Effect of Ti addition on the microstructure and mechanical properties of cast Mg-Gd-Y-Zn alloys

The effect of Ti addition on the microstructure and mechanical properties of cast Mg-Gd-Y-Zn alloys was investigated. Ti exhibited a refinement effect comparable to that of Zr in the as-cast condition. With the increase of Ti content, the phenomenon of grain refinement and homogenization was more obvious, especially for solution-treated alloys. The finest grains were obtained with 0.8 wt% Ti and were composed of α-Mg and refined Mg5(Gd, Y, Zn) in the as-cast structure. Zr and Ti could be found along the grain boundaries. After solid solution treatment, Zr was still concentrated in the vicinity of the grain boundaries, while Ti was uniformly dispersed in the α-Mg matrix. The Ti-refined alloys exhibited superior grain size stability and mechanical properties. In addition, the mechanical properties of the considered alloys were improved due to the transition from Mg5(Gd, Y, Zn) to long-period stacking ordered (LPSO) phase during the heat treatment. Subsequent aging at 220 ℃ resulted in remarkable precipitation hardening and consequently the strength was significantly improved. The Ti-refined alloy exhibited higher ultimate tensile strength and elongation than the alloy containing the same amount of Zr, with an increase of 20 MPa and 29%, respectively.