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

• Precipitates of Mg-5Sm-0.6Zn-0.5Zr alloy were systematically investigated. • The dominant prismatic β 0 ′ phase was identified based on eight different SAED patterns. • The coexistence of basal γ " and prismatic β 0 ′ precipitates were revealed. • γ " precipitate is more stable than β 0 ′ precipitate in the present alloy. • Numerous β 0 ′ precipitates enhance the hardness and mechanical property. 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 β 0 ′ 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 β 0 ′ precipitate in the present alloy. β 0 ′ precipitates gradually coarsened and were even likely to transform into β 1 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. [ABSTRACT FROM AUTHOR]