Reducing the yield asymmetry in Mg-5Li-3Al-2Zn alloy by hot-extrusion and multi-pass rolling

Xioqing Li,Ling Ren,Qichi Le,?,Lei Bo,Peipeng Jin,Ping Wng,Chunlong Cheng,Xiong Zhou,Chenglu Hu

a Key Laboratory of Electromagnetic Processing of Materials,Ministry of Education,Northeastern University,Shenyang 110819,PR China

b Qinghai Provincial Key Laboratory of New Light Alloys,Qinghai Provincial Engineering Research Center of High Performance Light Metal Alloys and Forming,Qinghai University,Xining 810016,PR China

Abstract Reducing the yield asymmetry is very important concern for wrought Mg-Li alloys.In this study,Mg-5Li-3Al-2Zn(LAZ532)alloy was successfully produced by hot-extrusion followed by multi-pass rolling at 573K.Microstructure evolution,mechanical properties and yield asymmetry reducing of LAZ532 alloys at different rolling passes were studied.By observing microstructure using transmission electron microscopy showed that a small amount of ultra-fine AlMg4Zn11 and nano Li3Al2 phases existed in the alloy.With the increasing of rolling passes,the grains of the alloys were obviously refined,and comprehensive mechanical properties were dramatically improved.Meanwhile,it also showed an excellent tension and compression yield symmetry(TYS/CYS was about 1).The results showed that the combined action of the weak{0001}basal lamellar texture,grain refinement and addition of Li element could effectively improve the yield symmetry.Furthermore,based on theoretical analysis,the yield strength in the alloys mainly depended on the strengthening contributions of LAGBs and HAGBs,and strengthening effect of HAGBs most(～50%)to the yield strength improvement.? 2020 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Mg-Li alloy;Tension and compression yield asymmetry;Dynamically recrystallized;Texture evolution.

1.Introduction

Magnesium-lithium(Mg-Li)alloys are the lightest metal structure material to date and have been the subject of much study in the past,because of its potential application value in aerospace,automotive,electronics and other fields[4,5].At present,there are more and more basic researches related to it,and the mechanical properties of Mg-Li alloys are usually improved by adding alloying elements(Al,Zn and rare earth elements etc.)[6–8].

Up to now,the researches of reducing tension and compression yield asymmetry have been concentrated in AZ31,Mg-Zn-Ca and ZK60 Mg alloys[9–11].However,it is rarely reported in Mg-Li alloys.Generally speaking,the c/a ratio of Mg lattice is 1.624,which is easy to cause serious tension and compression yield asymmetry of Mg alloys because of forming strong{0001}basal texture after severe plastic deformation(SPD)[12].Nevertheless,the addition of Li element can not only effectively reduce the density of Mg alloys to 1.3～1.65g/cm3,but also change the phase structure[13].According to the Mg-Li binary phase diagram[14],when the Li content is less than 5.7wt%,there is only a singleα-Mg phase in the alloys;When the Li content is between 5.7wt%and 10.3wt%,α-Mg phase andβ-Li phase coexist in the alloys;When the Li content is higher than 10.3wt%,there is onlyβ-Li phase in the alloys.Especially,when the Li content is lower than 5.7wt%,the c/a ratio of Mg lattice can be effectively reduced,thus improving the symmetry of Mg lattice[14].Li et al.[15]showed that the axial ratio c/a of AZ31 Mg alloy was as low as 1.608 when the content of Li was 5wt%.It is often considered that the alloy with a lower c/a value contributes to a higher tension and compression yield symmetry,and as a result,the variation of c/a will vary the yield asymmetry.For instance,the extruded Mg-3Li and Mg-3Li-2Zn alloys prepared by Dong et al.[16]showed excellent tension and compression yield symmetry,and the TYS/CYS ratio was close to 1～1.1.However,in our previous studies,it was first time found that there was also serious the tension and compression yield asymmetry in extruded LAZ532 alloy because of existence of a strong{0001}basal fiber texture(<0001>⊥ED),and the TYS/CYS ratio was as high as 1.46[17].Moreover,at present,there are few reports on reducing the yield asymmetry of Mg-Li alloy.This remains to be further studied.

In conventional Mg alloys,relevant studies have showed that the weak{0001}basal texture,precipitates and grain refinement could reduce the yield asymmetry of tension and compression by hindering the occurrence of twins and basal slips[10,18,19].For instance,Wang et al.[20]reported that in ZK60 alloy,ultra-fine grains not only inhibited the occurrence of{102}<101>extension twinning,but also weakened the{0001}basal texture,thus reducing the yield asymmetry of tension and compression.In addition,Jain et al.[21]found that the second phase at grain boundaries in Mg–8Al–0.5Zn alloy could also reduce the yield asymmetry by inhibiting twin’s expansion.In conventional deformation processes,grain refinement,precipitation of second phase and texture weakening were achieved by SPD,addition of alloying elements and heat treatment[22–24].Therefore,in this study,the extruded LAZ532 alloy was deformed by secondary rolling.It is hoped that reducing the yield asymmetry of tension and compression by further refining the grains and weakening the{0001}basal texture,and improving mechanical properties.Meanwhile,microstructure evolution and cause of yield asymmetry reducing of the alloys at different rolling passes were studied,and the strengthening mechanism of dislocation and grain boundary for the yield strength of the alloys was also analysed.

2.Experiment method

The initial materials used in the work were commercial pure Mg ingots(99.9%),pure Li strips(99.9%),pure zinc ingots(99.9%)and pure Al ingots(99.9%).The extruded Mg-5Li-3Al-2Zn(LAZ532)alloy(?12 mm)was produced by casting followed by hot-extrusion at 573K,and the process was specifically described in literature[17].Finally,extruded squares rods were kept for 30 min at 573K,then rolled at different passes at 573K,and the rolled plates with a thickness of 3mm were obtained.The rolled plates were put into the resistance furnace and kept warm for 5～10 min at 573K,and then quenched by water to release internal stress.

Mechanical properties of the alloys were measured using an Instron-5982 testing machine,and both stretching and compressive rates were 0.2 mm·min?1.Loads were along the rolled direction.In order to ensure the accuracy of mechanical properties,the mechanical properties of samples of the alloy with different rolling passes were tested repeatedly.The phase composition and macro-texture were analysed by XD8ADVANCE-A25X-ray diffraction(XRD).The microstructure was observed by transmission electron microscopy(TEM,JEM-2100F)and scanning electron microscopy(SEM,ZEISS-6035 field-emission).Electron backscattered diffraction(EBSD)was performed on the characteristic region by SEM.The data was analysed by using an Oxford HKL Channel 5 software.

3.Results and discussion

3.1.Microstructure of the extruded LAZ532 alloy

Fig.1 shows that SEM image and XRD pattern of the extruded LAZ532 alloy(before rolling).As shown in Fig.1(a)and(b),a bit of white precipitates were diffusely distributed in the Mg matrix,and no corresponding diffraction peaks of the precipitates were observed,exceptα-Mg,due to the low content of the precipitates.To further calibrate the precipitates,the TEM images and corresponding selected area electron diffraction(SEAD)maps of the precipitates are shown in Fig.2.As can be from Fig.2(a)and(b),these fine white precipitates(～1μm)proved as AlMg4Zn11phase.Besides,as shown in Fig.2(c),the few finer precipitates(～100nm)could be also observed,which proved as Li3Al2phase.The highlighted region in Fig.2(d)is a dark field image of(0)plane of Li3Al2phase.

3.2.Mechanical properties of the rolled LAZ532 alloy

Fig.3 shows true stress-strain curves of LAZ532 alloys at different rolling passes and corresponding statistical chart of mechanical properties.The mechanical properties of the alloys are given in Table 1.As can be seen from Fig.3(a),(b)and(c),with the increasing of rolling passes,comprehensive mechanical properties of the alloys were improved markedly,that is,the TYS and UTS increased from 122.5±2.9MPa and 235.1±3.3MPa to 153.2±2.1MPa and 269.9±3.0MPa,respectively,and the CYS and UCS increased from 116.3±3.7MPa and 365.4±3.0MPa to 139.3±1.7MPa and 387.1±5.2MPa,respectively.As shown in Fig.3(d),the values of TYS/CYS were basically remained near 1,and the values of UTS/UCS increased slightly to 0.7 from 0.64.Compared with the serious tension and compression yield asymmetry(TYS/CYS=1.46)and low mechanical properties of extruded LAZ532 alloy in previous studies[17],the tension and compression yield symmetry and mechanical properties of the alloy were significantly improved via hot-extrusion followed by multi-pass rolling in this study.

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Fig.1.SEM image(a)and XRD map(b)of the extruded LAZ532 alloy.

Fig.2.TEM image of the extruded LAZ532 alloy:(a)bright field image of the precipitated phase;(b)corresponding SAED of the highlighted region(red circle)in Fig.2(a);(c)bright field image of the precipitated phase and corresponding SAED of the highlighted region(red circle)and(d)dark field image of(0)plane of the Li3Al2 phase.(For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

Table 1Mechanical properties of LAZ532 alloys at different rolling passes.

Fig.3.Mechanical properties of LAZ532 alloys at different rolling passes:(a)tensile properties;(b)compressive properties;(c)statistical chart of mechanical properties and(d)stress ratio.

3.3.The macro-texture evolution of the rolled LAZ532 alloy

Fig.4 shows macro-texture of LAZ532 alloys at different rolling passes.RD,TD and ND are the rolling direction,transverse direction and normal direction,respectively.As can be seen from Fig.4(a),(b)and(c),the pole density points of all(0001)and(100)pole figures are distributed along the center and edge,respectively.That is,the(0001)plane of the grains is approximately perpendicular to the ND.Furthermore,with the increasing of rolling passes,the maximum pole density values of(0001)pole figures decreased slightly to 3.91 from 4.94.The above results manifest that the rolled Mg-5Li-3Al-2Zn alloys have a weak{0001}basal lamellar texture(<0001>//ND).

3.4.EBSD analysis of the rolled LAZ532 alloy

Fig.5 shows band contrast images and grain size distribution maps of LAZ532 alloys at different rolling passes.The grain boundaries are very clear in all band contrast images.As shown in Fig.5(a)–(d),both coarse grains and fine grains coexist in the alloys rolled 3 and 6 passes,and the average grain sizes are about 10.3μm and 10.1μm,respectively.As can be seen from Fig.5(e)and(f),with the increasing of deformation level,the grains in the alloy rolled 9 passes are further refined,and the average grain size is about 4.5μm.This is mainly due to that under the same rolling temperature,large deformation can increase lattice distortion energy stored in the material and form large amount of deformed grains,which makes the nucleation points of recrystallization increasing;meanwhile,high deformed temperature causes low angle grain boundaries(LAGBs)to transform rapidly to high angle grain boundaries(HAGBs)in a relatively short time,thus forming fine recrystallized grains[25].

Fig.6 shows EBSD maps,misorientation angle distribution maps and inverse pole figures of LAZ532 alloys at different rolling passes.As shown in Fig.6(a),(d)and(g),the grain orientation of the alloys is greatly obvious,and the colors of orientation distribution maps are mainly red.This indicates that the orientation of the grains is mainly in the direction of[0001].It is also consistent with the information of inverse pole figures in Fig.6(c),(f)and(i).As shown in Fig.6(b),(e)and(h),misorientation angle distribution of the alloys are bell-shaped and focus on the HAGBs.This indicates that dynamically recrystallized(DRXed)is main microstructure in all the alloys,because the HAGBs through grain boundary bulging can usually become the potential nucleation sites of DRXed[26].In addition,when the rolling passes increases to 9 from 3,the proportion of HAGBs decreases gradually,meanwhile,the proportion of LAGBs increases slightly,indicating a slight decrease in the content of DRXed.

In order to analyze the evolution of DRXed microstructure.As can be seen from Fig.7(a)～(c),the DRXed microstructure accounts for a large proportion,and some undynamically recrystallized(un-DRXed)grains are distributed piecemeal in the matrix.Based on the analysis of Mg-Li binary phase diagram,the recrystallization temperature of Mg-Li alloy is lower,because the melting point of Mg-Li alloy gradually decreases with the increasing of Li content[27].Therefore,in this study,there are high volume fraction of DRXed in all the rolled LAZ532 alloys.Moreover,related studies have also showed that temperature and strain are main factors affecting recrystallization[28,29].However,strain is the dominant factor affecting recrystallization process because the rolling temperature is constant in this study,and the more serious the grains breakage and the easier the sub-grains formation with the increasing of deformation level.Therefore,as shown in Fig.7(d)～(f),when the rolling passes increases to 9 from 3,the proportions of DRXed decrease to 72%,and the proportions of un-DRXed increase to 27%.This indicates that the level of the original grain breakage is more serious with the increasing of deformation,because the rate of grain boundary migration and the ability of recrystallization nucleation are the same under the same rolling temperature[30].Besides,it can be observed from Fig.7(g)～(l)that the texture types for DRXed and un-DRXed are the{0001}basal lamellar texture(<0001>//ND).With the increasing of rolling passes,the recrystallization texture intensity increases slightly due to recrystallization microstructure can weaken the basal texture[30],meanwhile,the non-recrystallization texture intensity gradually weakens.

Fig.4.Macro-texture of LAZ532 alloys at different rolling passes:(a)rolled 3 passes;(b)rolled 6 passes and(c)rolled 9 passes.

In order to further intuitively observe the content of the{0001}basal lamellar texture,EBSD maps show grain orientation(<0001>//ND)distribution of LAZ532 alloys at different rolling passes,as shown in Fig.8.The colored grains in Fig.8 indicate that their〈0001〉are approximately parallel to the ND,and the color from blue to red indicates that the deviation angle of between the〈0001〉of grains and the ND gradually increases.The deviation angle was less than 20 ° in this study.As can be seen from Fig.8(a)～(f),The number of colored grains accounts for about 1/3 of the total number of grains in all rolled LAZ532 alloys,and the deviation angles of most grains are distributed in the range of 10°～20° This further proves that the alloys have a weak{0001}basal lamellar texture.It is beneficial to improve the tension and compression yield symmetry of the alloys[20].Furthermore,with the increasing of rolling passes,there are no obvious change for the content of{0001}basal lamellar texture,which is consistent with the results in Figs.4 and 6.

Fig.9 shows the local misorientation maps and corresponding local misorientation angle distribution maps of LAZ532 alloys at different rolling passes.Any local misorientation angle calculated less than 5° were included in this study.Local misorientation was measured by EBSD to study the geometrically necessary dislocations(GND)evolution[31,32].Therefore,to quantitatively study the dislocation density evolution of LAZ532 alloys at different rolling passes,a simple method according to the strain gradient theory was used to calculate GND density[33,34]:

whereρGNDis the GND density at the point of interest;Δθirepresents the local misorientation;uis the unit length of the point(400nm);b is the Burgers vector(Mg=3.21×10?10m).The calculated mean GND density of LAZ532 alloys at different rolling passes is shown in Fig.10,with the increasing of rolling passes,the mean GND density gradually increases from 9.4×1013m?2to 1.2×1014m?2.That means,there is a strong connection between the deformation of the material and the dislocation density.

Fig.5.Band contrast images and grain size distribution maps of LAZ532 alloys at different rolling passes:(a,b)rolled 3 passes;(c,d)rolled 6 passes and(e,f)rolled 9 passes.

3.5.Discussion

The studies showed that twin,second phase,grain size and texture type are the main factors affecting the yield asymmetry[10,18,19].In this study,all of rolled LAZ532 alloys have excellent yield symmetry of tension and compression.Based on the analysis of experimental results(Fig.3(d)),the following three factors can be considered to contribute to the improvement of tension and compression yield symmetry:(1)refinement of grain;(2)DRXed microstructure and(3)addition of Li element.According to the results in Fig.5,all of the rolled LAZ532 alloys have finer grain microstructure than that extruded LAZ532 alloy in previous study[17].It was reported that twinning activation is more difficult than dislocation slip in fine-grained Mg alloys due to the fact that with decreasing of grain size,the increase rate of critical resolved shear stress(CRSS)for twinning was much higher[35].Therefore,in this study,the twin is difficult to be activated in fine grains during the deformation before yielding due to the large constraint of the fine grains by adjacent grains.This can reduce the yield asymmetry to some extent.Generally,the Mg alloys have a strong{0001}basal texture after SPD,and the deformation is dominated by(100)prismatic slip in the process of axial stretching,while the deformation is dominated by(0001)basal slip in the process of axial compression[11].According to the results in Figs.4,6 and 8,all of rolled LAZ532 alloys have weak{0001}basal lamellar texture due to the existence of plenty DRXed[20].The weak{0001}basal lamellar texture can weaken the(100)prismatic slip in the axial stretching process and the(0001)basal slip in the axial compression process[11],thus reducing the yield asymmetry of tension and compression.As shown in Fig.11,there are large intersection zones between SF of basal slip and SF of prismatic slip when tensile and compressive deformation along the RD.This also proves the above inference.Furthermore,it is well known that the c/a ratio affects the CRSS of basal slip and non-basal slip in Mg alloys[15].In general,the c/a ratio in Mg is 1.624,which leads to low CRSS of basal plane favoring basal slip[12,15].The addition of Li can effectively reduce the c/a ratio of Mg alloy and improve the symmetry of Mg lattice.As the c/a ratio decreases,the slip of〈a〉dislocations on the prismatic plane of Mg alloys can be alleviated[36,37].It contributes to further reduce the yield asymmetry of tension and compression.Therefore,in this study,TYS/CYS ratios of all of rolled LAZ532 alloys were nearly 1.Besides,as shown in Fig.3(d),with the increasing of rolling passes,the values of TYS/CYS increased slightly from 1.05 to 1.13.In the case of the same Li content,the DRXed fraction and grain size are the main influencing factors.The smaller the grain size is,the more obvious the inhibition on twin is,and it is contributing to reduce the yield asymmetry of tension and compression[20].On the contrary,the decreasing of DRXed fraction can make the yield asymmetry of tension and compression more severe[20].Thereby,the decreasing of DRXed fraction is the major reason that the values of TYS/CYS increase slightly when the rolling passes increase.

Fig.6.EBSD maps,misorientation angle distribution maps and inverse pole figures of LAZ532 alloys at different rolling passes:(a,b,c)rolled 3 passes;(d,e,f)rolled 6 passes and(g,h,i)rolled 9 passes.

Fig.7.maps showing grain morphology,frequencies of the microstructures and inverse pole figures of LAZ532 alloys at different rolling passes:(a,d,g,j)rolled 3 passes;(b,e,h,k)rolled 6 passes and(c,f,i,l)rolled 9 passes.

Based on the above analysis,an excellent tension and compression yield symmetry of the rolled LAZ532 alloys has been clarified.The change in yield strength of metal material is affected by grain refinement,texture change and precipitates.In this study,the effects of texture strengthening and precipitates strengthening can be ignored because of the inconspicuous change in texture and the low content of the precipitates.In addition,the effect of twins can also be ignored,because previous studies have showed that twins were not observed in the deformation stage before the tensile and compressive yielding[17].Therefore,in this study,the modified Hall-Petch relationships was used to calculate the change in yield strength of metal materials,which considers dislocation strengthening due to the presence of LAGBs and grain boundary strengthening due to medium to high angle grain boundaries,the equation as following[38,39]:

Fig.8.Grain orientation(<0001>//ND)and orientation deviation angle distribution maps of LAZ532 alloys at different rolling passes:(a,b)rolled 3 passes;(c,d)rolled 6 passes and(e,f)rolled 9 passes.

WhereσYSis yield strength,σ0is the friction stress for a dislocation glide on a slip plane,σLAGBsandσHAGBsare the strengthening contributions of LAGBs and HAGBs,respectively.The strengthening effect of LAGBs can be correlated with the dislocation density as following[40]:

where M is the average Taylor factor(i.e.,～2.5)[41],a is a constant(=0.2),G is the shear modulus(16.6GPa),b is the Burgers vector of the gliding dislocations(3.2×10?10m),ρis the dislocation density,fis the number fraction of HAGBs,dis average grain size,is average misorientation angle of LAGBs.

In addition,the strengthening contribution of the HAGBs may thus be expressed by a Hall-Petch equation[38]:

Fig.9.Local misorientation maps and local misorientation angle distribution maps of LAZ532 alloys at different rolling passes:(a,b)rolled 3 passes;(c,d)rolled 6 passes and(e,f)rolled 9 passes.

Therefore,Modified Hall-Petch relationships can be given as:

By substituting parameters as listed in Table 2,strengthening contributions of LAGBs and HAGBs can be calculated,respectively.

According to the Eq.(1.5),the expression of yield strength of LAZ532 alloys at different rolling passes can be obtained:

The least-square fitting results of Eq.(1.6)are shown in Fig.12,the calculated values ofkTen.andkCom.are 176.7 MPa·m1/2and 120.0 MPa·m1/2during stretching and compression deformation,respectively.Besides,the calculated strengthening contributions of LAGBs and HAGBs are also given in Tables 3 and 4.In this study,for the rolled LAZ532 alloys,the calculatedkTen.is far greater thankCom..This also further illustrates that thekvalue in Mg-Li alloy with HCP structure is closely related to the texture types.The influence of texture onkis often attributed to the variation of the predominant deformation mode with texture[42].It is considered that the mode with a higher CRSS contributes to a higherk[43],and as a result,the variation of deformation mode will vary thekvalue.For Mg alloys,CRSS of prismatic slip is much larger than that of basal slip.Therefore,in this study,because of the existence of(0001)basal texture,which resulted inkTen.>kCom..In addition,as shown in Tables 3 and 4,the strengthening contribution of HAGBs is about twice that of LAGBs when stretching along the RD,however,the strengthening effect of HAGBs is slightly higher than that of LAGBs when compressive along the RD.Related research showed that the strengthening essence of LAGBs is dislocation strengthening,and the force required to overcome dislocation movement is equal in the process of tension and compression deformation[39].However,the strengthening essence of HAGBs is grain boundary strengthening[39],which is related to the hardness or softness of grain orientation.It is considered that the grains with a harder orientation contribute to a higher strengthening effect.Therefore,in this study,due to the existence of(0001)basal texture in the recrystallization microstructure(Fig.7),the strengthening effect of HAGBs during deformation before tensile yielding is much higher than that during deformation before compressive yielding.

Table 2Structural parameters and yield strength of LAZ532 alloys at different rolling passes.

Table 3Strengthening contributions of LAGBs and HAGBs during tensile deformation.

Fig.10.The calculated mean GND density of LAZ532 alloys at different rolling passes.

Fig.11.Schmid factor(SF)distribution maps of LAZ532 alloy at different rolled 9 passes:(a)tensile deformation along the RD and(b)compressive deformation along the RD.

Fig.12.The least-square fitting betweenσYS?σLAGBs and(d/f)?1/2.

4.Conclusions

Based on the experimental results of this work,the following conclusions were drawn:

(1)In the LAZ532 alloys,the ultrafine AlMg4Zn11phase and nano Li3Al2phase were found.

(2)In the rolled LAZ532 alloys,DRXed microstructure,grain refinement and addition of Li element could effectively improve the tension and compression yield symmetry,and TYS/CYS ratios of the alloys were nearly 1.

(3)9 passes rolled LAZ532 alloy had excellent comprehensive mechanical properties,and tensile and compressive yield strengths were 153.0±2.1MPa and 139.3±1.7MPa respectively.The high yield strength of the alloy was mainly attributed to the dislocation strengthening and grain boundary strengthening.

(4)In the rolled LAZ532 alloys,the existence of(0001)basal texture not only led tokTen.>kCom.,but also resulted in the high strengthening contribution of HAGBs during deformation before tensile yielding.

Acknowledgments

The work was financially supported by the National Key Research and Development Program of China(No.2016YFB0301104),National Natural Science Foundation of China(No.51771043)and Programme of Introducing Talents of Discipline Innovation to Universities 2.0(the 111 Project 2.0 of China,No.BP0719037).