Microstructure and texture evolution in LZ91 magnesium alloy during cold rolling

Wen-hui Liu,Xio Liu,∗,Chng-ping Tng,Wei Yo,Yng Xio,Xu-he Liu

aKey Laboratory of High Temperature Wear Resistant Materials Preparation Technology of Hunan Province,Hunan University of Science and Technology,

Xiangtan,Hunan 411201,China

bDepartment of Light Metal,Zhengzhou Non-ferrous Metals Research Institute Co.Ltd of CHALCO,Zhengzhou 450041,China

1.Introduction

Magnesium alloys as very light metals can be used for structural application.However,they have poor formability at room temperature owing to their hexagonal close-packed(HCP)crystal.To make up for the poor formability and further reduce weight,Mg-Li alloys are generally considered[1-8].Mg-Li alloys have good formability.Especially,when Li content between ∼5wt.%to 11wt.%,BCC-structureβphase of Li(β-Li)can co-exist with the HCP-structureαphase of Mg(α-Mg)[4,9].This dual phase structure is related to superplasticity,for example,Mg-9wt.%Li alloys have high elongation of 460%[10].However,Mg-Li alloys do not have high strength[11].For this reason,the third elements,such as Zn,Al and Mn,are added[4,12].Chang et al.[4]investigated five alloys with Li content of 9 and 11wt.%plus various three kinds of elements,Zn,Al and Mn and found that Mg-9Li-1Zn alloy(LZ91)had the excellent tensile strength,which was around 41.8MPa.

Magnesium alloy sheets are generally produced by rolling.It is well known that grain orientation distribution(texture)and microstructure play important role during deformation[7,13].Therefore,it is essential to investigate the texture and microstructure evolution during rolling.In the present study,a multipass rolling without intermediate annealing at room temperature was chosen for as-extruded LZ91 alloy.The microstructure evolution was detected.The texture evolutions ofα-Mg andβ-Li were also investigated.The Schmid factor was calculated to analyze the deformation modes under deformation processing.An unusual texture component was studied in the term of the orientation relationship betweenα-Mg andβ-Li.

2.Experimental method

The alloy used in the present research was LZ91 alloy(Mg-8.85Li-0.92Zn).The alloy was received in the form of extruded sheets with a width of 260mm and a thickness of 14mm.The LZ91 sheets was rolled along extrusion direction(ED)for consecutive passes of 10%-15%per pass to differ-ent final thicknesses(h)with reductions of 49%,61%,78%,85%and 95%without intermediate annealing at room temperature with a rolling speed of 13.8m/min.Cold rolling was performed on a rolling mill with Ф220mm in diameter and 600mm in width.

In order to investigate the optical microstructure,the rolled sheets were sectioned in the rolling-normal direction(RDND)plane.The specimens were mounted and polished to a 1200 grit surface finish using SiC papers.Polishing was then carried out with diamond paste through the sequence of 3,1 and 0.05μm.The polished samples were etched with a solution consisting of picric acid(0.2g),ethanol(25ml),acetic acid(1ml)and water(5ml)for times that varies from 15 to 30s.Then,the optical structure was measured by OM.

Aiming to measure the macrotexture,the rolling-transverse direction(RD-TD)cross-sections were cut from the rolled sheets.These were polished to a 1200 grit surface finish using SiC papers.Polishing was then carried out with diamond paste through the sequence of 3,1 and 0.05μm.Then,the macrotexture was measured by XRD.

3.Results and discussion

The microstructures under different reductions are displayed in Fig.1.The white color phase is corresponding toα-Mg,and the dark grey phase is related toβ-Li.Theα-Mg phase of initial sample with irregular shape distributes inβ-Li matrix(in Fig.1(a)).Theα-Mg andβ-Li phases were elongated along RD during rolling process(Fig.1(bf)).When LZ91 sheets were subjected to reductions of 85%and 95%,α-Mg andβ-Li phases are elongated to thin lamellas along RD and twoβ-Li phase lamellas carry oneα-Mg phase lamella in the middle that seems like a “sandwich”structure.The thickness ofα-Mg andβ-Li phases along ND is only around 2μm and 1.5μm,respectively(Fig.1(e,f)).At a reduction of 95%,the volume fraction ofα-Mg phase increases.

Fig.1.Optical structures under different reductions:(a)0%;(b)49%;(c)61%;(d)78%;(e)85%;(f)95%.

Fig.2.Pole figures of α-Mg in LZ91 alloy under different reductions:(a)0%;(b)61%;(c)95%.

Table 1 Schmid factor(m)of different deformation modes for the main texture components under different deformation conditions in α-Mg.

Table 2 CRSS/m of different deformation modes for the main texture component under different deformation conditions in α-Mg.

Fig.3.Pole figures of β-Li in LZ91 under different reductions:(a)0%;(b)61%;95%.

Fig.4.The schematic of phase interface between α-Mg and β-Li in LZ91 alloy and related indices of crystal face.

The pole figures ofα-Mg in LZ91 alloy under different reductions are exhibited in Fig.2.It can be seen from Fig.2(a)that c-axes of initial texture is approximately mainly parallel to the TD whose Euler angle is around(0 90 90).In this type of texture,basal slip is suppressed under both tensile and compressive loading condition since there is little or no resolved shear stress on the basal planes[14].Rolling can be approximately considered as subjecting tensile in RD and compressive in ND.Thus,the basal slip is not favored during rolling.At a reduction of 61%,the Euler angle of main texture component is approximately(0 45 90)and an RD texture component whose c-axes is closely parallel to RD,is also detected.At 95%,the RD texture component strengthens and becomes the main texture.This is different from the general condition that c-axes of main texture component should be parallel to ND[15-18].The secondary important texture component is close to the orientation of(0 50 90).

To identify the deformation modes ofα-Mg phase during rolling,the Schmid factors(m=0.5×(cosαcosβ-cosγcosδ),hereαandβare the angles between(i)the RD and slip direction and(ii)the RD and the slip plane normal,respectively;γandδare the angles between(i)the ND and slip direction and(ii)the ND and the slip plane normal,separately[19])for main texture components under different reductions were calculated and are illustrated in Table 1.The activation of deformation modes depends on critical resolved shear stress(CRSS)/m.According to Ref.[20],the CRSS values at room temperature for basal slip,prismatic slip,＜c+a＞pyramidal slip,extension twinning and contraction twinning are 0.4MPa,45MPa,40MPa,3MPa and 28MPa,respectively.The values of CRSS/m are illustrated in Table 2.It can be concluded from Table 2 that extension twinning will be firstly initiated at the beginning of rolling.At a reduction of 61%,basal slip is the main deformation mode,followed by extension twinning and contraction twinning.At a reduction of 95%,Euler angle of the main texture components are(90 90 0)and(90 90 30).The extension twinning has related low CRSS/m,suggesting that extension twinning should be activated to aid uniform plastic deformation.It can be concluded that twinning plays an important role during rolling process.However,it can be seen from Fig.1 that twinning is not initiated during the whole rolling process.

LZ91 is dual phase microstructure that consists of the BCC-structure solid solution(β-Li phase)and HCP-structure solid solution(α-Mg phase).BCC-structure has 12 slip systems,which could easily satisfy the requirement of homogeneous deformation.HCP-structure has limited slip systems,resulting in the poor formability at room temperature.When BCC-structured phase co-exist with HCP-structured phase,BCC-structure may aid the HCP-structure slip.Agnew et al.[13]investigated pure Mg and Mg-15 at pct Li bypostmortemtransmission electron microscopy(TEM)after small deformation.They inferred that Li additions might lower the{11-2-2}stacking fault energy for glissile dissociation,contributing to the enhancement of＜c+a＞slip.In the present study,β-Li andα-Mg phases gradually form a “sandwich”structure during rolling process(Fig.1(b-f))andβ-Li andα-Mg phase lamellas are pretty thin.Kral et al.[21]pointed out that theα/βinterface orientation is close to a Burgers orientation relationship([0001]α//[0-11]β).Therefore,the closepacked plane(011)ofβ-Li phase is parallel to the closepacked plane(0001)ofα-Mg in theα/βinterface.It is pos-sibly concluded that thin lamellarβ-Li phase may act as sliding plate,helping thin lamellarα-Mg phase glide during deformation.This could mean that twinning is not required to accommodate the uniform deformation.

In Fig.2,an unusual RD texture component is observed at severe plastic deformation during cold rolling.With the aim of analyzing formation of unusual RD texture component,the texture evolution ofβ-Li was measured and is illustrated in Fig.3.As shown from Fig.3,β-Li phase exhibits a typical rolling texture{001}＜110＞.The close-packed plane(011)ofβ-Li phase is parallel to the close-packed plane(0001)ofα-Mg in theα/βinterface.The schematic of phase interface betweenβ-Li andα-Mg and related indices of crystal face are illustrated in Fig.4.The orientation between(001)plane ofβ-Li phase and close-packed plane(0001)ofα-Mg are close to 45°.When a{001}＜110＞texture forms inβ-Li,the angle between the normal of(001)plane inβ-Li phase and RD are close to 45°(see(200)pole figure in Fig.3).Then,the close-packed plane(011)ofβ-Li phase is vertical to RD(see(110)pole figure in Fig.3).According to the Burgers orientation relationship inα/βinterface,the c-axes ofα-Mg around the phase interface will be rotated to become parallel to RD.Finally,this contributes to the usual RD texture component.

4.Conclusions

In summary,theα-Mg andβ-Li phases are gradually elongated along RD and form a thin lamellar“sandwich”structure during rolling process.The thin lamellarβ-Li in “sandwich”structure can possibly act as a slide plane and aids the thin lamellarα-Mg gliding during deformation.This means that twinning is not required to accommodate plastic deformation,even though CRSS/m of twinning is pretty low and the basal slip is not favored.A near Burgers orientation relationship is betweenα-Mg andβ-Li phases in phase interface,contributing to an unusual RD texture component.

Acknowledgments

The authors gratefully acknowledge research support from the National Natural Science Foundation of China(Grant No.51601062,51605159 and 51475162).

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