Phase equilibria of the long-period stacking ordered phase in the Mg–Ni–Y system

Abstract

Phase equilibria in the Mg-rich Mg–Ni–Y system at 300, 400 and 500 °C have been experimentally investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), electron probe micro-analyzer (EPMA) and transmission electron microscopy (TEM). The results show that a long-period stacking ordered (LPSO) phase with 14H structure is thermodynamically stable in the Mg–Ni–Y system in a wide temperature range, but it dissolves varying from 492 to 559 °C depending on the alloy composition. The equilibrium 14H phase has a very limited solid solution range, and can be nearly regarded as a ternary stoichiometric compound with a formulae as Mg91Ni4Y5. The isothermal sections of the Mg-rich Mg–Ni–Y system at 300, 400 and 500 °C have been finally established, and a eutectic reaction, Liquid ↔ α-Mg + 14H + Mg2Ni, has been determined occurring at 492 °C with a liquid composition about Mg84.8Ni12.0Y3.2.