Effect of lattice disregistry variation on the late stage phase transformation behavior of precipitates in NiAlMo alloys

Abstract

A variety of experimental techniques have been applied to examine how compositional variations in lattice disregistry (δ) affect the coarsening kinetics and particle morphology evolution of γ′ particles in several Ni-Al-Mo alloys. Mo segregates to and increases the lattice parameter of the γ matrix in two phase Ni-base Al alloys. Therefore, controlled ternary additions of Mo to dilute, binary Ni-Al alloys permits variation of the lattice parameter disregistry between the Ni-base solid solution and the Ni 3Al ordered precipitates. Compositions were selected to maintain an approximately constant 10% volume fraction of the second phase. The coarsening rate of the γ′ phase is observed to decrease with decreasing lattice disregistry and decreasing Al Mo ratio. The implications of this observation are discussed with reference to differences in solid solubility, diffusivity and interfacial energy. The γ′ particle shape in low coherency strain alloys is observed to be spherical and independent of particle size. In an alloy exhibiting appreciable δ and coherency strain, the γ′ particles evolve from randomly distributed spheres to sharply aligned cubes and rods as particle size increases. There is also an increase in δ with aging time in this alloy. Compression creep of [100] single crystals at 750°C did not affect the shape of particles in the low coherency strain alloys while in a higher disregistry alloy, a highly oriented array of plates grew with long edges perpendicular to the applied stress direction. These findings are discussed with regard to stress and thermodynamic considerations.