Microstructure and texture evolution in LZ91 magnesium alloy during cold rolling

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ScienceDirect Journal of Magnesium and Alloys ■■ (2017) ■■–■■ www.elsevier.com/journals/journal-of-magnesium-and-alloys/2213-9567

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Microstructure and texture evolution in LZ91 magnesium alloy during cold rolling Wen-hui Liu a, Xiao Liu a,*, Chang-ping Tang a, Wei Yao a, Yang Xiao b, Xu-he Liu b a

Key Laboratory of High Temperature Wear Resistant Materials Preparation Technology of Hunan Province, Hunan University of Science and Technology, Xiangtan, Hunan 411201, China b Department of Light Metal, Zhengzhou Non-ferrous Metals Research Institute Co. Ltd of CHALCO, Zhengzhou 450041, China Received 30 June 2017; revised 12 December 2017; accepted 13 December 2017 Available online

Abstract LZ91 magnesium alloy extruded sheets were subjected to cold rolling. The microstructure and texture evolution were tracked using optical microscopy (OM) and X-ray diffraction (XRD). The α-Mg and β-Li phases were elongated along rolling direction (RD), contributing to the formation of a thin lamellar “sandwich” structure. This “sandwich” structure is favored for glissile dislocation of α-Mg phase during rolling. The α-Mg and β-Li phases are near Burgers orientation relationship, resulting in an unusual RD texture component in (0002) pole figure. © 2018 Production and hosting by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: LZ91; Microstructure; Texture; Burgers orientation relationship

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 ~5 wt.% to 11 wt.%, BCC-structure β phase of Li (β-Li) can co-exist with the HCP-structure α phase of Mg (α-Mg) [4,9]. This dual phase structure is related to superplasticity, for example, Mg-9 wt.% 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 11 wt.% 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 41.8 MPa. Magnesium alloy sheets are generally produced by rolling. It is well known that grain orientation distribution (texture) and * Corresponding author. Key Laboratory of High Temperature Wear Resistant Materials Preparation Technology of Hunan Province, Hunan University of Science and Technology, Xiangtan, Hunan 411201, China. E-mail addresses: [email protected]; [email protected] (X. Liu).

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 of α-Mg and β-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 between α-Mg and β-Li. 2. Experimental method The alloy used in the present research was LZ91 alloy (Mg8.85Li-0.92Zn). The alloy was received in the form of extruded sheets with a width of 260 mm and a thickness of 14 mm. The LZ91 sheets was rolled along extrusion direction (ED) for consecutive passes of 10%–15% per pass to different final thicknesses (h) with reductions of 49%, 61%, 78%, 85% and 95% without intermediate annealing at room temperature with a rolling speed of 13.8 m/min. Cold rolling was performed on a rolling mill with Ф220 mm in diameter and 600 mm in width. In order to investigate the optical microstructure, the rolled sheets were sectioned in the rolling-normal direction (RD-ND) plane. The specimens were mounted and polished to a 1200 grit surface finish using SiC papers. Polishing was then carried out

https://doi.org/10.1016/j.jma.2017.12.002 2213-9567/© 2018 Production and hosting by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: Wen-hui Liu, Xiao Liu, Chang-ping Tang, Wei Yao, Yang Xiao, Xu-he Liu, Microstructure and texture evolution in LZ91 magnesium alloy during cold rolling, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2017.12.002

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with diamond paste through the sequence of 3, 1 and 0.05 µm. The polished samples were etched with a solution consisting of picric acid (0.2g), ethanol (25 ml), acetic acid (1 ml) and water (5 ml) for times that varies from 15 to 30 s. 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 0.05 µm. 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 to α-Mg, and the dark grey phase is related to β-Li. The α-Mg phase of initial sample with irregular shape distributes in β-Li matrix (in Fig. 1(a)). The α-Mg and β-Li phases were elongated along RD

during rolling process (Fig. 1(b–f)). When LZ91 sheets were subjected to reductions of 85% and 95%, α-Mg and β-Li phases are elongated to thin lamellas along RD and two β-Li phase lamellas carry one α-Mg phase lamella in the middle that seems like a “sandwich” structure. The thickness of α-Mg and β-Li phases along ND is only around 2 µm and 1.5 µm, respectively (Fig. 1(e,f)). At a reduction of 95%, the volume fraction of α-Mg phase increases. The pole figures of α-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

Fig. 1. Optical structures under different reductions: (a) 0%; (b) 49%; (c) 61%; (d) 78%; (e) 85%; (f) 95%. Please cite this article in press as: Wen-hui Liu, Xiao Liu, Chang-ping Tang, Wei Yao, Yang Xiao, Xu-he Liu, Microstructure and texture evolution in LZ91 magnesium alloy during cold rolling, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2017.12.002

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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. 0% 61% 95%

(0 90 90) (0 45 90) (90 90 0) (90 90 30) (0 50 90)

Basal

Prismatic

pyramidal

Extension twinning

Contraction twinning

0 0.5 0 0 0.49

0.22 0 0.44 0 0.04

0.39 0.20 0.23 0.11 0.26

0.50 0.03 0.25 0.13 0.12

0.42 0.28 0.21 0.105 0.2

Please cite this article in press as: Wen-hui Liu, Xiao Liu, Chang-ping Tang, Wei Yao, Yang Xiao, Xu-he Liu, Microstructure and texture evolution in LZ91 magnesium alloy during cold rolling, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2017.12.002

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Table 2 CRSS/m of different deformation modes for the main texture component under different deformation conditions in α-Mg. Basal 0% 61% 95%

(0 90 90) (0 45 90) (90 90 0) (90 90 30) (0 50 90)

– 0.8 – – 0.82

Prismatic 204.5 – 102.3 – 1125

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].

pyramidal

Extension twinning

Contraction twinning

102.6 200 174 363.6 153.9

6 100 12 23.18 25

66.7 100 133.3 254.5 140

The secondary important texture component is close to the orientation of (0 50 90). To identify the deformation modes of α-Mg phase during rolling, the Schmid factors ( m = 0.5 × (cos α cos β − cos γ cos δ ),

Fig. 3. Pole figures of β-Li in LZ91 under different reductions: (a) 0%; (b) 61%; 95%. Please cite this article in press as: Wen-hui Liu, Xiao Liu, Chang-ping Tang, Wei Yao, Yang Xiao, Xu-he Liu, Microstructure and texture evolution in LZ91 magnesium alloy during cold rolling, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2017.12.002

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

here α and β are the angles between (i) the RD and slip direction and (ii) the RD and the slip plane normal, respectively; γ and δ are 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, pyramidal slip, extension twinning and contraction twinning are 0.4 MPa, 45 MPa, 40 MPa, 3 MPa and 28 MPa, 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 BCCstructure 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 BCCstructured phase co-exist with HCP-structured phase, BCCstructure may aid the HCP-structure slip. Agnew et al. [13] investigated pure Mg and Mg-15 at pct Li by post-mortem transmission 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 slip. In the present study, β-Li and α-Mg phases gradually form a “sandwich” structure during rolling process (Fig. 1(b–f)) and β-Li and α-Mg phase lamellas are pretty thin. Kral et al. [21] pointed out that the α/β interface orientation is close to a Burgers orientation relationship ([0001]α//[0–11]β). Therefore, the close-packed plane (011) of β-Li phase is parallel to the close-packed plane (0001) of α-Mg in the α/β interface. It is possibly concluded that thin lamellar β-Li phase may act as sliding plate, helping thin lamellar α-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 of β-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) of β-Li phase is parallel to the close-packed plane (0001) of α-Mg in the α/β interface. The schematic of phase interface between

Please cite this article in press as: Wen-hui Liu, Xiao Liu, Chang-ping Tang, Wei Yao, Yang Xiao, Xu-he Liu, Microstructure and texture evolution in LZ91 magnesium alloy during cold rolling, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2017.12.002

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β-Li and α-Mg and related indices of crystal face are illustrated in Fig. 4. The orientation between (001) plane of β-Li phase and close-packed plane (0001) of α-Mg are close to 45°. When a {001} <110> texture forms in β-Li, the angle between the normal of (001) plane in β-Li phase and RD are close to 45° (see (200) pole figure in Fig. 3). Then, the close-packed plane (011) of β-Li phase is vertical to RD (see (110) pole figure in Fig. 3). According to the Burgers orientation relationship in α/β interface, the c-axes of α-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, the α-Mg and β-Li phases are gradually elongated along RD and form a thin lamellar “sandwich” structure during rolling process. The thin lamellar β-Li in “sandwich” structure can possibly act as a slide plane and aids the thin lamellar α-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 between α-Mg and β-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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Please cite this article in press as: Wen-hui Liu, Xiao Liu, Chang-ping Tang, Wei Yao, Yang Xiao, Xu-he Liu, Microstructure and texture evolution in LZ91 magnesium alloy during cold rolling, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2017.12.002