Formation and microstructural stability of multiphase intermetallics based on NiAl

Multiphase NiAl(Fe)(Cr)-based intermetallics with β(B2) + [γ(A1) + γ′(L12)], β + γ or β + γ′ microstructures can offer significant tensile ductilities. In the case of NiCrAl-based alloys, microstructural development is complicated by the precipitation of α-Cr, which can supplant the γ phase during aging of three-phase β + (γ + γ′) microstructures. This paper presents an investigation of microstructural development in cast NiCrAl(Co) alloys which begin to solidify in the β-phase field and subsequently form interdendritic γ + γ′ regions. As-cast microstructures and the effect on microstructural stability of aging at 850 and 1100 °C for 140 h are discussed. In the as-cast condition, the materials investigated consisted of a dendrite matrix comprised entirely of L10-type martensite, with interdendritic regions composed principally of cuboidal γ′ in γ. The two-phase γ + γ′ interdendritic microstructure was retained after aging of an Ni-24at.%Al-21at.%Cr alloy at 850 or 1100 °C. Extensive α-Cr formation was observed in the as-cast condition, but aging did not result in fresh α-Cr precipitation.