An alternating ageing-annealing process for enhancing strength and ductility of a Mg-Gd-Y-Zn-Zr alloy

Abstract In this paper, an alternating ageing-annealing process was proposed for an extruded Mg-9.5Gd-4Y-2.2Zn-0.5Zr (wt.%) alloy, and then the effects of this process on the microstructure, mechanical properties and fracture mechanism of the alloy were investigated. The results showed that short rod-shaped particles with long period stacking ordered (LPSO) phases and β′ phases were densely and uniformly distributed in the sample and exhibited a homogeneous microstructure after alternating ageing-annealing treatment, while after the traditional ageing treatment only basal γ′ phases were sparsely distributed in the fine grain region of the sample with a bimodal microstructure. Deformed grains with strong basal textures and deformation incompatibility between different regions (deformed grain region, fine grain region and bulk-shaped LPSO phase) were prone to high local stress concentrations, which caused the alloy to fracture without full work hardening. The sample possessed an excellent strength-ductility synergistic effect after alternating ageing-annealing treatment. This is mainly attributed to the coordinated deformation of the homogeneous microstructure and the pinning effect of the short rod-shaped LPSO phases and the prismatic β′ phases on dislocations and grain boundaries, which significantly improved the strength and inhibited the initiation of cracks. In summary, the mechanical properties of the sample were improved after heat treatment, with a yield strength (YS) of 425 MPa, an ultimate tensile strength (UTS) of 493.2 MPa, and an elongation (EL) of 11.2%.