Effects of Zn content on microstructures and mechanical properties of as cast Mg-Zn-Y-Zr alloys

In the present work, the effects of Zn content on the microstructures and mechanical properties of as cast Mg-xZn-5Y-0.6Zr alloys (x=2, 5, 8 and 13 wt-%) have been investigated. The results show that the ternary Mg-Zn-Y phase compositions change with Zn/Y ratios induced by the change in Zn content. It is found that the fracture is mainly decided by the characteristics and distribution of second phase rather than the grain size. The influences of these phases, especially the W phase, on the mechanical properties of the alloys have been discussed. Both ultimate tensile strength (UTS) and elongation decrease with the increase in Zn content, while the instance of yield strength (YS) is just the reverse. The W phase is easily cracked because of its brittleness and easy to result in decohesion from the matrix because of the weak atomic bonding, which greatly degrade the UTS and elongation. It can be concluded that the YS closely depends on the grain size, while UTS and elongation closely depend on the volume fraction of eutectic compound (alpha-Mg+W phase).