The effect of uniaxial stress on the periodic morphology of coherent gamma prime precipitates in nickel-base superalloy crystals

Abstract

The periodic morphology of cube shaped, coherent γ ′ [Ni3(Al, Ti)] precipitates in a representative nickel-base superalloy single crystal is shown to be influenced by the application of a 〈100〉 oriented uniaxial stress during annealing at elevated temperature. Furthermore, the stress annealed γ′ morphologies depend on the stress sense. Tensile and compressive stress annealing result in a directional coarsening of precipitates, and then agglomeration to form γ ′ precipitate plates with broad faces aligned perpendicular to the stress axis and precipitate parallelepipeds with long axes parallel to the stress axis, respectively. In explaining the morphological changes that occur during stress annealing, we consider two driving forces: i) a driving force due to changes in the stress-free shape of the specimen, and ii) a driving force due to changes in the effective modulus of the specimen. The first, which depends on a difference in the elastic constants and the lattice parameters of the two phases, is apparently the dominant driving force for the observed morphological changes in a stress annealed superalloy. The second, which depends only on a difference in elastic constants, may be important in alloys with elastically soft, incoherent inclusions such as pores.