On the modulated structure of aged Ni-Al alloys: with an Appendix On the elastic interaction between inclusions by J. D. Eshelby

Abstract

Studies of the early stages of ageing in a Ni-6.71 wt.% Al alloy have shown that the modulated (periodic) arrangement of the γ′ precipitate evolves from an initially random spatial distribution of γ′ nuclei by a process of alignment during coarsening. It is concluded that spinodal decomposition is not responsible for the modulated structure under the ageing conditions employed. The ageing kinetics support the conclusion that growth is due to coarsening, since the mean particle size increases according to a (time) 1 3 law. The experimentally determined coarsening parameters are in excellent agreement with the values in the literature. It is shown that there exists an elastic interaction between the γγ particles arising from the difference between the elastic moduli in the precipitate and the matrix. The strength of the interaction is considered and it is postulated that it affects the coarsening process by selecting favourably located particles for growth rather than dissolution, all other factors being equal. Since the experimentally determined γ′-matrix interfacial energy is small (~30 erg/ cm 2), the coarsening rates are sufficiently slow to allow time for the selection process to occur.