Using elastic energy considerations to explain rafting in Ni-based superalloys with a high gamma volume fraction

Abstract

A simple three-dimensional model of a high y' volume fraction Ni-based alloy is developed. The model is based on the idea that a unit volume in the superalloy can be represented by a cuboidal precipitate with thin 'slabs' of matrix material 'stuck' onto its faces in such a way that coherency is maintained. Rafting is investigated by considering the changes in total energy whon the cuboidal precipitates start to flatten into plate shapes or lengthen into rod shapes. It is assumed that inelastic effects are negligible. Internal and external stresses and strains are determined. Expressions for the derivative of the total energy with respect to a shape parameter are then calculated in the absence of applied stress and in the presence of applied stress. Predictions of rafting behaviour are made for six alloys. Excellent agreement is found with experimental evidence but it is suggested that the agreement is fortuitous.