The phase equilibria and thermal stability of the long-period stacking ordered phase in the Mg–Cu–Y system

•14H LPSO structure has been confirmed to be stable in the Mg–Cu–Y system.•Partial isothermal sections of the Mg–Cu–Y system from 300 to 450 °C have been established.•Reaction L + α-Mg ↔ 14H + Mg2Cu has been determined in the Mg–Cu–Y system.•The thermal stability of the 14H phase in the Mg–Cu–Y system has been well studied.

Phase equilibria in the Mg-rich Mg–Cu–Y system at 300, 400 and 450 °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 microscope (TEM). The results show that a long-period stacking ordered (LPSO) phase 14H is stable in the Mg–Cu–Y system, which is the only one ternary intermetallic compound that gets a thermodynamic equilibrium with the a-Mg phase. 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 Mg91Cu4Y5. The thermal stability of the 14H phase in the Mg–Cu–Y system has been well studied, which shows that the 14H phase disappears varying from 442 °C to 490 °C depending on the alloy composition. The isothermal sections of the Mg-rich Mg–Cu–Y system at 300, 400 and 450 °C have been finally established, and moreover, a quasi-peritectic reaction L + α-Mg ↔ 14H + Mg2Cu has been determined occurring at 442 °C with an estimated liquid composition of Mg77Cu18Y5.