The Co–Ni–Zr Phase Diagram in the Zr–ZrCo–ZrNi Region I. Phase Equilibria in the Zr–ZrCo–ZrNi System at Subsolidus Temperature, 900°C, and 800°C

Abstract

The phase equilibria in the Zr–ZrCo–ZrNi system at subsolidus temperature and at 900 and 800°C were first studied using physicochemical analysis methods, and solidus surfaces and isothermal sections at 900 and 800°C were constructed. Isomorphic Zr2Co and Zr2Ni compounds with a tetragonal crystal structure of AlCu2 (θ) type form a continuous series of solid solutions dividing the Zr–ZrCo–ZrNi system into two subsystems: Zr–Zr2Co–Zr2Ni and ZrCo–ZrNi–Zr2Ni–Zr2Co. The equilibria on the solidus surface of the Zr–Zr2Co–ZrCo–Zr2Ni system and at 900°C differ significantly. This is associated with the Zr3Co-based η phase formed by peritectoid reaction 〈β − Zr〉 + 〈Zr2Co〉 → η at 980°C, being close to theL ↔ θ + β eutectic crystallization temperature (986°C), in the binary Zr–Co system. At 900 and 800°C, the 𝜂 phase dissolves up to 14.5% Ni. The solidus surface of the ternary Zr2Co–Zr2Ni–ZrCo–ZrNi system shows a three-phase equilibrium of the θ phase with the ZrCo (δ) and ZrNi (δ2) phases of the ZrCo–ZrNi quasibinary section: δ + δ2 + θ. The plane of this tie-line triangle extends significantly at 900 and 800°C as the solubility of nickel in the cubic ZrCo-based phase of CsCl type changes. At room temperature, all alloys of the ZrCo–ZrNi–Zr2Ni–Zr2Co subsystem should contain three phases: δ + δ2 + θ. The solidus surface of the Zr–ZrCo ZrNi system is thus completed by surfaces corresponding to the homogeneity regions of the δ, δ2, δ, and β phases, θ + δ + δ2 tie-line triangle plane, and ruled surfaces representing the upper boundary of the two-phase θ + δ2, θ + δ, and θ + δ volumes. At 900 and 800°C, two three-phase equilibria, η + θ + β and δ + δ2 + θ, are observed in the system.