Influence of pre-strain on de-twinning activity in an extruded AM30 magnesium alloy

Abstract

This study was aimed at examining the compressive deformation behavior of an extruded AM30 alloy along the normal direction (ND) after different amounts of pre-strain along the extrusion direction (ED), with a focus on the strain hardening behavior, texture evolution, and deformation mechanisms. Compressive loading directly in the ND gave a slip-dominated flow behavior due to the presence of two sets of basal textures with c-axes aligned almost parallel to the ND, which was unfavorable for extension twinning. In the two-step ED–ND compression, the compressive yield strength decreased, while the ultimate compressive strength increased with increasing pre-strain. At the lower pre-strain levels two hardening stages of B and C occurred, while three hardening stages of A, B and C were present at the higher pre-strain levels. The peak value between stages B and C and the slope of strain hardening rate in stage B linearly decreased with increasing pre-strain. The phenomenon of de-twinning (or back-twinning) was observed, and the de-twinning activity decreased with increasing pre-strain in the ED while keeping the re-compression amount in the ND constant. Texture measurements revealed that the c-axes of hcp unit cells were always rotated towards the anti-compression direction, regardless of compression in the ED or ND. Texture weakening was achieved via the pre-compression in the ED and subsequent re-compression in the ND.

Introduction

Lightweighting of ground vehicles is today considered one of the most important strategies to improve fuel economy and reduce climate-changing and environment-damaging emissions [1], [2], [3], [4], [5], [6], [7]. It has been reported that the fuel efficiency of passenger vehicles can be enhanced by 6–8% for each 10% reduction in weight [8], [9], [10]. Thus magnesium alloys, as the lightest structural metallic materials, have recently drawn a lot of interest in the transportation industry to reduce the weight of vehicles due to their high strength-to-weight ratio, dimensional stability, and good machinability and recyclability. Wrought magnesium alloys have in general better tensile properties [11], [12] and fatigue resistance [13], [14], [15], [16], [17], [18], [19] compared to cast magnesium alloys [20], [21]. However, wrought magnesium alloys normally form a strong texture during manufacturing processes and exhibit a high degree of anisotropy and poor room temperature formability due to the limited slip systems in the hexagonal close-packed (hcp) crystal structure and the polar characteristics of deformation twinning.

During the manufacture of magnesium components (via rolling, leveling, coiling, bending, stretching, etc.) the change in strain path could have a significant influence on the mechanical behavior of magnesium alloys. The flow stress may increase or decrease when such alloys are subjected to strain reversals [9], [22], [23]. It is therefore important to identify the effect of reversing the strain path on the stress–strain response, as well as the role of twinning and de-twinning in bringing this about. The reverse motion of {10$\overline{1}$2} twin boundaries or de-twinning was observed by Molnár et al. [24], during successive compression of a hot-rolled AZ31 alloy at 3.5% strain along the rolling direction (RD) and then along the normal direction (ND). During the initial compression along the RD, {10$\overline{1}$2} twinning was the main deformation mode, whereas a twin-free microstructure was observed after subsequent compression along the ND which restored the initial shape of the sample. Xin et al. [25] examined the plastic deformation behavior of a hot-rolled AZ31 alloy under 6 passes of plain compression along the transverse direction (TD) and ND alternatively, and reported that at 10% compression along the TD most grains were nearly twinned and extension twinning took place during subsequent compression along the ND without any de-twinning. However, Proust et al. [26] studied the effect of strain path change on twinning and de-twinning where an AZ31-H24 alloy was pre-strained in plane and then reloaded along the through-thickness direction, and observed that the twinned region of the grain that had been created during the loading phase of the deformation was allowed to shrink until it disappeared totally and the grain was then constituted entirely of matrix. Similar results of de-twinning were observed by Brown et al. [27] in rolled beryllium which was initially deformed in-plane and then in the through-thickness direction.

Twinning and de-twinning phenomena in hcp materials such as magnesium [24], beryllium [27] and zirconium [28] have been reported. The occurrence of twinning and de-twinning has a significant impact on the mechanical properties of magnesium and its alloys [24], [26], [27]. The de-twinning of Mg alloys has been reported to increase fatigue resistance [29], [30], improve tensile and compressive properties [20], [31], and decrease tension–compression yielding asymmetry [32]. Thus Mg alloys have been recently referred to as a smart material [24]. However, it is unclear how de-twinning occurs in a previously twinned sample with a change of strain path, especially how the pre-strain amount affects the subsequent strain hardening behavior, de-twinning tendency, and the associated texture change. The objective of this study was, therefore, to explore the relationship between the hardening behavior, texture evolution, and de-twinning in an extruded AM30 alloy upon changing the strain path.