Formation of γ'-Ni₃Al via the Peritectiod Reaction: γ + � (+ Al₂O₃) = γ' (+ Al₂O₃)

Abstract

The activities of Al and Ni were measured using multi-cell Knudsen effusion-cell mass spectrometry (multi-cell KEMS), over the composition range 8 -32 at.%Al and temperature range T = 1400 − 1750 K in the Ni-Al-O system. These measurements establish that equilibrium solidification of γ′-Ni 3 Al-containing alloys occurs by the eutectic reaction, L (+ Al 2 O 3 ) = γ + β (+ Al 2 O 3 ), at 1640 ± 1 K and a liquid composition of 24.8 ± 0.2 at.%Al (at an unknown oxygen content). The {γ + β + Al 2 O 3 } phase field is stable over the temperature range 1633 − 1640 K, and γ′-Ni 3 Al forms via the peritectiod, γ + β (+ Al 2 O 3 ) = γ′ (+ Al 2 O 3 ), at 1633 ± 1 K. 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 steadystate solidification structures reported previously and provides a much simpler reaction scheme in the vicinity of the γ′-Ni 3 Al phase field.