The influences of Sr on the microstructure and mechanical properties of Mg-5Zn-2Al alloy

The effects of Sr on the microstructure and mechanical properties of Mg-5Zn-2Al alloy are studied. The results indicate that the as-cast ZA52 alloy mainly consists of α-Mg matrix, τ-Mg32(Al, Zn)49 and MgZn phases. Sr addition results in the precipitation of Al4Sr and suppression of MgZn phase. In high Sr-containing alloys, the Mg5Zn2Al2 phase can also be observed. Both the dendrite and grain size of the alloys can be refined by 1.0% Sr addition. However, the 0.2% Sr addition results in the grain coarsening in the alloy due to the decrease of contribution of Al and Zn solutes on the grain refinement. Both extrusion process and Sr addition give a response in refined microstructure. The yield strength and ultimate tensile strength of extruded alloys are increased, respectively, by about 167 MPa and 135 MPa in compared with the as-cast state. The elongations to failure enhanced by about 1 time, reaches 9.9–16.5%, with a strength of 322–338 MPa. Go along with Sr increase, yield strength increases and elongation decreases gradually in as-extruded alloy. The elongation of all the alloys decreases rapidly when Sr content exceeds 0.5% due to the formation of Mg5Zn2Al2 phase.