THERMODYNAMIC ASSESSMENT OF THE Mg–Ni–Si SYSTEM

Abstract

The thermodynamic assessment of the Mg–Ni–Si system has been developed in the framework of the CALPHAD method. The critical review of the literature data has been provided. A set of self-consistent thermodynamic parameters for the phases in this ternary system is obtained using the phase diagram data available in the literature. The excess Gibbs energy term of solution phases is described using the Redlich-Kister polynomial. The sublattice model is used to describe the MgNi₂ intermetallic phase having homogeneity range in the ternary system. Ternary intermetallic compounds are modelled as line compounds. Isothermal sections and liquidus surface, as well as the coordinates of the invariant reactions, are calculated. The calculated results are in satisfactory agreement with the experimental data. The calculated liquidus surface is extremely complex and contains 38 invariant reactions including 8 degenerated reactions and 10 invariant maxima. Most of the presented invariant reactions must be considered as approximate. The τ₃ and τ₅ phases melt congruently at 1544 K and 1487 K, respectively. The τ₄ forms via the peritectoid τ₅ + NiSi + βNi₂Si ↔ τ4 reaction at a temperature of 1193 K. According to our calculations, τ₁ and τ₈ melt at 1359 K (the L + τ₅ ↔ τ₁ reaction) and 1267 K (the L + τ₁ ↔ τ₈ reaction). The τ₂ and τ₆ are formed via the peritectic reactions at 1317 K (the L + τ₃ + MgNi₂ ↔ τ₂ reaction) and at 1229 K (the L + τ₅ + Mg₂Si ↔ τ₆ reaction), and the τ₇ is formed via peritectoid Mg₂Si + τ₁ + τ₅ ↔ τ₇ reaction at 1279 K.