Microstructure evolution during heat treatment and mechanical properties of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr cast alloy

Microstructure evolution during heat treatment and mechanical properties of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr cast alloy were investigated experimentally in different stages of heat treatment process. As-cast alloy was found to be consisting of dendritic α(Mg) grains, interdendritic α(Mg) + Mg₁₂Nd eutectic. Within α(Mg) grains, needle-like Mg₁₂Nd phase, quadrate-like Mg₅Gd phase and α(Zr) particles were also observed. After a solution treatment at 515 °C for 18 h, α(Mg) + Mg₁₂Nd eutectic was almost dissolved into α(Mg) grain, but a new Zn₂Zr₃ phase was observed within α(Mg) grains and along grain boundaries. After an aged treatment at 205 °C for 18 h, more dispersed interdendritic Mg₁₂Nd precipitates, coarsened Zn₂Zr₃ and rod-shaped metastable precipitates of β″(Mg₃Nd) and β′(Mg₇Nd) were observed within α(Mg) grains. The ultimate tensile strength and the yield strength of the experimental alloy after the aged treatment were measured to be 287 MPa and 206 MPa at room temperature, 240 MPa and 175 MPa at 200 °C, and 220 MPa and 165 MPa at 250 °C, respectively. The improved mechanical properties were also discussed based on the observed microstructure evolution.