Xin Che,Qiang Wang,Beibei Dong,Mu Meng,Zhi Gao,Kai Liu,Jin Ma,Fulai Yang,Zhiming Zhang

College of Materials Science and Engineering,North University of China,3 Xueyuan Road,Taiyuan 030051,PR China

Abstract This study proposed an effective plastic deformation technique,rotating backward extrusion(RBE),for producing high performance AZ80 magnesium alloy cup-shaped pieces.The RBE process was carried on the Gleeble-3500 test machine at 653K,and the conventional backward extrusion(CBE)was also conducted for comparison.A detailed microstructure analysis was performed using the optical microscopy(OM)and electron back-scatter diffraction(EBSD).The results shown that the equivalent strain and deformation uniformity of the cup pieces could be substantially increased by the RBE process compared with the CBE process.Furthermore,the RBE process could significantl improve the grain refinin capacity and the proportion of dynamic recrystallization(DRX),of which the maximum reduction of grain size was 88.60%,and the maximum increase of DRX proportion was 55.30% in the cup bottom.The main deformation mechanism of the RBE process was the discontinuous DRX(DDRX),while the continuous DRX(CDRX)was also occurred in the cup transition.Compared with the CBE sample,the texture of the cup bottom was weakened for the RBE sample.The microhardness value of the RBE sample was higher than that of the CBE sample,which can be attributed to the grain refinemen strengthening.? 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:Rotating backward extrusion;AZ80 magnesium alloy;Dynamic recrystallizaton;Texture.

1.Introduction

In recent years,the interest in wrought magnesium(Mg)alloys has been increased due to their outstanding performance in weight saving,high specifi strength and excellent mechanical ability,etc.[1,2].However,a limited number of slip systems can be activated at the lower temperature due to the hexagonal close packed(HCP)crystal structure,which results in poor ductility and formability of Mg alloys at room temperature and hinders their application as structural parts.Therefore,in order to utilize the advantages of the Mg alloys,the disadvantages of poor ductility and formability need to be improved.And refinin the grains and modifying the texture may be two effective ways for solving this problem[3-5].

It is reported that the severe plastic deformation(SPD)method has the ability to produce ultra-fin grain(UFG)and modify the texture,so that the mechanical properties of the material can be significantl improved according to the Hall-Patch relationship[6].Nowadays,the most widely used SPD technologies are equal channel angular pressing(ECAP)[7-9],high pressure torsion(HPT)[10],accumulative rolling bonding(ARB)[11],multi-directional forging(MDF)[12]and repetitive upsetting and extrusion(RUE)[13].In addition,there are some SPD methods specificall for producing tubular specimens.To′th et al.[14]proposed a new SPD method named high-pressure tube twisting(HPTT)for producing UFG tubes without changing their dimensions.But this method had two drawbacks:one was that the device required a large external force to maintain a significan hydrostatic pressure,and the other was the difference in the microstructure and mechanical properties between the inside and outside of the tube.Then,Arzaghi et al.[15]proposed a modifie HPTT method,which could substantially improve the process efficien y and deformation uniformity of the HPTT process.Mohebbi et al.[16]developed an accumulative spin bonding(ASB)method based on the ARB process,which could produce long nanostructured tubes with uniform distribution of hardness along the tube wall.However,it needed multi-passes for production and the interlayer defects was prone to occur,which was labor intensive and time consuming.Korbel et al.[17,18]designed a new rotary extrusion(RE)process,named the‘KOBO’process,through the combination of torsion and forward extrusion for producing rods and tubes.And the grains could be refine by the additional shear strain and the cyclic changed deformation path.However,one problem with the RE process was that not all rotary work could be transferred into the materials,which was resulted from the relative slippage between the rotating die and billet.And this drawback would substantially reduce the production efficien y of the process and hinder its industrial applications.

Fig.1.Schematic illustrations of the RBE process.

Recently,we had developed a novel backward extru-sion process,entitled as rotating backward extrusion(RBE),through rotating die and open punch[19,20].The schematics of the RBE process are shown in Fig.1.The RBE process can apply a strong compressive and shear stress to the billet and bring a continuous cumulative plastic deformation to the billet.Different from other RE processes,an innovated open punch with transverse groove at the end face is provided,as shown in the magnificatio in Fig.1.The open punch can effectively improve the fluidit of metal,resulting more metal to undergo the shear deformation.Thus,the grain refinemen and mechanical properties of the materials can be substantially enhanced.

However,our previous researches have only concentrated on the formability of the RBE process,the evolution of the microstructure and texture of the Mg alloy during the RBE process is still unclear,which will limit the further development and exploration of the RBE process.In addition,the AZ80 Mg alloy is one of the most promising metal materials for commercial application because of its good plastic processing performance and low production price.Therefore,thispaper will investigate the microstructure and texture evolution of the AZ80 magnesium alloy during the RBE process,and compare it with the conventional backward extrusion(CBE)to obtain the deformation mechanisms of the RBE process.

2.Experiment procedures

2.1.Material and extrusion process

The initial material used in this investigation was commercially available as-cast AZ80(Table 1),prepared from the Wenxi Yinguang of Magnesium Industry Co.,Ltd.(Shanxi,China)by semicontinuous casting process.A cylindrical billet of 21mm in diameter and 27mm in length was machined from the as-cast ingot and then homogenized at 688K for 16h to eliminate the composition segregation and shrinkage porosity in the cast structure,and finall quenched into water(shown in Fig.2).Then the homogenized billet was performed at 653K with a die speed of 0.05mm/s on the Gleeble-3500 thermal simulation test machine.The temperature control precision could reach±274K.And the temperature of the billet was controlled by the thermocouple wires fi ed on the billet bottom(indicated by red circle in Fig.2).The samples were heated at a heating rate of 278K/s,and then held for 10min to obtain heat balance before experiment.The rotating speed of the open punch was 0rad/s(marked as CBE sample)and 0.52rad/s(marked as RBE sample).The total stroke of the moveable die was 20mm,and after deformation the processed samples needed to be quenched into water immediately to avoid the degree of annealing during cooling.An oil-based graphite was used on the billet surface and the inner wall of the die to reduce the friction.

Table 1Chemical composition of the as-cast AZ80(wt%).

2.2.Microstructure observation

The metallographic observation sample was taken from the symmetrical center surface of the extrusion sample(shown in Fig.2),mechanically polished and then etched with the chemical reagent(0.3g oxalic acid,3ml acetic acid,20ml distilled water).The obtained microstructure was examined by optical microscope(OM,Zeiss Axio Imager-A2m,Germany).The samples for electron backscattered diffraction(EBSD)analysis were ground and pre-polished on a precision grinding machine(Leica EM-TXP,Germany),and fina polished on an argon ion polishing machine(Leica EMS-102,Germany)at 6keV and 2.5mA with a milling angle of 3° and a polishing time of 30min.EBSD observation was performed on the SEM(Hitachi-SU5000,Japan)equipped with an EDAX-TSL EBSD system operating at 15kV with a working distance of 15mm.For clear image and further detailed analysis,the areas of interest were selected and examined with smaller step size of 0.7-2.0μm.The grain size,textures and other related microstructural features were acquired from EBSD data using the OIM analysis software.And the dynamic recrystallization(DRX)fraction(area fraction)was measured by an Image-Pro-Plus 6.0 software.The fraction of recrystallized grains was calculated according to the following assumptions[9]:(i)If the boundaries angle in a grain exceeded 2° the grains were classifie as“deformed”.(ii)If the grains consisting of subgrains,which had an internal misorientation less than 2° and a misorientation between subgrains greater than 2°?15°,the grains were classifie as“substructure”.(iii)The remaining grains were considered to have been recrystallized.The phase analysis of the extruded samples was conducted on the SmartLab-SE X-ray diffractometer(XRD)with a Cu-Kαradiation(λ=0.154nm),operated at 40kV and 40mA with a 2θrange of 20-80° The Vickers microhardness(HV)was measured under the load of 200 gf for 15s,and 10 points were tested at each region of the cup sample.

Fig.2.The schematics of the initial billet and the longitudinal section of the extruded samples.

2.3.Simulation procedures

In order to investigate the strain distribution difference between the CBE and RBE process,the DEFORM-3D V12.0 finit element method(FEM)simulation software was used.The geometrical dimensions of the die and billet in the simulation were identical to the experiment.In the simulation model,the billet and the die were assumed to be elastoplasticity and rigid model,respectively.The relative element type was used and the initial mesh number was 50,000.The friction behavior between the die and billet,i.e.,the shear friction coefficient was supposed to be 0.3.The extrusion parameters were given in Table 2.

Table 2Extrusion parameters used in simulation.

3.Results and discussion

3.1.FEM simulation

Fig.3 displays the equivalent strain distribution between the CBE and RBE process.It can be seen that the RBE sample exhibits a higher equivalent strain value than the CBE sample.And the color between the inner and outer walls of the RBE sample is closer than the CBE sample,which means that the strain is close and also proves that the RBE process can effectively improve the deformation uniformity.In order to quantify the difference of equivalent strain between the CBE and RBE process,eight typical regions are selected for analysis as shown in Fig.3c.It can be seen that the equivalent strain difference at the cup bottom is the largest,while the strain difference at the cup wall is the smallest.It suggests that the cup bottom is the most severe deformed area in the RBE deformation,where the metal is subjected to both compression and torsional deformation.At the same time,some metals will fl w from this dramatic plastic deformation area into the cup wall,so the strain difference in the wall of the RBE sample is lower than the bottom area.In addition,it can be observed that the strain difference of the inner wall(region A,D,F)is significantl larger than the outer wall(region C,E,H),which indicates that the inner wall is more obviously affected by the combined effect of compression and torsional deformation during the RBE process.

Fig.3.Distribution of equivalent strain of the AZ80 alloy cup-shaped pieces during(a)the CBE process and(b)the RBE process;(c)the equivalent strains and its strain difference at the regions A-H for CBE and RBE samples(the regions are highlighted in(a)).

3.2.Microstructure evolution

Fig.4a shows the as-homogenized microstructure of the AZ80 alloy.It can be seen that some undissolved eutectic phases are still distributed at the grain boundaries,but few phase exists inside the grains,and the initial grain size is about 310μm.The XRD result in this paper(Fig.4b)shows that the AZ80 alloy after homogenization is mainly composed ofα-Mg matrix andβ-Mg17Al12phase,which is same with the results of the Yu et al.[21]and Zhao et al.[22].Meanwhile,it is also worth noted that the CBE sample has the same composition as the RBE sample,indicating that the structure of the phase cannot be changed during the RBE process under the conditions given in this paper.

The optical microstructure(OM)of the CBE and RBE samples at different regions are presented in Figs.5 and 6.Different microstructure characteristics can be seen from the beginning to the end of the deformation.As can be seen from Fig.5i that the microstructure of the CBE sample can be divided into three main areas,the compression area(regions A-C),the shear area(regions D-E)and the deformed area(regions F-H).For the compression area,it can be observed that the grain size firs decreases and then increases from the inside to outside along the cup wall.The average grain size of the region A is about 94.93μm,which implies that the metal in this region cannot fl w easily and the deformation is uneven.The proportion of dynamic recrystallization(DRX)grains in the region B is the highest with the grain size of 25.66μm,indicating that the metal of this region is subjected to a large strain.Furthermore,it can be observed lots of DRXed grains distributed along the coarse deformed grains in the region C,showing a typical bimodal microstructure(Fig.5c).During the extrusion,part of the metal at the region C will fl w into the region B,and the deformed grains will be replaced by the DRXed grains.Finally,the metals of the regions B and C will fl w into the shear area,i.e.region D and E,respectively.

Fig.4.(a)The initial microstructure of the homogenized AZ80 alloy.(b)the XRD diffraction patterns of the AZ80 alloy during the CBE and RBE process.

Fig.5.The optical microstructures(OM)of the CBE sample:(a-h)the high magnifie microstructure of the regions A-H and(i)the low magnifie microstructure of the cup-shaped piece.

For the shear area,the grain size gradually increases from the inner wall to the outer wall(Fig.5d and e).The inner wall(region D)has a high proportion of DRX with an average grain size of 19.97μm.For the forming area,it can be seen that the grain size also gradually increases from the inside to the outside of the cup wall.The average grain size of the region F is about 18.50μm,which is similar to that of the region D.It also can be noted that the regions G and H both exhibit a bimodal microstructure,while the DRX proportion of the region G is higher than that of the region H.It can be inferred that the strain distribution of the cup wall is not uniform in the CBE sample and the inner wall is subjected to more strain compared to the outer wall.

Fig.6.The optical microstructures(OM)of the RBE sample:(a-h)the high magnifie microstructure of the regions A-H and(i)the low magnifie microstructure of the cup-shaped piece.

The OM of the RBE sample can also be divided into three areas,the compression-torsion area(regions A-C),the shear area(regions D-E)and the deformed area(regions F-H),as shown in Fig.6.It can be seen that the grain size difference between the inner and outer walls of the cup is reduced,indicating that the deformation uniformity of the RBE sample is improved.And the grain size of the RBE sample is signifi cantly smaller than that of the CBE sample(Fig.5).In addition,it is worth noting that the average grain size of the region A in the RBE sample is about 10.51μm,which is reduced by nearly 88.9% compared to the corresponding region in the CBE sample.And the microstructure difference of the region A is the most significan between the CBE and RBE samples,which is consistent with the FEM results of the largest strain difference in the region A(Fig.3c).And it can be attributed to the large cumulative plastic deformation caused by the additional torsion deformation and the increase of metal fluidit during the RBE process.It also can be observed that the grain size of region F with 16.98μm is slightly larger than the grain size of region D with 14.98μm,which may be attributed to the growth of static recrystallization(SRX)caused by the deformation heating and frictional heating[23].Therefore,it can be inferred that the RBE process can significantl refin the grains and increase the deformation uniformity of the sample comparing with the CBE deformation.In order to thoroughly investigate the characteristics of texture evolution in the RBE process,six typical regions are selected(regions A-B,D-E,F-G)for studying in the next section.

3.3.Texture development

Fig.7 shows the inverse pole figur(IPF)maps and the corresponding misorientation angle(MA)distribution of the CBE and RBE samples.Fig.8 displays the proportion of low angle grain boundaries(LAGBs,2-15°)and high angle grain boundaries(HAGBs,>15°)between the CBE and the RBE samples at different regions.It can be obtained from the IPF maps(Fig.7)that the grain refinemen ability of RBE process has been improved significantl compared to the CBE process in each region.It can be seen that the grain size of the RBE sample is significantl smaller than that of the CBE sample at each regions.In addition,for the cup bottom,the average MA of the RBE sample(Fig.7s)in region A is significantl larger than that of the CBE sample(Fig.7g),indicating that more DRXed grains are formed[24,25].However,for the region B,although the RBE sample has a larger DRX ratio,the average MA is smaller than the CBE sample.This is due to the RBE sample have a higher LAGBs fraction of 34.3% than the CBE sample of 19.3%(as shown in Fig.8)showing an intensive dislocation activity[12,26].The same phenomenon can also be observed in regions D and G.It’s known that the formation of LAGBs is related to the piling up of dislocations in the grain,when the dislocations density exceeds the critical value,it will cause a local orientation rotation in the grain,and eventually form a subgrain with some LAGBs[27-32].Thus,the phenomena suggest that the RBE process can significantl increase the deformation amount of materials,while promoting more activation of slip systems,which in turn promotes the generation and migration of dislocations to achieve uniform deformation.In the region F,the RBE sample have a higher HAGBs fraction of 71.6%(Fig.8)and average MA value of 38.39°(Fig.7w),indicating that the LAGBs generated in the region D will gradually be changed to HAGBs and eventually promoted the occurrence of DRXed grains[26,29-32].Thus,it can be inferred that the RBE process can significantl increase the deformation amount,promote the occurrence of DRX,and finall achieve grain refinement In order to systematically investigate the DRX mechanisms of the RBE process,six unDRXed regions(R1-R6 in Fig.7m-r)are selected using EBSD characterizations.

Fig.7.The inverse pole figur(IPF)maps of(a-f)CBE samples and(m-r)RBE samples;the corresponding misorientation angle(MA)distribution of the(g-l)CBE samples and(s-x)RBE samples.

Fig.8.The length fraction of LAGBs and HAGBs between the CBE and RBE samples at different regions.

It is well known that there are two main DRX mechanisms in the plastic deformation of Mg alloys based on the nucleation and growth characteristics,which are continuous DRX(CDRX)and discontinuous DRX(DDRX)[26,28,29].The CDRX[30,31]is a process that the LAGBs will be gradually transformed into the HAGBs through continuous absorption of dislocations,finall forming a new grain.In contrast,the DDRX[28,32-36]is a process with nucleation at serrated HAGBs by bulging and grain growth through the grain boundary(GB)migration.It is seen from Fig.9 that for all regions,the GBs of the deformed grains are serrated,and part of the GBs are bulged outward with a large number of LAGBs formed in the bulging grains inside,which is a typical DDRX deformation mechanism.In addition,from the variation of GBs fraction(Fig.8),it can be obtained that the fraction of LAGBs are decreased from 41.1% in region D to 28.4% in region F,indicating a typical CDRX behavior that lots of LAGBs are transformed to HAGBs during the continuous deformation.Thus,it can be concluded that the cup bottom is mainly controlled by the DDRX deformation.However,when the metal fl ws from the cup bottom to the cup wall,it is determined both by the DDRX and CDRX deformation.Moreover,this deduction is also confirme by the occurrence of new DRXed grains D1-2,D5 in the region D(Fig.9g),as well as the new DRXed grains D1-6 in the region E(Fig.9j).

Until now,there are still some controversy about the effect of DRX on the grain orientation and texture.Some studies have suggested that the occurrence of DRX can change the texture of alloys due to the random orientation of DRXed grains[37,38].Other researchers,however,believe that the new formed DRXed grains is consistent with the orientation of the parent grains,so that the texture of the alloy will not be changed[39,40].In this paper,it can be obtained from the(0001)pole figure(PF)and the corresponding IPF in Fig.9 that the subgrains formed by the DDRX have the similar orientations with their parent grains.Furthermore,it also should be noted that some DRXed grains share the same orientations with their parent grains,such as D1 in R1(Fig.9b,c),D3,4 in R2(Fig.9e,f),D4-8 in R3(Fig.9h,i)and D1,3,6,7 in R4(Fig.9k,l),while some of them exhibit random distributions that are completely different from the orientation of their parent grains,such as D2 in R1(Fig.9b,c),D1-2,5 in R2(Fig.9e,f),D1-3 in R3(Fig.9h,i)and D2,4,5 in R4(Fig.9k,l).Therefore,it can be inferred that the new formed subgrains always have a similar orientation to the parent grains,but some orientations of the DRXed grains will be changed with further deformation.

Fig.9.The DRX behavior of the RBE sample in the unDRXed region(a-c)R1,(d-f)R2,(g-i)R3,(j-l)R4,(m-o)R5 and(p-r)R6 selected in Fig.7(i-l):(a,d,g,j,m,p)inverse pole figur maps,the corresponding orientation highlighted in(b,e,h,k,n,q)(0001)pole figur(PF)and(c,f,i,l,o,r)inverse pole figur(IPF).(S:subgrain,D:DRXed grain).

Fig.10 shows the texture development during the CBE and RBE process.It can be seen from the(0001)PF and the IPF with normal direction(ND)that the texture evolution during the RBE process is obviously different from that of the CBE process.In the region A(Fig.10a,g),both the CBE and the RBE sample have a texture with the c-axis near the extrusion direction(ED).And the c-axis of some grains in the CBE sample shows a about 45° deviation from ED on the ED-ND plane.But such texture can not be found in the RBE samples,and the c-axis of the grains is distributed in a range of about 60° around the ED on the ED-radial direction(RD)plane.In addition,the maximum pole intensity(MPI)of(0001)PF is decreased from 19.121 of the CBE sample to 3.719 of the RBE sample,indicating that the texture can be weakened by the RBE process.Furthermore,it can also be seen from the IPF that the CBE process has a[-12-12]//ND and[2-1-10]//ND preferred orientation,while the RBE process exhibits a high intensity of[10-10]fibe component.It is reported[41-43]that in the plastic deformation,the slip planes will gradually deflec towards the direction that is perpendicular to the external force due to the activation of the slip systems.And the basal slip systems are ready to be activated due to its lower critical resolved shear stress(CRSS)[44].Thus,most of the c-axis will gradually rotate to the direction that is parallel to the ED.However,due to the continuous torsional deformation in the RBE process,the direction of resultant force will be rotated in the ED-RD plane,resulting thec-axis of the basal plane rotates around the ED in the ED-RD plane and finall weakening the strong texture in the CBE sample at the region A.

Fig.10.The(0001)PF and corresponding IPF maps of(a-f)the CBE samples and(g-l)the RBE samples at different regions.

At the region B(Fig.10b,h),the c-axis of the grains is distributed sparsely on the ED-ND plane with a preferred orientation of[2203]//ND and[-12-14]//ND in the CBE sample.And the RBE sample has a strong[10-10]-[2-1-10]double fibe texture with the c-axis paralleling to the ED.Furthermore,the MPI of the RBE sample with 19.247 is slight higher than that of the CBE sample with 11.126,which may be due to the large strain amount in the RBE sample.The same results can be obtained from the research of Arzaghi et al.[15],which found that the main texture intensity was continuously strengthened with the strain increasing in the HPPT process.

Fig.12.(a)The area fraction of recrystallization grains,sub-grains and deformed grains at different regions.(b)The average grain size distribution of recrystallization grains and deformed grains at different regions.

Fig.13.The kernel average misorientation(KAM)maps of(a,c,e)the CBE samples and(b,d,f)the RBE samples.

The texture of the RBE sample and the CBE sample in the region D(Fig.10c,i)are significantl different.There are two kinds of textures in the CBE sample.Both textures have a caxis deflectio of about 45° with the ED,and one of them is the basal plane of grains perpendicular to the shear direction(denoted by T1),and the other is parallel to the shear plane(denoted by T2).However,there is only one texture of T1 in the RBE sample.The same phenomenon was observed in the ECAP process[8],which founded that a texture that the basal plane of grains was inclined by about 55° from the processing direction was developed after the ECAP deformation,which could be attributed to the activation of basal slip.During the backward extrusion,the materials will be subjected to the shearing stress when it fl ws into the shear area,resulting the basal plane to turn parallel with the shear plane under the effect of basal slip.It can be seen from the OM(Figs.5i and 6i)that the metal at the region B will gradually fl w into the region D during the continuous deformation.For the CBE sample,thec-axis of some grains is distributed at about 45° with the ED in ED-ND plane in the region B(Fig.10b),which is favorable for the activation of the basal slip systems[13,25,27,44],thus a texture with basal plane parallel to the shear plane(T2)is formed in the region D.However,for the RBE sample,the basal plane of most grains is perpendicular to the ED(Fig.10g),which is not favorable for the activation of the basal slip systems.Thus,there is only the T1 texture at the region D.And the formation of T1 texture is related to the reverse fl w of metal.The same phenomenon is occurred at the region E in both CBE and RBE samples.

It can be observed from the region F(Fig.10e,k)that thec-axis of the grains is rotated from the ED to the RD both in the CBE and RBE samples.Moreover,the MPI value of region F is smaller than the corresponding sample at the region D,which indicates that the texture is further weakened.Finally,the CBE sample forms a[10-10]-[2-1-10]double fibe texture,while the texture of the RBE sample shows a strong[2-1-10]//ND preferred orientation.As for the region G,it can be seen from the(0001)PF(Fig.10f,l)that both the CBE and the RBE specimens retain the texture of region E,but the MPI value is decreased.This result indicates that when the metal fl ws from the region E to region G,the metal is less affected by external forces.This also corresponds to the change trend of the strain value,and the equivalent strain values of regions E and G are significantl lower than those of other regions.

Fig.14.The microhardness measurements of the CBE and RBE samples at different regions.(Each error bar is the standard deviation resulting from 10 indentation measurements).

In order to investigate the DRX behaviors of the CBE and RBE process,the area fraction and orientation distribution of DRX grains(blue color),sub-grains(yellow color)and deformed grains(red color)are calculated from the EBSD data in Fig.11.The area distribution proportions and average grain size of the above three kinds of grains are plotted in the Fig.12.It can be seen from the(0001)PF that the orientation distribution of the DRXed grains are more random compared with that of the deformed grains and sub-grains.In addition,the DRX fraction of the RBE specimens is higher than that of the CBE specimens in the corresponding region.Especially,in the region A,the DRX proportion of the RBE sample is nearly 72.6%,which is more than twice as much as that of the CBE sample.Furthermore,the DRX proportion of the CBE and RBE samples increased from 72.0% and 75.7% of region B to 77.4% and 82.8% of region F,respectively,which is opposite to the change of MPI values(Fig.10).Tong et al.[26]had investigated the evolution of the DRX and texture of Mg-Y-Zn alloy during hot extrusion process and found that a higher proportion of DRXed grains will lead to a weak basal texture.Thus,the decrease trend of MPI value is related to the increase of the DRX ratio.Fig.12b shows the average grain size of the CBE and RBE samples.It can be seen that the average grain size of the RBE sample is smaller than that of the CBE sample in all regions.The grain size difference between the CBE and RBE process is gradually decreased from the region A to the region F,which is corresponded to the variation trend of the strain difference(Fig.3).Therefore,it can be inferred that the RBE process can not only improve the occurrence of DRXed grains and the ability of grain refinement but also weaken the basal pole intensity,especially in the region A.

Fig.11.The EBSD maps distinguished by recrystallization region(blue area),subgrains region(yellow area)and deformed region(red area)and corresponding orientation distributions in(0001)pole figures(a-f)the CBE samples and(g-l)the RBE samples at different regions.

Fig.13 shows the kernel average misorientation(KAM)maps of the CBE and RBE samples.The KAM maps can indirectly reflec the strain distribution,and a high KAM value means a large strain gradient and dislocation density,which is conductive to the occurrence of DRX[28].It can be obtained that the RBE samples have a larger proportion of areas with high KAM values than the CBE samples in region A,B and D,indicating a large cumulative strain inner grain,which is corresponding to its simulation results of the larger strain value(Fig.3)and deformation behaviors with a larger DRX fractions(Fig.12).

3.4.Microhardness evolution

Based on the microhardness value in Fig.14,it can be obtained that the RBE process hardens the AZ80 alloy cup samples more effectively than the CBE process.The largest increase in microhardness is in the region A,which is increased from 65.8 HV of the CBE sample to 74.6 HV of the RBE sample with an increase of 11.8%.It is reported that the fin grains with complex GBs can effectively hinder the dislocations movement,which can increase the microhardness[45].Therefore,the increase in hardness value under the RBE process can be attributed to its smaller grain size compared with the CBE process.

4.Conclusion

The RBE process was successfully applied to produce AZ80 alloy cup-shaped pieces.The evolution of microstructure and texture were systematically investigated and compared with those of the CBE process.The results are concluded as follows:

1.The RBE process could increase the equivalent strain and deformation uniformity of the materials than the CBE process.And the largest strain difference between the CBE and RBE process was occurred in the region A at cup bottom.

2.The grain refinemen and DRX proportion could also be significantl enhanced by the RBE process.The largest decrease in grain size and the largest increase in DRX fractions were both in region A at cup bottom,which were 88.6% and 55.3%,respectively.

3.The main deformation mechanism of the RBE process was the DDRX,while the CDRX was also occurred in the cup transition.

4.Comparing with the CBE sample,the texture of the RBE sample in the region A at cup bottom could be weakened due to the additional torsion deformation.The pole intensity of(0001)basal plane was decreased from the region B at cup bottom to the region F at cup wall,attributing to the change of strain and DRX proportions.

5.The microhardness value of the RBE sample was higher than that of the CBE sample,with the maximum increase of 11.8% in the region A at cup bottom,which could be attributed to the grain refinemen strengthening.

Declaration of Competing Interest

All authors declare that they have no conflic of interest.

Acknowledgment

This work was financiall supported by the National Natural Science Foundation of China(No.51775520),the National Key Research and Development Program(No.2016YFB0301103-3),the Key R&D program of Shanxi Province(International Cooperation)(No.201903D421036)and the Scientifi and Technological Innovation Programs of Higher Education Institutions in Shanxi(No.2018002).