Improved strength and ductility of AZ31B Mg alloy sheets processed by accumulated extrusion bonding with artificia cooling

Tingzhuang Han,Jingfeng Zou,Guangsheng Huang,Lifeng Ma,**,Chaojie Che,Weitao Jia,Lifei Wang,Fusheng Pan

aCollege of Materials Science and Engineering,Taiyuan University of Science and Technology,Taiyuan 030024,China

b Heavy Machinery Engineering Research Center of the Ministry of Education,Taiyuan University of Science and Technology,Taiyuan 030024,China

c State Key Laboratory of Mechanical Transmission,College of Materials Science and Engineering,Chongqing University,Chongqing 400044,China

d College of Materials Science and Engineering,Taiyuan University of Technology,Taiyuan 030024,China

Abstract In this study,accumulated extrusion bonding(AEB)process with application of artificia water cooling was successfully performed to fabricate fined-grain AZ31B Mg alloy sheets at 150°C,200°C and 250°C.The resultant microstructure and mechanical properties are systematically investigated.It reveals that the processing temperature has an important effect on the microstructural evolution during extrusion.During AEB process at 150°C and 200°C,{10-12}tensile twinning was activated at early stage of extrusion,and subsequently continuous dynamic recrystallization(CDRX)occurred and dominated the further deformation.However,for the sample extruded at 250°C,hardly any twins can be observed,and new fine dynamic recrystallized grains were found along grain boundaries.Artificia cooling was utilized to reduce the rate of grain growth out of the extrusion die,resulting the grains significantl refine from 11μm to 2.5μm.Local high dislocation density region was also observed in the microstructure of sample processed at 150°C in artificia cooling condition,and the degree decreased with the processing temperature increase.The results summarized from tensile tests indicated that due to the grain refinemen the strength and ductility was significantl enhanced(YS of 186MPa vs.145MPa,UTS of 391MPa vs.336MPa and FE of 31.5%vs.24.5% compared with the as-received sample).Subsequently,annealing treatment at different temperatures was applied to eliminate the high dislocation density.The sample annealed at 200°C exhibited the best comprehensive mechanical property with YS of 179MPa,UTS of 390MPa and FE of 33.0%.As the annealing temperature increasing,the dislocation density was reduced by static recrystallization(SRX)and grain growth,leading to a decreased strength and ductility.? 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;Accumulated extrusion bonding;Artificia cooling;Microstructure;Texture;Mechanical properties.

1.Introduction

Considering the energy conservation and environment protection,the requirement of lightweight components for aerospace and automotive applications have attracted considerable attention[1-3].As the lightest structural metal material,magnesium(Mg)and its alloys with excellent properties,such as good damping,high specifi strength,casting capability,strong electromagnetic shielding and good recyclability,attained increasing interest over recent years[4-6].However,Mg alloys generally suffer from low strength and ductility at room temperature,limiting the applicability in various practical application as structure components.Therefore,lots of research work had been carried out to improve their mechanical properties.

According to the Refs.[7-9],Mg alloys with ultra-fin grained(UFG)microstructures showed a good combination of high strength and ductility.Up to now,severe plastic deformation(SPD)processes have been known as the most important method to produce UFG materials,including high pressure torsion(HPT)[8,10],cyclic extrusion compression(CEC)[11-13],multidirectional forging(MDF)[14-16],constrained groove pressing(CGP)[17,18],equal channel angular pressing(ECAP)[19-23],accumulative roll bonding(ARB)[24-28],and so on.Bry?a et al.[10]indicated that grains of EZ33A alloy was greatly refine to～127nm by HPT process and an enhancement of both tensile strength and uniform ductility was obtained.Liu et al.[20]reported that after 16 passes ECAP process,Mg94Y4Zn2 alloy with refine-graine(0.85μm)microstructure exhibited outstanding compressive strength(611MPa)and fracture strain(20.1%).The average grain size of AZ61 alloy processed by MDF was reduced to 0.8μm,and the alloy showed excellent comprehensive mechanical properties with ultimate tensile strength of 440MPa and plastic strain to fracture of over 20%[16].Wang et al.[17]had investigated the influence of CGP process on microstructure and mechanical properties of AZ31B magnesium alloy sheets at 423K.They indicated that the grains were refine to 2.81μm,and the excellence balance of yield strength(240MPa)and elongation(18.5%)were achieved.As mentioned above,it can be seen that grain refinemen achieved by SPD processes can enhance the strength without sacrific ing the ductility.However,most of the SPD techniques are still at laboratory scale and not suitable for industrial production[29].Among them,ARB process has been proved to be the most promising SPD technique in industrial scale up for continuous manufacturing bulk UFG magnesium alloy sheets[24,30].While due to the poor formability of Mg alloy sheets,higher temperatures over 300°C were usually adopted to prevent the occurrence of edge cracking during rolling.The higher processing temperatures generally led to a grain growth and weakened the grain refinin effects of ARB process,limiting its wide application.

Our previous studies have come up with a new technique of accumulate extrusion bonding(AEB)[31-33].Due to the good plastic deformation capacity under hydrostatic stress induced by extrusion,AZ31B alloy sheets can be processed at lower temperatures without edge cracking.During the investigation of microstructure evolution,we found that the matrix grains were completely transformed into dynamic recrystallized(DRXed)grains with average grain size of 1.4μm at sizing band.While owing to the heating induced by the plastic deformation and friction,the actual temperature near the die exit was higher than the initial set temperature,resulting that grains significantl grew to 4.5μm after the sheet exited from extrusion die.This subdued the effect of AEB process on grain refinemen in a large extent.In order to fully utilizing the AEB process,further work is mainly focused on retarding grain growth out of the extrusion die.Bohlen et al.[34]firstl applied artificia water cooling on the hydrostatic extrusion of AZ31B magnesium alloy,and reported that a representative further refine microstructure with average grain size of 2μm was obtained.Then Kim et al.[35-37]and Park et al.[38,39]investigated the microstructure evolution and mechanical properties of AZ31B,ZK60,AZ91 and Mg-7Sn-1Al-1Zn alloys processed by indirect extrusion utilizing artificia water cooling.They found that the actual extrusion die temperature could be reduced from 350°C to 260°C and 213°C,respectively,by directly spraying water onto the extruded rod at the die exit[37,39].This led to a remarkable grain refine ment from 5.5μm to 1.8μm and a great enhancement in yield strength as much as 50MPa with almost same ductility.Thus,aiming at reducing rate of grain growth out of the extrusion die,artificia water cooling was performed to AEB process in this present work.The microstructural evolution and mechanical properties were systematically investigated.In addition,the processing temperature was out of consideration in our previous studies.Variations in the microstructural characteristics and mechanical properties of AZ31B magnesium alloy sheets processed by AEB at different temperatures were also examined and discussed herein.

2.Material and methods

Rolled AZ31B magnesium alloy sheets with 1.5mm thickness was selected for experiments in this work.The actual chemical composition of AZ31B magnesium alloy was measured by X Ray Fluorescence(XRF,1800CCDE),as shown in Table 1.The initial microstructure of as-received sheet was measured by electron backscatter diffraction(EBSD)on a JEOL JSM-7800F fiel emission scanning electron microscopy,and the results are shown in Fig.1.The sheet shows a microstructure consisted of fully recrystallized grains with a statistical average grain size of～11μm.The texture observation measured from(0002)pole figur exhibited a characteristic of rolling basal texture with basal planes of most grains nearly parallel to the RD-TD plane.Pieces with dimensions of 25mm(rolling direction,RD)×15mm(transverse direction,TD)×1.5mm(normal direction,ND)were cut from the asreceived sheets.Before stacking,the surfaces of pieces were wire-brushed and cleaned by alcohol.Then ten pieces were stacked and put into the container with RD parallel to the extrusion direction(ED),and hold for 20min at 150°C,200°C and 250°C,respectively.The schematics of AEB process with artificia water cooling is shown in Fig.2.In order to obtain better cooling effect,the mold for holding water was placed under the sizing band and the extrusion immediately started.The plunger speed was set as～6 mm·s?1.The extrusion ratio was 12.8:1 and the equivalent strain was～2.55(ε=lnλ,λwas the extrusion ratio).

Table 1Chemical composition of the AZ31B alloy(wt.%).

The interrupted samples were taken from the extrusion die and sectioned for six different locations parallel to ED to investigate the microstructural characteristics at different regions,as shown in Fig.3.It can be seen that locations of 1mm,5mm and 9mm were in the container,and 11.5mm and 15mm were in the cone die and sizing band,respectively.The microstructures at location 25mm were used for distinguishing the differences among the samples processed by AEB with and without artificia cooling.The samples processed at different temperatures without artificia cooling are marked as 150°C,200°C and 250°C samples,respectively.While the samples with artificia cooling are labeled as AC-150°C,AC-200°C and AC-250°C samples,respectively.

Fig.1.The(a)EBSD map and(b)(0002)pole figur of as-received sheet.

Fig.2.The schematics of AEB process with artificia cooling.

Fig.3.Microstructural observation locations of the interrupted extruded sample along ED.

The microstructures of various samples were observed by using optical microscope(OM)and scanning electron microscope(SEM,TESCAN VEGA 3).Grain sizes were acquired from optical microstructures by using a linear intercept method.Mechanical properties were examined by tensile tests carried out on a CMT6305-300KN electronic universal testing machine at room temperature.Tensile samples were machined for gauge length of 8mm(ED)×6mm(TD).Three tests were conducted under each condition to get the representative results.

3.Results and discussion

The microstructures at different locations during AEB process at 150°C and 200°C are shown in Figs.4 and 5[31].Compared with the microstructure characteristics in Figs.4 and 5,it suggested that the 150°C and 200°C samples exhibited a similar microstructural evolution from start to end of AEB process.At 1mm,a large number of twins were observed in the matrix grains.According to the EBSD data in our previous study[31],the boundaries of twinning lamella belonged to the crystallographic characteristics of{10-12}tensile twin boundaries.With the strain increasing,a few new ultra-fine grains distributed along twin boundaries and grain boundaries were observed,forming a typical‘necklace’grain structure as illustrated by blue arrows in Fig.4b.This process exhibited a typical characteristic of continuous DRX(CDRX).As reported in previous studies that the critical resolved shear stress(CRSS)for non-basal slips decreased with the temperature increase,leading to a high activity of prismatic and pyramidalslips during high temperature deformations[40-42].Then the slips were piled up along grain boundaries and rearranged themselves into low angle grain boundaries(LAGBs)[31].As the strain increasing,LAGBs continued to absorb the dislocations and transformed into high angle grain boundaries(HAGBs),and finall converted into new DRXed grains[43].At 9mm,more fin grains were formed by the means of CDRX and some initial grains were consumed by the fin DRXed grains.For the further deformation at cone die and sizing band,an almost DRXed microstructure was obtained with an average grain size of～0.9μm for the 150°C sample.When the extrusion temperature increased to 200°C,the average grain size gradually increased to～1.4μm.This is consistent with the results reported by the Refs.[44-46],indicating that the grain size of DRXed grains enlarged with the extrusion temperature increasing.At 25mm,grains grew to～3.8μm and 4.5μm for 150°C and 200°C samples,respectively.This originated from the fact that the actual die exit temperature increased because the heat generated by plastic deformation and friction during extrusion[37-39].Park et al.[39]reported that the actual die exit temperature increased from 350°C(the set temperature)to 414°C,resulting in coarsening of the DRXed grains.

Fig.4.The microstructures at different locations during AEB at 150°C:(a)-(f)from 1mm to 25mm,and enlarged images at cone die and sizing band.

Fig.5.(a)-(f)the microstructures at different locations from 1mm to 25mm during AEB at 200°C[29].

Fig.6.The KAM maps of 200°C interrupted sample at(a)1mm,(b)5mm,(c)9mm,(d)11.5mm,(e)15mm and(f)25mm.

According to the EBSD data reported in our previous study[31],the Kernel Average Misorientation(KAM)maps at different locations of 200°C interrupted extruded sample were obtained and shown in Fig.6.As it is well known,KAM maps are calculated based on the local misorientation level between individual point(kernel)and all its surrounding points,which is generally used to represent internal plastic strain of the materials[47-49].Three neighbor points were measured to defin the kernel in the present work.According to the legend bar,the local residual plastic strain increased with the color changing from blue to red,which indicated the enlarged dislocation density[50-52].As seen from Fig.6a,the grains in green and yellow colors exhibited a high level of dislocation density and plastic strain,resulting from the high activity of tensile twinning and non-basal slips at initial stage of extrusion.It is evident that the fraction of green grains decreased from location 1mm to 9mm.This may be related to the occurrence of DRX in the vicinity of grains.The regions with high KAM value accumulated a large number of subgrain boundaries,which could offer sufficien strain for triggering CDRX[42]and release dislocations[53].It also can be observed that higher dislocation density mainly focused on the coarse deformed grains and the fin DRXed grains were presented in blue at 9mm.At locations of cone die and sizing band,laurel-green grains distributed homogeneously in the microstructures.After the sheet exited the extrusion die,static recrystallization(SRX)and grain growth occurred and eliminated the strain generated in the deformation process.

Fig.7 shows the microstructures observed at different locations of interrupted sample processed by AEB at 250°C.Compared with that of 150°C and 200°C samples at 1mm,the microstructure consisted of a few fin grains along grain boundaries(indicated by yellow arrows)without any twins being observed,suggesting the onset of DRX process at initial extrusion stage(as shown in Fig.7a).The absence of twins might be owing to the fact that the high stored energy induced by plastic deformation facilitated the nucleation of DRXed grains[54].For the further deformation at 5mm and 9mm,the newly formed DRXed grains exhibited a larger grain size and volume fraction compared samples processed at 150°C and 200°C.DRX process completed in the cone die and a bimodal microstructure consisted of coarser grains(5-6μm)and fine grains(1～2μm)was obtained.The microstructure became more homogeneous with average grain size of 2.9μm at sizing band.Grains grew up to～6.1μm at location of 25mm.

The locations at 25mm along ED of 150°C,200°C and 250°C samples were obtained to reveal the effect of processing temperature on the texture evolution.The relevant EBSD maps and(0002)pole figure are shown in Fig.8.As can be seen from the EBSD maps,the grain sizes of AEBed samples enlarged with the processing temperature increase.This is consistent with the results of optical microstructure observation.According to the(0002)pole figures three AEBed samples exhibit similar double peak textures with the basal poles inclined from ND towards ED,which might be mainly owing to the high activity ofpyramidal slips during extrusion.Agnew et al.[55]and Zhou et al.[56]indicated that the increased tendency of RD-tilt texture of hot rolled Mg alloys showed a good correlation with the increased activity ofslips.In addition,a close texture intensity was obtained in AEBed samples and slightly increased from 10.83 to 11.97 compared with that of as-received sample.This indicated that the processing temperature had little effect on the difference of texture evolution.

It is well known that the mechanical properties of Mg alloys strongly depend on the grain size.As mentioned above,due to the higher exit extrusion temperature,a significan grain growth was observed,weakening the effects of AEB process on grain refinemen and strength enhancement.In order to limit grain coarsening of AZ31B alloy sheet,the additional heat generation induced by plastic deformation and friction should be reduced.In this regard,artificia cooling had been proved to be an effective method in suppressing excessive temperature rise during extrusion[37-39].Therefore,artificia cooling was performed during AEB process at 150°C,200°C and 250°C in the present work.As shown in Fig.2,because the water cooling worked outside the extrusion die,the microstructures at 25mm were observed by optical microscopy and EBSD technique to investigate its effects on microstructural evolution and the results are shown in the Fig.9.From the optical microstructure in Fig.9a,it can be seen that the grains in～1μm were observed distributed around coarser grains(3～5μm)and the average grain size decreased from 3.8μm to 2.5μm compared with that of sample without artificia cooling.Due to the higher heat capacity of water than that of air,the excessive heat could be rapidly absorbed by water,reducing grain growth rate after the sheet exited from the die.As seen from the KAM map,a small amount of green area was mainly distributed in the fine grains observed in the AC-150°C sample.This means that dislocations formed during hot extrusion was not completely annihilated by SRX and grain growth out of extrusion die.For the AC-200°C and AC-250°C samples,the grains were refine to 3.4μm and 3.9μm,respectively.With the processing temperature increasing,the green area of high strain energy reduced in a certain extent and the location of high dislocation density was mainly distributed along grain boundaries.In addition,the artificia cooling had little influ ence on the crystal orientations,as illustrated by the(0002)pole figures Various AC samples also exhibited double peak texture with a broader spread of basal poles towards ED.The texture intensity of AC-150°C greatly increased from 11.97 to 18.58,which might be owing to the high dislocation density in the fine grains.

Fig.7.(a)-(f)the microstructures at different locations from 1mm to 25mm during AEB at 250°C.

Fig.8.The EBSD maps and(0002)pole f gures of(a)150°C,(b)200°C and(c)250°C samples.

Fig.9.The optical microstructure,KAM maps and(0002)pole figure of(a)AC-150°C,(b)AC-200°C and(c)AC-250°C AEBed sample.

The true stress-strain curves of various samples are represented in Fig.10,and the corresponding mechanical properties including yield strength(YS),ultimate tensile strength(UTS)and fracture elongation(FE)are summarized in Table 2.As shown in Fig.10,the black solid line represents the as-received sample and other color solid line represent AEBed samples without artificia cooling.It is clearly observed that the sheets processed by AEB exhibited an increasing trend in YS,UTS and FE comparing with that of as-received sample.Among the AEBed samples,150°C sample showed a highest YS(178MPa vs.145MPa,increased by 22.8%),UTS(391MPa vs.336MPa,increased by 16.4%).The enhanced strengths might be attributed to the refine grains and ED-tilt texture.The influenc of grain size could be concluded by the Hall-Petch relationship(formula(1))as follows:

Table 2The mechanical properties of various samples.

Fig.10.The true stress-strain curves of various samples.

whereσyrepresents the yield strength,σ0represents the yield stress of a single crystal,d is the average grain size,k is the Hall-Petch coefficien of 300 MPa·μm1/2[57](280-320 MPa·μm1/2for Mg alloys[58]).According to the Hall-Petch relational expression,the YS of magnesium alloys was inversely proportional to the square root of grain size[12].Therefore,the increased YS value originated from grain refinemen of AEBed samples can be calculated by the following formula(2):

According to the grain size measured from the microstructures,the contribution of grain refinemen to the YS is～63MPa for the 150°C sample.While the actual difference between 150°C and as-received samples was 33MPa,lower than the calculated value,which was related to the texture evolution.As reported by Tong et al.[59],the tensile deformation was mainly dominated by basal slip system,largely depending on the texture.The results obtained from Schmid factor of basal slip in our previous study[31]indicated that the AEBed sheet exhibited a larger Schmid factor to facilitate the activation of basal slip during tensile tests,and this led to a decrease trend in YS.For the finall increased YS as shown in Fig.10,it can be concluded that grain refine ment played a more important role in strength enhancing than texture evolution.Due to the increased grain size of 200°C and 250°C samples,the YS and UTS decreased.In addition,higher FEs(31.5% vs.24.5%,increased by 26.5%)of AEBed samples were obtained.This related to the combination effects of grain refinemen and texture evolution:on one hand,the fine-graine microstructure with more grains in per unit area,so the same plastic strain amount could be dispersed into more grains during deformation,resulting in more uniform deformation and a lower degree of stress concentration[60].As a result,the FEs of AEBed samples were enhanced by the refine microstructure.On the other hand,it is known that{10-11}compression twins and{10-11}-{10-12}could act as microcracking sites during plastic deformation,causing localized deformation and early failure[61].The easy activation of basal slip owing to the ED-tilt texture could effectively retard the occurrence of{10-11}twinning,leading to the enhancement of ductility during tension[62].Moreover,compared solid lines and dash lines in Fig.10,it also can be observed that the YS of extruded samples in artificia cooling condition would be further increased.As is summarized,the average YS of AC-150°C sample increased to 186MPa,while the FE was no more improved.The strength enhancement is likely caused by the fine grains and local high dislocation density.While for the AC-200°C and AC-250°C samples with further refine microstructure,the strength and ductility were simultaneously improved to 178MPa and 31.0%,respectively.

Fig.11.The EBSD maps,KAM maps and(0002)pole figure of(a)200°C-1h,(b)250°C-1h and(c)300°C-1h.

In order to investigate the effects of local high dislocation density on mechanical properties,the AC-150°C sample was performed to annealing treatment at 200°C,250°C and 300°C for 1h.The annealed samples were named as 200°C-1h,250°C-1h and 300°C-1h samples,respectively.The microstructure evolution was measured by EBSD technique and the results are shown in Fig.11.As shown in Fig.11a,the microstructure did not change obviously before and after annealing with the average grain size of 2.4μm,similar to that of AC-150°C sample.The degree of internal strain decreased after annealing at 200°C,and the high dislocation density region mainly distributed along grain boundaries and fine grains.This may be due to the occurrence of static recovery at 200°C,which cannot completely eliminate the dislocations.As the annealing temperature increasing,SRX and the grains grew to 3.8μm and 4.5μm for the 250°C-1h and 300°C-1h samples,respectively.This is consistent with the results reported by the Refs.[56,63],indicating high annealing temperature promoted recrystallization nucleation and growth.It can be seen from the KAM maps in Fig.11b and c that the dislocations formed during extrusion was almost eliminated by recrystallization,and resulted in forming stress-free grains within 250°C-1h and 300°C-1h samples.In addition,the results of texture evolution measured by(0002)pole figure showed that the annealed samples exhibited ED-tilt double peak texture with decreased texture intensities.The texture intensity slightly decreased from 18.58 to 18.16 for the sample annealed at 200°C,and greatly decreased to 11.21 and 11.62 for the 250°C-1h and 300°C-1h samples,respectively,which may be related to the SRX and grain growth.Previous studies[56,63,64]reported that high annealing temperature promoted recrystallization,and the texture intensity decreased with the increase of the area faction of recrystallization grains.

Fig.12.The(a)true stress-strain curves of various samples,(b)corresponding mechanical properties.

The true stress-strain curves of annealed samples are shown in Fig.12a and the average values of YS,UTS and FE are presented in Fig.12b.It can be observed that the various samples in annealing condition exhibited a decreasing trend in YS and UTS,and an upward trend followed by a downward trend in FE.Among the annealed samples,200°C-1h sample showed the best comprehensive mechanical properties with a slightly decreased YS of 179MPa and increased FE of 33.0%compared with that of AC-150°C sample.Because the two samples had the similar grain size and texture,the difference in the mechanical properties could be mainly attributed to the level of dislocation density reduction by the lower temperature annealing.For the 250°C-1h and 300°C-1h samples,the average YS,UTS and FE continued to decrease with the annealing temperature increasing.The reason responsible for the decreasing trend is suggested to the grain growth during higher temperature annealing.

4.Conclusion

In this work,AZ31B alloy sheets were subjected to AEB process with artificia cooling at different temperatures.A detailed analysis of the plastic deformation characteristics and the effects of processing parameters on mechanical properties were investigated.The main conclusions drawn from the experimental study are as follow:

(1)The processing temperature has a great effect on the microstructure evolution.{10-12}twin boundaries were observed at initial stage in samples processed at 150°C and 200°C.After twinning,the microstructures of the two samples continuously developed through CDRX process,forming an ED-tilt double peak basal texture.While for the 250°C sample,hardly any twins were observed and new fin DRXed grains were found at the early stage of extrusion.After the sheets exited from the extrusion die,grain growth was observed,and the grain size increased with the extrusion temperature increase.

(2)Obvious mechanical properties enhancement was obtained in the AEBed samples.Compared with the as-received sample,the YS,UTS and FE improved significantl from 145MPa,336MPa and 24.5% to 178MPa,391MPa and 31.5%,respectively.The strength enhancement was mainly attributed to the grain refinement while the improved ductility was due to the combination effect of fine grains and texture evolution.

(3)In order to reduce the rate of grain growth out of the extrusion die,artificia cooling was performed during extrusion.As a result,grains of sample processed at 150°C could be further refine to 2.5μm;residual strain was not completely released and mainly distributed in the fine grains;the degree of internal residual strain decreased with the extrusion temperature increasing.Application of artificia cooling during extrusion will enhance the mechanical properties.Due to the further grain refinement the yield strength of AC-150°C sample was improved to 186MPa with almost same ductility to the sample without artificia cooling.

(4)Annealing treatment was performed to the AC-150°C sample with the aiming of investigating the effects of local high dislocation density on mechanical properties.The degree of dislocation density in sample annealed at 200°C reduced,leading to the best comprehensive mechanical properties,such as YS of 179MPa,UTS of 390MPa and FE of 33.0%.Owing to the SRX and grain growth after annealing at 250°C and 300°C,the local high dislocation density was relieved,resulting in a decreased strength and ductility.

Acknowledgment

The authors are grateful for the National Natural Science Foundation of China(No.51905366,51901149,51671041,51604181,U1910213 and U1610253),the Key Research and Development Program of Shanxi Province(No.201603D111004),the Fund for Shanxi“1331 Project”Key Subjects Construction,Scientifi and Technological Innovation Programs of Higher Education Institutions in Shanxi(No.2019L0632),Taiyuan University of Science and Technology Scientifi Research Initial Funding(20182030,20192003 and 20192008).