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 ^oC, AND 800 ^oC

Abstract

The phase equilibria in the Zr–ZrCo–ZrNi system at subsolidus temperature and temperatures of 900 and 800 °C were first studied using physicochemical analysis methods, and the solidus surfaces and isothermal sections of the system 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 forms by peritectoid reaction áβ-Zrñ + áZr₂Coñ → η at 980 °C, being close to the L ↔ θ + β eutectic crystallization temperature (986 °C), in the binary Zr–Co system. At 900 and 800 °C, the η phase dissolves 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 widens 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: d + d₂ + q. The solidus surface of the Zr–ZrCo ZrNi system is thus completed by surfaces corresponding to the homogeneity regions of the δ, d₂, q, and β phases, the q + d + d₂ tie-line triangle plane, and the ruled surfaces bounding the two-phase q + d₂, q + d, and θ + β volumes from above. At 900 and 800 °C, two three-phased equilibria, η + θ + β and δ + δ₂ + θ, are observed in the system.