Phase Equilibria in the Ti-Zr-Ni System Part 2: Isothermal Sections and Solid-State Transformations in the ZrNi–Ni–TiNi Subsystem

Abstract

Phase equilibria and solid-state transformations in the Ni-rich region of the Ti–Zr–Ni system were investigated through a combined application of scanning electron microscopy, electron probe microanalysis, x ray diffraction, and differential thermal analysis. Comprehensive analysis enabled the construction of isothermal sections at 960 °C and 750 °C, revealing substantial differences in phase topology across this temperature interval. At 960 °C, the ternary system exhibits equilibria that differ markedly from the Zr–Ni binary: the intermetallic compounds ZrNi₃ and Zr₉Ni₁₁ remain stable due to the stabilizing effect of Ti, forming isolated phase fields that are not present in the binary diagram. The formation temperature of ZrNi₃ was established as 1000 °C, while Zr₉Ni₁₁ decomposes below 750 °C. At 750 °C, the phase relations significantly differ from those at 960 °C as a result of a sequence of solid-state reactions. The most noteworthy of these is the decomposition of the ternary compound τ₂, producing Zr₈Ni₂₁, Zr₇Ni₁₀, and TiNi₃, and fundamentally altering the topology of the isothermal section. Intermediate annealing at 900 °C provided insight into the transitional behavior between the two isothermal sections, revealing incomplete transformations and confirming the peritectoid formation of ZrNi₃ from Zr₂Ni₇, τ₃, and Zr₈Ni₂₁. The resulting reaction scheme for the ZrNi–Ni–TiNi subsystem integrates high-temperature stability, intermediate transformations, and low-temperature decomposition pathways. These findings establish a coherent thermochemical framework for the Ni-rich portion of the Ti–Zr–Ni system and provide essential reference data for thermodynamic modeling and the design of Ti–Zr–Ni intermetallic alloys.