The role of Yb content on the microstructural evolution and mechanical characteristics of cast Mg-9Gd-0.5Zn-0.2Zr alloy

Abstract

Inspections on microstructural design suggest that a high-strength Mg alloy can be achieved via the generation of co-precipitation of prismatic and basal precipitates with high number density and large aspect ratios. Currently, there is a lack of feasible approaches to generating such a microstructure. This work investigated the specific role of various Yb contents on the precipitation behavior, solute segregation and mechanical performance of cast Mg-9Gd-xYb-0.5Zn-0.2Zr alloys (x = 0, 0.5, 1 wt%). It was found that, the age hardening response of Mg-9Gd-0.5Zn-0.2Zr alloy could be significantly improved as the Yb addition increased, primarily ascribed to introduction of dense distribution of both prismatic and basal matrix precipitates involving γ′′, β′ and β1. Solute segregation of Yb had been examined utilizing high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), revealing the evidence of strong Yb segregation in both prismatic β′, β1 and basal γ′′ precipitates. Quantitative TEM analysis indicated that the increasing Yb addition was found to improve the number density of γ′′, β′ and β1 during isothermal ageing at 225 °C. The substantial improvement on yield strength (~84 MPa) in peak-aged alloy was thereby achieved due to the co-precipitation of dense prismatic and basal strengthening phases associated with Yb addition (1 wt%). However, in addition to the promoted precipitation of these strengthening phases, Zn–Zr particles were confirmed to act as favorable nucleation sites for prismatic β1 phases in the under-aged condition. During prolonged ageing time, the addition of 1 wt% Yb was found to greatly accelerate the growth of these preferentially formed β1 phases and part of β1 transformed in situ into β phases, accompanied by the gradual developments of matrix precipitate-free zones (PFZs). The greatly improved uneven distribution of these coarse β-type phases concomitant with widened PFZs primarily led to the significant reduction in ductility as the Yb content increased up to 1%.