Coarsening of cobalt grains dispersed in liquid copper matrix

Abstract

Coarsening of spherical Co-rich grains dispersed in Cu-rich liquid matrix is investigated. The specimen compositions are 50 pct Co-50 pct Cu, 40 pct Co-60 pct Cu, and 30 pct Co-70 pct Cu by weight. The annealing temperatures have been varied between 1150 and 1300 °C. The specimens have been prepared by usual powder metallurgy technique from fine Co and Cu powders. Due to the equal density of Co and Cu, the Co-rich grains remain uniformly dispersed in the liquid matrix. The increase of average grain size with annealing time,t, follows closely thet 1/3 law predicted for diffusion controlled mechanism by Lifshitz, Slyozov, Wagner (LSW) and Ardell. The observed increase of the growth rate with increasing solid grain fraction is a clear evidence for diffusion controlled mechanism. The linear intercept distribution of the grains agrees closely with the predictions for reaction controlled growth in LSW theory, but the result is also consistent with Ardell’s prediction that when the grains grow with small inter-grain distance under diffusion control, the grain size distribution is almost identical to that of the reaction controlled growth in LSW theory. The estimated values of the diffusion constant and its activation energy agree in order of magnitude with the typical values for diffusion in liquid metal.