Effects of minor Sr addition on the microstructure, mechanical properties and creep behavior of high pressure die casting AZ91 0.5RE based alloy
Abstract 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 a Mg and eutectic compound b 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 b 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 degC/30 70 MPa and 50 MPa/140 160 degC 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.