Influence of additions of Sb and/or Sr on microstructure and tensile creep behaviour of squeeze-cast AZ91D Mg alloy

The effects of individual and combined additions of Sb and Sr on microstructure and creep properties of the squeeze-cast AZ91D alloy have been investigated. For comparison, the same has also been studied on the as-cast AZ91D alloy without any addition. The results indicate that both individual and combined additions refine the grain size and β-Mg17Al12 phase, which is more pronounced with combined addition. Besides α-Mg and β-Mg17Al12 phases, a new rod-shaped Mg3Sb2 and an irregular shaped Al4Sr phases are formed following individual additions of Sb and Sr in the AZ91D alloy. With combined additions, both Mg3Sb2 and Al4Sr phases are formed in the AZ91D alloy. All the alloys have been creep tested at a temperature of 175 °C and at an initial stress of 70 MPa. All the modified AZ91D alloys exhibit superior creep resistance as compared to the base AZ91D alloy. Individual addition of Sb is more effective as compared to the individual addition of Sr in improving creep resistance of the AZ91D alloy owing to the higher thermal stability of the Mg3Sb2 phase. Among the modified alloys, the best creep resistance is obtained in the AZ91D alloy pertaining combined addition of both Sb and Sr owing to the reduced amount of β-Mg17Al12 phase and increased amount of high melting point Mg3Sb2 and Al4Sr intermetallic phases.