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 Zn2Zr phase, [0001]Mg rods (β′1) MgZn2 Laves phase, (0001)Mg plate (β′2) MgZn2 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 Zn2Zr phase was also observed, which is proposed to hinder the coarsening of Zn2Zr phase. No further change in average size of the Zn2Zr 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.