The Co–Ni–Zr Phase Diagram in the Zr–ZrCo–ZrNi Region II. Liquidus Surface of the Phase Diagram. Interaction of Alloys with Hydrogen

The results of microstructural, X-ray diffraction, differential thermal, and electron microprobe analyses of Zr–ZrCo–ZrNi cast alloys were used to first construct the liquidus surface of the Zr–ZrCo–ZrNi system on the composition triangle. The liquidus surface was found to consist of four primary crystallization surfaces of the phases: a solid solution of β-Zr and phases based on ZrCo (δ) and ZrNi (δ₂) compounds and a θ phase (continuous solid solutions between Zr₂Co and Zr₂Ni isostructural compounds of AlCu₂ type). In crystallization of the alloys, an invariant four-phase transition equilibrium with participation of the liquid phase, L + δ ↔ δ₂ + θ, was observed at 1025°C. The constitution of the quasibinary sections of the ZrCo–ZrNi and Zr₂Co–Zr₂Ni systems, showing a decrease in the solidus and liquidus temperatures of the alloys with increasing nickel content, determined the type of invariant equilibrium. The study of cast alloys confirmed that the phase based on Zr₃Co (η) compound formed by peritectoid reaction. A scheme of reactions occurring in Zr–ZrCo–ZrNi alloys in the temperature range from crystallization to solid-state transformations associated with the peritectoid formation of the η -phase and with the martensitic transformation of β ↔ α zirconium was developed. Two vertical sections were constructed using data on phase equilibria in the Zr–Co and Zr–Ni boundary binary systems and data on equilibria on the solidus surface and at 900 and 800°C in the Zr–ZrCo–ZrNi ternary system. Data on the interaction of individual alloys in the system with hydrogen were obtained. Significant rates of absorption and release of hydrogen from the hydrides were observed above room temperature.