Study on the investigation of creep behaviour in squeeze cast Ca-modified AZ91 magnesium alloy

The AZ91 (Mg-Al) alloy is the most commonly used Mg-cast alloy with high specific strength and excellent die-castability; however, its poor creep performance limits its applications at elevated temperatures. In the present investigation, the effect of Ca addition on the microstructure development and creep performance of squeeze-cast AZ91 alloy recognised as AZX911 alloy has been investigated at 150 degrees C-250 degrees C temperature under 65 and 80 MPa stress. The addition of Ca in the AZ91 alloy results in the precipitation of the thermally stable Al2Ca intermetallic phase, which partially suppresses the precipitation of the thermally unstable beta-Mg17Al12 phase and significantly improves the creep resistance of the AZ91 Mg alloy. The governing creep mechanism of AZX911 alloy is found to follow power law creep with n = 3.49 at 150 degrees C that mainly results from dislocation climb. In contrast, a transition to power-law breakdown is observed at 175 degrees C. The resistance to creep deformation of AZX911 further improves with heat treatment at 450 degrees C as compared to the as-cast specimen of the same alloy due to the complete dissolution of the beta-Mg17Al12 phase. The activation energy of phase dissolution, estimated using the Kissinger method, is found to be higher (923 kJ/mol) for the Al2Ca intermetallic phase as compared to that for beta-Mg17Al12 phase (664 kJ/mol) in AZX911 alloy.