Microstructure and mechanical properties of aged Mg-Gd-Y-Zr-Ag alloy sheets processed by different rolling routes

Abstract

Microstructure and mechanical properties of aged Mg-10Gd-2Y-0.4Zr-0.4Ag alloy sheets prepared by different rolling routes were investigated. The results showed that the cross rolling aged (CRA) sheet possesses larger grain size than unidirectional rolling aged (URA) sheet due to the occurrence of dynamic recovery during rolling which reduces the dislocation density and delays dynamic recrystallization (DRX). The URA sheet has basal texture and RD-favored texture while CRA sheet has multiple-peak texture. Both sheets precipitate β' phase and CRA sheet exhibits a stronger aging response. The CRA sheet has higher yield strength and tensile strength than URA sheet, with reduced yield strength anisotropy but increased tensile strength anisotropy. Taking into account different strengthening mechanisms, although the finer grain size of URA sheet enhances grain boundary strengthening, CRA sheet is more responsive to aging, leading to superior aging-precipitated phase strengthening and consequently higher yield strength.

1. Introduction

Owing to their strong strength, stiffness and damping-capacity, Mg alloys have gained widespread attention in aerospace, military and automotive sectors [1−3]. However, the hexagonal close-packed (HCP) structure of Mg alloys implies poor plastic deformation capability at ambient temperature, which impedes their development. To enhance formability, Mg alloys are normally prepared by hot processing, such as hot extrusion [4−6] or hot rolling [7−9]. Conventional unidirectional-rolling of plates is prone to develop a specific texture, resulting in anisotropy in the plate plane or tension-compression asymmetry along the normal direction of the rolled plate. Cross-rolling via different rolling paths mitigates the intense texture developed through unidirectional rolling, thereby achieving a more homogenized distribution of grain orientations. Therefore, cross-rolling [10−12] was proposed to modulate the deformation texture and reduce the anisotropy of plates.

In addition to cross-rolling, the addition of rare-earth (RE) elements can also reduce the texture intensity [13, 14] of Mg alloys. Meanwhile, many researchers have found that adding RE to Mg alloys can effectively improve their mechanical properties [15, 16] and thermal stability [17]. The solubility of Gd in Mg alloys is up to 23.5 wt%, which declines rapidly with reducing temperature, the precipitation of massive scattered high melting points and thermally stable phases can enhance strength and heat resistance [18]. Furthermore, adding Y lowers the critical resolved-shear stress (CRSS) of non-basal slip, thus reducing CRSS gap with basal slip and mitigating the mechanical anisotropy [19] in deformed Mg alloys. Alloying element Zr acts to pin grain boundaries [20], so as to refine grain size and bolster its tensile strength and resistance to creep.