Effects of heat treatments on microstructure and mechanical properties of Mg–11Y–5Gd–2Zn–0.5Zr (wt.%) alloy

The microstructure and mechanical properties of Mg–11Y–5Gd–2Zn–0.5Zr (wt.%) (WGZ1152) alloy during different heat treatments were investigated. Almost all the Mg24(GdYZn)5 eutectic phases dissolved into the α-Mg matrix after solution treatment at 535 °C for 20 h. After ageing at 225 °C for 24 h (T6 state), a great amount of fine β′ precipitates formed. Both the 18R-type long period stacking ordered (LPSO) Mg12YZn phase and 6H′-type LPSO phase exhibit good thermal stability during the high-temperature heat treatments process. The 18R-type LPSO Mg12YZn phases are much harder than α-Mg matrix and have a volume fraction of ∼16%. The ultimate tensile strength at the room temperature of the peak-aged alloy (T6 state) is 307 ± 6 MPa and elongation is 1.4 ± 0.3%. The alloy in T6 state shows anomalous positive temperature dependence of the strength from room temperature to 250 °C, and maintains a strength of more than 260 MPa up to 300 °C (0.64Tm). The excellent strength of the WGZ1152 alloy at both room and elevated temperatures is mainly attributed to the solid solution strengthening, β′ precipitates strengthening and LPSO strengthening. Slip line observations suggest a transition from basal to non-basal slip with increasing temperature.

Research highlights▶ High strength can be obtained after optimization of heat treatment parameters. ▶ Morphology and microhardness of phases during heat treatments were determined quantitatively. ▶ Slip lines observations reveal a transition from basal to non-basal slip with increased temperature. ▶ The long period stacking ordered structures have good thermal stability and high hardness. ▶ Solution, precipitate and LPSO strengthening are all effective strengthening sources.