KINETICS OF DISCONTINUOUS PRECIPITATION IN COPPER-INDIUM ALLOYS

Supersaturated α phase copper-indium alloys decompose by both a continuous and discontinuous mode. An additional fine precipitate, called δ-, forms in the 10 at.% In alloy. The general precipitate does not usually interfere with the cellular reaction. The growth rate, ν, and lamellar spacing, S, of the discontinuous precipitate were measured, as well as the average a phase composition for some heat treatments. For a given initial composition the reciprocal of spacing extrapolates to the value zero at a temperature below the solvus. Transmission electron micrographs of the advancing interface region are presented. Various theories are evaluated using the kinetic data. Turnbull's interface diffusion model yields a reasonable growth rate, but the activation energy of the interface diffusion coefficient, bDB, appears high, 36.5 kcal/mole. The large S values support Cahn's boundary diffusion and reaction control model. However, the boundary mobility required here is about 1/1000 ofthat estimated from mobility-controlled reactions in other copper alloys. Kirkaldy's proposal that discontinuous precipitation is a metastable monotectoid reaction controlled by interface diffusion is tested. An activation energy for bDB of 25 kcal/mole is obtained and a plausible explanation is given for the abnormally large spacings near the solvus temperature. Sluggish volume diffusion ahead of the interface (required by the local equilibrium hypothesis) may account for the relatively large spacings at all temperatures. © 1968.