Kinetics of discontinuous coarsening of cellular precipitate in a Cu-15 wt% in alloy

Abstract

The morphology and growth kinetics of the cellular precipitate and discontinuously coarsened cellular precipitate have been studied in the temperature range 573–731 K by utilizing optical and scanning electron microscopy and X-ray diffraction. In order to avoid precipitation of the Widmanstatten precipitate phase, which has a retarding effect on the rate of growth of primary cells, isothermal aging of the alloy was preferred. The Cu-In alloy was observed to decompose completely by cellular precipitation reaction into a lamellar structure consisting of alternate lamellae of the α and δ phases at all aging temperatures. The fine lamellar structure of the primary cells decomposed into a coarse lamellar structure consisting of the same two phases by a discontinuous coarsening or secondary reaction. Lattice parameter measurements indicated that whereas the depleted matrix was richer in solute than the equilibrium solvus during the primary reaction, it was very close to the equilibrium solvus during the secondary reaction. Analysis of the growth kinetics both of the primary and secondary cellular reaction indicated that the transformations are controlled by diffusion through the cell boundaries.