Thermodynamic re-assessment of the Mg–Y binary system coupling with the precipitation sequence during aging process

Abstract

The experimentally observed aging precipitation sequence of the Mg–Y alloys from literatures is α(Mg)SS(supersaturated)→ β''(c-bco like GP zones)→ β′(c-bco, metastable coherent Mg7Y)→ β(bcc, stable incoherent Mg24Y5). Compared with the equilibrium phase diagram, Mg7Y is not a stable precipitate, but from the view point of the crystal structure, Mg7Y is an ordering product of HCP_A3(Mg). The Mg–Y binary system has been thermodynamically re-assessed by means of CALPHAD technique in order to study its aging precipitation sequence in the present work. Three solution phases, HCP_A3, BCC_A2 and Liquid, are modeled with the Redlich-Kister polynomial. The intermetallic compounds Mg2Y and Mg24Y5 have been described by the following formulas, (Mg, Y)0.6667(Mg, Y)0.3333 and Mg0.8276(Mg, Y)0.1379 Y0.0345, respectively. Especially for order-disorder transitions, the following two-sublattice models (Mg, Y)0.5(Mg, Y)0.5 and (Mg, Y)0.875(Mg, Y)0.125 have been used to describe the stable MgY/BCC_A2 and the metastable Mg7Y/HCP_A3, respectively. Based on the satisfied optimization of equilibrium phase diagram, the order-disorder transition between the metastable intermediate Mg7Y and the stable phase HCP_A3 is assessed to ensure that Mg7Y has a big enough driving force but not a lowest Gibbs free energy change to precipitate preferentially from the supersaturated α(Mg). Meanwhile, in order to match the aging precipitation sequence of the Mg–Y alloys, the effective nucleation driving forces for the precipitated phases are calculated by subtracting the interfacial energy and the elastic strain energy from the thermodynamic driving force. A set of self-consistent thermodynamic parameters has been obtained to agree satisfactorily with the reported experimental data from literatures, including the phase diagram, the thermochemical properties, and the aging precipitation sequence of the typical Mg–Y alloys with x(Y) = 0.0232 at 473 K.