Jingrn Li ,Aiyu Zhng ,Huhng Pn,* ,Yuping Rn ,Zhuorn Zng ,Qiuyn Hung ,Chnglin Yng,Ling M,Gowu Qin,*

aKey Laboratory for Anisotropy and Texture of Materials (Ministry of Education),School of Materials Science and Engineering,Northeastern University,Shenyang 110819,China

b State Key Laboratory of Rolling and Automation,Northeastern University,Shenyang,110819,China

c Department of Materials Science and Engineering,Monash University,Vic.3800,Australia

d Institute of Metal Research,Chinese Academy of Sciences,Shenyang 110016,China

e State Key Laboratory of Solidificatio Processing,Northwestern Polytechnical University,Xi’an,Shaanxi 710072,China

fShanxi Provincial Key Laboratory of Metallurgical Equipment Design and Technology,Taiyuan University of Science and Technology,Taiyuan 030024,China

Abstract This work reported the effect of extrusion speeds on the microstructures and mechanical properties of Mg-Ca binary alloy.The results showed that yield strength of the as-extruded Mg-1.2wt.% Ca alloys decrease from～360MPa to～258MPa as the ram speed increases from 0.4mm/s to 2.4mm/s,and the elongation increases from～3.9% to～12.2%.The microstructure changes from bimodal grain feature to the complete dynamical recrystallization (DRX) with increase of the extrusion speed.The ultrafin DRXed grains in size of～0.85μm,the numerous nano-Mg2Ca particles dispersing along the grain boundaries and interiors,as well as the high density of residual dislocations,should account for the high strength.It is believed that the high degree of dynamic recrystallization and the resulting texture randomization play the critical roles in the ductility enhancement of the high-speed extruded Mg alloys.

Keywords: Extrusion speed;Microstructure;Mechanical properties;Mg-Ca alloy.

1.Introduction

As the lightest metallic alloy,Mg alloys have drawn great interests in the past decades,especially in the field of automotive and aerospace industries [1-5].However,the widespread applications of Mg alloys are restricted by their poor workability,which is caused by the few independent slip systems at room temperature [6,7].Alloying and the severe plastic deformation(SPD)process are effective methods to increase the mechanical properties [8-19].Rare earth elements(including Y,Sc,Nd,Er,Ce,Sm and Gd) have been reported to improve both ductility and absolute strength of Mg alloys[20-22].Zhang et al.[23] found that the elongation to failure (EL) of ZK60-4Sm (wt.%) alloy is increased by 100%compared with that of ZK60 alloy,reaching～28.1%.

Fig.1.(a) The tensile engineering stress-strain curves of the as-extruded Mg-Ca alloys.(b,c,d) The OM image of the as-extruded X12-0 alloy,X12-1 alloy,X12-2 alloy,respectively.The extruded direction and the radial direction are denoted as ED and RD,respectively.

Recently,it is becoming a new trend to develop of the low-cost Mg alloys for wider industrial applications.For example,Kang et al.[16] reported a low-alloyed Mg-1.38Zn-0.17Y-0.12Ca (at.%) alloy,which was extruded at 190°C with ram speed of 0.1mm/s-1,showing the ultimate tensile strength (UTS) of～357MPa,tensile yield strength (YS)of～317MPa and EL of～6.4%.Zhou et al.[24] reported that with only 3% yttrium added,a high EL of～33% was obtained in Mg-Y alloy.In view of the similarity to the RE elements,the cheap price and low density,Mg-Ca alloys have received extensive attentions [25-28].For example,Pan et al.[18] reported a Mg-Ca alloy which exhibits high YS of～377MPa and UTS of～392MPa.As the extrusion temperature increases from 230 °C to 250 °C,the EL has enhanced by six times (13.2%).Chai et al.[29] also reported that the UTS of Mg-1.0Sn-0.5Zn alloy increases from～261MPa to～300MPa as the Ca added from 0 to 2wt.%.However,there are few researches on effect of extrusion speed on microstructure and mechanical properties of the binary Mg-Ca alloys,which will be the focus of present study.

2.Materials and methods

Mg-1.2wt.% Ca was prepared by melting pure Mg(99.99wt.%) and pure Ca (99.90wt.%) in the electronic resistance furnace,under the protection of mixture gas of CO2and SF6(100:1).Then the as-cast ingots were homogenized at 500 °C for 24h.The billets were indirectly extruded at 350 °C with extrusion ratio of 20 and ram speed of 0.4mm/s,1.0mm/s,2.4mm/s (named X12-0,X12-1 and X12-2,respectively).The mechanical properties of the as-extruded bars were measured at a strain rate of 10-3s-1on universal Material Testing Machine (Shimazu AG-X Plus) at room temperature,with the dog bone specimens in gage length of 25mm and the diameter of 5mm.The tensile yield strength,ultimate tensile strength and elongation to fracture are the average values of at least three individually repeated tests.Themicrostructures of the alloys were characterized via optical microscopy (OM),electron backscattered diffraction (EBSD,MAIA3 model 2016) and transmission electron microscopy(TEM,JEOL-2100F,operated at 200kV).The volume fraction of un-DRXed region was measured by image J software for the OM images and channel 5 for EBSD images.

3.Results and discussion

Fig.1 shows the tensile engineering stress-strain curves of present Mg-Ca alloys,which were extruded at different ram speeds.The mechanical properties are summarized in Table 1.As compared with the X12-1 and X12-2 alloys,the low-speed extruded X12-0 alloy exhibits higher engineering stress,with YS of～360MPa,UTS of～370MPa and the EL of～3.9%.With the increasing of ram speed,the UTS decreases from～370MPa to～266MPa and the EL increases from～3.9% to～12.2%.The YS,UTS and EL of X12-1 alloy are measured to be～326MPa,～319MPa and～8.3%,respectively.Fig.1b-d display the OM images of the asextruded X12-0,X12-1,X12-2 alloys.The un-recrystallized regions can be found in the OM images of X12-0 and X12-1 alloys (as marked by the red arrows in Fig.1b,c) and the volume fraction of unDRXed regions are estimated to be～64.5% and～12.1%,respectively.With the increase of ram speed,the complete recrystallization microstructure can be obtained in the X12-2 alloy,and the average grain size has grown to be～2.95μm.

Table 1 Mechanical properties of as-extruded Mg-Ca binary alloys.

Fig.2.(a) The band contrast (BC) map of X12-0 alloy,(b) the BC and grain boundaries(GB) map,the black line indicates the high angle grain boundaries(HAGB) and the red line represents the low angle grain boundaries (LAGB),(c) the inverse pole figur (IPF) of X12-0 alloy.(d,e,f) The BC,BC+GB and IPF images of X12-1 alloy.(g) The pole figur (PF) of X12-0 alloy and (h) the PF image of X12-1 alloy.(For interpretation of the references to colour in this figur legend,the reader is referred to the web version of this article.)

The microstructures of the alloys extruded at ram speed of 0.41mm/s and 1.09mm/s were investigated,and the results are presented in Fig.2.For the X12-0 alloy,it can be seen the un-recrystallized regions are separated by a number of low angle grain boundaries (LAGB,as indicated by the red lines in Fig.2b,c).The number fraction of un-DRXed region is calculated to be～19.6%,which indicates the low degree of DRX.The un-DRXed grains usually exhibit a typical fibe texture [30],with<10-10>//ED,which is exactly the case in present Mg-Ca alloys.Consequently,a strong texture with a～11.51 multiple random distribution (mrd.) is obtained in the X12-0 alloy,due to the low degree of DRX(Fig.2g).By the way,according to the band contrast image in Fig.2a,the average grain size is estimated to be～0.85μm.As compared with the X12-0 alloy,the X12-1 alloy displays higher degree of DRX,as shown in Fig.2D-f.The almost completely DRXed microstructure can be found in X12-1 alloy (Fig.2e),and a much weaker texture of～5.17 mrd.,than that in X12-0 alloy can be detected due to the high degree of recrystallization,as illustrated by the pole figur in Fig.2h.The deviation of recrystallization volume fraction between the OM and EBSD results was ascribe to the EBSD image area is too small.In this sense,the DRXed grains have also apparently grown due to the DRX and the average grain size can be estimated to be～1.58μm (Fig.2d).

Figs.3 and 4 show the typical bright-fiel TEM images of as-extruded X12-0 alloy.The DRXed grains with the average grain size of 300～500nm are observed (Fig.3a),and profuse dislocation accumulate in front of the bulk Mg2Ca phase(Fig.3b),which is exactly the same case reported in Ref.[31,32].It suggests that the DRXed grains prefer to nucleate at the second-phase boundariesviathe well-known particle simulation nucleation (PSN) mechanism and the particles can also retard the grain growth to some extent [33].By the way,the nano-phases are also seen to densely distribute in the Mg matrix in Fig.3c,and d is the higher-magnificatio image.The number density of the nano-particles is high,and the average size is estimated to be～25nm.Besides,the second phases can be determined to be Mg2Ca,according to phase diagram of the Mg-Ca binary system [34].

Fig.3.(a-c) The TEM images of as-extruded X12-0 alloy,and (d) the high magnificatio of the area marked by the red rectangle in Fig.3c.(For interpretation of the references to colour in this figur legend,the reader is referred to the web version of this article.)

Fig.4.(a-d) TEM images of the as-extruded X12-0 alloy,and (d) the high magnificatio of the area marked by red rectangle in Fig.4c,under the two-beam condition of (a) g=10-10,(c,d) g=0001.(For interpretation of the references to colour in this figur legend,the reader is referred to the web version of this article.)

Particularly,a high volume fraction of un-DRXed grains exists in the as-extruded X12-0 alloy,and the detailed subgrain and dislocation structures are shown in Fig.4.It can be seen that the dislocations are invisible in Fig.4a,under the two beam condition ofg=10-10,while the dislocations can be detected in Fig.4c,d,in the condition ofg=0001.According to the g·b=0 invisible criterion [35],the dislocation lines distributed in theα-Mg matrix (Fig.4c),belong to the non-basal types and come from the dissociation of perfect 〈c+a〉 dislocations [36].These partial dislocations lying on the basal planes have recently been confirme by both experiments and simulations [37,38].Besides,the sub-grains are detected to exist in grain interiors of unDRXed regions,as marked by the green lines in Fig.4c,and the Fig.4d is the high magnificatio of the rectangle in Fig.4c.These low angle grain boundaries should have been formed during DRX processing and further separate the Mg matrix into lamellae (Fig.4d).In fact,the present authors have reported the similar sub-grain lamellae in Mg-2Sn-2Ca alloys [39].More interesting,plenty of nano-particles,as marked by the blue circles in Fig.4d,are found to distribute along the residualdislocations,which are believed to hinder movement of the newly activated dislocations and can effectively enhance the strength.

Fig.5.(a-d) The typical TEM images of the as-extruded X12-1 alloy,and the Fig.5d is viewed under the two-beam condition of g=0001.

With increasing of the extrusion speed,the as-extruded X12-1 alloy shows a much higher degree of recrystallization,as compared with the X12-0 alloy,and the corresponding TEM images are shown in Fig.5.The grain size of X12-1 alloy is determined to be～1.43μm,and the average size of nano-spherical phase is estimated to be～75nm,which has increased by three times.The nano-phases disperse along both the grain boundaries and grains interiors,as shown by the blue circles in Fig.5c.By the way,the number density of residual dislocations has dramatically decreased due to the high degree of DRX (Fig.5d).

In present Mg-Ca binary alloy,the grain size is obviously refine after extrusion processing,and both the Ca atoms dissolved in Mg matrix and the bulk Mg2Ca phases should play the important roles.Firstly,a great number of 〈c+a〉 dislocations should have been activated in Mg-Ca matrix,and considerable amount of residual dislocations can still be found in as-extruded samples (Fig.4d).The decreasing of stacking fault energy of Mg matrix due to the Ca addition is considered as the main reason [40].Secondly,the coarse Mg2Ca phases can also induce a high density of dislocations around the interphase between Mg2Ca and Mg matrix,which would provide potential nucleation positions for DRX.Readily,the rearrangement of high density dislocations leads to the formation of LAGBs (Fig.2b and Fig.4d).More importantly,Pan et al.[18] found the segregations of Ca atoms at the LAGBs,and such segregation would not only decrease the boundary energies but also strongly hinder the migration of LAGBviapining effect,which thereby lead to the obvious grain refine ment.Besides,the dispersive nano-particles distributing along dislocations can also impede the migration of LAGBs and lead to the formation of fin grains.With increasing of extrusion speed,the grain size becomes larger and excess heat generated during high-speed extrusion should be the one reason.Nevertheless,the average grain size is still low,around～1.5μm in the case of X12-1 alloy,which demonstrates the high thermal stability in present Ca-containing Mg alloys.

Moreover,it is reported that grain refinemen [41,42] and precipitations [43,44] should account for the improvement of mechanical properties in Mg alloys.Consequently,in the present Mg-Ca alloys,the existence of ultrafin grain size,the sub-grain lamellae,and the dispersive nano-particles can effectively hinder movement of the newly activated dislocations and enhance the absolute strength.Besides,the ductility also increases from～3.9% to～8.3% as the ram speed improve from 0.4 to 1.0mm/s.It is seen that the X12-1 alloy undergoes a higher degree of DRX and exhibits more uniform grains than those in X12-0 alloy.The high degree of DRX leads to the low intensity of fibe texture,as displayed in Fig.2h.Texture randomization is important for enhancing the ductility of Mg alloys,since more grains are suitable for activations of both basal and non-basal slips and thevon Misescriterion would be more readily satisfie [13].Besides,the fin recrystallized grains distribute homogeneously within the matrix of X12-1 alloy.Consequently,the lattice strain can be more uniformly accommodated in the X12-1 alloy,and the internal stress concentration can be largely decreased,as compared with X12-0 alloy,which is also beneficia for the ductility.

Conclusion

This work focuses on the effect of extrusion speed on microstructure and mechanical properties of the Mg-Ca binary alloys,and the following conclusions can be drawn.

(1) As ram speed increases from 0.4 to 2.4mm/s,the UTS of alloys decreases from～370MPa to～266MPa and the elongation increases from～3.9% to～12.2%;

(2) The ultrafin grain size,the sub-grain lamellae,and the dispersive nano-particles can effectively enhance the absolute strength of the low-speed extruded samples;

(3) The strength decreases and ductility enhancement in the high-speed extruded samples can be attributed to the higher degree of DRX,more uniform grain structure and the weaker texture,as compared with low-speed extruded alloy.

Acknowledgment

This work is supported by National Natural Science Foundation of China (Nos.51525101,U1610253,51701211,and 51971053),and funded by the Project of Promoting Talents in Liaoning province (No.XLYC1808038).H.C.Pan acknowledges the financia assistance from the State Key Laboratory of Solidificatio Processing in NPU (No.SKLSP201920),the Fundamental Research Funds for the Central Universities (No.N2002011) and joint R&D fund of Liaoning-Shenyang National Research Center for Materials Science(No.2019JH3/30100040).