Phase equilibria in Ni-rich region of Ni–Al–Hf system

Abstract

Phase equilibria have been investigated in the Ni-rich portion of the Ni–Al–Hf system at 1473 and 1273 K, using electron microprobe analysis, X-ray diffraction analysis, and optical metallography. The phases involved are the Ni-rich solid solution (γ), Ni₃Al (γ′), NiAl (β), Ni₇Hf₂, Ni₅Hf, Ni₃Hf, Ni₇Hf₃, and Ni₂HfAl. A large extension of the Ni₇Hf₂ compound into the ternary system was observed and the following three-phase regions existed at 1473 K: γ + Ni₅Hf + Ni₇Hf₂, γ + γ′ + Ni₇Hf₂, γ′ + β + Ni₇Hf₂, and β + Ni₂HfAl + Ni₇Hf₂. On cooling from 1473 to 1273 K a peritectoid reaction γ + Ni₇Hf₂ → γ′ + Ni₅Hf occurs. A liquidus plan has been proposed for the section Ni–Ni₃Al–Ni₇Hf₂. Differences between the present and previous work are discussed.