Experimental Study on Phase Equilibria in the Vicinity of X, W and H Phases in the Mg-Zn-Y Ternary System

Three Mg-Zn-Y ternary alloys in the vicinity of X, Wand H phases were prepared and were isothermally heat treated at 833, 793, 723 and 673K to attain thermodynamic equilibrium. The microstructure was observed using electron probe microanalysis and transmission electron microscopy, and the chemical compositions of the equilibrium phases were analyzed using wavelength dispersive X-ray spectroscopy. The crystal structure of the Wand H phases were analyzed using X-ray diffraction, electron back scattering diffraction and electron diffraction pattern obtained by transmission electron microscopy. The Mg-Zn-Y ternary phase diagrams of the isothermal section were also calculated using the Thermo-Calc to compare with the experimental results. The X phase was found to be solidified in high-Mg alloys not in a manner of eutectic reaction as reported previously but in a manner of peritectic reaction. The W phase has a Heusler (L2(1)) type crystal structure with a stoichiometric composition of Mg1Zn2Y1, and fine mesh texture composed of alpha (Mg) and W phases was occasionally observed in non-equilibrium solidified parts, implying that the transient W phase has potential to be utilized as an additional strengthening phase. The equilibrium chemical composition of the H phase with a hexagonal crystal structure (P6(3)/mmc) was found to be different from that of the previous reports.