Alloy development of high toughness Mg-Gd-Y-Zn-Zr alloys

The effects of zinc addition on microstructure evolution and mechanical properties of Mg-Gd-Y-(-Zr) based alloys are investigated in details using OM, TEM, Vickers hardness tests and tensile tests. Specific line-shaped structure is formed inside of matrix grain in the as-cast This structure is dissolved after a solution-treatment at 773 K. Instead of it, in the 0.3-1Zn alloy, the 14H LPSO structure is observed at grain boundaries of Mg matrix phases after the solution heat treatment. Metastable beta' phase is formed during subsequent aging treatment at 498 K. The 14H LPSO structure is stable and remained even after the aging treatment. These structure and phase coexist at the peak-aged condition in the microstructure. In cast- specimens, 0.2% proof stress and tensile strength slightly decreases with an addition of zinc up to 0.75 mol% and rapidly decreases over I mol% zinc addition. On the contrary, in rolled-specimens, addition of 0.3-1 mol% zinc improves mechanical properties in both strength and ductility significantly. It is found that the UTSs reach more than 400 MPa in the 0.3-1Zn alloys. This characteristic effect is considered to be the contribution of LPSO structure and its specific orientation relationship with matrix phase.