Phase equilibria among γ-Fe, TCP (Fe2Nb-ε), GCP (Ni3Nb-δ, hP24) and liquid phases in Fe-Ni-Nb ternary system at 1473 K and 1373 K were experimentally examined. At 1473 K, a ternary compound hP24 phase exists in the composition range of Fe-21.5Nb-(56.8~59.8) Ni. Liquid phase also exists around the composition of Fe-17Nb-50Ni. At 1373 K, the hP24 phase remains stable, whereas the liquid phase disappears due to a temperature invariant reaction (L→γ+ε+hP24), resulting in the three three-phase coexisting region around the hP24 phase: γ+hP24+ε, γ+hP24+δ, ε+hP24+δ. The interaction parameters for thermodynamic calculation to reproduce the experimental phase diagrams at both temperatures were successfully evaluated. The calculated isothermal section at 973 K using the parameters revealed the ternary eutectoid decomposition of the hP24 phase, leaving a γ+ε+δ three-phase coexisting region extended toward Fe-rich side. The results suggest a possibility to design austenitic heat resistant steels strengthened by both TCP and GCP phases.
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