Grain refinemen impact on the mechanical properties and wear behavior of Mg-9Gd-3Y-2Zn-0.5Zr alloy after decreasing temperature reciprocating upsetting-extrusion

Wenlong Xu,Jianmin Yu,Leichen Jia,Chang Gao,Zhan Miao,Guoqin Wu,Guojun Li,Zhimin Zhang

School of Material Science and Engineering,North University of China,Taiyuan 030051,China

Abstract Based on the deforming technique of severe plastic deformation(SPD),the grain refinemen of a Mg-9Gd-3Y-2Zn-0.5Zr alloy treated with decreasing temperature reciprocating upsetting-extrusion(RUE)and its influenc on the mechanical properties and wear behavior of the alloy were studied.The RUE process was carried out for 4 passes in total,starting at 0 °C and decreasing by 10 °C for each pass.The results showed that as the number of RUE passes increased,the grain refinemen effect was obvious,and the second phase in the alloy was evenly distributed.Room temperature tensile properties of the alloy and the deepening of the RUE degree showed a positive correlation trend,which was due to the grain refinement uniform distribution of the second phase and texture weakening.And the microhardness of the alloy showed that the microhardness of RUE is the largest in 2 passes.The change in microhardness was the result of dynamic competition between the softening effect of DRX and the work hardening effect.In addition,the wear resistance of the alloy showed a positive correlation with the degree of RUE under low load conditions.When the applied load was higher,the wear resistance of the alloy treated with RUE decreased compared to the initial state alloy.This phenomenon was mainly due to the presence of oxidative wear on the surface of the alloy,which could balance the positive contribution of severe plastic deformation to wear resistance to a certain extent.? 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

Key words:Mg-9Gd-3Y-2Zn-0.5Zr alloy;Reciprocating upsetting-extrusion;Grain refinement Texture;Mechanical properties;Wear behavior.

1.Introduction

Sustainable development is a timely theme,and magnesium(Mg)alloys have received widespread attention as an ideal material for industrial lightweight components and energy savings[1-3].However,its poor wear resistance and high temperature resistance have become serious obstacles to the application of Mg alloys in the fiel of tribology[4,5].Certain Mg alloy parts undergo friction and wear in application,which affect the service life of Mg alloy parts.Therefore,it is essential to improve the wear resistance of Mg alloys to increase their service life.Therefore,how to simultaneously improve the mechanical properties and wear resistance of Mg alloys to extend their service life has become a current research hotspot.Mg-Gd-Y-based alloys are widely used in aerospace,automotive,military and other field due to their good mechanical properties and high-temperature strength[6-10].This is because the addition of rare earth(RE)elements can produce significan solid solution strengthening of the Mg matrix and slow down the diffusion rate of Mg atoms,thereby improving the heat resistance of the Mg alloy.At the same time,Mg alloys containing RE elements can precipitate high-melting-point nano-second phases during aging,so that the Mg alloys have significan aging strengthening[11].Furthermore,the addition of RE elements can weaken the basal texture of the wrought Mg alloy,make it form a RE texture,and significantl improve the forming properties of the Mg alloy[12].In addition,the study has shown that Mg-Zn-2Y alloy has better wear resistance than AZ91 alloy[13].In recent years,researchers have found that adding Zn to Mg-Gd-Y-based alloys would produce long-period accumulation order(LPSO)phase precipitation.While retaining the good mechanical properties and high-temperature strength of Mg-Gd-Y-based alloys,LPSO phases can be used as a hardened structure to improve the overall mechanical properties of the alloy effectively further[14-17].In addition,a large number of studies have shown that the LPSO phase would undergo a kink phenomenon of coordinated deformation during thermal deformation,which will further improve the strength and ductility of the alloy[18-20].Therefore,Mg-RE-Zn series alloys containing LPSO phases have a wide range of application prospects.However,the poor mechanical properties and wear resistance of cast Mg-RE-Zn alloys are difficul to meet a variety of service conditions,which has become the fundamental problem restricting its wider application.

Existing studies have shown that surface treatment technologies,such as microarc oxidation(MAO)[21]and laser surface melting[22],can markedly further the wear resistance of Mg alloys.However,the results are limited to the surface treatment layer.Extrusion processing can markedly refin grains and increase the hardness of alloys,thereby improving the wear resistance of the alloy[23].Therefore,it is believed that the wear resistance of Mg alloys can be improved by grain refinemen and a uniform distribution of the second phase[24],which can be achieved by a severe plastic deformation(SPD)method,namely,RUE[25-27].Compared with the capabilities of other SPD methods,such as equal channel angular extrusion[28],high-pressure torsion[29],and multidirectional forging[30],which cannot be used for mass production of large-size billets because of its limitations,but RUE can be used to deform large-size billets,which is suitable for industrial production.Existing studies have shown[31]that decreasing the RUE temperature is an effective way for large billets to achieve a cumulative strain,which can produce ultrafin grains that strengthen the alloy.RUE can effectively break block-shaped LPSO strengthening phases to achieve an ideal dispersion distribution.At the same time,scholars carried out RUE tests on 2A66 Al-Li,AZ80 and other materials,and all obtained fin and uniform microstructures[32-34].In addition,as the deformation temperature gradually decreases as number of RUE passes increases,the solid solubility of RE atoms in the alloy gradually decrease,which induces an increase in the precipitated phase and improved the alloy performance.

An improvement in the mechanical and wear properties of Mg-Gd-Y-Zn-Zr alloys would increase their service life.Therefore,in this work,we took a Mg-9Gd-3Y-2Zn-0.5Zr alloy as the research object to explore the influenc of grain refinemen on the mechanical properties and wear behavior of the alloy after decreasing the RUE temperature.

2.Experimental procedures

The Mg-9Gd-3Y-2Zn-0.5Zr(wt%)alloy ingot of fabricated by semicontinuous casting was selected as the experimental alloy.After being homogenized at 520°C for 16h,it was subjected to conventional extrusion with a cumulative strain of 4.57.And then,the cylindrical billet of RUE(50mm in diameter and 230mm in height)experimental alloy was sampled from the edge of the extruded rod(φ330 mm×2040mm).According to the universal ASTM naming standard for Mg alloys[35],the Mg-9Gd-3Y-2Zn-0.5Zr(wt%)alloy is abbreviated as GWZ932K.

The RUE experiment was carried out for a total of 4 passes.The experimental temperature was reduced from 420°C to 390 °C successively,and each pass dropped by 10 °C(see Fig.1(b)).The strain rate was 0.002 s-1.Fig.1(a)shows a schematic diagram of the working principle of the RUE experiment.The billet was upset in the mold,and the diameter of the billet wasD=70mm after upsetting.After that,the upset billet was kept warm,and the ejecting rod was removed to facilitate the next extrusion operation.After extrusion,the billet diameter becamed=50mm again.The above experiment was repeated until the deformation was completed in 4 passes.Through the formulaε=4n ln D/d(n is the number of passes through the extrusion die)[36],the cumulative strain after 4 RUE passes during the deformation experiment was calculated to be approximately 5.38.

To control the temperature of the mold and ensure that the billet was uniformly deformed during RUE,the mold and the billet were lubricated with an oil-based graphite lubricant before each pass during the RUE experiment,and they were preheated at 450 °C for 2 h.A total of 4 RUE deformation passes were performed in the experiment,and the billet was placed in cold water to cool immediately after each pass to prevent dynamic recrystallization(DRX)grain growth.

Samples were taken for microscopic analysis at a distance of 110mm from the head of the extruded material and 25mm from the edge.A plane parallel to the extrusion direction(ED)was selected for analysis.The sample used for microscopic analysis was polished step by step with sandpaper,polished to a mirror finis on a polishing machine,etched with an acetic acid and picric acid etchant(1g picric acid,2ml acetic acid,2ml distilled water and 14ml alcohol)and then the microstructure was observed with optical microscopy(OM;DM2500M,Leica Microsystems,Wetzlar,Germany).X-ray diffraction(XRD;Rigaku D/MAX2500PC,Rigaku,Tokyo,Japan)was used to analyze the phase composition in the alloy.Electron backscatter diffraction(EBSD,EDAX Inc,Mahwah,NJ,USA)was used to characterize the grain size refinemen effect of each RUE pass.To highlight the influenc of grain refinemen on the experimental results,the initial alloy state and the alloy that was deformed by 1,2,and 4 passes were selected for microscopic analysis.

In addition,to determine the influenc of the RUE deformation on the mechanical properties of the alloy,the alloys in the initial state and after a different number of RUE passes were processed into a dog-bone shape along the direction parallel to the extrusion direction(the sample size is shown in Fig.1.c.).At room temperature(RT),the tensile properties of the alloy were obtained by using an Instron 3382 tensile testing machine(INSTRON,Norwood,MA,USA)at a strain rate of 0.01mm/min.To avoid the contingency of the experimental results,three tensile specimens were processed for testing in the initial state and different numbers of RUE passes of the alloy.

Fig.1.(a)Working principle of RUE:(b)fl w diagram of the RUE experiment;and(c)dimensions of the specimens for the tensile test.

Before the reciprocating friction test,we used an analytical balance to test the quality of the polished sample.A multifunctional material surface performance tester(MFT-4000,Lanzhou Huahui Instrument Technology Co.,LTD.China)was used to perform the reciprocating friction tests at RT with loads of 10N,20N,and 30N.The friction speed was 50mm/min,the reciprocating sliding distance was 5mm,and the friction time for each sample was 30min.After the test was completed,the sample was placed in absolute ethanol,cleaned by an ultrasonic wave,dried,and then weighed to calculate the amount of wear(m).Scanning electron microscopy(SEM;SU5000,Hitachi,Tokyo,Japan)was used to observe the wear morphology of the sample,and energy dispersive spectrometry(EDS)was used to analyze the composition of the wear area of the alloy.As we all know,the wear rate is an important indicator reflectin the wear resistance of alloys.Under the same conditions,the greater the wear rate,the worse the wear resistance of the material.The Archard formula[37]was used to calculate the wear rate of the alloy:

where Wr(mm3/m)is the wear rate,m(mg)is the amount of wear,ρ(g/cm3)is the alloy density,and L(mm)is the sliding distance of the friction ball.

3.Results

3.1.Microstructures of the alloys

Fig.2 shows the microstructure of the GWZ932K alloy in the initial state(Fig.2a)and the SEM-BSE(backscattered electron)(Fig.2b)results.Fig.2c is an enlarged view of the area in the blue box in Fig.2b.The grains in the GWZ932K alloy in the initial state were relatively coarse,and numerous lamellar phases can be observed inside the grains(shown in the green circle in Fig.2).Scholars have reported that these lamellar LPSO phases had been confirme as 14H structures[27].In addition,bright precipitates can be seen at the edges and grain boundaries of the block-shaped phase(shown by point A in Fig.2c).According to our previous research,the bright precipitates were RE-rich phases[38].

Fig.3 shows the optical microstructure of the GWZ932K alloy with different numbers of RUE passes.After 1 RUE deformation pass,DRX grains appeared at grain boundary triple points.At this time,the alloy exhibited a typical bimodal structure.This was because during RUE deformation,the dislocation density at the grain boundaries was high,the degree of deformation was large,and the dislocations near the grain boundaries rearranged to form low-angle boundaries(LABs),thereby forming subgrains[39].At the same time,it can be clearly seen that the grain size in the GWZ932K alloy gradually decreased with RUE processing.

Fig.2.OM and SEM-BSE images of the initial state alloy.

Fig.3.OM images of longitudinal sections of the alloys in different states:(a)after 1 pass;(b)2 passes;and(c)4 passes.

Fig.4.XRD patterns of the alloys in different states.

According to the XRD results(Fig.4)and other research reports,it can be determined[40]that during the RUE process,the interior of the GWZ932K alloy was mainly composed of anα-Mg matrix,Mg5(Gd,Y,Zn)particles and different Mg12(Gd,Y,Zn)phases(block-shaped and lamellar).There were no new precipitated phases during the RUE process.After 1 RUE deformation pass,the block-shaped LPSO phase was deformed,broken,and refine(shown in the blue dashed box in Fig.3b),and the number of coarse grains containing the lamellar LPSO phase was markedly reduced.Some lamellar LPSO phases were kinked(shown in the green arrows in Fig.3),and DRX grains were generated around them.With a continuous increase in the number of RUE passes,the grains were further refine(Fig.3b,c).When 4 RUE deformation passes were completed(Fig.3c),many fin Mg5(Gd,Y,Zn)particles and DRX grains precipitated inside the alloy and were difficul to distinguish.

Fig.5 shows the SEM-BSE images of the GWZ932K alloy after RUE deformation along the longitudinal section.Fig.5(b,c,e,f,h,i)shows a high-magnificatio SEM-BSE image of the area in the blue box in Fig.5(a,c,e).Because the contrast of different phases was different,theα-Mg matrix appeared dark gray,the lamellar LPSO phase was distributed inside the grains,and the block-shaped LPSO phase was bright.After 1 RUE deformation pass,the block-shaped LPSO phase was fragmented along the extrusion direction(indicated by the red arrow in Fig.5),and the lamellar LPSO phase was kinked(indicated by the green arrow in Fig.5).As the accumulated strain gradually increased,the lamellar LPSO phases need to be kinked at a large angle to coordinate with the plastic strain(green line marked in Fig.5f).During the 4 passes RUE deformation,the lamellar LPSO phases could not further coordinate the plastic strain through large Angle twisting,and the lamellar LPSO phase twisted in some areas was broken(area 2 in Fig.5i).In addition,some fin Mg5(Gd,Y,Zn)particles precipitated at grain boundaries(indicated by the yellow arrow in Fig.5)and some fin plate-like precipitates were observed inside some specifi grains(area 1 Fig.5c in)after 1 pass RUE deformation due to the influenc of strain-induced precipitation.It had been reported in our previous studies that during the hot deformation process of Mg-Gd-Y-Zn-Zr alloy,Mg5(Gd,Y,Zn)phases of two shapes would precipitate in the grain,the platelike Mg5(Gd,Y,Zn)phase is mainly distributed in the inner grain,and the fin granular Mg5(Gd,Y,Zn)phase is distributed in the grain boundary or the interior of the grain[41].Both Mg5(Gd,Y,Zn)and Mg5(Gd,Y,Zn)have significan second-phase strengthening effect on the mechanical properties of the alloy.A large number of Mg5(Gd,Y,Zn)particles was precipitated and uniformly dispersed after 4 passes(shown by yellow arrows in Fig.5),which could pin slip of the grain boundaries and inhibit growth of the DRX grains[42].Studies have shown that for Mg-RE-Zn alloys,the DRX process can be activated through the particlestimulated nucleation(PSN)mechanism,which has a positive effect on grain refinemen[43].Because of the large difference between the elastic modulus of the LPSO phase andα-Mg[44],the inconsistent deformation of theα-Mg and LPSO phases led to stress concentration at theα-Mg/LPSO interface during the RUE deformation process.The stress concentrations promoted the formation of DRX grains and could further refin the grains[45].With continued RUE deformation,the fragmentation degree of the block-shaped and lamellar LPSO phase gradually increased,and many Mg5(Gd,Y,Zn)phases became dispersed in the alloy.There were many defects in these regions,which were conducive to atomic diffusion and led to the accelerated precipitation of Mg5(Gd,Y,Zn)phases.As the accumulated strain reached 4 passes,the grains in the GWZ932K alloy were obviously refined and the second phase was uniformly dispersed in the alloy.An improved microstructure morphology was obtained.

Fig.5.SEM-BSE micrographs of longitudinal sections of the alloys in different states:(a,b,c)after 1 pass;(d,e,f)2 passes;and(g,h,i)4 passes.

To characterize the grain refinemen during the decreasing temperature RUE process,EBSD was used to analyze each RUE deformation pass.Fig.6 shows the grain orientation distribution and average grain size of the GWZ932K alloy in the initial state and different RUE passes.Different colors represent different grain orientations.In the black area in Fig.6,because the confidenc index(CI)of this area was very low(CI＜0.1),the Kikuchi pattern map was not obtained,so the area was not calibrated.

The average grain size of the GWZ932K alloy in its initial state was approximately 90.81μm.After 1 RUE deformation pass,the average grain size was reduced to 15.92μm,the degree of refinemen was obvious,and the average grain size was reduced by approximately 82.5%.At this time,the inside of the alloy was mainly composed of coarse original grains and fin DRX grains generated at the grain boundary triple points.The overall appearance of the alloy showed an obvious bimodal structure.At the same time,DRX grains were observed inside the original grains.This was because the kinks of the lamellar LPSO phase promoted DRX in the vicinity of the kinks[42].The fin DRX grains bent into the deformed matrix,causing the grain boundaries of the original grains to appear jagged.However,most of the grain boundaries edges remain straight.This was because the expansion of fin DRX grains was hindered by the lamellar LPSO phase in the grains[42].A color change was found inside the grains,which was due to the lattice rotation inside the grains[31].From the grain size,it can be seen that the refinemen after 1 pass was the most obvious,and it became increasingly weaker as the number of passes increased.During the RUE deformation process,both the deformation temperature and the accumulated strain affected the grain size.As the RUE process progressed,the increase in accumulated strain induced additional DRX.However,due to the thermal deformation process,grain growth also occurred at the same time.Therefore,grain refine ment mainly occurred in the early stage of RUE deformation.The influenc of grain refinemen was weakened in the late stage of RUE deformation.After 4 RUE deformation passes,a relatively uniform fine-graine structure was obtained,and the average grain size was refine to 5.89μm.At this time,most areas in the alloy were replaced by DRX grains,and dislocation multiplication was suppressed.When the dislocation density was lower than that required for subcrystalline nucleation,it became very difficul to continue to refin the grains through DRX.The grain refinemen during the RUE process showed that SPD methods cause the nucleation of new grains,and the elongated grains gradually transform into fin and uniform equiaxed grains.

Fig.6.OIM maps and grain size distribution of the alloy of longitudinal sections of the alloys in different states:(a)the initial state and after(b)1 pass;(c)2 passes;and(d)4 passes.LAGB with misorientation＜15° were removed for better observation.

3.2.Texture evolution of the alloys with different states

In the case of Mg alloy,its texture significantl changes during the metal forming process and affects greatly the mechanical properties of the processed alloys.Fig.7 shows{0001},{10-10},{11-20}pole figure of the GWZ932K alloy in different states.It can be noted that the basal texture of the alloy in the initial state shows a typical bimodal type,and the basal texture strength is 5.08.the texture of the initial extruded GWZ932K alloy has a strong ED-oriented texture;namely,the c-axes of most grains are oriented parallel to the ED.This ED texture is considerably different from the typical basal fibe texture of extruded Mg alloys,whose caxes are oriented perpendicular to the ED[46].This is mainly due to the initial state alloy extrusion deformation belongs to large size billet deformation(φ330 mm×2040 mm),and the subsequent RUE experiments using small size bar are in the large size of extrusion bar sample of edge,the edge by friction and the influenc of uneven deformation leading to initial extruded alloy base texture angle deflection After 1 pass RUE,the traditional extrusion texture is not formed inside the alloy,and the texture strength of{0001},{10-10},{11-20}decreases compared with the initial state alloy.At the same time,after 1 pass,the direction of the maximum polar density changes,which is because the loading direction alternately changes between axial and radial directions in the process of RUE deformation[31].The texture strength of{0001},{10-10},{11-20}in the alloy decreases gradually with the RUE deformation,which is mainly because the alloy will produce many DRX grains during the RUE process.And the DRX grains show random grain misorientation and have a significan weakening of the texture[43].In addition,existing studies have shown that the dynamic precipitate particles during deformation can significantl weaken the texture strength[40].On the one hand,the precipitated phase particles located at the grain boundary can pin the boundary and hinder the growth and coalescence of grains.On the other hand,the presence of precipitated particles can also pin dislocations and hinder the directional slip and rotation of grains,which ultimately weaken the texture of the material[40].It is well known that the weakening of the texture of Mg alloy is beneficia to eliminate the anisotropy and improve the plasticity of the material at RT[47].At present,RUE process can significantl weaken the texture and effectively improve the properties of Mg alloy.

Fig.7.{0001},{10-10},{11-20}pole f gures of the GWZ932K alloy in different states:(a,b,c)initial state;and after(d,e,f)1 pass;(g,h,i)2 passes;and(j,k,l)4 passes.

3.3.RT mechanical properties of the alloys with different states

Fig.8 shows the influenc of the number of RUE passes on the mechanical properties of the GWZ932K alloy at RT.Obviously,the mechanical properties of the GWZ932K alloy were effectively improved after the RUE process due to grain refinemen and second-phase dispersion.The ultimate tensile strength(UTS),tensile yield strength(TYS)and elongation of the GWZ932K alloy in the initial state were 245.1MPa,189.1MPa and 4.5%,respectively.After 1 RUE deformation pass,the UTS,TYS and elongation of the GWZ932K alloy increased to 274.3MPa,203.5MPa and 5.7%,respectively,which were markedly higher than those of the alloy in its initial state.With a continuous increase in the strain during the RUE process,the UTS,TYS and elongation of the GWZ932K alloy gradually increased.After 4 RUE deformation passes,the UTS,TYS and elongation of the alloy reached 331.2MPa,263.6MPa and 7.1%,respectively,which were 1.35 times,1.39 times and 1.57 times higher than those of the alloy in its initial state,respectively.

Fig.8.Tensile properties and microhardness of the GWZ932K alloy in different states.

Table 1 shows the vickers hardness values of the GWZ932K alloy in different states.The average vickers hardness of initial state,1 pass,2 passes,and 4 passes alloys are 93.6(±1.5)HV,106.3(±2.9)HV,121.2(±2.1)HV and 112.4(±3.3)HV,respectively.It should be noted that in the early stage of RUE deformation,the hardness of the alloy increases with the gradual increase of the RUE pass,which mainly due to the grain refinemen and the fragmentation of the LPSO phase as well as the precipitation of a large amount of dispersed Mg5(Gd,Y,Zn)precipitated phases work together,so the hardness of the alloy increases[48].However,compared with the 2 passes RUE alloy,the microhardness after 4 passes RUE deformation showed a slight decrease,which is due to during the entire deformation process,the internal work hardening and DRX softening of the alloy are in a dynamic equilibrium competition.As the accumulated strain gradually increases,the DRX content inside the alloy gradually increases,and the DRX softening effect gradually replaces the work hardening effect,which leads to a slight decrease in the average microhardness of the alloy after 4 passes of RUE treatment[49].

3.4.Tribological properties of the alloys with different states

RUE can markedly refin the grains and improve the overall performance of alloys.Existing studies show that grain refinemen is a valid way to improve the wear resistance of alloys[49,50].Fig.9 shows the fluctuatio of the coefficien of friction(COF)with time for the GWZ932K alloy in different states under different load conditions.COF refers to the ratio of the friction force between two surfaces to the vertical force acting on one surface.The COF on the surface of the material can reflec the wear resistance to a certain extent,and a higher COF indicates that the material has poor wear resistance.In addition,the COF at the initial stage of the reciprocating friction experiment was low,which was due to the poor contact between the GWZ932K alloy and the indenter during the initial stage of the experiment.When the load was 30N,the COF values of all samples gradually decreased with the reciprocating friction experiment(see Fig.9c).

Table 1Vickers hardness values of the GWZ932K alloy in different states.

Fig.9.COF fluctuatio with time under different load conditions:(a)10N;(b)20N;and(c)30N.

Fig.10 shows the effect of the number of RUE passes and different load conditions on the average COF(Fig.10(a))and wear rate(Fig.10(b))of the GWZ932K alloy.It was observed that the wear rate of the alloy that underwent RUE had a lower wear rate than the alloy in its initial state when the load was 10N and 20N.However,when the load was 30N,the alloy that underwent RUE had a higher wear rate than the alloy in its initial state.Kim et al.studied the negative effect of equal channel angular extrusion(ECAP)treatment on the wear rate of an AZ61 alloy,which may have been the result of grain boundary sliding[50].As shown in Fig.10(a),the average COF also decreased with increasing cumulative strain when the load is 10N or 20N,which is mainly due to the grain refinement However,when the load is 30N,the average COF of the alloy shows the opposite law.Under 10N and 20N load conditions,the wear rate of the alloy that underwent RUE was lower than that of the alloy in its initial state.As the alloy surface load increased,the area of interaction between the indenter and the alloy surface increased.Therefore,the friction between the alloy surface and the indenter increased,which increased the wear.

Generally speaking,there is a positive correlation between the wear resistance of an alloy and its surface hardness[50].Therefore,the wear resistance of GWZ932K alloy after RUE treatment should be slightly higher than that of the initial state alloy.However,when the load is 30N,the wear resistance of the alloy treated with RUE is not as good as that of the initial state alloy,which shows that the hardness is not the main factor determining the wear resistance of the alloy after RUE deformation.When the load is 30N,the wear resistance of the RUE-treated alloy is worse than that of the initial alloy.This phenomenon is attributed to the influenc of the alloy surface wear mechanism,which will be further discussed in the next section.

4.Discussion

4.1.Grain refinemen rules in the Rue process

Fig.11 shows the misorientation angular distribution after the different numbers of RUE passes of the GWZ932K alloy.Fig.11(a)shows that after 1 RUE deformation pass,the misorientation(less than 5°)of the alloy had an obvious distribution peak.At this time,due to a small cumulative strain,the inside of the alloy was mainly composed of coarse original grains and a small amount of DRX grains.Discontinuous dynamic recrystallization(DDRX)was the main recrystallization mode[51].With a continuous increase in the accumulated strain during the experiment,the degree of plastic deformation in the alloy was intensified grains underwent dislocation multiplication,movement and formation of tangles,so that the grains were stretched,broken and fibrillated and the subgrains formed by LABs further developed.After that,the subgrains continued to absorb dislocations and changed from LABs to high-angle boundaries(HABs).There were many LABs inside the original coarse grains,which indicated that the internal dislocation density of these grains was relatively high,and these grains were transformed into a chain-like distribution of LAB subgrains[52].

Fig.10.Influenc of the number of RUE passes on the(a)average COF and(b)wear rate.

Fig.11.Misorientation angle distribution of the alloys in different states:1 pass;(b)2 passes;and(c)4 passes.

With the progress of the RUE process,the misorientation of the LABs gradually decreased,while the misorientation of the HABs gradually increased.DRX grains were formed by the continuous accumulation of dislocations,which gradually deflecte the grains and transformed subgrains into DRX grains.This process is a typical CDRX process[51].When the RUE deformation progressed to 4 passes,the inside of the alloy was mainly HABs,the microstructure comprised uniform and fin equiaxed grains,and the grain refinemen was obvious.

With the progress of RUE deformation,CDRX and DDRX within the GWZ932K alloy occurred simultaneously.The fundamental difference between CDRX and DDRX is that CDRX has no obvious nucleation and growth stages,and new grains are mainly formed by a gradual increase of the LAGBs misorientation.However,DDRX has obvious nucleation and growth stages,and the DRX core was mainly produced by the grain boundary arching nucleation method,that is,strain-induced grain boundary movement[53-55].In order to further clarify the internal CDRX and DDRX deformation processes of the alloy during the RUE deformation process,typical grains are selected for analysis from Fig.6(b).During the hot deformation of the alloy,due to the rotation of the crystal lattice inside the crystal grains,misorientation inside the crystal grains changes,and the cumulative misorientation from point c to d reaches 36.A large number of LAGBs are distributed in the grain interior,which is due to the work hardening effect that produces a large number of dislocations in the alloy.Under the action of stress,the dislocations slip along the basal or non-basal,and generates dislocation plugs when they slip to the initial grain boundary.When the dislocation plugs accumulate to a certain extent,they rearrange and merge,resulting in dislocation cells and subgrain boundaries.Subgrain boundaries can increase their misorientation by absorbing lattice dislocations,and then transform LAGBs into HAGBs.Then,HAGBs migrate,eliminate part of the subgrain boundaries and grain boundaries,and produce equiaxed recrystallized grains(see in blue arrow in Fig.12(a))[56,57].Specificall,the sharp increase in the misorientation curve(Fig.12(b))indicates the occurrence of CDRX in the RUE process.In Fig.12,there are serrated grain boundaries at the boundary of typical grains,and some grain boundaries are arched toward adjacent grains,gradually forming subgrains(see in black arrow in Fig.12(a)).This process has obvious nucleation and growth stages,which are the typical DDRX characteristic[58].

Fig.12.(a)Typical grain selected from 1 pass RUE alloy(marked with white wire frame in Fig.6(b)and line graph of misorientation angle along the arrows AB(b)and CD(c)in(a).

4.2.Mechanism of improving the RT mechanical properties of alloys

The RT mechanical properties of the GWZ932K alloy markedly increased after RUE deformation,which was mainly attributed to the grain refinemen of the RUE process and the dispersion of the second phases.In crystal defects,dislocations are known to be three-dimensional,and the line segments in the dislocation networks inside the alloy on the slip plane could be a dislocation source.Under the action of stress,the dislocation source could continuously release dislocations,causing the crystal to slip.During dislocation movement,the hindrance of the dislocation network must be overcome first When dislocations move to the grain boundaries,the grain boundary obstacles must be overcome so that the deformation could be transferred from one grain to another.The TYS of metals should depend on the minimum stress required to activate the dislocation source,as well as the resistance of the dislocation network to the mobile dislocations and the resistance of the grain boundaries.Therefore,the smaller the grains in the alloy are,the more grain boundaries there are inside the alloy,and the greater the force that is required to operate the dislocation source,leading to an increase in the strength of the material.This could be explained by the Hall-Petch formula derived based on the in-place dislocation plug model[59]:

whereσsrepresents the yield limit of the material,which is the yield stress when the material is deformed by 0.2%;σ0represents the lattice friction resistance generated when a single dislocation is moved;Kis related to the nature of the material and the grain size,which is a constant;and d is the average grain size of the material.Therefore,the strength of the material is inversely proportional to the grain size,and the smaller the grain size is,the higher the strength of the material.It should be noted that the relationship between the strength from the Hall-Petch formula and the grain size does not extend to nanoscale materials.This is because when the grain size is at the nanoscale,there may be very few dislocations in the grains,or even only one.Therefore,the Hall-Petch formula is no longer applicable.However,after 4 RUE deformation passes,the grain size of the alloy was still at theμm-scale(Fig.6d),so the Hall-Petch formula could be used to qualitatively analyze the relationship between strength and grain size of alloy[60].

Studies have shown that Mg-Gd-Y-Zn-Zr alloys precipitate a denseβ’phase,which can markedly prevent sliding of dislocations,especially basal dislocations,thereby increasing the strength of the alloy[61].After 4 RUE deformation passes were completed,the block-shaped and lamellar LPSO phases were uniformly broken,and a large amount of Mg5(Gd,Y)phases that precipitated at the grain boundaries were evenly dispersed in the alloy,which could pin dislocation slip.The grain boundary deformation was hindered.Therefore,the dispersion of the second phases inside the alloy could also play an important role in the improvement of the mechanical properties of the material.This can be explained by the second-phase dispersion strengthening mechanism[62].When the alloy undergoes plastic deformation,the dislocation line cannot directly cut through the second-phase particles,but under the action of external force,the dislocation line can move around the second-phase,and as the applied stress increases,the dislocations are forced to move forward in a continuously bending manner.Finally,a dislocation ring is left around the second-phase particles to realize the increment of the dislocation.Dislocation bending increases the lattice distortion energy in the dislocation-affected zone and increases the resistance of the dislocation line movement.It becomes more difficul for the subsequent dislocation line to bypass the particles,thereby increasing the strength of the material.

In addition,the texture of Mg alloy can affect the mechanical properties of the alloy at RT by changing the schmid factor(SF)of slip system,especially{0 0 0 l}[1 l-2 0]basal slip system[63].Fig.13 shows the SF distribution maps and histograms of the alloy of longitudinal sections of the alloys in different states.The alloy exhibits a higher SF(0.36)at initial state,which indicates that the basal slip is easily activated during the deformation process of the initial state,and the basal slip is the main deformation mechanism at the initial stage of tensile deformation at RT[64].The SF value also decreases gradually with the RUE process,which indicates that the basal slip in the alloy is gradually inhibited.Therefore,the TYS of the alloy shows an upward trend with the increase of RUE process[63].In addition,since Mg alloys have fewer slip systems,theoretically speaking,when the SF of the applied stress on the basal slip system is high,the basal slip is easier to start and the alloy has better plasticity.From the perspective of texture analysis,the plasticity of the alloy should gradually decrease with the increase of RUE passes.However,from Fig.8 RT tensile test results show that the plasticity of the alloy gradually increases with the progress of the RUE process.This may be due to the positive influenc of grain refinemen and second phase dispersion on plasticity to a certain extent to offset the negative influenc of texture weakening to suppress basal slip on plasticity.Therefore,RUE process can change the activity of basal slip by weakening the texture and thus affect the mechanical properties of Mg alloys.

Fig.13.SF distribution maps and histograms of the alloy of longitudinal sections of the alloys in different states:(a)the initial state and after(b)1 pass;(c)2 passes;and(d)4 passes.LAGB with misorientation＜15° were removed for better observation.

In short,the remarkable improvement in the mechanical properties of the GWZ932K alloy after RUE deformation was due to the combined effect of grain refinement a dispersed second phase and texture weakening.

4.3.Worn surface of alloys with different states

The RUE process could markedly refin the grains,which improved the wear resistance.Therefore,in theory,the alloys that undergo RUE should have a higher wear resistance than the alloy in its initial state.However,under higher load conditions(Load＞30N),the GWZ932K alloy that underwent RUE had a higher wear rate than the alloy in its initial state(see Fig.10b).To clarify the wear mechanism of the GWZ932K alloy during the reciprocating friction experiment after the RUE deformation,the wear surface of the alloy in different states was observed under SEM-SE.Fig.14 shows the microstructure morphology of the worn samples under 10N,20N,and 30N loads,and we selected an area(blue box)for EDS element analysis.Additionally,debris and delamination were observed on the wear surface of the GWZ932K alloy under different conditions.In Fig.14(a,b,c),we can see many debris and groove-like wear marks in the SEM-SE images of the alloy after reciprocating friction under different load conditions.The groove-shaped wear marks were caused by the abrasive particles that were ground into the contact surface under the action of a load,followed by shear and microcutting of the worn surface during the friction process.Finally,groove-shaped wear marks were produced on the grinding surface.Ploughing wear is a kind of abrasive wear mechanism,where debris is formed in the abrasive wear process,and delamination is caused by plastic deformation during reciprocating friction,which indicates that the alloy surface is severely worn[65,66].The plow wear and delamination wear of the GWZ932K alloy surface after 1 RUE deformation pass was more severe than that of the alloy in its initial state(Fig.14g,h,i).With the progress of the RUE process,additional delamination and debris appeared on the surface of the GWZ932K alloy.Alloys after 4 RUE passes produced the most debris under the same loading conditions.EDS elemental analysis of all worn surfaces found that there was O,which proved that surface oxidation occurred in the sample.Therefore,the GWZ932K alloy had oxidative wear during the wear process.The oxidative wear was caused by friction and heat generation during the reciprocating friction experiment on the alloy surface under high load conditions,and a thin oxide layer was formed on the alloy surface.The oxide fragments were removed during the wear process to produce oxidative wear.The formation of the oxide layer had a negative impact on the wear resistance of the alloy.Studies have shown that the oxide layer can eliminate the positive impact of SPD on the alloy wear resistance[67].When the formation rate of the oxide fil on the alloy surface was greater than the wear rate,the wear surface quickly formed an oxide film This oxide fil hindered the wear of the material,and the wear rate was relatively small.When the wear rate was greater than the formation rate of the oxide film the oxide fil was worn away before it could be formed,resulting in a higher wear rate of the material[48].This also indicated that the oxide layer on the surface of the GWZ932K alloy after RUE treatment was under a low load,such as 10N and 20N.At this time,the negative effect of the oxide layer on the wear resistance of the alloy was less than the positive effect of the grain refinement Macroscopically,the wear resistance of the GWZ932K alloy treated by RUE improved.However,when the load was large,such as more than 30N,the negative impact of the oxidation layer on the wear resistance of the alloy was greater than the positive impact of the grains that were refine during RUE.Therefore,under 30N loading conditions,the surface of the alloy treated by RUE had a higher wear rate than the alloy in its initial state.

Fig.14.SEM-SE image of the wear surface of alloys with different states and the EDS element analysis of the corresponding blue box:(a,b,c,d,e,f)initial state;and after(g,h,i,j,k,l)1 pass;(m,n,o,p,q,r)2 passes;and(s,t,u,v,w,x)4 passes.(For interpretation of the references to colour in this figur legend,the reader is referred to the web version of this article.)

5.Conclusion

In this study,we investigated the improvement in the mechanical and wear properties of a GWZ932K alloy as a result of grain refinemen and LPSO phase dispersion after decreasing temperature RUE.The following conclusions were drawn from this study:

(1)After 4 RUE deformation passes of the GWZ932K alloy,the grain refinemen is obvious,and the average grain size reaches 5.89μm.The block-shaped and lamellar LPSO phases are broken,and a large amount of Mg5(Gd,Y,Zn)phase is precipitated and uniformly dispersed inside the alloy to obtain an improved microstructure.

(2)After 4 RUE deformation passes are completed,the tensile properties of the alloy are markedly improved.The UTS,TYS and elongation reach 331.2MPa,263.6MPa and 7.1%,respectively,which are 1.35 times,1.39 times and 1.57 times higher than those for the alloy in its initial state,respectively;this behavior is due to grain refinement second-phase dispersion and texture weakening during the RUE deformation process.

(3)The RUE process can markedly increase the average COF of the alloy.As the load and contact area increase,the fluctuatio of the COF decreases.Under 10N and 20N load conditions,the sample wear rate increases with increasing number of RUE passes.However,under higher loads(＞30N),the initial state specimens have better wear resistance than the alloys that underwent RUE.

(4)The wear morphology of the alloy under different loading conditions is observed in SEM-BSE images.The alloy samples undergo abrasive wear and oxidative wear during the wear process.It is precisely due to the existence of oxidative wear that the alloys subjected to RUE treatment under larger load conditions show a higher wear rate than the initial state alloys.

Acknowledgments

This research was financiall supported by the Natural Science Foundation of Shanxi Province(No.201901D111176),the Joint Funds of the National Natural Science Foundation of china(Grant No.U20A20230),the Bureau of science,technology and industry for National Defense of China(No.WDZC2019JJ006),the Key R&D program of Shanxi Province(International Cooperation)(No.201903D421036),the National Natural Science Foundation of China(Grant No.52075501),Scientifi and Technological Innovation Programs of Higher Education Institutions in Shanxi(No.2018002)