Coarsening Behavior of Co Precipitates in Cu-Co Alloys

The coarsening of Co precipitates in Cu alloys containing 1, 2, and 4 wt pct Co during aging at 873 to 973 K has been examined by transmission electron microscopy (TEM) and electrical resistivity. The precipitate shape transitions from a sphere to a {001}-faceted cuboid and from the cuboid to a {111}-faceted octahedron cause no change in the coarsening rate of Co precipitates. Application of the Lifshitz–Slyozov–Wagner (LSW) theory has enabled independent calculation of the Cu/Co interface energy, γ, and volume diffusion coefficient, D, of Co in Cu during the coarsening of precipitates. The value of γ is estimated as 0.15 J m⁻² using data on coarsening alone. The pre-exponential factor and activation energy for diffusion of Co in Cu, experimentally corrected for the precipitate volume fraction, are determined as 1.2 × 10⁻⁵ m² s⁻¹ and 208 kJ mol⁻¹, which are in agreement with those for diffusion of Co in Cu obtained from tracer diffusion measurements. An analysis using the generalized Gibbs–Thomson equation has shown that the estimates of γ are 0.17, 0.18, and 0.11 J m⁻² for the sphere, {001} and {111} interfaces. A free energy analysis for coarsening accounts for the lack of change in coarsening rate by the shape transitions.