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

Highlights: • Yb atoms are significantly enriched in the β′, β₁ and γ′′ phases. • Concurrently dense precipitation of β′, β₁ and γ′′ phases is generated due to Yb addition. 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%.