The (γ + γ′)/γ′ phase boundary in the Ni–Al phase diagram from 600 to 1200°C

Abstract

Abstract The phase boundary separating the γ + γ′ and γ′ regions of the Ni–Al phase diagram (the γ solvus) was established over the temperature range 600–1200°C. The γ phase is the Ni–Al solid solution, and γ′ is (non-stoichiometric) Ni3Al. Cast alloys containing 22.0, 22.2, 22.4, 22.6, 22.8 and 23.0 at.% Al, all compositions being within ±0.03 at.%, were used in the experiments. At temperatures from 1000 to 1200°C, the γ solvus was determined using aging treatments designed to eliminate the dendritic structure in the as-received castings. At lower temperatures, from 600 to ≈ 900°C, dissolution experiments were conducted. The method takes advantage of the absence of a barrier to the dissolution of a dispersed phase. Specimens of the supersaturated γ′ phase were aged at low temperatures, 600 or 650°C, for long times (up to 633 h), to produce fine dispersions of the coherent γ phase precipitates. These specimens were then re-aged at 10–20°C intervals of temperature and re-examined to determine whether the γ...