Microstructures and mechanical properties of a hot extruded Mg–4.45Zn–0.46Y–0.76Zr alloy plate

Highlights

• A mixed grain structure was obtained after extrusion.

• The texture in the as-extruded ZWK401 alloy plate was analyzed.

• Mechanical properties of the as-extruded alloy plate along ED, 45° direction and TD were investigated.

Abstract

The microstructure, texture and mechanical properties of the as-extruded Mg–4.45Zn–0.46Y–0.76Zr (ZWK401) alloy plate were investigated on specimens with the extrusion direction (ED), 45° direction and the transverse direction (TD), respectively. The as-extruded alloy showed a mixed grain structure composed of long elongated unrecrystallized grains (LEGs) which contained both stringer LEGs and elliptical LEGs, fine equiaxed recrystallized grains (FEGs) and peculiarly arranged equiaxed grains referred as row stacked grains (RSGs). The texture in the as-extruded ZWK401 alloy plate was formed by six components: $\{11\overline{2}0\}〈10\overline{1}0〉$, $\{11\overline{2}0\}〈10\overline{1}3〉$, $\{11\overline{2}0\}〈0001〉$, $\{10\overline{1}0\}〈11\overline{2}0〉$, $\{10\overline{1}0\}〈0001〉$ and $\{0001\}〈10\overline{1}0〉$. In particular, the predominance of a $\{11\overline{2}0\}〈10\overline{1}0〉$ texture strongly affects twinning and slipping behavior, thus affects the mechanical anisotropy. The ZWK401 alloy plate showed excellent mechanical properties in the ED sample with the UTS of 331 MPa, YS of 278 MPa and elongation to failure of 12.3%.

Introduction

Lightweight magnesium alloys have attracted significant interest in the last decade due to their potential applications in automotive, personal electronics and aerospace industries [1]. However, extensive application of magnesium alloys is strongly restricted by their limited strength and ductility. This is mainly attributed to the fact that the hexagonal close-packed crystal (hcp) structure of Mg has only two independent slip systems at room temperature, whereas five are required for generalizing homogeneous plasticity. Generally, both the strength and ductility can be improved by refining grain size through wrought processing such as rolling and extrusion. However, due to wrought processing, magnesium alloys would have an orientation distribution referred to as crystallographic texture, which leads to mechanical anisotropy.

In general, dynamic restoration behavior is dependent on the stacking fault energy (SFE) and lattice diffusivity [2]. An increase in SFE or a decrease in lattice diffusivity facilitates the dynamic recovery process, which will suppress dynamic recrystallization (DRX). Since the SFE of magnesium (78 mJ m⁻²) is smaller than that of a typical light metal, such as aluminum (200 mJ m⁻²) [2], grain refinement of magnesium by DRX occurs quite readily. It is important to consider grain size strengthening for magnesium alloys, as finer grains lead to a relatively higher YS due to high Hall–Petch coefficient comparing to aluminum [3]. Besides, because of grain boundary sliding (GBS), activating non-basal slip systems and restricting twin forming, the ductility of magnesium alloys will be better improved with the decrease in grain size. Additionally, yield strength anisotropy can also be reduced with the decrease in grain size [3]. Thus, hot deformation accompanied by DRX is essential for attaining superior properties, such as greater strength and ductility [4].

Binary Mg–Zn alloys suffer from grain coarsening and micro-porosity [5]. It is an effective method to improve their mechanical properties by adding rare earth (RE) metals in Mg–Zn alloys, such as Ce, Gd, Y and so on [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Y is a widely used rare earth alloying element for commercial Mg alloys. Investigations [11], [12], [13], [17] have shown that Y improves the mechanical properties of magnesium alloys with Zn. A small volume fraction of intermetallic particles at grain boundaries can prevent grain growth during thermo-mechanical processing by pinning the boundaries [18], [19]. To facilitate the wider application of wrought Mg alloys as structural materials in industries, low-cost RE-low Mg alloys with improved mechanical properties and minimized anisotropy are much more desirable. Further thermal treatment subsequent to hot deformation processing are not used due to the fact that the Mg–Zn system shows a relatively low precipitation hardening response [1]. In this work, we investigates the microstructures and mechanical properties of as-extruded Mg–4.45Zn–0.46Y–0.76Zr (wt.%) alloy plate to explore the possibility of new magnesium alloys with low-cost micro-alloying elements.