Qingshn Yng ,Bin Jing ,Bo Song ,Zujin Yu ,Dwi H ,Ynfu Chi ,Jinyu Zhng,Fushng Pn

a School of Metallurgy and Material Engineering,Chongqing University of Science and Technology,Chongqing 401331,China

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

c School of Materials and Energy,Southwest University,Chongqing,400715,China

d MagIC -Magnesium Innovation Centre,Helmholtz-Zentrum Geesthacht,Max-Planck-Stra?e 1,Geesthacht 21502,Germany

e School of Engineering Technology,Purdue University,West Lafayette,IN 47907-2021,United States

Abstract The grain orientation control via twinning activity on deformation features is of great significanc to offer a key insight into understanding the deformation mechanism of Mg alloy sheets.The {10-12} twinning were performed by pre-strain paths,i.e.,tension (6%)and compression(5%) perpendicular to the c-axis along extrusion direction (ED),to investigate the microstructural evolution and mechanical properties of AZ31 Mg alloy sheets.The distinction in the texture evolution and strain hardening behavior was illustrated in connection with the pre-strain paths for the activities of twinning and slip.The result shows that the activation of the deformation mode was closely bound up with the grain orientation and the additional applied load direction.The {10-12} twin-texture components with c-axis//ED were generated by precompression,which can provide an appropriate alternative to accommodate the thin sheet thickness strain and enhance the room temperature formability of Mg alloy sheet.

Keywords: Mg alloy;Orientation control;Microstructural evolution;Texture;Stretch formability.

1.Introduction

With the rapid development of automobiles,track traffi and aerospace vehicles towards light weight,low energy consumption and the high-performance lightweight materials are urgently required [1-4].As the lightest metal structural material,Mg alloy has the advantages of high specific strength,excellent damping performance and recyclability [5,6].It can be considered as an important supplement to traditional metal materials such as steel,aluminum alloy,titanium alloy,etc.to enhance the energy efficiency in transportation [7-9].However,the room temperature formability of wrought Mg alloy sheets is restricted due to the limited number of slip systems inherent in the hexagonal close-packed (hcp) structure of Mg alloys,which further limited the practicability of strain accommodation during subsequent plastic forming of the semifinishe components [10-12].Therefore,improving the room temperature strength and formability becomes more critical for the comprehensive application of magnesium alloy sheet.

Orientation control is a key measurement in the development of wrought Mg alloy sheets for wider application.In the last decades,a great number of experimental studies related with alloying additions and plastic working method have been developed to overcome this problem[13-17].Zhao et al.[18] reported that the ductility of the extruded Mg-1Gd alloy sheet increased with increasing the Zn content to 0.5 and 1.0 wt.%,which can result in weakened ED-split texture and more grains with the c-axis tilted preferential towards TD.Mao et al.[19] has studied that the rolling paths and pass reductions can be used to tailor microstructure to obtain the asymmetrical texture distribution to enhance the room temperature ductility.However,for the above-mentioned methods,the high cost and complex processing equipment are required,which became the obstacle for industrial applications and unfi for the extensive promotions of Mg alloys.

Recently,the grain orientation control via twinning deformation has been considered as an effective approach to improve the room temperature stretch formability [20-23].The deformation of Mg alloy usually resulted in the alignment of the basal planes parallel to the main direction of material flow,which can easily produce a strong (0002) basal texture during the common processing methods [24-26].In such case,the twinning can be related to the preferential activation of foundation slip.The twinning can offer a key insight into understanding the deformation mechanism of Mg alloys.Meanwhile,{10-12}tension twinning usually results in about 86.3°lattice rotation,which can give a rise to a new texture component with twin-texture [27,28].Thus,the pre-strain path via the twinning deformation can be considered to be an effective and low-cost way for orientation control to optimize the texture and formability of Mg alloy sheets.In this study,the pre-strain path combined with tension and compression was applied to induce twinning behavior and the microstructure evolution of the thin AZ31 Mg alloy sheets was investigated.The main purpose of the present study is aimed to explore an appropriate twinning deformation,which would facilitate the grain orientation control to adjust mechanical responses of Mg alloy sheets.

2.Material and methods

The material used in the present work was AZ31 (Mg-3 Al-1Zn in wt.%) Mg alloy sheet with a thickness of 1mm and a width of 56mm (termed as-received sample).The asreceived samples processed by hot extrusion were achieved with extrusion speed of 10mm/s and extrusion ratio of 101:1 at the temperature of 693K.The as-extruded sheets were machined into a rectangle shape with a cross section of 56mm(TD)×100mm(ED).Here,ED,TD and ND are the extrusion direction,transverse direction and normal direction.Fig.1 shows the schematic diagram and strain-loading curves of pre-tension/compression paths of AZ31 Mg alloy sheets.The pre-tension test was performed at a CMT6305-300KN universal testing machine at a constant strain rate of 10-3s along ED with 6mm displacement at room temperature (termed PT sample).To avoid the effect of internal stress and dislocations,PT samples were performed by the recrystallization annealing at 623K for 1h (termed PTA sample).Subsequently,the PTA samples were pre-compressed by a previous method [29] with 5mm displacement along ED at the same strain rate(termed PTC sample).The PTC samples were annealed at 623K for 1h (termed PTCA sample).After the pre-tension/compression deformation,the displacements were re-measured and the defamation strain was 6% (PT sample)and 5% (PTC sample),respectively.

To investigate the mechanical properties (strength and ductility) of the processing,the tensile samples of Mg alloy sheets were machined into the nominal gage dimensions of 12mm×6mm×1mm.The mechanical tests were measured by the universal testing machine at a constant strain rate of 10-2s-1along ED and TD,respectively.For all mechanical results discussed in this work,at least three stress-strain curves were chosen for the evaluations.Then-value was calculated from the uniform plastic deformation region of the stress-strain curves.Standard Erichsen (IE) tests were performed by a hemispherical punch to examine the room temperature stretch formability of Mg alloy sheets.The punch diameter and speed were 20mm and 5mm/min,respectively.The blank holder force was 10 KN and silicon oil was used as a lubricant.The Mg alloy sheets were electro-polished at 20V for～150s in the AC2 solution at -10°C.The crystal orientation and texture were examined by electron backscatter diffraction (EBSD) method in a JEOL 7800F-SEM equipped with an HKL Channel 5 System.The Kernel average misorientation (KAM) was also calculated from the EBSD data.

3.Results

3.1.Microstructure evolution during pre-tension/compression deformation

Fig.2 shows the microstructure and texture evolution of as-received Mg alloy sheets examined by EBSD method (EDND plane).The as-received Mg alloy sheets reveal the mixed microstructure with a mass of the dynamically recrystallized grains of about 7μm around the comparatively coarse grains of about 30μm along ED.Fig.2(b) displays a strong basal texture which the c-axis of grains were parallel to the ND owing to the deformation flow orientation of Mg alloy during the hot extrusion.The Microstructure evolution of PT and PTA samples are shown in Fig.3.A pre-tension strain of～6%was carried out at room temperature along ED.PT sample exhibits the deformed elongated grains and without the appearance of twinning.After the annealing,the PTA sample reveals an inhomogeneous structure of profuse equiaxed fine grains of about 10μm embedded in the large grains of about 70μm,as shown in Fig.3(b).The degree of deformation is enough to trigger the abnormal grain growth during static recrystallization as a result of the strain induced the grain boundary migration.The selective grain growth may be due to the activation of strain induced grain boundary migration which was stimulated by the high tension strain gradient related with the pre-tension strain paths [19,30,31].

Fig.1.(a) Schematic diagram and (b) strain-loading curves of pre-tension/compression paths of AZ31 Mg alloy sheets.

Fig.4 shows the EBSD results of PTC and PTCA samples.It is interestingly noted that a large amount of {10-12}twins were formed after pre-compression of 5% strain level with a fixed compression die along ED.The area fraction of{10-12} twins of PTC samples came up to～65%.After annealing at 623K for 1h,most of the grains were recrystallized.In this work,the grains with internal average misorientation angle below 1° were define as recrystallized grains.After annealing,the area fraction of recrystallized grains is increased from 4% (PTC sample) to 49% (PTCA sample).As shown in Fig.4(b),the twins and dislocation were largely removed and the grains were coarsened to～20μm.The primordial {10-12} twins structure promoted grain rotation during the thermal process.Moreover,the grain orientation of PTCA samples remarkably became more dispersive.

The (0002) basal texture evolution of PT,PTA,PTC and PTCA samples presented in IPF maps by EBSD on the EDND plane are seen in Fig.5.It can be found that the texture of PT sample exhibited the strong (0002) basal texture after pre-tension at ambient temperature.It was not much different from the featured texture in Mg alloy sheets processed by the common deformation process path such as rolling.The(0002)basal plane of PT samples inclined about 20° away from TD.After annealing,a large number of the (0002) basal plane of PTA samples tilted towards TD.It is suggested that the pre-tension paths can stimulate the rotation of the c-axis toward the imposed strain accommodation direction under static recrystallization,which may be attributed to the activation of prismatic 〈a〉 slip and pyramidal 〈c+a〉 slip at the high temperature [32-34].In addition,some grains with special orientations,e.g.the faster recovery rates,can also consume the rest of grains modifying the texture.PTC sample formed a new {10-12} twin-orientation,as shown in Fig.5(c),related to c-axis//ED texture component.The remaining grains generated a weak bimodal texture,where the {0002} basal texture was symmetrically distributed.The c-axis//ED texture of PTCA sample was further reinforced after the annealing treatment,which is due to the strain-induced boundary migration during the static recrystallization.It can be noted that these recrystallized grains have an orientation similar to the{10-12} twinning activity.

Fig.2.EBSD results of (a) microstructure and (b) texture presented as IPF maps for as-received Mg alloy sheets.

3.2.Mechanical properties and stretch formability at room temperature

The true stress-strain curves of various samples tensioned along ED and TD at room temperature are presented in Fig.6.The PTCA samples exhibit the highest uniform elongation(Eu) and lowest yield strength (YS) under ED-loading,which illustrates that the PTCA sheets have the highest strain hardening rate along ED related to the pre-compression strain deformation.The concave shape and rapid increase of the tensile curve along ED clearly states that the primary deformation mechanism was {10-12} twinning.Moreover,the pre-strain with pre-tension/compression paths in the same direction has little effect on PTCA and as-received samples,as shown in Fig.6(b).This indicates an evident distinction between the mechanical behaviors of these three deformed sheets as a result of the differences of their (0002) basal texture features.

The mechanical properties,i.e.,YS,ultimate tensile stress(UTS),Eu,n-value and IE value of various samples are listed in Table 1.It can be seen that the YS in TD loading return to the original value of as-received samples,whereas an obvious reduction to 58MPa for PTCA samples in ED direction.It is suggested that the advantageous activation of {10-12} twins can lead to a low tensile YSEDand high in-plane anisotropy of PTCA sample.Moreover,PTCA sample shows the larger peak strength and comparable ductility in comparison with as-received sheets.The room temperature ductility was also improved by pre-compression paths,where the PTCA sample was achieved to about 29.7%.In addition,nvalue of Mg alloys was about 0.3,while that of PTCA samples in ED loading was enhanced to 0.84.Fig.7 shows the stretch formability of annealed samples measured by the Erichsen test.The as-received sheets exhibit a limited stretch formability of about 2.3mm.The IE value of PTCA sample was about 6.9mm,three times that of the as-received samples,whereas the pre-tension path had no influence on the stretch formability.During the pre-compression process with strain//ED strain,a massive the grains of PTA possess a hard orientation with a high CRSS value,which facilitates 〈c+a〉pyramidal slip and {10-12} twinning [28,35].After annealing at 623K for 1h,PTCA can conserve the twin structure,thereby stimulating the reorientation of grains to soft orientation.

Fig.3.EBSD mapping of (a) PT and (b) PTA samples.

Fig.4.EBSD observation of (a) PTC and (b) PTCA samples.

Fig.5.(0002) basal texture evolution of (a) PT,(b) PTA,(c) PTC and (d) PTCA samples presented in IPF maps by EBSD.

Table 1Yield strength (YS),ultimate tensile stress (UTS),uniform elongation (Eu), n-value and IE value of as-received,PTA and PTCA samples.

Fig.8 shows the curves of work hardening rate and flow stress (σ-σ0.2) of Mg alloy sheet.Generally,the strain hardening curve with dislocation slip usually shows three stages as the main deformation mode of Mg alloy with hcp crystallographic texture:(I) short elastic-plastic transition stage;(II)almost constant strain hardening stage;(III) degressive linear strain hardening stage [36,37].The strain hardening rateθ=dσ/dε,whereσandεare the true stress and the true strain,respectively.The as-received samples reveal a smooth stage ofθIIand a linear stage ofθIII,which diminishes after yielding owing to a short elastic-plastic transition.However,for PTCA samples,the length and strain hardening rate of stage II was remarkably increasing,which was due to the lowest yield strength and a {10-12} twin-texture induced by precompression path.Because of the twin-texture with grain orientation,the SF factor was also changed.It indicates that the effect on the length of stage II was enhanced with increasing SF factor.Furthermore,the strain hardening behavior was laid on the distinction between the CRSS on (0002) basal plane that can be converted by the grain orientation.The orientation of the{10-12}tension twins induced by pre-compression was beneficial for the basal slip,which can increase the contribution of the basal slip to the plastic strain,and gradually resulted in the flow stress (σ-σ0.2).In addition,the PTCA samples also have a highestθIIIunder the ED-loading.It was interrelated with the different dislocation densities and {10-12} tension twin-texture with c-axis// ED,which could offer an additional hardening effect and become an obstacle for the dislocation slipping.Moreover,it was related to the flow stress applied to the Mg alloy sheets with the c-axis perpendicular to the tensile axis in the tensile test.As above mentioned,the pre-strain path has little impact of the strain hardening behaviors under TD-loading.As seen in Fig.3(c),PTA samples show the mixed microstructure of equiaxed fine and coarse grains.It was speculated that coarse grains would obviously affect the strain hardening where the non-basal slips were promoted in the coarse grains [38].Thus,the PTA sample has the largest value ofθIIIalong TD.

Fig.6.Tensile true stress-strain curves of various samples:(a) ED-loading;(b) TD-loading.

4.Discussions

Fig.9 reveals the effect of pre-strain and annealing on the grain nucleation and growth.In the EBSD result mapping of different types of grains,the blue,yellow and red areas express the recrystallization grain,sub-grain and deformation grain,respectively.Blue grains represent recrystallized grains with internal average misorientation angle below 1°;Yellow grains represent the sub-structured grains containing sub-grains,the internal average misorientation angle of the sub-grain is below 1° and the misorientation angle between sub-grains is above 7.5°;and red blue grains are deformed grains with internal misorientation greater than 7.5°Moreover,the corresponding fractions of different grains and(0001) pole figure are also summarized in Fig.9.The EBSD results show the pre-strain path can observably increase the deformed grains fraction of Mg alloys in Fig.9(a)and(c),and the annealing can enhance the recrystallization and sub-grains in Fig.9(b) and (d).It indicates that the pre-strain can provide the effective nucleation site by increasing the dislocation density and the annealing was conducive to the recrystallized behavior.The storage energy processed by the pre-strain paths was enough to achieve the formation of grain nucleation and met the grain growth along ED.

Fig.7.Erichsen value (IE) of as-received,PTA and PTCA samples.

The twin boundaries were removed owing to the consuming of recrystallized grains under annealing treatment simultaneously.For the Mg alloy with the strong basal texture,the extra deformation mode,i.e.,{10-12} extension twin or pyramidal 〈c+a〉 slip,needs to be taken into account[39-41].In the case of ED compression,extension twin variants were favored for many grains to serve on the primary deformation mode.The {10-12} tension twins also offer the profuse grain boundaries to form the nucleation cores,thus resulting in accelerating the recrystallization.In addition,the pre-compression that has their c-axis aligned close to the loading direction may facilitate the prior grain growth and lead to the weakening of the recrystallized texture.It was worth noting that the distribution of recrystallized grains of PTCA samples in (0002) pole figure were more dispersed,which was attributed to the weak global twin-texture,making the grain orientation related with deformed grains decrease rapidly [42,43].

To understand of the effect of the pre-strain on deformation features,the dislocation accumulation was measured by KAM.Fig.10 displays the KAM maps and corresponding histogram of KAM value of various samples,which qualitatively reflect the uniformity of plastic deformation.The KAM mapping signifie the average angles between the crystallographic orientation of each pixel and their eight nearest neighbors,meanwhile,the misorientation＜5° was included in the KAM calculation [44].It can be observed that the an average KAM value of PT,PTA,PTC and PTCA samples were 1.46,1.14,1.32 and 0.69,respectively.High value of KAM represented the large strain deformation or dislocation accumulation [45].It makes clear that the pre-strain samples have more dislocation accumulation than those of the annealing samples.In addition,the KAM diagram shows that the local dislocation was mainly focused around the grain boundary in the comparatively early stage of the plastic deformation.

When the Mg alloy sheets are undergoing the pre-strain process,the extra stress will be produced at grain boundary to guarantee the strain consistency.Thus,the largest KAM value of PT sample is due to the highest pre-strain of 6% along ED.Barnett [46] reported that the activity of dislocation slip was increased with increasing the strain deformation.The pre-compression deformation along ED was dominated by {10-12} twinning.For PTCA sample,the decrement of KAM value was two times than that of PTA sample during the annealing at 623K for 1h,which indicates the static recrystallization was fully activated.That is to say,the characteristics of {10-12} twinning variants are entirely eliminated.In general,the thermal stability of twin variant was relied on the dislocation accumulation in Mg alloys [27,47].Improving the dislocation accumulation would lessen the thermal stability of Mg alloy with {10-12}twin-texture.Therefore,the tendency of recrystallization may be enhanced by the high dislocation accumulation in PCTA samples.

Fig.8.Strain hardening behavior during tensile tests of various samples:(a) ED-loading;(b) TD-loading.

Mg alloys were extruded to sheets of 1mm with strong basal texture in relation to the c-axis of grains parallel to ND,as shown in Fig.11.Subsequently,the as-extruded Mg alloy sheets were pre-stretched of 6% strain with c-axis perpendicular to the tension tress direction,which resulted in the basal plane of PT samples orientating about 20° to ED.It indicates that the prismatic＜a＞slip activity seems to play a key part in promoting the orientation of (0002) basal plane,where the c-axis deflect toward the pre-tension stress on account of the reduction in the Schmid factor [48,49].After annealing,the orientation of grains in PT samples may be different from the parent grains as a result of the static recrystallization and nucleation of new grains.A large amount of the recrystallized grains mainly came from pre-tension deformation and dislocations,which can generate a great number of strain gradients along ED and accumulated the local recrystallized grains and migrating grain boundaries.

The PTA samples were pre-compressed of 5% strain,where the compression stress remarkably affected the rotation of c-axis of grains and the {10-12} twinning was the main deformation mode.This promoted the incline of c-axis of the grains to about 86.3°.Such variation in grain orientation can lead to the weakening effect on the basal texture.Furthermore,{10-12} twin boundaries were removed owing to the growth consumption of recrystallized grains after annealing.Meanwhile,the pre-compression strain can simulate the local (0002) basal slip activities to achieve in the repetitive recrystallization during annealing.The twin-texture with c-axis//ED consumption conserved the texture components along ED.As mentioned above,the PTCA sample had the high anisotropy,which was because of the fact that CRSS value of {10-12} twins was much smaller than that of prismatic slip [50,51].This signifie that the tension twins can produce a strong refinement and hardening effect on prismatic slip.For the twin-texture with c-axis// EDpreferred orientation,the basic slip and de-twinning adapted to the thickness strain during the sheet stretch forming process.Therefore,the c-axis// ED texture induced by the combination of pre-tension and compression strain paths can significantly improve the deformability of Mg alloy sheet.

Fig.9.Different types of grains of (a) PT,(b) PTA,(c) PTC and (d) PTCA samples:blue recrystallized,yellow-sub-structured,red-deformed,respectively.(For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

Fig.10.KAM maps and corresponding histogram of KAM value of (a) PT,(b) PTA,(c) PTC and (d) PTCA samples.

Fig.11.Schematics showing grain rotation evolution under pre-tension-compression paths.

5.Conclusions

The pre-strain paths combined with tension and compression have been conducted to facilitate the grain orientation control to optimize the microstructural evolution and mechanical responses of the thin AZ31 Mg alloy sheets.The main conclusions are drawn as follows:

(1) The AZ31 Mg alloy sheet processed by pre-tension strain of～ 6% along ED exhibits deformed elongated grains and without the appearance of twinning.An inhomogeneous structure of fine grain embedded in the large grains was formed during annealing because of the tension strain gradient induced by pre-tension path.

(2) A large amount of {10-12} twins were produced after pre-compression of～5% strain with a fixed compression die along ED.The area fraction of {10-12} twins came up to～65%.The twins and dislocation were removed and the grains were coarsened to～20μm during annealing.The primordial {10-12} twins structure reinforced the recrystallized behavior.

(3) The basal plane of pre-tension samples inclined about 20° away from TD.A new {10-12} twin-orientation related to c-axis//ED texture component was formed in PTCA samples.Moreover,the remaining grains generated a weak bimodal texture.

(4) The advantageous activation of {10-12} twins can lead to a low tensile yield strength and high in-plane anisotropy of PTCA sample with the high ultimate tensile strength and comparable ductility along ED.The IE value of about 6.9mm was three times that of the as-received samples.In addition,the strain hardening behavior was laid on the distinction in the CRSS on(0002) basal plane that can be converted by the grain orientation.

(5) The compression stress remarkably affected the rotation of c-axis of grains.{10-12} twinning was the main deformation mode during the pre-compression process.The twin-texture with c-axis// ED-preferred orientation can produce a strong refinement and hardening effect on prismatic slip,which can adapt to the thickness strain during the sheet stretch forming process.Thus,the caxis// ED texture induced by the combination of pretension and compression strain paths can significantly improve the deformability of Mg alloy sheet.

Acknowledgments

This project was financially supported by National Natural Science Foundation of China (51701033,51701035),Chongqing Municipal Education Commission(KJQN201901504,KJZD-K202001502),Chongqing Science and Technology Commission (cstc2018jcyjAX0022).