Effects of minor Sr addition on the microstructure, mechanical properties and creep behavior of high pressure die casting AZ91-0.5RE based alloy

Effects of minor Sr (x=0, 0.2, 0.6, 1.0 wt%) addition on microstructure, mechanical properties and creep behavior of high pressure die casting AZ91-0.5RE based alloys were thoroughly investigated. Experimental results show that the addition of Sr to HPDC AZ91-0.5RE based alloys can refine the microstructure including externally solidified crystal α-Mg and eutectic compound β-Mg₁₇Al₁₂. On the other hand, when the content of Sr reaches 1.0%, a small amount of plate-like Al₄Sr phase can be formed. Moreover, minor of Sr addition can significantly decrease the volume fraction of β-Mg₁₇Al₁₂ precipitates of AZ91-0.5RE based alloys. In addition, the mechanical properties of AZ91-0.5RE based alloys can be improved remarkably with Sr addition, and the alloy containing 0.2% Sr exhibits the best mechanical properties at ambient temperature. Fractographic analyses demonstrate that the fracture modes of the alloy without and with Sr are quasi-cleavage fracture and ductile fracture, respectively. Furthermore, Creep properties of HPDC AZ91-0.5RE based alloys were obviously improved by adding Sr element. And creep mechanisms of the alloys with and without Sr at 150 °C/30–70 MPa and 50 MPa/140–160 °C are both considered to be dislocation climb controlled creep based on the analyses of the stress exponent and apparent activation energy. The AZ91-0.5RE-1.0Sr alloy presents the best creep resistance among the studied alloys, which is mainly attributed to the presence of thermally stable intermetallic compound of Al₄Sr phase at the grain boundaries.