Precipitation hardening of an Mg–5Zn–2Gd–0.4Zr (wt. %) alloy

Abstract Mg–5Zn–2Gd–0.4Zr alloy (wt. %) shows a significant age hardening response with a hardness increment from 62 HV to 72 HV by ageing at 200 °C for up to 80 h. Transmission electron microscopy and atom probe tomography characterizations reveal that precipitates with different morphologies and habit planes form in the alloy, including triangular-shaped ZnZr phase, rectangular-shaped Zn 2 Zr phase, [0001] Mg rods ( β ′ 1 ) MgZn 2 Laves phase, (0001) Mg plate ( β ′ 2 ) MgZn 2 Laves phase, and coherent GP zones. Furthermore, a significant partitioning of Gd into Zn-rich precipitates ( β ′ 1 and β ′ 2 ) was, for the first time, observed, which is considered to be responsible for improving the thermal stability of Zn-rich precipitates and enhancing the precipitation hardening of the alloy. A core–shell structure with the shell of Gd (∼2 at%) and the core of Zn 2 Zr phase was also observed, which is proposed to hinder the coarsening of Zn 2 Zr phase. No further change in average size of the Zn 2 Zr particles was observed after ageing over 15 h. Better understanding the partitioning of Gd into the Zn-rich precipitates and the formation of the core–shell structures are essential to control the precipitation microstructure and develop high performance Mg alloys.