Yingze Meng,Jinmin Yu,Gunshi Zhng,Yojin Wu,Zhimin Zhng,Zheng Shi

aCollege of Mechanical and Electrical Engineering,North University of China,Taiyuan 030051,China

b College of Materials Science and Engineering,North University of China,xueyuan road 3,Taiyuan,Shanxi,Taiyuan 030051,China

Received 2 August 2019;received in revised form 25 November 2019;accepted 4 December 2019 Available online 5 August 2020

Abstract Gleeble-3500 thermal simulator was applied to realize the rotary backward extrusion forming of Mg-13Gd-4Y-2Zn-0.5Zr(wt%)alloy at different circumferential strain rate from 0.009 s?1 to 0.027 s?1 at 400°C and the dynamic recrystallization mechanism and texture evolution were studied.The results show that the grain size of the alloy was obviously refined after rotary backward extrusion.As the circumferential strain rate increased,the dynamic recrystallization fraction gradually increased causing the grain size decreased and the distribution of microstructure became more uniform.At the same time,the texture of{0001},{10-10},{11-20}was weakened and the grain orientation distribution became more random.With the increase of circumferential strain rate,the discontinuous dynamic recrystallization mechanism became dominant,which promoted the weakening of texture and grain refinement of the alloy.

Keywords:Mg-Gd-Y-Zn-Zr alloy;Rotary backward extrusion;Dynamic recrystallization;Texture.

1.Introduction

Magnesium alloy is the most potential light metal.It has many advantages such as good electrical conductivity,high specific stiffness and specific strength,giving it great prospects for development[1–3].However,the lattice structure of the magnesium alloy is close-packed hexagonal structure with low symmetry,and its slip systems that can be initiated at room temperature are less.These conditions limit the application of magnesium alloy in various aspects[4,5].

Many studies have shown that the addition of rare earth(RE)elements to magnesium alloys can significantly improve high temperature mechanical properties,effectively expanding the application of magnesium alloys[6–9].Among various Mg-RE alloys,the Mg-Gd-Y-Zn-Zr alloy have attracted the more and more attention because long period stacked ordered(LPSO)phases it contains and its unique kink phenomenon that adapts to deformation,which effectively improve the ductility and strength of the alloy[10–14].

In order to obtain more excellent properties of the magnesium alloy,some scholars have used severe plastic deformation(SPD)to fabricate ultra-fine grain(UFG)structures in recent years[15].Typical SPD processing techniques include high pressure torsion(HPT)[16],equal channel angular pressing or equal channel angular extrusion(ECAP or ECAE)[17],accumulative roll-bonding(ARB)[18],multi-axial forging(MAF)[19],cyclic extrusion compression(CEC)[20],etc.The grain refinement effect caused by SPD is better than that of the traditional deformation,so that the properties of the alloy are enhanced to varying degrees[21].Yu et al.[22]have put forward a new SPD forming process of rotary backward extrusion,combined with reverse extrusion and high-pressure torsional process,in order to achieve grain refinement and higher properties.

The machinability of magnesium alloys at high temperature is usually enhanced,and dynamic recrystallization(DRX)is easier to start,leading to continuous microstructure refinement.By different nucleation mechanisms and growth processes of grains,DRX mechanisms could be divided into two types:continuous dynamic recrystallization(CDRX)and discontinuous dynamic recrystallization(DDRX)[23–26].Different DRX mechanisms are initiated according to different deformation conditions such as different deformation temperature,stain rate and strain amount[23,24].

Table 1Chemical compositions of the alloy(wt%).

Galiyev et al.[24]found that CDRX was the dominant DRX mechanism under the condition of thermal compression from 200°C to 250°C,while DDRX dominated deformation from 300 to 450°C.A large number of research results have shown that the properties of materials are 20–50% affected by texture[27,28].Different DRX mechanisms have different contributions to texture.Jiang et al.[29–32]proposed that CDRXed grains have the characteristics of maintaining the original texture,while DDRXed grains have a distinct rotational orientation,which contributes to the randomization of texture.

For the new rotary backward extrusion process,Qu et al.[33]studied the effect of circumferential strain rate on the microstructure and properties of magnesium alloys,but little research has been done on texture and dynamic recrystallization.Therefore,it is useful to study the effect of rotary backward extrusion on the deformation mechanism and the texture of Mg-Gd-Y-Zn-Zr alloys to control and weaken the deformation texture,which will bring great significance for theory and practical application.

The purpose of this paper is to study the effect of different circumferential strain rates on the microstructure and texture of Mg-13Gd-4Y-2Zn-0.5Zr(wt%)alloy prepared by rotary backward extrusion,and further reveal the change of recrystallization behavior and the mechanism of grain refinement at different circumferential strain rate.

2.Materials and methods

The material used in this experiment was Mg-13Gd-4Y-2Zn-0.5Zr(wt%)as-cast alloy.The chemical compositions of the alloy are listed in Table 1.Rotary backward extrusion experiment was carried out in the unique torsion unit of the Gleeble 3500 thermal simulator[22].The schematic diagram of the process is shown in Fig.1(a)and the billet used in the experiment is shown in Fig.1(b).

Before the experiment,welded the thermocouple wires at the bottom of the billet,then the initial billet was placed into the female die.Thermocouple wires were pierced from the bottom hole of the female die and connected to the measuring terminal for real-time monitoring of temperature.The temperature control precision could reach±1°C.Mold and billet were preheated to 400 °C at the heating rate of 40°C/min and held for 5 min in the induction furnace of Gleeble 3500 thermal simulator.

During the experiment,the temperature was kept at 400°C.The material was backward extruded along the gap between the punch and the female die.The grinding depth of the punch was 20mm,and the axial extrusion speed was 0.067mm/s.At the same time,the punch was rotated by the motor driver,providing the shear stress to the billet.Different rotational speeds of the punch provided different circumferential strain rates.

The deformation process was completed in 5 min and the number of punch revolutions was 3,6,and 9 turns respectively,corresponding to the circumferential strain rate˙εCwas 0.009 s?1,0.018 s?1,0.027 s?1,respectively.

Fig.1(c)shows the cup-shaped piece of magnesium alloy after the rotary backward extrusion.The samples were then cut by wire cutting,and the cross section which was perpendicular to extrusion direction(ED)was taken as the observation surface.The inner deformation area was near the punch,as shown in the dark gray area in Fig.1(d).The surface of the pattern was mechanically ground with sandpapers and polished on MP-2A grinder.The corrosive with the ratio of 5g picric acid,4mL acetic acid,10mL distilled water and 120mL ethanol was used to corrode the surface.Then the microstructure was investigated using optical microscopy(OM,Zeiss Axio Imager A2m,Oberochen,Germany).Subsequent electron backscattered diffraction(EBSD)was performed using a scanning electron microscopy(SEM,Hitachi SU5000,Tokyo,Japan)in combination with the EDAX-TSL EBSD system at 20kV,70° tilt angle and 15mm working distance.The number of grains in the scanning region exceeded 200.Data processing software TSL OIM was used to analyze features such as grain size,grain boundary angle,orientation difference angle and texture.The DRXed grains in this study were defined as grains with grain orientation spread(GOS)less than 2°.

3.Results

3.1.Evolution of microstructure

Fig.2 shows microstructure of the cross section in the radial direction at 0.009 s?1.It was observed that the grain refinement was obvious at the 0–1mm internal deformation area.The 1–2mm deformation area appeared as the transitional area,gradually distributing more coarse grains.Along the radial direction,the coarse grains become more numerous,showing a typical gradient microstructure from the inside to the outside.Due to the difference in the degree of deformation from inside to outside,the main innermost 0–1mm significant deformation area at different circumferential strain rates was observed emphatically.

Fig.3 shows the inside microstructure on the 0–1mm area of the cup-shaped pieces at different circumferential strain rates.It can be observed from the low expansion optical microscope images(Fig.3(a),(c)and(e))that when˙εCwas 0.009 s?1,there were many large grains elongated along the rotating direction,and the grain size distribution was uneven.As the circumferential strain rate increased,the deformation amount gradually increased,leading to increase of the dynamic recrystallization degree in the radial direction.At the same time,the grain size decreased and the microstructure distribution became more uniform.

Fig.1.schematic diagram of rotary backward extrusion

Fig.2.OM image of gradient microstructure at 0.009 s?1.

Fig.3(b),(d)and(f)are the microstructures under 500-times optical microscope.As˙εCwas 0.009 s?1,there were still some large LPSO phases on the inside of the sample,and the distribution was not uniform(Fig.3(b))[22,34].The kink could be observed in the lamellar phases to participate in the coordination of grain deformation.The surrounding block-shaped phases were broken up,where were more likely to produce large quantity of dislocation pile-up and induce dynamic recrystallization[35].Many fine DRXed grains were formed,represented by the bimodal structure in which coarse unDRXed grains and fine DRXed grains coexisted.

With the strain rate increased,the lamellar LPSO phases decreased.The block-shaped phases were further broken down and decomposed,gradually refined into irregular short stick-shaped and rod-shaped phases,showing a more uniform dispersion distribution,and there was a tendency to be chained along the direction of rotation(Fig.3(d)and(f)).As the inside degree of deformation increased,the deformation amount increased and dislocations piled up in a larger range,which increased the dislocation density.This increased the DRXed region and the degree of DRX became more complete,thereby increasing the grain refinement range.

3.2.Evolution of texture

Electron backscatter diffraction(EBSD)orientation imaging techniques were performed on the inner 0–1mm area of the cross section of the cup-shaped piece prepared at different circumferential strain rates at 400°C.The observation direction was parallel to the ED.The orientation imaging microscopy(OIM)maps(Fig.4(a),(d)and(g))were obtained by EBSD results.The same or similar colors indicated the same or similar grain orientation.

Through the EBSD data analysis,the grain boundary angle was counted.Since a small misorientation angles may be caused by instrument error,the misorientation of less than 2°was not counted in the statistics.The red lines are used to represent the low angle grain boundaries(LAGBs)with misorientation angles of 2?15°,and the black lines are used to represent the high angle grain boundary(HAGBs)with misorientation angles larger than 15°.

When the circumferential strain rate˙εCwas 0.009 s?1(Fig.4(a)),coarse grains were visible in the inner region of the sample,in which the color gradually changed from the grain boundary to the center.Combined with the grain boundary map(Fig.4(b)),it can be found that there were a large number of LAGBs inside,indicating that a large amount of dislocation accumulated in the coarse grains due to the increase of strain.DRX occurred around the coarse grains,and the grain size was not uniform.

Fig.3.OM images at different circumferential strain rates

As the deformation rate up to 0.018 s?1and 0.027 s?1(Fig.4(d)and(g)),the strain amount continued to increase and the critical strain required for DRX was reached in a larger region.The dislocation density within the grains was increased,and many dislocations were piled up at grain boundaries.When the dislocation density reached the critical value required to initiate DRX again,the newly formed crystal grains re-generated DRX.The DRXed regions gradually expanded into the inside of the grains from the initial grain boundary,gradually phagocytizing the coarse grains.The coarse grains could barely be observed and the grain size became more uniform.This indicated that the rotary backward extrusion process can increase the uniformity of the strain.

It can be seen that the HAGBs consisted of the original grain boundaries and the newly formed DRXed grain boundaries,as shown in Fig.4(b),(e)and(h).As˙εCreached to 0.009 s?1(Fig.4(b)),the fraction of LAGBs was up to 0.32,most of which were distributed along the original grain boundaries.These LAGBs were usually produced by the accumulation of the dislocations or sub-grain boundaries in deformed alloy[36,37].With the increase of the strain rate,the fraction of LAGBs decreased,and more appeared inside the small grains.The dynamic recrystallization of the alloy became more and more sufficient.

Combined with the distribution of misorientation angle(Fig.4(c),(f)and(i)),the misorientation of alloys in different states all had extreme values in the range of less than 5°.The HAGBs distributions were relatively uniform and increased slightly.The extreme value decreased and the average misorientation angle gradually increased with the increase of the circumferential strain rate.The fraction of LAGBs and HAGBs were mainly determined by the degree of dynamic recrystallization[38].

Fig.4.OIM maps,grain boundary maps and misorientation distributions at different circumferential strain rates

The pole figures of the rotary backward extrusion samples of Mg-13Gd-4Y-2Zn-0.5Zr(wt%)at different circumferential strain rates are shown in Fig.5.When the circumferential strain rate was 0.009 s?1(Fig.5(a)–(c)),most of the(0001)basal planes were parallel to the ED,and the texture intensity was at most 6.47.Most of the<10–10>and<11–20>crystal orientations tended to be parallel to the ED,while some of the crystal grains were slightly deflected.The maximum polar density values were 2.93 and 2.73,respectively.

As the strain rate increased to 0.018 s?1the texture intensity of the basal plane decreased to 2.69.Most of the grain basal planes remained in the initial direction parallel to the ED,and the partial grain basal planes were deflected and became perpendicular to the ED.The{10–10}and{11–20}planes no longer had a preferred orientation.The density distribution of the pole figures were more dispersed and the maximum density value decreased.

4.Discussion

4.1.Effect of dynamic recrystallization on texture

According to whether the GOS was less than 2°,the grains were classified into“unDRXed grain”and“DRXed grain”,respectively,which were represented in the orientation imaging microscopy(OIM)maps,and the(0001)pole figures(Fig.6)and the grain size distributions(Fig.7)were separately counted.

It can be seen from Fig.6(a),(e)and(i)that as the strain rate increased,the fraction of unDRXed grains gradually decreased from 0.27 to 0.16,and the number of un-DRXed grains with large grain size decreased significantly.More unDRXed grains underwent dynamic recrystallization,the grain size gradually decreased and the distribution was more concentrated(Fig.7(b)).

The DRX fraction increased from 0.73 to 0.84(Fig.6(c),(g)and(k)),while the DRXed grain size decreased(Fig.7(c)).A larger range of DRX promoted the overall grain refinement,and the grain size distribution became narrower,showing an approximately normal distribution(Fig.7(a));the average grain size decreased from 4.36μm to 3.69μm,which was consistent with the microstructure evolution in 3.1.

Fig.5.pole figures of{0001},{10–10},{11–20}at different circumferential strain rates

The unDRXed grain orientation of the alloy at different circumferential strain rates tended to be parallel to the ED(Fig.6(b),(f)and(j)).The texture intensity was up to 37.53 at the circumferential strain rate of 0.009 s?1.The density value decreased with increasing strain rate but still remained above 15.This type of basal texture has been reported in many magnesium alloys by extrusion deformation[32].

However,the texture feature of the DRXed grains showed a significant difference.The peak of the DRXed grain exhibited a fairly dispersed distribution,and the maximum texture intensity was only 5.30 when the circumferential strain rate was 0.009 s?1(Fig.6(d)).As the strain rate increased,the texture intensity of DRXed grains decreased to 4.05(Fig.6(h)and(l)).This indicated that the unDRXed grains in the alloy had stronger fiber texture than the DRXed grains,and the formation of DRXed grains can greatly promote the random development of grain orientation in the alloy.

With the increase of the circumferential strain rate,the extreme density values of unDRXed and DRXed grains were reduced.The continuous progress of DRX made the DRXed grains had a more random crystal orientation.

4.2.Evolution of dynamic recrystallization mechanism

During the rotary backward extrusion process,the grains were continuously refined due to the occurrence of DRX.From the detailed EBSD observations,it was found that CDRX and DDRX appeared in different regions,and different dynamic recrystallizations had different effects on the newly formed grain orientation.Therefore,we selected the typical region R1-R4 from the OIM maps(Fig.4(a),(d)and(g))for analysis,and discussed the changes in DRX mechanism at different strain rates in detail.

To analyze the dynamic recrystallization behavior at 0.009s?1strain rate,we selected two typical regions R1 and R2 from Fig.4(a)for further analysis,as shown in Figs.8 and 9.

Fig.6.OIM maps and(0001)pole figures of unDRXed grains and DRXed grains at different circumferential strain rates

Fig.7.Grain size distribution at different circumferential strain rates

In unDRXed grain,showed in Fig.8(a),many sub-grain boundaries formed,subdividing R1 into P1,P2,P3 and P4.The line graph(Fig.8(b))represented the misorientation distribution from point to origin and point to point along the arrow direction of black line AB.Almost all the points of the point-to-point line were above 0 and the point-to-origin line continued to rise with the accumulated misorientation angle gradually increasing to 40,indicating that the crystal inside the grain rotated continuously and the dislocations had high activity.At the same time,it was observed that some fine DRXed grains in Fig.8(a)were formed in the region where the sub-grain boundaries were heavily accumulated,as indicated by white arrows.These sub-grain boundaries can continuously absorb dislocations during the deformation process,to which the formation of crystal grains may be related.

The distribution of DRXed grains in the R1 region were not concentrated,but the common feature was that there were a large number of sub-grain boundaries around the DRXed grain boundaries,and the grains were surrounded by sub-grain boundaries.With increasing strain,the dislocations were plied-up and rearranged along the grain boundary,its density increased at the same time.This phenomenon formed LAGBs and reduced the local stress concentration.During the deformation process,the LAGBs absorbed the surrounding dislocations continuously,then transformed into HAGBs.The orientation of newly formed grains was similar to the original grain orientation.The formation of these DRXed grains appeared as a typical CDRX mechanism[27].The new grains were gradually formed by the sub-grains,which further confirmed that CDRX achieved an in-situ transformation from sub-grains to new DRXed grains through the continuous increase in the misorientation angle.

Fig.8.the typical unDRXed region R1 selected in Fig.4(a)

Fig.9.the typical DRXed region R2 selected in Fig.4(a)

In the DRXed region R2(Fig.9),parent grains,sub-grains and DRXed grains were represented separately.As shown in Fig.9(a),the parent grains had serrated grain boundaries.The existing grain boundaries bowed toward the adjacent grains,the sub-grains S1-S3 were formed in these regions,which were separated from the parent grain by LAGBs.Different degrees of expansion occurred at the grain boundaries of the parent grains.A large number of fine DRXed grains nucleated and grew along the serrated grain boundary.The nucleation by bows in pre-existing serrated HAGBs appeared as a typical DDRX process[26].

The corresponding crystal orientations of the parent grain and DRXed grains in the selected region were marked with hexagonal prisms,respectively.The sub-grains were found to had similar orientations to the parent grains,while most newly formed DDRXed grains had different angles of deflection compared with the parent grain.The distribution of unDRXed and DRXed grains was represented by the(0001)pole figure(Fig.9(b))with different colors.The(0001)basal plane of DRXed grain was significantly deviated from the unDRXed grain.There was no preferred orientation and the crystal orientation tended to be randomly distributed.

Therefore,the DDRX process contributed to grain refinement and randomization of the basal texture,confirming the conjecture in 4.1.Figs.8 and 9 showed different dynamic recrystallization mechanisms,indicating that CDRX and DDRX were initiated simultaneously at a circumferential strain rate of 0.009 s?1.

As˙εCwas increased to 0.018 s?1and 0.027 s?1,in order to show the ongoing DRX behavior,the typical DRXed regions R3 and R4 were marked from Fig.4(d)and(g),respectively,as shown in Figs.10 and 11.It can be observed that there were a large number of LAGBs inside the original grains.The line graph of point-to-origin and point-to-point along the arrow direction of black line AB(Fig.10(a),Fig.11(a))showed that the original grains had a high dislocation density,similar to the observation in Fig.8,indicating the occurrence of CDRX.

Fig.10.the typical DRXed region R3 selected in Fig.4(d)

Fig.11.the typical DRXed region R4 selected in Fig.4(g)

As the strain increased,there were significant bowings at the grain boundaries.Some LAGBs formed near the bulged grain boundaries and divided them to produce many subgrains,forming the distribution of DRXed grains along the serrated grain boundaries,as shown in Figs.10(c)and 11(c).The newly generated DRXed grain orientations were greatly different from the original grain orientation,appearing as a random distribution.This phenomenon was consisted with the observations in Fig.9,indicating that DDRX occurred continuously in the DRXed region of R3,R4.

As the fraction of DRX increased,the number of unDRXed grains decreased,causing DDRX to become the main mechanism for adapting to deformation.When the DRXed grains were in contact with each other,the already formed DRXed grain boundaries provided nucleation sites for the new DRX process,new DRXed grains were generated by the DDRX mechanism.The DDRX was continuously carried out,so that the fraction of DRX continued to rise and the grains continued to be refined.

However,in high-temperature extruded magnesium alloys,the increase in DRX fraction was often accompanied by larger grain size[39,40],while during the rotary backward extrusion process,the high DRX fraction and the fine grain size were simultaneously obtained.The reason for this phenomenon was that the rotary feed process of the punch imposed more complex three-dimensional stress on the material compared to the conventional backward extrusion.In the internal deformation area of the cup-shaped piece near the rotating punch,more strain was accumulated and finer grains were obtained.Lee et al.[41]found the similar phenomenon in the process of caliber rolling,which was also attributed to the threedimensional stress to make the material with higher plastic strain.Therefore,due to the increase DRX fraction,grains were refined continuously.DDRX played an important role in the microstructure evolution in the late extrusion stage,as Zhang et al.[42–44]studied that the random orientation of the DRXed grains in hot-deformed magnesium alloys counteracted the strong basal texture,which promoted the weakening of texture.

5.Conclusions

The dynamic recrystallization mechanism and texture evolution of Mg-13Gd-4Y-2Zn-0.5Zr(wt%)alloy were investigated at different circumferential strain rates from 0.009 s?1to 0.027s?1.The main conclusions are summarized as follows:

1.After the rotary backward extrusion deformation,DRX occurred to refine the grains.With the increase of the circumferential strain rate,the DRX fraction increased,while the grain size decreased.The uniformity of the microstructure was improved.

2.As the circumferential strain rate increased,the texture of the alloy was weakened and the grain orientation distribution was more random.The maximum density of the basal texture decreased from 6.47 to 2.00.The increase of the fraction of DRX was the main reason for the weakening of the texture.

3.The CDRX mechanism and DDRX mechanism were activated simultaneously during rotary backward extrusion.With the increase of circumferential strain rate,DDRX became dominant.The newly formed grain orientation was different from the parent grain orientation.The new grains rotated uncertain angles,effectively refining the grain and weakening the texture.

Acknowledgements

This research was financially supported by the National Natural Science Foundation of China(Grant No.51775520),and the National Key Research and Development Plan(Grant No.2016YFB0301103-3).