Solidification behavior of γ′-Ni3Al-containing alloys in the Ni–Al–O system

The chemical activities of Al and Ni in γ′-Ni3Al-containing alloys were measured using the multi-cell Knudsen effusion-cell mass spectrometry technique, over the composition range 8–32 at.% Al and temperature range T = 1400 to 1750 K. From these measurements a better understanding of the equilibrium solidification behavior of γ′-Ni3Al-containing alloys in the Ni–Al–O system was established. Specifically, these measurements revealed that (i) γ′-Ni3Al forms via the peritectiod reaction, γ + β (+Al2O3) = γ′ (+Al2O3), at 1633 ± 1 K; (ii) the {γ + β + Al2O3} phase field is stable over the temperature range 1633–1640 K; and (iii) equilibrium solidification occurs by the eutectic reaction, L (+Al2O3) = γ + β (+Al2O3), at 1640 ± 1 K and a liquid composition of 24.8 ± 0.2 at.% Al (at an unknown oxygen content). When projected onto the Ni–Al binary, this behavior is inconsistent with the current Ni–Al phase diagram and a new diagram is proposed. This new Ni–Al phase diagram explains a number of unusual steady-state solidification structures reported previously and provides a much simpler reaction scheme in the vicinity of the γ′-Ni3Al phase field.