Structural heat-resistant β-NiAl + γ′-Ni3Al alloys of the Ni–Al–Co system: I. Solidification and structure

Abstract

When analyzing the ternary Ni–Al–M phase diagrams, where M is a group VI–VIII transition metal, we chose the Ni–Al–Co system, where the γ′ and γ phases are in equilibrium with the β phase, as a base for designing alloys with the following physicochemical properties: a moderate density (≤7.2 g/cm³) and satisfactory heat resistance at temperatures up to 1300°C. The structure formation in heterophase β + γ′ alloys during directional solidification is studied. It is found that, in contrast to cobalt-free β + γ′ alloys (where the γ′-Ni₃Al aluminide forms according to the peritectic reaction L + β ⇄ γ′), the alloys with 8–10 at % Co studied in this work during directional solidification at 1370°C contain the degenerate eutectic L ⇄ β + γ. The transition from the β + γ field to the β + γ′ + γ field occurs in the temperature range 1323–1334°C, and the γ′ phase then forms according to the reaction β + γ ⇄ γ′.