FORMATION AND MICROSTRUCTURAL STABILITY OF MULTIPHASE INTERMETALLICS BASED ON NIAL

Multiphase Ni-Al-(Fe)-(Cr)-based intermetallics with beta(B2)+[gamma(A1)+gamma'(L1(2))], beta + gamma or beta + gamma' microstructures can offer significant tensile ductilities. In the case of Ni-Cr-Al-based alloys, microstructural development is complicated by the precipitation of a-Cr, which can supplant the gamma phase during aging of three-phase beta + (gamma + gamma') microstructures. This paper presents an investigation of microstructural development in cast Ni-Cr-Al-(Co) alloys which begin to solidify in the beta-phase field and subsequently form interdendritic gamma + gamma' regions. As-cast microstructures and the effect on microstructural stability of aging at 850 and 1100 degrees C for 140 h are discussed. In the as-cast condition, the materials investigated consisted of a dendrite matrix comprised entirely of L1(o),-type martensite, with interdendritic regions composed principally of cuboidal gamma' in gamma. The two-phase gamma + gamma' interdendritic microstructure was retained after aging of an Ni-24at.%Al-21 at %Cr alloy at 850 or 1100 degrees C. Extensive alpha-Cr formation was observed in the as-cast condition, but aging did not result in fresh alpha-Cr precipitation.