Microstructure,texture evolution and yield strength symmetry improvement of as-extruded ZK60 Mg alloy via multi-directional impact forging

Cho Cui,Wencong Zhng,b,c,Wenzhen Chen,b,c,?,Jin He,Xuemin Chen,Jibin Hou

a School of Materials Science and Engineering,Harbin Institute of Technology,Weihai 264209,China

b Weihai Lightweight Materials and Forming Engineering Research Center,Weihai 264209,China

c Harbin Institute of Technology-Weihai Innovation Park,Weihai 264209,China

Abstract Multi-direction impact forging(MDIF)was applied to the as-extruded ZK60 Mg alloy,and the microstructure,texture evolution and yield strength symmetry were investigated in the current study.The results showed that the average grain size of forged piece was greatly refined to 5.3μm after 120 forging passes,which was ascribed to the segmenting effect of{10–12}twins and the subsequent multiple rounds of dynamic recrystallization(DRX).A great deal of{10–12}twins were activated at the beginning of MDIF process,which played an important role in grain refinement.With forging proceeding,continuous and discontinuous DRX were successively activated,resulting in the fully DRXed microstructure.Meanwhile,the forged piece exhibited a unique four-peak texture,and the initial＜10-10＞//ED fiber texture component gradually evolved into multiple texture components composed of＜0001＞//FFD(first forging direction)and＜11–20＞//FFD texture.The special strain path was the key to the formation of the unique four-peak texture.The{10–12}twinning and basal slip were two dominant factors to the evolution of texture during MDIF process.Grain strengthening and dislocation strengthening were two main strengthening mechanisms of the forged piece.Besides,the symmetry of yield strength was greatly improved by MDIF process.

Keywords:Multi-directional impact forging;Twinning;Dynamic recrystallization;Texture;Yield strength symmetry.

1.Introduction

Magnesium(Mg)alloys possess certain extraordinary physical properties,such as low density,high specific strength and high specific stiffness,and therefore are becoming increasingly attractive for potential use in the automotive,aerospace and communicated industries[1,2].However,Mg has a hexagonal closed-pack(hcp)crystal structure with an insufficient number of operative slip systems,leading to limited ductility at room temperature.In addition,the formation of strong basal texture during hot working process will bring about the severe asymmetry in yield strength[3–13],including tension/compression yield strength asymmetry(Table 1)and tension yield strength anisotropy(Table 2).These defects limit the widespread industrial applications of Mg alloys.

Table 1Literature review of the tension/compression yield strength asymmetry in Mg alloys fabricated by different hot working processes.

Table 2Literature review of the tension yield strength anisotropy in Mg alloys fabricated by different hot working processes(LD:longitudinal direction;TD:transverse direction).

Grain refinement and texture modification are considered as two effective ways to improve the mechanical properties of Mg alloys.Recently,some severe plastic deformation(SPD)techniques,such as friction stir processing(FSP)[14],different speed rolling(DSR)[15],high-pressure torsion(HPT)[16],multi-directional forging(MDF)[17,18],etc.,were used to improve strength and ductility of Mg alloys by grain refinement and texture modification.Among these SPD methods,MDF has the advantages of simple operation,low cost,and suitable for large-scale industrial products.Moreover,MDF has greater potential for weakening the basal texture of thewrought Mg alloy due to the continuously changing external load direction on the workpiece.In previous research,MDF has been applied to different Mg alloys to obtain finegrained microstructure and excellent mechanical properties.Miura et al.[19]fabricated an ultra-fine grains AZ61 Mg alloy with the average grain size of 0.8μm by MDF under decreasing temperature conditions,and the MDFed AZ61 Mg alloy exhibited a superior balance of strength and ductility,with ultimate tensile strength(UTS)of 440MPa and plastic strain to fracture of over 20%.Wu et al.[20]reported that high strain rate multiple forging(HSRMF)was successfully carried out on ZK60 Mg alloy.The initial grains were extensively refined after HSRMF due to dynamic recrystallisation(DRX)and the elongation as well as ultimate tensile strength was significantly improved.Shah et al.[21]studied the effect of multi-direction impact forging(MDIF)on solutionized Mg-Gd-Y-Zr alloy.They reported that the initial coarse grains of Mg-Gd-Y-Zr alloy were significantly refined from 165μm to 5μm after 50 forging passes,and both strength and ductility of GW94 alloy were enhanced by the MDIF process.From the above,these previous researches have shown that MDF can significantly refine grains,often attributed to DRX process,and improve mechanical properties for both traditional commercial Mg alloy and new rare earth(RE)Mg alloy.However,the effect of various DRX mechanisms on the crystallographic orientation evolution was not clear.Also,the enhanced mechanical properties were often attributed to the grain refinement,whereas texture effect was not taken into consideration.Moreover,the effect of MDF on yield strength asymmetry of Mg alloys has not been reported so far.

In this work,a multi-directional impact forging(MDIF)technology was applied to the as-extruded ZK60 Mg alloy and a small pass strain of about 0.05 was employed.The small pass strain was designed to avoid the formation of strong compressed type basal texture.Different from the conventional MDF performed on the hydraulic press machine,MDIF used industrial air pneumatic hammer machine for high strain rate forging,which could effectively improve forging efficiency and reduce processing costs.From MDIF technology,the yield strength asymmetry of as-extruded ZK60 Mg alloy has been significantly improved due to the grain refinement and texture modification.The evolution of microstructure and texture during MDF process will be discussed in detail in this paper,as well as the effect of various DRX mechanisms on the crystallographic orientation evolution.It is hoped that this paper will provide useful information for manufacturing large block ZK60 Mg alloy with yield strength symmetry and expanding the application range of Mg alloy in a simpler process.

2.Experimental procedures

A commercial ZK60 Mg alloy(Mg-6.63 wt%Zn-0.56 wt%Zr)was used in this work.Cubic block samples were machined from the as-extruded ZK60 Mg bar stock with dimensions of 60×60×60mm3.The cubic block samples were firstly heated to 400°C in an electric resistance furnace and kept for 30min.Then,the MDIF process was performed using industrial air pneumatic hammer machine with load gravity of 150 Kg.To avoid a rapid drop in the sample temperature during MDIF process,the top and bottom anvils contacting the sample were also heated to 400°C before forging.As shown in Fig.1a,the initial forging direction is parallel to the extrusion direction,and between the continuously forging passes,the loading direction was changed by 90° pass by pass(i.e.,X to Y to Z to X···)and no reheating process was carried out during the whole MDIF process.In order to study the microstructural evolution during MDIF process,the samples were forged to different forging passes(1,3,6,12,30,60,120 passes)which were named MDIF1,MDIF3,MDIF6,MDIF12,MDIF30,MDIF60,and MDIF120,respectively.After MDIF process,the surface temperature of each forged piece was detected by a TES 1310 Type-K thermometer and listed in Table 3,and then these forgings were quickly cooled down in water.As shown in Fig.1b,the forged piece with 120 passes was free from any surface defects.

Table 3The final surface temperatures of each samples.

As shown in Fig.1c,the microstructures were observed at the central part of the FFD-LFD plane(FFD:the first forging direction,LFD:the last forging direction)using electron back-scatter diffraction(EBSD)and scanning electron microscopy(SEM).Sample was prepared for EBSD observation by mechanical grinding with 3000-grit SiC paper,followed by electro-chemical polishing in a 5:3 solution of ethanol and phosphoric acid for 8min at 0.25 A.EBSD was carried out at 20kV,15mm working distance,a tilt angle of 70° and ascan step of 0.5μm.The scans results were analyzed using TSL OIM Analysis 7.2.1 software.For SEM observation,the samples were polished and etched with a solution of 4.0g picric acid,50ml alcohol,20ml acetic acid and 20ml distilled water.The tensile specimens(17.5mm in gage length and 3mm in diameter of gage section)and compressive specimens(7.5mm in length and 5mm in diameter)were extracted from the central part of FFD-LFD plane,as shown in Fig.1c.Tension and compression properties at room temperature were tested by Instron 5967 testing machine with an initial strain rate of 1×10-3s-1.

Fig.1.(a)Schematic illustrations of the MDIF process,(b)macroscopic morphology of forged piece after MDIF120 passes and(c)the position of microstructure,tension and compression specimens in the forged pieces.

3.Results and discussion

3.1.Microstructure characteristics of the as-extruded ZK60 mg alloy

Fig.2a shows the microstructure characteristics of the asextruded ZK60 Mg alloy in the inverse pole figure map on ED-TD plane(ED:extrusion direction,TD:transverse direction).It was clear that the initial material had a bimodal grain size,consisting of small DRXed grains and large deformed grains elongated along the extrusion direction.The average grain size of small equiaxed grains(＜40μm)was about 18.6μm(Fig.2b),while the grain size of some large deformed grains could not be accurately measured from the present EBSD.As shown in Fig.2c and d,the initial material exhibits a strong(0002)basal texture with maximal pole intensity of 9.60 existing typical＜10-10＞//ED fiber texture component[22].As shown in Fig.2e,low angle grain boundaries(LAGBs,2–15°)and high angle grain boundaries(HAGBs,15–100°)accounting for 31.9% and 68.1% respectively,and these LAGBs were concentrated in the coarse deformed grains(Fig.2a).

3.2.Microstructure evolution during MDIF process

In order to analyze the evolution of microstructure during MDIF process,the MDIFed pieces with different forging passes were examined in detail using EBSD and the corresponding results are presented in Figs.3-9,respectively.LAGBs,HAGBs and twin boundaries of different types including{10–12}extension,{10–11}contraction and{10–11}-{10–12}double twin boundaries were highlighted withdifferent colors.The angular deviation to identify the twin boundaries was within 5° of the ideal values.

Fig.2.EBSD observation results of the as-extruded ZK60 Mg alloy:(a)inverse pole figure map,(b)distribution histogram of grain size,(c)(0002)basal pole figure,(d)inverse pole figure and(e)distribution histogram of misorientation angle.

Fig.3 shows EBSD observation results of the ZK60 alloy after the first forging pass.As shown in the inverse pole figure map(Fig.3a),plenty of twins were activated in initial coarse grains,and dividing them into many irregular grains by these twins.According to the corresponding boundary misorientation map(Fig.3b),all of these twins were identified to be{10–12}twins.Besides,it can be clearly found that these{10–12}twins had different morphologies and paralleled or intersected to each other(Fig.3a),which was related to the activation of various twin variants.The distribution histogram of misorientation angle(Fig.3c)displays three obvious peaks for LAGBs,as well as HAGBs close to 60° and 86° From the corresponding axis distributions inserted in Fig.3c,it could be seen that the distinct peak at 86°±5° shared a strong〈11–20〉rotation axis,which corresponded well to{10–12}twins.However,the peak at 60°±5° shared a strong〈10–10〉rotation axis.In Mg alloys,different misorientation relationships can be generated between different{10–12}twin variants:7.4° about〈11–20〉axis between the same twin variant pair and 60° about〈10–10〉axis between different twin variant pairs[23,24].Hence,the distinct peaks close to 60° were mainly ascribed to the intersection of different twin variant pairs.In order to clarify the effect of{10–12}twins on the texture evolution at the early stage of the MDIF process,the crystallographic orientation of{10–12}twins were separately highlighted in blue as shown in Fig.3e and f.It could be found that most of the{10–12}twins displayed the basal planes perpendicular to the FFD and exhibited＜0001＞‖F(xiàn)FD crystallographic orientation.Fig.3d shows the distribution of grain size after the first forging pass,and the average grain size was reduced to 23.5μm due to the segmenting effect of twins on the initial coarse grains.

In order to specifically demonstrate{10–12}twinning behavior after the first forging pass,the typical initial coarse grains(G1-G4)containing twins with different morphologies were extracted from Fig.3a.Besides,to distinguish the twin variants activated in G1-G4,six possible variants of each{10–12}twin are coded as T1-T6 in advance and the relative geometrical relationships of the six variants are reflected in the schematic diagram(Table 4).In the following sections,the analysis of the selection mechanism for various twin variants would use such code.

Table 4The codes and indices of six possible{10–12}twin variants and the illustration of their geometrical relationships.

Fig.3.EBSD observation results of the ZK60 Mg alloy after the first forging pass:(a)inverse pole figure map,(b)corresponding boundary misorientation map,(c)distribution histogram of misorientation angle with axis distributions for the angles of 60°±5° and 86°±5°,(d)distribution histogram of grain size,(e)(0002)pole figure and(f)inverse pole figure.

The various twin variants activated in G1-G3 were clearly identified based on the particular misorientation angles and rotation axes,including 86.3±5° about〈11–20〉axis between{10–12}twin and its parent grain,7.4±5°about〈11–20〉axis between a twin variant pair and 60±5° about〈10–10〉axis between two different twin variant pairs.Table 5 gives the misorientation angles of points at the junction of twins with different morphologies indicated in Fig.4a,e,and i.Besides,the orientation of parent grains(blue square)in G1-G3 andcorresponding six possible{10–12}twin variants(red circles)were marked in a(0002)pole figure(Fig.4b,f and j).Interestingly,the activation of twin variants in grain G1-G3 was identical,i.e.,two pairs twin variant T1,T4 and T3,T6 were activated.The crystallographic orientation relationship between parent grains and various twin variants was shown in(0002)pole figure(Fig.4b,f and j)and corresponding inverse pole figure(Fig.4c,g and k).In G1,the parent grain belonged to[11–20]fiber orientation with a slight deviation of～12° from[11–20]//FFD,and the orientation of all twin variants was between[0001]//FFD and[11–24]//FFD.In G2,the orientation of parent grain had a slight deviation from[11–21]//FFD,while the orientation of all twin variants was between[0001]//FFD and[10–12]//FFD.The crystallographic orientation of both parent grain and twin variants in G3 was very similar to those in G1.Fig.4d,h and l clearly depict the three-dimensional crystallographic relationship between parent grains and various variants in G1-G3.

Table 5The misorientation angles of points at the junction of twins with different morphologies indicated in Fig.4a,e,and i.

Table 6Schmid factor values of six{10–12}twin variants and their activations in the four grains(G1-G4)of MDIF1 sample.

Fig.4.The details of{10–12}twinning behavior after the first forging pass in the initial coarse grains(a-d)G1,(e-h)G2 and(i-l)G3 selected in Fig.3a:(a,e,i)inverse pole figure maps,(b,f,j)(0002)pole figure,(c,g,k)inverse pole figure and(d,h,l)three-dimensional crystallographic relationship between various variants and their parent grain.(P:parent grain,T:various variants).Blue squares and red circles indicating parent grains and six potential twin variants,respectively.(For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

Clearly,the initial coarse grain G4 shown in Fig.3a has obviously different microstructure characteristics compared to the above grains G1～G3,i.e.,parent grain was almost swallowed by{10–12}twins and some fine DRXed grains were observed in G4.To study the effect of{10–12}twins on the subsequent DRX process,the initial coarse grain G4 was extracted from Fig.3a.The various variants activated in G4 was identified in the same way,and the results showed that two different{10–12}twin variants T2 and T5 were activated in G4.As shown in corresponding(0002)pole figure(Fig.5b)and inverse pole figure(Fig.5c),the parent grain had a[10–10]fiber orientation,while these two variants T2 and T5 exhibited[0001]//FFD crystallographic orientation.Meanwhile,according to the location of DRXed grains and the color shown in the inverse pole figure maps(Fig.5a),these DRXed grains could be roughly divided into two categories:(i)concentrated in the interior and boundary of twins and had a blue or purple color;(ii)scattered in the parent grain and possessed a red or orange color,and the above two kinds of DRXed grains were named TDRX grains and PDRX grains,respectively.From the corresponding(0002)pole figure(Fig.5d)and inverse pole figure(Fig.5e),it could be found that TDRX grains showed obvious preference orientation,which was very close to T2 and T5;although the orientation distribution of PDRX grains was more diffuse,it also showed obvious preference orientation.The orientation distributions of DRXed grains were quantified by the spread angle away from the[0001]direction in inverse pole figures.Clearly,the spread angles of TDRX grains and PDRX grains were 0–35° and 72–90°,respectively.The misorientation angles developed along the red arrow AB in{10–12}twins showed that the point-to-origin misorientation gradually increased up to 25°(Fig.5f),indicating the high activity of dislocations in{10–12}twins.A previous study[25]reported that the highly incoherent twin boundaries have a strong obstacle effect on the dislocations and contribute significantly to the DRX behavior and formation of DRX chains inside the grain.Therefore,we can infer that{10–12}twins facilitated the subsequent DRX process and formed the above TDRX grains.Besides,a continuous increase misorientation angles along the red arrow CD in parent grain were observed(Fig.5g),which indicated the dislocation slip inthe parent grain was also active.Hence,it could be speculated that while the{10–12}twins continued to expand and swallow the parent grain,DRX process was taking place in part of parent grain,so as to generate the above PDRX grains.

Fig.5.The effect of{10–12}twins on the subsequent DRX process:(a)inverse pole figure maps,(b,d)(0002)pole figure,(c,e)inverse pole figure and(f,g)the misorientation angles developed along the red arrows AB and CD indicated in(a).Blue square and red circles indicating parent grains and six potential twin variants,respectively.(For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

Generally,the selection mechanism for various{10–12}twin variants strongly depends on the strain path,i.e.,compression perpendicular to the c-axis or tension parallel to the c-axis[26].Hong et al.[27]suggested that all six twin variants were equally possible to be activated due to the same Schmid factor when tension parallel to the c-axis,while for compression perpendicular to the c-axis,the twin variants with higher Schmid factor were activated preferentially.Therefore,we analyzed the selection mechanism for various{10–12}twin variants using the Schmid law in this work.The Schmid factor of six possible{10–12}twin variants in G1-G4 were calculated and displayed at the Table 6.Those bold black fonts highlighted in Table 6 corresponded to the activated twin variants.We could see that the twin variants with the Schmid factor ranking in top four were activated in G1-G3,and the T2 and T5 with the Schmid factor significantly higher than other twin variants were activated in G4.Obviously,the selection mechanism of various twin variants is governed by the Schmid law during the MDIF process of ZK60 Mg alloy.In addition,it could be found that the morphology and growth mechanism of{10–12}twins also seem to be related to the Schmid factor.The twin variants with relatively low Schmid factor in G2 showed a typical lenticular morphology and these twins did not grow further(Fig.4e),while the T2 and T5 in G4 with the highest Schmid factor obviously grew up and the parent grain was almost swallowed by these twins(Fig.5a).Therefore,we can infer that the Schmid factor of{10–12}twins is positively correlated with its growth rate.

Fig.6 shows EBSD observation results of the ZK60 alloy after 3,6 and 12 forging passes.The inverse pole figure maps(Fig.6a,d and g)show an inhomogeneous microstructure,consisting of equiaxed DRXed grains and irregular grains.Through careful observation,it could be found that a large number of LAGBs were mainly concentrated in these irregular grains,as shown in R1,R2 and R3 in Fig.6a,d and g.According to relevant literature[28,29],the existence of abundant LAGBs is a typical feature of continuous dynamic recrystallization(CDRX).Therefore,it can be speculated that the grain refining mechanism was dominated by CDRX process in this period.In the corresponding boundary misorientation maps(Fig.6b,e and h),all the twins were still found to be{10–12}twins.Besides,the fraction of{10–12}twins were gradually reduced with the increase of forgingpasses.Fig.6c,f and i show the distribution of grain size after forging 3,6,and 12 passes respectively,the average grain size did not change significantly due to the small cumulative strain during this period.

Fig.6.EBSD observation results of the ZK60 Mg alloy after 3(a-c),6(d-f)and 12(g-i)forging pass:(a,d,g)inverse pole figure map,(b,e,h)corresponding boundary misorientation map and(c,f,i)distribution histogram of grain size.

Fig.7 shows the details of CDRX behavior and corresponding effect on the crystallographic orientation evolution in R1-R3 selected in Fig.6a,d and g,respectively.As shown in Fig.7a,e and i,there are several similarities as follows.First,lots of LAGBs was observed in parent grains.Second,sub-grains formed subdividing these parent grains into several parts.Third,fine DRXed grains formed surrounding parent grains boundaries or situated inside parent grains.Fig.7d,h and l respectively show the misorientation angles developed along the blue arrows AB,CD and EF indicated in(a,e,i).It can be found that there is a large orientation gradient in parent grains,which suggests the high activity of dislocations in the parent grains.Previous literatures[30,31]had shown that the accumulated dislocations on dislocation walls would rearrange or merge with each other to generate sub-grains,and eventually formed new DRXed grains by continuous absorption of the dislocations in sub-GBs.The corresponding(0002)pole figure(Fig.7b,f and j)and inverse pole figure(Fig.7c,g and k)exhibit the crystallographic orientation of parent grains,parts of sub-grains and DRXed grains.We could see that the orientation of the sub-grains was very close to that of the parent grains,while the orientation distribution of the DRXed grains was more diffused,but it also showed a clearly preferred orientation related to the parent grains.

Fig.8 shows EBSD observation results of the ZK60 Mg alloy after 30,60 and 120 forging passes.As shown in the inverse pole figure maps(Fig.8a,d and g),the irregular grains almost disappeared and the microstructure became much more homogeneous.Compared to the previous period,although the fraction of LAGBs was significantly reduced,it was still at a higher level.A careful examination on Fig.8a and d shows that these LAGBs were mainly concentrated in the relatively large DRXed grains,indicating that CDRX was still the dominant mechanism for grain refinement in this period.And the relatively large DRXed grains were further refined by a new round of CDRX process.However,it should be noted thatCDRX was not the only grain refinement mechanism during MDIF process.As shown in R4 and R5 in Fig.8a and d,some small grains(＜5μm)formed along the serrated grain boundaries of relatively large DRXed grains,which was a discontinuous dynamic recrystallization(DDRX)process with nucleation at serrated HAGBs by bulging and grain growth by grain boundaries migration[32,33].The corresponding boundary misorientation maps(Fig.8b,e and h)shows that the microstructure was almost twin-free.In the distribution histogram of grain size(Fig.8c,f and i),the average grain size was effectively refined with the continuous increase of cumulative strain.Especially after 120 forging passes,the average grain size was refined to 5.3μm.It should be pointed out that after 120 forging passes,the significant refinement of grain size was unnegligiblely influenced by the temperature,in addition to the contribution of high cumulative strain.Generally,the DRXed grain size largely depends on the Zener-Hollomon parameter Z(Z=˙εexp(Q/RT)),where˙εis the strain rate,Qis the activation energy,Ris the gas constant andTis the deformation temperature,and a finer DRXed grain size will be obtained by increasing the Z-parameter[34,35].

Fig.7.The details of CDRX behavior and corresponding effect on the crystallographic orientation evolution in R1(a-d),R2(e-h)and R3(i-l)selected in Fig.6a,d and g,respectively:(a,e,i)inverse pole figure maps,(b,f,j)(0002)pole figure,(c,g,k)inverse pole figure and(d,h,l)the misorientation angles developed along the blue arrows AB,CD and EF indicated in(a,e,i).(For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

Fig.9 shows the details of DDRX behavior and corresponding effect on the crystallographic orientation evolution in R4 and R5.Compared to the previous CDRX,the formation of new DRXed grains by DDRX process had a distinct characteristic,i.e.,the DRXed grains nucleated along serrated HAGBs of parent grains by bulging.Sitdikov et al.[36]suggested that the interaction between non-basal slip and basal slip dislocations yields the formation of sub-GBs,which cut off the protrusion from the grain body,and the misorientation of such sub-GBs increase with increasing strain by continuous gathering dislocations,finally leading to the formation of new DRXed grains.It is well known that non-basal slip system is difficult to be operated at room temperature due to the high critical resolved shear stress(CRSS).However,the CRSS for non-basal slip system significantly decreases as the temperature increases[37].Besides,non-basal slip system becomes easier to be activated with the decrease of grain size due to grain-boundary compatibility stress[38,39].Therefore,the combined effect of grain refinement and high temperature makes non-basal slip systems active,which contributes to the DDRX process.The crystallographic orientation relationship between parent grains and DRXed grains in R4 and R5 is shown in Fig.9b-c and e-f.Obviously,the DRXed grains show a randomly distributed orientation,which are not related to their parent grains.

Fig.8.EBSD observation results of the ZK60 Mg alloy after 30(a-c),60(d-f)and 120(g-i)forging pass:(a,d,g)inverse pole figure map,(b,e,h)corresponding boundary misorientation map and(c,f,i)distribution histogram of grain size.

From the above discussion,it can be concluded that the grain refinement in the process of MDIF was ascribed to{10–12}twins and multiple rounds of DRX(including CDRX and DDRX),and CDRX was the dominant grain refinement mechanism.Galiyev et al.[40]have compared the DRX behavior of ZK60 Mg alloy at temperatures ranging from 150 to 450 °C and strain rates between 10-5and 10-1s-1.It was found that the mechanisms of DRX depended on the operating deformation mechanisms which changed with temperature.The CDRX and DDRX were observed at intermediate temperature of 200-250 °C and high temperatures of 300-450 °C,respectively.However,in current study the CDRX is the dominant grain refinement mechanism during the whole MDIF process,even if the temperature of the forging is always higher than 350°C(Table 3).This unusual phenomenon may be related to the high strain rate(～10 s-1)of MDIF.On one hand,the high strain rate promoted the CDRX process by activating a large number of{10–12}twins.On the other hand,the high strain rate limited the nucleation of DDRX to a certain extent.

3.3.Texture evolution during MDIF process

Fig.10 shows the evolution of(0002)pole figures and corresponding inverse pole figures in the ZK60 Mg alloy during MDIF process.After the first forging pass,one strong basal texture was formed with most basal poles rotating towards FFD,and the maximal intensity of basal texture increased to 15.89(Fig.10a).According to the corresponding inverse pole figures(Fig.10h),the texture component was mainly＜0001＞‖F(xiàn)FD texture and still contains the residual〈10–10〉fiber texture.As mentioned above,the＜0001＞‖F(xiàn)FD texture was obviously related to lots of activated{10–12}twins after the first forging pass.In addition,basal slip was also nonnegligible to the formation of this texture component.Although the CRSS of non-basal slip decreased significantly at high temperature,basal slip was still the dominant mechanism in the deformation process[13,41–42].The residual〈10–10〉fiber texture could attribute to the small strain in single forging pass.After 3 forging passes,the basal texture was significantly weakened,and the maximal intensity of basaltexture reduced to 6.37 as shown in Fig.10b.Interestingly,the predominant texture component was＜10–10＞‖F(xiàn)FD texture once again,while＜0001＞‖F(xiàn)FD texture component decreased a lot(Fig.10i),which was probably related to detwinning behavior.As reported[43],the pre-existing twins would become narrower or disappear when the strain path changes.As shown in Fig.10c,d,j and k,with the continuous forging,the basal texture does not change much,and its texture component mainly consists of＜0001＞‖F(xiàn)FD and＜10-10＞‖F(xiàn)FD texture.Meanwhile,in addition to the two peaks in the FFD direction,another two peaks were gradually formed in the LFD direction,which rotated about 30°～50°away from the center of the(0002)pole figure.Previous studies showed that the equal channel angular extrusion(ECAE)process could apply the simple shear to the material at the 90°angled channel portion,leading to the basal plane with the majority of grains rearranged along the shear plane[44,45].Fig.11 shows the schematic diagram of metal flow on LFDND section.Clearly,two shear planes along the diagonal of LFD-ND plane were produced during MDIF process,which would cause the basal plane to rotate in the direction parallel to these two shear planes,forming the above two peaks on the(0002)pole figure.After 30 forging passes,with annihilation of the original microstructure(Fig.8a),the initial〈10–10〉fiber texture disappeared at the same time,replaced by a unique four-peak texture(Fig.10e).With the continuous increase of forging passes,despite that the unique four-peak texture state has been maintained,the basal texture has been weakened continuously(Fig.10f and g).As mentioned above,the DDRX process became active after 30 forging passes,in addition to the CDRX process.The correlation between the orientation of DRXed grains,especially DDRXed grains,and the parent grains diminished rapidly.Therefore,the progressively increased fraction of DRXed grains resulted in a weakening texture.Besides,the＜10-10＞‖F(xiàn)FD texture gradually transformed into＜11-20＞‖F(xiàn)FD texture as shown in Fig.10l,m and n.From Table 3,we could know that the sample temperature was always higher than 350°C during the whole MDIF process,which was very beneficial to the activation of non-basal slip.This may be the reason for the transformation of texture components.From the above,it can be concluded that the special strain path(i.e.,the continuous changes in the forging direction)is the key to the formation of the unique four-peak texture.In addition,the activation of different deformation modes has a significant influence on the texture evolution of the MDIF process.At the beginning of MDIF process,the activated{10–12}twinning led to the formation of＜0001＞//FFD texture component.With the continuous forging,especially after 30 forging passes,the basal slip played a dominant role in texture evolution.Finally,it is noteworthy that the small strain in single forging pass is crucial for the formation multiple texture components during MDIF process.

Fig.9.The details of DDRX behavior and corresponding effect on the crystallographic orientation evolution in the different regions(a-c)R4 and(d-f)R5 selected in Fig.8a and d,respectively:(a,d)inverse pole figure maps,(b,e)(0002)pole figure and(c,f)inverse pole figure.

3.4.Mechanical properties

Fig.12 displays room temperature tension and compression stress-strain curves of the as-extruded and MDIF120ed ZK60 Mg alloy with different direction,and the corresponding data for the yield strength(YS),ultimate strength(US)andfracture elongation(FE)are listed in it.For tension properties,the tensile yield strength(TYS)along the FFD decreased from 251.9MPa to 221.5MPa,while the TYS along the LFD increased from 124.5MPa to 210.6MPa.For compression properties,the compressive yield strength(CYS)along the FFD and LFD increased to 215.8MPa and 198.2MPa,respectively.

Fig.10.The evolution of(0002)pole figures and corresponding inverse pole figures in the ZK60 Mg alloy during MDIF process.

Fig.11.Schematic diagram of metal flow on LFD-ND section(the orange arrows point to the direction of metal flow).

It is well known that the main strengthening mechanisms of Mg alloys include,but are not limited to,grain strengthening,precipitation strengthening,dislocation strengthening and texture strengthening.According to the well-known Hall-Petch relationship[46],grain refinement serves to enhance yield strength.Thus,the fine and homogeneous grains of the MDIF120ed Mg alloy had an important contribution to the improvement of yield strength.Fig.13 shows the SEM images of the as-extruded and MDIF120ed alloy,respectively.It can be clearly seen that second phase particles with dimensions in the approximate range 0.5–3μm precipitate along ED in the as-extruded alloy(Fig.13a),and the EDS results exhibited that these particles are MgZn2and MgZn phase.After 120 forging passes,the size of second phase particles was reduced to less than 1μm,and the distribution of these second phase particles was more random(Fig.13b).Regrettably,the number of second phase particles was very limited.Therefore,the effect of precipitation strengthening on the yield strength of the MDIF120ed ZK60 alloy was almost negligible.The kernel average misorientation(KAM)map was usually used to reflect an orientation gradient and the residual dislocation density distribution in the grains,i.e.,the higher KAM value indicated the higher dislocation density[47].As shown in Fig.14,the average misorientation values in as-extruded and MDIF120ed alloys were 0.84° and 0.93°,respectively,indicating the density of residual dislocations in as-extruded alloy was higher than that of MDIF120ed alloy.Obviously,dislocation strengthening had a certain contribution to the improvement of yield strength for MDIF120ed alloy.

Fig.12.Room temperature tension and compression stress-strain curves of the ZK60 Mg alloy:(a)as-extruded;(b)MDIF120ed(Ten:tension test;Com:compression test;YS:yield strength;US:ultimate strength;FE:fracture elongation).

Fig.13.SEM images of the ZK60 Mg alloy:(a)as-extruded,(b)MDIF120.

However,the TYS along the FFD decreased significantly,which was mainly related to the textural softening after the MDIF process.It is well known that basal slip and{10–12}twinning are the dominant deformation mechanism at room temperature.Thus,referring to the researches of Koike et al.[48]the average Schmid factor of basal slip(mbas.)and{10–12}twinning(mtwin)in tension and compression along ED/FFD and TD/LFD were calculated from inverse pole figures for as-extruded and MDIF120ed ZK60 Mg alloy as shown in Fig.15.The Schmid factor of basal slip and{10–12}twinning for tension along FFD respectively increase from 0.115 and 0.006 to 0.209 and 0.156,which means that the basal slip and{10–12}twinning are easier to be activated when tension along FFD.And it seems that the effect of texture softening overcomes the grain and dislocation strengthening,so the TYS along FFD is significantly reduced.Besides,it is worth pointing out that the difference in the yield strength of MDIF120ed alloy decreased significantly,which was closely related to the unique four-peak texture obtained after 120 forging passes.Previous study[41]had shown that the alloy usually exhibited high yield strength(or low yield strength)when the initial deformation was dominated by crystallographic slips with high CRSS(or{10–12}twinning with low CRSS).Thus,the difference of{10–12}twinning activation at the initial deformation of the as-extruded alloy was the main reason for the tension/compression asymmetry and anisotropy.Compared with the as-extruded alloy,the difference ofmtwinwas obviously reduced when the MDIF120ed alloy was stretched and compressed in different directions,which made the difference in yield strength mainly caused by{10–12}twinning also decrease.

Fig.14.The KAM maps of the ZK60 Mg alloy:(a)as-extruded,(b)MDIF120.

Fig.15.Schmid factors of basal slip(mbas.)and{10–12}twinning(mtwin)in tension and compression along ED/FFD and TD/LFD calculated from inverse pole figures for as-extruded and MDIF120ed ZK60 Mg alloy.

Fig.16 shows the comparison of yield strengths between the present study and other previous literatures[3–13].It is clear that the wrought Mg alloy processed by the traditional extrusion or rolling process often shows severe yield strength asymmetry due to the formation of strong basal texture.As shown in Fig.16a,the tension/compression yield strength symmetry of Mg alloy rods was improved by CEC and ECAP process compared to traditional extrusion process.Zhang et al.[12]reported that the tension yield strength anisotropy of ZK60 Mg alloy sheets was improved by extrusion and multi-pass rolling with lowered temperature(Fig.16b).For ease of presentation,ED/FFD are collectively called LD,and TD/LFD are collectively called TD in the present study.At the same time,the tension/compression yield strength asymmetry was quantified by the ratio of the tension/compression yield strength along the LD(i.e.,TYS-ED/CYS-EDand TYS-FFD/CYS-FFD).Similarly,the ratio of the tension yield strength along LD and TD(i.e.,TYS-ED/TYS-TDand TYS-FFD/TYS-LFD)was used to quantify the tension yield strength anisotropy.According to the experimental results,the above four ratios are respectively 1.86 and 2.02 in the initial material as well as 1.02 and 1.05 in the MDIF120ed ZK60 Mg alloy,i.e.,the tension/compression yield strength symmetry and the tension yield strength isotropy are significantly improved through MDIF process,which is beneficial for the application of Mg alloys under complex loads.

Fig.16.Comparison of yield strengths between the present study and other previous literature[3-13]:(a)TYS vs.CYS;(b)TYS-LD vs.TYS-TD.

4.Conclusions

The present study confirms that it is feasible to regulate the microstructure and improve yield strength symmetry in the as-extruded ZK60 Mg alloy by multi-directional impact forging(MDIF).The forged piece with average grain size of 5.3μm,unique four-peak texture and improved yield strength symmetry were obtained after MDIF process.The following conclusions can be made:

(1)A great amount of{10–12}twins were activated at the beginning of MDIF process.The selection mechanism of various twin variants was governed by the Schmid law.Besides,these activated twins play an important role in grain refinement.On one hand,the{10–12}twins segmented the initial coarse grain.On the other hand,the{10–12}twins facilitated the subsequent DRX process.

(2)Multiple rounds of DRX existed in grain refinement with dominant mechanism of CDRX during the whole MDIF process and DDRX turning active after 30 forging passes.The CDRXed grains showed a clearly preferred orientation related to the parent grains,while the orientations of DDRXed grains were completely different from their parent grains.

(3)The special strain path is the key to the formation of the unique four-peak texture.The{10–12}twinning and basal slip were two dominant factors to the evolution of texture during MDIF process.At the beginning of MDIF process,the activated{10–12}twinning led to the formation of＜0001＞//FFD texture component.With forging proceeding,the amount of{10–12}twinning decreased and the basal slip played a dominant role in the texture evolution.After 30 passes,the initial〈10–10〉fiber texture was completely replaced by a unique four-peak texture.With the forging passes increase to 120,the basal texture has been weakened continuously due to DRX process.

(4)Through 120 passes MDIF,the yield strength of alloy had been significantly improved under the combined effects of grain strengthening and dislocation strengthening,except for the TYS along FFD.Besides,the symmetry of yield strength was greatly improved by MDIF process.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by National Natural Science Foundation of China(Grant No.51975146),Key Research and Development Plan in Shandong Province(Grant No.2018JMRH0412,2019JZZY010364),National Defense Basic Scientific Research of China(Grant no.JCK2018603C017)