Discontinuous coarsening of discontinuous precipitate in a Ni-7.5 at.% In alloy

Abstract

The morphology and growth kinetics of the discontinuous coarsening of discontinuous precipitate in a Ni-7.5 at.% In alloy have for the first time been investigated at temperatures ranging from 667 to 1030 K by light and scanning electron microscopy and X-ray diffraction. At all aging temperatures the alloy was observed to decompose completely by discontinuous precipitation into a fine lamellar structure of nickel rich solid solution and θ (Ni 3In) precipitate phase. This lamellar structure was then decomposed at all aging temperatures by a discontinuous coarsening reaction. This reaction occurred at a much slower rate than the first reaction and resulted in a much coarser lamellar structure of the same phases. Lattice parameter measurements showed the Ni rich solid solution in the product of the first reaction to be far from equilibrium while that of the second reaction was close to equilibrium. Analysis of the growth rates, lamellar spacings and phase compositions for both the discontinuous precipitation and discontinuous coarsening reactions showed that they were controlled by grain boundary diffusion. In analyzing the results a generally applicable procedure for calculating the driving force is presented. In this calculation the solid solution is treated as a regular solution and the actual thermodynamic data are used for the precipitate phase. The driving forces calculated in this way should be more reliable than those calculated with the approximations based on Raoult's and Henry's laws.