Effect of Sn addition on the microstructure and mechanical properties of AZ31 alloys

An Mg-3Al-1Zn-xSn (x = 0, 3, 6, 9) alloy was prepared by die-casting and analyzed by XRD, SEM, and EBSD. The microstructure, second phase, and grain orientation of the AZT31x alloy were characterized. As the Sn content increased from 3 wt.% to 9 wt.%, the tensile and yield strength of the alloy were effectively improved. With the addition of Sn, the grain size of alloys decreases gradually blocking the dislocation by the grain boundary and the dispersion of the Mg2Sn second phase in the AZT31x(x = 3, 6, 9) alloys contributes to the strength via grain boundary pinning. According to theoretical analysis and calculation, the high strength of AZT319 alloy is partly attributed to the grain fine strengthening (sigma HP = 13.76 Mpa) and second phase strengthening (Delta sigma = 9.27 similar to 12.67 MPa). The total increased strengthening value is lower than experimental value (Delta sigma = 31.82 MPa). The ratio about tau(prism)/tau(basal) and tau(< c+a >)/tau(basal) in die-cast alloys tensioned to 0.08 deformation gradually decrease, which can reflect that high-Sn content contributes to the strain hardening behavior. The ductility of AZT313 alloy was lightly improved due to the {10-12} tensile twins. When excessive Sn was added, the Mg2Sn second phase coarsened and acted as the nucleus of micro-cracks during the stretching process, thereby reducing the ductility of the alloy.