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.