Effect of Li addition on the mechanical behavior and texture of the as-extruded AZ31 magnesium alloy

Abstract

Lithium alloying additions are verified to be an effective way to enhance the room temperature formability of magnesium. In this work, AZ31 (Mg–3 wt% Al–1 wt% Zn) alloys with different lithium additions (0–5 wt%) were melted and extruded to 2 mm thick sheets at 380 °C. The microstructure and texture evolution were investigated by optical microscopy, X-ray diffraction (XRD) and electronic backscattered diffraction (EBSD). Tensile tests along three directions were carried out at room temperature, to access the mechanical properties and anisotropy. It was found that the mechanical anisotropy of the as-extruded AZ31 alloy was modified remarkably with lithium additions and AZ31 alloy with 5% Li content was found to have the smallest planar anisotropy and enhanced elongation. Lithium additions also increased the rotation of basal poles in the transverse direction, which was attributed to decreased c/a ratio and refined recrystallized structure. The thickness reduction and width reduction during tensile test were also measured and discussed.

Introduction

Recently, magnesium alloys are attracting more and more attention in automobile and aerospace applications for potential improvement in fuel economy by efficient weight reduction [1], [2], [3]. However, the hexagonal close packed (hcp) structure of magnesium and its alloys indicates that they have crucial drawbacks of limited ductility and poor formability, particularly at room temperature, compared with aluminum and its alloys [4], [5] since dislocation slip at low temperatures (<250 °C) occurs mostly on the basal planes, leading to the development of a preferred orientation with the alignment of the hexagonal c-axis perpendicular to the metal flow direction during deformation [6], [7], [8]. Thus, the extruded or rolled sheets of magnesium and its alloys generally show a strong anisotropy in plastic deformation behavior at room temperature.

It is also well known that grain orientation distribution (texture) plays a critical role in meliorating mechanical properties and formability of the hexagonal metals and alloys [9], [10], because these materials usually have quite high anisotropy in their crystals. The typical basal texture exists in the vast majority of grains in commercial AZ31 alloys [11], i.e. basal planes aligned parallel to extrusion direction (ED) or rolling direction (RD). It is difficult to deform at lower temperature with such an orientation. Al-Samman and Gottstein [12] reported that strongly textured AZ31 alloy failed earlier during channel-die compression tests; meanwhile, the intensity of the basal texture increased even during hot rolling [13]. Flow curve anisotropy was observed in rolled AZ31 alloys during room temperature compression test [14], which was found to originate from the primary twin formation during plastic deformation. Masoumi et al. [15] reported that the texture of AZ31-H24 sheet was strongly basal; even after annealing at 420 °C, the strong basal texture still persisted.

Several methods have been proven useful in controlling the texture development. Moderate annealing can permit grain growth with random orientation, thus weakening the basal texture of magnesium alloy [16]. However, the result may not be quite as obvious, and sometimes even be on the contrary [17]. Equal channel angular pressing (ECAP) has been successfully used in producing materials with ultra-fine grains [18] and weakening basal texture [19], but this technique cannot be used to produce magnesium sheets. Otherwise, asymmetric rolling, such as single roller drive rolling (SRDR) [20] and differential speed rolling (DSR) [21], can be effective to change the orientation of basal plane in the rolling plane and to enhance the press formability compared with normal (symmetric) rolling. But it is costly and not easy for the practical production of magnesium sheets. Alloying with other element is also a way to alter the basal texture of magnesium alloy. Of the elements that are known to weaken the texture, most of them have large atomic radii. They are Ca, Sr, rare earth element, etc., which are likely to be effective texture modifiers. Ca-alloyed Mg yielded a small rise and then a drop in the extruded texture strength [22]. The addition of strontium can reduce the intensity of basal texture of magnesium alloys [23], which is attributed to particle stimulated nucleation (PSN). Magnesium alloy with Y addition obtained its increased ductility from a significantly larger amount of slip with 〈c+a〉 components [24]. The effects of those elements on the intensity of the basal texture of Mg have been investigated to some degree, and the rotation of c-axis is not clear yet which is a key factor that influences the formability of Mg and its sheets.

As is known, dramatic improvement of ductility is ensued in Mg due to Li addition [25], [26]. As shown in Mg–Li binary phase diagram [27], when Li content in Mg alloys exceeds over 5.5 wt%, a new phase of body center cubic structure will be formed and thus the ductility of the alloys will improve obviously, but their mechanical properties are lower and the applications are limited [4], [26], [28]. As for the alloys with 0–5.5 wt% Li content, a Mg-rich α-phase of hcp structure exists. Compared with the typical Mg alloys of hcp structure, like pure Mg and AZ31, these Mg–Li alloys of hcp structure have better ductility due to the decrease of axial ratio (c/a) of the α-phase. Hauser et al. [29] have reported that the axial ratio (c/a) of Mg containing Li decreases from 1.624 for pure Mg to 1.607 at the solid-solubility limit. This change is largely associated with a decrease in the (0002) lattice spacing, which results in the increased critical resolved shear stress (CRSS) for basal slip while the CRSS of the prismatic 〈a〉 slip mode may be decreased relative to basal 〈a〉 slip in Mg alloys containing Li [30], which may cause the slip between crystal planes less difficult [31]. Mg–4Li alloy, introduced by Al-Samman. [32], had a prominent ductility enhancement over AZ31 magnesium alloy, and Ando et al. [33] reported that the fourth pyramidal slip $\left\{11\overline{2}2\right\}〈11\overline{2}3〉$ could be started up at 77–293 K in Mg–3.5Li alloy, although their phase composition is the same α-solid solution of hcp structure. Nevertheless, the Mg–(0–5)Li alloy has relatively low yield strength and corrosion resistance, and thus limited practical use of Mg–Li alloys. In order to improve the ductility and formability of AZ31 Mg alloys with good mechanical properties, much work has been done. Mackenzie and Pekguleryuz [34] reported that the rolled AZ31–3Li sheet exhibited a large split of basal poles in the rolling direction and a smaller increased rollability, which was attributed to the increased activity of prismatic planes owing to lithium addition. But it is not known whether the mechanical properties and anisotropy of AZ31 Mg alloy with lithium improved. So far, very few studies on the mechanical properties and anisotropy of the extruded AZ31 sheets with lithium additions have been carried out.

Compared with the rolling process, the extrusion of Mg sheets is a short procedure and of lower cost because 1–3 mm thick sheet can be directly prepared from cast ingot. In the present work, the AZ31 sheets with different lithium additions were prepared through the extrusion process, and the effect of lithium additions on deformation behavior and anisotropy of the as-extruded AZ31 sheets was also investigated.