Hardening effect and precipitation evolution of an isothermal aged Mg-Sm based alloy

The age-hardening behavior and precipitation evolution of an isothermal aged Mg−5Sm−0.6Zn−0.5Zr (wt.%) alloy have been systematically investigated by means of transmission electron microscopy (TEM) and atomic-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The Vickers hardness of the present alloy increases first and then decreases with ageing time. The sample aged at 200 °C for 10 h exhibits a peak-hardness of 90.5 HV. In addition to the dominant β0′ precipitate (orthorhombic, a = 0.642 nm, b = 3.336 nm and c = 0.521 nm) formed on {11-20}α planes, a certain number of γ” precipitate (hexagonal, a = 0.556 nm and c = 0.431 nm) formed on basal planes are also observed in the peak-aged alloy. Significantly, the basal γ” precipitate is more thermostable than prismatic β0′ precipitate in the present alloy. β0′ precipitates gradually coarsened and were even likely to transform into β1 phase (face centered cubic, a = 0.73 nm) with the increase of ageing time, which accordingly led to a gradual decrease in number density of precipitates and finally resulted in the decreased hardness and mechanical property in the over-aged alloys.