the critical resolved shear stresses(CRSSs)for nonbasal slip systems and contraction twinning gradually decrease,while those remain relatively constant for basal slip and extension twinning[9].Thus,the degree to which different nonbasal slip contribute during deformation is enhanced with increasing temperature and the deformation is progressively accommodated by multiple slips[10].So,it is expected that the anisotropy in mechanical behavior to be reduced at high temperatures.However,due to anisotropic nature of crystal structure in magnesium,dislocation-dislocation and dislocation-boundary interactions describe the hardening behavior of the material along different directions.Moreover,magnesium alloys undergo dynamic recrystallization(DRX)during deformation at high temperatures,which is influence by multiple system activities[11].Therefore,the anisotropy in microstructural evolutions is complicated by diverse influenc of temperature and texture-loading orientationship.Al-Samman et al.[12]investigated the tension-compression anisotropy of an extruded AZ31 alloy at high temperatures.Their result implied that compression at 200 °C and 300 °C parallel to the extrusion axis favors tensile twinning over prismatic slip,which gives rise to high hardening rates.At 400 °C the deformation is carried out by a combination of prismatic slip,〈a〉pyramidal slip and basal slip.The study of high strain rate deformation of an AZ31 sheet at 250 °C showed that the highest resistance to dynamic recrystallization was found when the material was tested in tension along the rolling direction[13].Liu et al.[14]Studied deformation behavior of as-pressed AZ31 magnesium at 300 °C along different directions.They believed that it is crucial to have significan local dislocation accumulation to promote DRX,which is achieved by extensive dislocation activities starting from the early stages of deformation.However,the works by Li et al.[15]and Wang et al.[16]claimed that no effect of initial texture could be traced microstructures recrystallized during extrusion and rolling experiments,respectively.The results reported by Sabat et al.[9]implied that the anisotropy in recrystallization is remarkable during static annealing,where the rate of subgrain formation was enhanced in the grains of orientations＞40° from the normal direction.The aforementioned researches mainly dealt with the operation of different deformation mechanisms and related fl w stresses during loading at a given temperature along different direction.Nevertheless,rare researches could be found in the literature that systematically study the anisotropy in deformation behavior and in development of DRX at different temperatures in wrought magnesium alloys.Such information can shed light on the challenges of the workability of magnesium alloys at high temperatures.Moreover,pre-existing twins have been shown,in recent years,that could considerably influ ence the slip and twinning activities during deformation of magnesium[5,17-20].Accordingly,the material anisotropy may be varied by the presence of pre-existing twins.Inplane deformation characteristics of rolled AZ31 alloy containing pre-existing twins were investigated at room temperature using in-situ electron backscatter diffraction technique(EBSD)[21].The results implied that pretwins gave rise to in-plane anisotropic activities of slips and twinning controlled the combined effect of the texture and effective grain size.

Present work was aimed to investigate the evolutions of fl w stress as well as progression of DRX during compressive deformation along different directions of an AZ31 rolled alloy at a temperature range of 25-350 °C.The anisotropic characteristics were also evaluated after introducing pre-existing extension twins in the experimental material at the same conditions.

2.Experimental procedure

A commercial rolled AZ31 magnesium alloy(Mg-2.9Al-0.9Zn-0.7Mn,%wt.),with 22 mm thickness,was employed as the experimental alloy for this research.The as-received material was annealed for 1 h at 350 °C,after that it exhibited a fully recrystallized equiaxed microstructure without twins.The ND inverse pole figur(IPF)map of the initial material was given in Fig.1.The mean grain size was measured to be 45μm.Cylindrical compression samples with 10 mm diameter and 15 mm height were cut from annealed and pretwinned materials along rolling,transverse and normal directions,which hereafter are denoted as RD,TD,ND,respectively.Teflo tape was used as lubricant between the specimens and the anvils.The compression experiments were performed using GOTECH A17000 universal testing machine equipped with programmable resistance furnace.Before testing,the specimens were soaked for 5 min at the predetermined temperature to allow equilibration of temperature throughout the specimens.The experiments were conducted at the temperatures range of 25,150,250 and 350 °C under initial strain rate of 0.01 s-1.Each compression test was repeated at least three times with three different samples.The compression trials were programed to apply up to strain of 0.6.The stress-strain data was corrected for friction between the samples and compression anvils.To trace microstructure evolutions,some extra tests were conducted and interrupted at different strains,after that the specimens were quenched in water immediately.Sections parallel to the deformation axis were cut from the deformed specimens for metallographic investigations.The dynamically recrystallized grains could be almost readily distinguished from the pre-existing grains.The dynamically recrystallized grain size was determined utilizing a powerful image analyzing system.Scanning electron microscopy(SEM)investigations were conducted by utilizing scanning electron microscope(FEGSEM,FEI Quanta 3D)furnished with electron back scattered diffraction(EBSD)facility.

To study the effect of pre-existing twins on the anisotropy of microstructural evolutions,precompression by strain of 6% was conducted on a part of annealed material along rolling direction.The magnitude of precompression was chosen according to the previous results[18,22],where it has been shown that 6% pstraining would produce a very high area fraction of twins.This could easily generate a high density of extension twins with 79% volume fraction.This yielded an effective mean grain size of 25μm(not presented here),considering initial grain boundaries as well as twin boundaries.The Series of compression tests along different directions as well as related microstructural investigations were performed for the material with pre-existing twins.

Fig.1.IPF map of the microstructure of the initial annealed material.

Fig.2.Pole figure of(0002),(10-10)and(11-20)planes of the initial experimental alloy.

3.Results and discussion

3.1.The effect of sample direction at different temperatures

Rolled samples after the annealing treatment is characterized by basal texture with the c-axis aligned parallel to the ND and randomized a-axis in the rolling plane,as shown in Fig.2.Assuming the later major component,the orientation factor and thereby activation stress as well as the extent of contribution to plastic fl w for different slip systems may be assessed using the standard stereographic projection for magnesium[23].The Schmid factors were calculated for different slip and twinning mechanisms during loading along different directions and given in Table 1.CRSS values of 10,55,30,and 19 MPa are considered for basal slip,prismatic slip,{10-12}twinning,and{10-12}detwinning,respectively[24].

The fl w curves of the experimental alloy deformed along different directions were plotted in Fig.3.Different shapes for the fl w curves,in terms of sigmoid trend,were observed for different directions up to 250 °C.At 25 and 150 °C,RD and TD samples show a rise in hardening rate at low strains,so called sigmodal hardening behavior,after that the hardening rate decreases.In contrast,the hardening rate of the ND sample at these temperatures is characterized with a monotonic decrease.However,it is interesting to note that a slight softening is realized in ND sample after reaching a peak stress at 150 °C.No softening regime could be distinguished in TD and RD samples.

Table 1The absolute Schmid factor for different deformation mechanism during compression along different directions.

RD samples locates basal planes parallel to the compression axis and this considerably reduces the contribution of basal slip.Thus,according to Table 1,the sample could be deformed at 25 and 150 °C primarily by twinning as well as prismatic slips.The{10-10}planes lie either perpendicular or at 30° with respect to the compression axis,where only the latter may contribute to a limited extent.Referring to Table 1,TD sample provides desired orientation for occurrence of extension twinning as well as prismatic slip.In agreement with the lower CRSS for twinning material fl w occurred at low stresses in RD and TD cases.At the late strains contraction twins with needle like morphology could be promoted in the work hardened material.The microstructures of the experimental alloy deformed along different directions at 150 °C are shown in Fig.4.A typical analysis for twin boundaries are presented in Fig.5 for the RD and TD samples,where boundaries corresponding to extension,contraction and double twins are showd in red,blue and green respectively..It is evident that the deformation of RD(and also TD)samples mainly is associated with the development of extension twinning.However,ND compression provides unfavorable orientation for extension twinning,and basal/prismatic slips.Therefore,the fl w stress is increased to activate non-basal slip,particularly〈c+a〉which assist in strain accommodating along c-axis.Owing to the high CRSS of nonbasal slip at low temperatures[25],the material hardenability is quickly exhausted and hardening rate dropped sharply.Thus,the critical stress for compression twins may be reached at low strain.Close observations indicated that localization of strain in the form of shear bands was developed in ND sample at late strains(Fig.4c),within which the occurrence of DRX within shear band might be triggered.Moreover,new grains may be geometrically generated through the intersection of compression twins.Thus,formation of shear regions,within which the deformation is localized,may also contribute to fl w softening in the ND sample at 150 °C.Though these evolutions led to the highest failure strain along ND,this could not provide enough hardenability to prevent cracking before targeted stain 0f 0.6.

Fig.3.True stress-strain curves obtained during compressive deformation of the experimental alloy along different orientations at a)25,b)150,c)250 and d)350 °C.

With increasing the temperature the CRSSs of basal slip and extension twinning remain relatively constant,while it decreases for non-basal slip systems in Mg[26],leading to substantial contribution of independent slip systems.Nonbasal slips are invoked for homogeneous deformation.Prismatic slip take a significan role at temperatures above beyond about 225 °C,while pyramidal slip may operates at temperature as high as 350 °C[10],although some previous works reported the activation of prismatic and second-order pyramidal slip at lower temperatures,due to compatibility stresses[27,28].Thus,the alteration of yield strength with temperature in the uniaxial loading is connected to the temperature dependence of the CRSS of the corresponding non-basal slip.Flow curves obtained at 250 °C show that the ND compression is still characterized with highest fl w stress.It is also surprising that a noticeable anisotropic behavior were obtained for RD and TD samples.The variation in the maximum anisotropy in yield and peak fl w stress of the material depicted in Fig.6.Accordingly,the anisotropy in fl w stress did not remarkably decrease with increasing temperature to 250 °C,while a sharp decrease in fl w stress anisotropy was realized as the temperature was raised to 350 °C.

For ND samples,according to Table 1,the second-order pyramidal plane is favorably oriented for activation.However,according to their high CRSS slip systems,it can hardly operate during deformation at 250 °C.The latter condition may provide multiple slip activity only close to the prior grain boundaries,leading to rapid exhaustion of material hardenability and thereby the promotions of fl w localization.Hardening behavior of ND sample,shown in Fig.7,better illustrates a continuous drop in hardenability of material with increasing strain.This may end up crack initiation and premature fracture of the sample.Moreover,as the deformation starts remarkable scope for cross slip of prismatic dislocations may occur according to its high stacking fault energy(173 mJ/m2)[25].The occurrence of multiple slip and following dynamic recovery may render nucleation of new grains at the vicinity of prior grain boundaries.McQueen et al.[29]emphasized the role of nonbasal slip,in the formation of a 3D network of recrystallization nuclei.

Fig.4.The microstructure of a)RD,b)TD and c)ND sample deformed at 150 °C.

Fig.5.Grain boundary map of a)RD and b)ND samples deformed at 150 °C showing different twin boundaries including extension twin(86°),contraction twin(56°),double{10-11}-{10-12}twins delineated as red,blue and green color,respectively.

For RD sample,the concave-up shape of the fl w curve represents that twinning,though at lower degree,yet take a respectful role during deformation of RD samples at 250 °C.Also,TD specimens present highly favorable orientation for prismatic slip and twinning,according to Table 1.The limited dynamic twinning could align the basal slip perpendicular to loading axis,resulting in a stage of work hardening,as obviously seen in Fig.7.The occurrence of twinning postponed the development of slip-dominated substructure preceding DRX nucleation.Therefore,the operation of prismatic slip as well as pyramidal slip could provide substructure condition for grain boundary nucleation of new grains at higher strains.Microstructure of the samples deformed along different directions at 250 °C were given in Fig.8.The evolutions of mean size and volume fraction of DRXed grains were given in Fig.9.It is evident that initial texture affect the DRX grain size and progression.Based on the author knowledge this is presented for the first-tim in magnesium alloys.Previous results[15,16]in these regards reported that initial texture does not have considerable effect on evolutions of DRXed microstructure.The largest grain size and DRX fraction at 250 °C were obtained after deformation of DR sample,while the minimum grain size and DRX fraction were obtained for ND one.The magnitude of the DRX fraction and fina grain size can be rationalized in terms of the effects of the various parameters on the nucleation and growth processes.Presumably,nucleation in magnesium is related to evolution of low angle boundary which,in turn,governed by climb and cross slip of nonbasal dislocations[28,30].Moreover,the deformation inhomogeneities may readily provide such nucleation conditions.However,growth rate of the nuclei,including diffusion of atoms to and from the adjacent grains,is controlled by driving force and mobility of grain boundary[31].Present results represent the lowest peak strain for ND direction,where,according to Table 1,just〈c+a〉slip with a high CRSS was favorable.However,it causes a more inhomogeneous deformation microstructure so that nucleation of new grains is triggered at pre-formed nuclei,i.e.areas with localized deformation.Recrystallization generally presents an incubation time,associated with the formation of a nucleus.Pre-existing nuclei are associated with a reduced DRX incubation time(and strain).Thus,it may be concluded that the initial texture-load orientationship,which stimulate inhomogeneous deformation such as ND compression,may promote the nucleation of DRX more quickly,than those facilitate the operation of different slip systems.The latter explanation is in line with the results reported on tension testing of AZ31 along different directions[32].They discussed that the activation of〈c+a〉slip render the occurrence of DRX because of their easy cross-slip and climb.However,nucleation of new grains during deformation along TD and RD invoke the time for the formation of a substructured nuclei with sufficien high mobility boundary.After the nucleus formation,recrystallization is controlled by growth,in which high angle grain boundaries migrate over the strained matrix(non-recrystallized),removing the crystalline defects until grains meet mutually.Major part of the energy spent during the deformation process is stored in the material in the form of dislocations.If the dislocation density during deformation(after nucleation)isρ,the driving force F is given by[33]:

Fig.6.The evolution of work hardening rate ND,TD and RD sample at 250 °C.

Fig.7.The variation of anisotropy in yield and peak fl w stresses of the experimental material with pre-existing twins at different temperatures.

where G is the shear modulus and b is the modulus of the Burgers vector.Thus,promoted multiple slip activities in TD and RD samples gave rise to larger DRXed grain size and fraction.It should be noted that higher Schmid factor in TD sample for multiple slip may cause an extended dynamic recovery occurring concurrently with recrystallization,reducing the available dislocation density and thus driving pressure for DRX.This provide a rational base for the lower DRX fraction developed during deformation of TD sample compared to RD.Consistently,previous work has pointed out that potential for intensive recovery may lead to a lower DRX rate[31].Del Valle et al.[34]have discussed that the effect of texture on DRX evolution in Mg alloys during tensile test could be attributed to the strain necessary to development a basal fibe with〈01-10〉parallel to the tensile axis.While Li et al.[15]believed the influenc of initial texture on DRX is not connected with the changes in deformation mechanisms.

At 350 °C,the anisotropy in fl w stress was significantl reduced,so that a yield and peak stress ratios of close to unity were achieved.The CRSSs for nonbasal slip systems decrease at higher temperatures[35],so that these may contribute more homogenously to deformation of samples with different orientation.Accordingly,the role of twinning is diminished at such high temperatures.The fina microstructures obtained after compression at 350 °C were illustrated in Fig.10.The size and fraction of DRX grains,given in Fig.9,indicate that anisotropy in DRX is almost vanished at 350 °C for RD and TD samples,while a lower DRX fraction(～60%)was observed after compression along ND.This is in agreement with the high strain required for achieving steady state stress in ND direction.The orientation relationship of the DRX grains formed at 350 °C with respect to the parent grains was also considered.Fig.11 and 12 depict the pole figure of(0002),(10-10)and(11-20)planes for DRXed and parent grains of the experimental alloy deformed along RD and ND samples,respectively.The results implied that DRXed grains adopted a texture corresponding to that of parent grains during deformation of AZ31 alloy along both RD and ND samples.

Fig.8.The microstructure of a)RD,b)TD and c)ND sample deformed at 250 °C.

Fig.9.Variations of DRX grain size and fraction and fl w stress softening during deformation along different direction at 250 and 350 °C.

Fig.10.The microstructure of a)RD,b)TD and c)ND sample deformed at 350 °C.

Fig.11.Pole figure of(0002),(10-10)and(11-20)planes for a)DRX grains,and b)parent grains of the experimental alloy deformed along RD at 350 °C.

To further study the influenc of sample direction on the material fl w behavior,the fractions of fl w softening were calculated.The latter was performed through dividing the peak stress by the fl w stress attained at true strain of 0.5,which is the maximum strain experienced in all specimens.The values are given in Fig.9 as interconnecting blue points.As is commonly reported in FCC and BCC metals,the softening fraction tends to reduce with increasing temperature[34].Similarly,the present results implied that the softening fraction dropped with increasing temperature from 250 to 350 °C,which is related to more efficien dynamic recovery.As is seen in Fig.9,the softening characteristic has been significantl influence by the sample orientation.The highest softening fraction was obtained for ND sample at both 250 and 350 °C.The evolution of softening fraction may be discussed relying on the amount of heterogeneity and strain energy accumulated in the deformed material that would be substituted by new soft grains.In the case ND sample,the inhomogeneous deformation trigger the DRX at lower strain and thus recrystallization of the regions with high stored energy would lead to a more effective softening in the fl w stress.More interestingly,the results denote a significan discrepancies between the DRX fraction in the microstructure and softening fraction of the fl w stress.It is in contrast to the model usually used for calculating the DRX fraction,in which the DRX volume fraction is considered to be equal to softening fraction in fl w stress[36,37].Chen et al.[38]discussed that such inconsistency is related to the strengthening effects of new fin DRXed grains.Nevertheless,for the present results this is not the case,because the higher DRX volume fraction at 250 °C was obtained for RD sample while the highest softening fraction was obtained for ND direction.

Fig.12.Pole figure of(0002),(10-10)and(11-20)planes for a)DRX grains,and b)parent grains of the experimental alloy deformed along ND at 350 °C.

Beside drop in dislocation density,two main factors may further contribute to the fl w softening fraction.The firs is the strain energy of deformed material which would be substitutes by new soft grains.So,if there could be a dynamic recovery prior to DRX(concurrent to deformation)the strain energy of the deformed material would be lower,and thereby its substitution with new grain give rises to a lower softening fraction.The second is the possible effect of development of a texture.There may be different texture hardening/softening effects during substitution of different deformed textures by DRX one.

3.2.Effect of pre-existing twins on anisotropic microstructural evolution

To study the effect of initial twins on the anisotropy evolution of fl w stress and DRX,the deformation regimes employed for the material with pre-existing twins.The texture components developed in the pretwinned material are presented in Fig.13 as pole figure for(0002),(10-10)and(11-20)planes.As the twinning occurred through 86° rotation about one a-axes of HCP crystal,the c-axis is oriented close to RD while one of the prismatic plane is located almost inclined to the rolling plane.

The fl w curves of the experimental alloy,with pre-existing twins were plotted in Fig.14.A close inspection of the curves implied that,in contrast to annealed material,TD and RD sample exhibited higher yield strength.Comparison of the curve appearance indicated that pretwins caused a remarkable changes in the shape of fl w curves,in terms of sigmoid trend,for ND samples at 25 and 150 °C.The pre-twinned ND samples show a rise in hardening rate at low strains,so called sigmodal hardening behavior,after that the hardening rate decreases.the maximum fl w stress ratios for yield and peak stress along different directions of the pretwinned material is given in Fig.15.It is interesting to note that the anisotropy in fl w stress has been remarkably reduced due to pre-existing twins.This is more pronounced at 250 °C.Using the main texture components,obtained in Fig.13,the values of Schmid factor for different deformation mechanisms in twinned area were given in Table 2.It should be noted that the twinned area provide relatively a higher orientation factors for deformation along ND directions which suffer from low Schmid factor in the untwinned areas.Therefore,it is rationalized that the twinned material(constituting both twinned and untwinned areas)exhibited less-anisotropic fl w stress.

Fig.13.The pole figure of different planes of the pretwinned alloy.

Fig.14.True stress-strain curves obtained during compressive deformation of the pretwinned experimental alloy along different orientations at a)25,b)150,c)250 and d)350 °C.

According to Table 2,the pretwinned regions oriented caxis of the crystal perpendicular to the ND direction and thus prismatic slip as well as extension twinning may be activated in pretwinned areas.However,such reverse loading,relative to preloading,may also cause detwinning or shrinkage of pretwins so that the grains shear back to their initial twin-free state[39].Detwinning with low CRSS happens by the twin dislocations with an opposite sign in the twin along the twin boundaries[40].Detwinning is assisted by back stresses generated by twin growth[17].As the deformation progressed compression twin may also be formed leading to a premature fracture.It should be noted that the occurrence of detwinning,through dislocation interaction with twin boundaries,includes either dissociating into interfacial defects or penetrating(transmitting)into the twin[39].The latter interactions are accompanied with a strain hardening effect in the fl w curve of ND sample,corresponding to a sharp rise in hardening rate at 25 and 150 °C.For RD sample,the twinned areas provide a hard orientation for extension twinning and basal slip.The occurrence of{10-12}twinning and basal slip in untwined grain parts is also important in deformation.However,as the critical stress for slip and twinning increased due to Hall-Petch effect of pretwins,a greater fl w stress was developed for RD samples.Also,RD samples are expected to be deformed at low strains by the growth of pretwins.It has been determined that twin growth occurs under stresses lower than those required for new twin nucleation[41].As pretwins continuation is exhausted the fl w stress is rapidly increased,developing a high hardening rate.Growth of pretwins give rise to dominance of the texture component with c-axis parallel to RD.

Table 2The absolute Schmid factor for different deformation mechanism in twinned area during compression along different directions.

Fig.15.The evolution in anisotropy of yeild and peak fl w stresses of material with pre-existing twins at different temperatures.

For TD samples,deformation at early deformation is governed by twinning in either pretwinned or untwined regions.The orientation of twinned regions is highly favorable for new twinning with variant different to that of pretwins.It should be noted that the occurrence of extension twinning in the narrow twinned area invoke a higher critical stress which further increase the yield strength through the Hall-Petch effect of pretwin boundaries.The prismatic slip direction lie either parallel or at 60° with respect to the compressive loading.Thus the latter orientation may contribute to prismatic slip.As the deformation is further progressed beyond yielding,the occurrence of basal and prismatic slips and then growth of pretwins provide respectful hardenability and gradual increase of fl w stress.The dominance of twin intersection justifie the greater yield stress in TD sample.

Fig.16.The evolution of work hardening rate during compression along ND,TD and RD at 250 °C.

With increasing the temperature the CRSSs of basal slip and extension twinning remain relatively constant,while it decreases for non-basal slip systems in Mg[26],leading to substantial contribution of independent slip systems.Nonbasal slips are invoked for homogeneous deformation.Prismatic slip take a significan role at temperatures beyond about 225 °C,while pyramidal slip may operates at temperature as high as 350 °C[10].Although,some previous works reported the activation of prismatic and second-order pyramidal slip at lower temperatures,due to compatibility stresses[27,28].Thus,the anisotropy of yield strength is connected to the different activated mechanisms.

The shape of the fl w curves were changed by pretwins for all of the RD,TD and ND samples at 250 °C,denoting the alteration of dominant deformation mechanism at low strains.While a concave-up fl w curve behavior for ND sample at low strains was obtained.Fig.16 compares the hardening behaviors of the material with pre-twins during compression at 250 °C where a stage of strain hardening was realized for ND sample and a monotonic descending hardening rates observed for RD and TD samples.ND sample exhibited the lowest fl w stress.This is related to operation of basal,prismatic and firs order pyramidal slips and〈c+a〉slip in twined regions,though new extension twins may be also formed in the twinned areas.Thus,texture softening resulted by pretwins was the prominent factor during deformation along ND direction of the pretwinned material.The microstructural investigation at low strain implied that the occurrence of detwinning was hindered at 250 °C due to stabilizing the twins against shrinkage or growth at such high temperatures[42].Thus,the high hardening rate for ND sample at early strains can be related to persistence of pretwin boundaries as obstacles for dislocation glides.Moreover,as the deformation starts remarkable scope for cross slip of prismatic dislocations may occur according to its high stacking fault energy(173 mJ/m2)[25].

For RD sample at 250 °C,in the untwinned regions the operation of new twinning as well as prismatic slip are hardened due to Hall-Petch effect of pretwins boundaries.The Schmid factor for contribution of slip system in the twinned area is very low.The growth of pretwins was also hindered at this temperature up to medium strain.So the yield strength of material was increased due to the pretwins.As the deformation progresses,the operation of nonbasal slip is facilitated at untwinned areas according to proper Schmid factor(see Table 2).Nevertheless,the effect of pretwins on work hardening of the material at low strain is not pronounced.However,pretwins hardening was significan during low strain deformation of ND sample.

Fig.17.The microstructure of the experimental material with pre-existing twins deformed along a)RD,b)TD and c)ND at 250 °C.

TD sample of the material with pre-existing twins presents a slip dominated shape of fl w curve.This is related to high Schmid factor for nonbasal slip(see Table 2),while extension twinning is hard to occur according to the Hall-Petch strengthening of pretwin boundaries.Thus,pretwins led to an increase in yield strength of TD sample at 250 °C due to higher CRSS of nonbasal slip.The contribution of different types of nonbasal slip and their following cross slip accelerated the microstructure evolution,preceding DRX nucleation.The occurrence of multiple slip and following dynamic recovery render nucleation of DRX at twin intersections,rather at the prior grain boundaries.Triggering of DRX is associated with a relatively low peak strain for TD sample.This caused the appearance of stress peak at lower strain,after that a gentle work softening trend is developed.

Fig.18.Variations of DRXed grain size and fraction,and fl w stress softening during deformation along different direction at 250 and 350 °C.

Microstructure of the samples deformed along different orientations at 250 °C were presented in Fig.17.The evolutions of mean size and volume fraction of DRXed grains were given in Fig.18.A relatively similar trend for evolution of size and fraction of DRXed grains is realized.Deformation along RD yielded the largest grain size and DRX fraction,while the minimum grain size and DRX fraction were obtained for ND and TD samples respectively.This observation is rationalized with high Schmid factor for slip in twinned areas of ND and both twinnwd and untwinned areas of TD,and resulting intensive dynamic recovery which limits the size of new grains.The lower softening fraction in pretwinned TD sample compared to RD is attributed to the extended recovery prior to DRX,considering the slip dominated fl w.Accordingly,a lower work hardening is developed before the peak stress,due to high Schmid factor for non-basal slips.This,in turn,may result in a lower softening ration in TD sample.

Fig.19.(a,b)Quaternion misorientation graphs and(c and d)misorientation distributions of the material with pre-existing twins deformed at 250 °C along ND and RD,respectively.

At 350 °C temperature,yield strength along different directions converge to about the same level.The CRSSs for nonbasal slip systems decrease at higher temperatures[35],so that they may contribute more homogenously to deformation of samples with different orientation.Accordingly,the contribution of twinning is diminished at such high temperatures.However,in case of pretwinned samples the stability of pretwins may be preserved at low strains leading to a minor anisotropy.The mean grain size and DRX fraction as well as the fl w softening fraction of the samples at 350 °C were included in Fig.18.Though the softening fraction was remarkably reduced at 350 °C,but there still is a slight anisotropy in the microstructure characteristics of the experimental material.Nevertheless,keeping in mind the overall microstructure evolutions one can infer that pretwins could mitigate anisotropy in microstructure evolutions in the experimental alloy.A slight increase in softening fraction with temperature in pre-twinned TD sample may be justifie considering the prevalence of non-basal slip in twinned and untwinned regions,where increasing the temperature to 350 °C can boost the activity of nonbasal slips with further straining.The latter may accelerate the progression of DRX and thereby greater fl w softening fraction.

To explore more details of anisotropy in microstructure evolutions,EBSD analysis were performed for the pretwinned material deformed at 250 °C along ND and RD.Fig.19a and b display the quaternion misorientation figur relative to the typical as-rolled texture component(0002)＜11-20＞.In the present study,although there is a significan difference in deformation mechanisms and DRX progression,no distinct difference in fina texture was observed.The majority of deformed and DRXed grains tend to align perpendicular(～90°)to the〈a〉direction of basal planes.The latter observation together with necklace like DRX structure signify the dominance of discontinuous DRX mechanism[33].Boundaries corresponding to extension,contraction and double twin relationship can be traced by the relative frequencies around 86°,56° and 38° grain boundaries.Distribution axes for different twin-related misorientations were included in Fig.19.A higher frequency for low angle boundaries(2-15°)indicates the occurrence of more extended dynamic recovery,corresponding to higher average Schmid factor along RD.A frequency peak at 86° and related high misorientation axis density clustering at the〈11-20〉for ND and RD samples is observed while no noticeable extension twins with lenticular morphologies is seen.It suggests that a portion of the DRXed grains satisfy the twin orientation relationship,which is an evidence for active contribution of twin recrystallization.It can be also deduced that pretwins would retain after early deformation,which confir our speculation on the cessation of detwinning at 250 °C.Deformation along RD gave rise to development of distribution peak around 30°with the range of 20-50°.Moreover,the recrystallized regions contained abundant traces of 40° misoriented boundaries.However,the rotation axes of the 30-40° boundaries are mainly concentrated around〈0001〉,excluding the possibility of{10-11}-{10-12}double twinning related boundaries.Boundaries which satisfy 30°[0001]correspond to the areas of the dynamically recrystallized grains,indicating that the formation of the 30°[0001]misorientation is associated with dynamic recrystallization.The formation of this type of grain boundaries was explained as a result preferential nucleation and growth of the recrystallized grains[43].The higher mobility and stability due to specifi slip configuratio in this misorientation were also discussed that stabilize such boundaries[44].

4.Conclusions

?A fl w softening was realized during deformation of ND sample at 150 °C,which is ascribed to formation of shear regions.

?The anisotropy in fl w stress did not remarkably decrease with increasing temperature to 250 °C,while a sharp decrease in fl w stress anisotropy was realized as the temperature was raised to 350 °C.

?The occurrence of twinning during deformation of RD and TD samples at 250 °C postponed the development of DRX and thereby fl w softening.

?The largest grain size and DRX fraction at 250 °C was obtained after deformation of DR.sample,while the minimum grain size and DRX fraction were obtained for ND one.

?Pre-existing twins could reduce the anisotropy of material in respect of fl w stress as well as DRX progression,where TD sample showed the lowest DRX fraction at 250 °C.

?A different misorientation distribution was realized after deformation of pretwinned material along ND and RD directions.

Acknowledgment

This work was supported in part by the National Science Center(Poland)under grant No.2018/31/B/ST8/00942.