Experimental Study on Phase Equilibria in the Vicinity of <i>X</i>, <i>W</i> and <i>H</i> Phases in the Mg–Zn–Y Ternary System

Abstract

Three Mg–Zn–Y ternary alloys in the vicinity of X, W and H phases were prepared and were isothermally heat treated at 833, 793, 723 and 673 K 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 W and 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 (L21) type crystal structure with a stoichiometric composition of Mg1Zn2Y1, and fine mesh texture composed of α (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 (P63/mmc) was found to be different from that of the previous reports.