Influence of the Cobalt Concentration and Chromium Alloying on the Structure of (β-NiAl + γ'-Ni3Al + γ-Ni) Ni–Al–Co Superalloys

Abstract

The influence of cobalt (5 and 16 at %) and chromium (4.25 at %) on the structure of a (β-NiAl + γ'-Ni₃Al + γ-Ni) alloy with 27 at % Al of the Ni–Al–Co system, which is chosen as the basis of alloys with given physicochemical properties, in particular, moderate density (≤7.2 g/cm³) and sufficient heat resistance at 1100–1300°C, is studied. A high stability of the high-temperature γ phase is found. It forms upon solidification according to the reaction L\( \rightleftarrows \) β + γ and is nonequilibrium at 1200 and 1300°C (below the temperatures of the transformations β + γ \( \rightleftarrows \) β + γ + γ' \( \rightleftarrows \) β + γ'). The volume fraction of the γ-Ni phase increases with the cobalt content or upon chromium addition in comparison with its fraction in the base alloy. The influence of alloying on the cobalt and chromium distribution in alloy phases is estimated. Additional alloying increases the differences in the phase compositions in cobalt and chromium.