Coarsening of Co-rich precipitates in a Cu–Co–Fe ternary alloy

The effect of coherency on coarsening of fcc Co–Fe precipitates in a Cu–1.47 wt.%Co–0.56 wt.%Fe (Co : Fe = 7:3 in atomic ratio) alloy aged at 873–973 K has been studied by measuring both the precipitate size by transmission electron microscopy and the solute concentration in the Cu matrix by electrical resistivity measurements. The precipitate phase consists of 7 parts of Co and 3 parts of Fe in atomic ratio, irrespective of the precipitate size. The precipitates smaller than about 8 nm in radius are coherent with the Cu-matrix. When the average precipitate radius is over 18 nm, all the precipitates become semi-coherent. The coarsening rates are not affected by the coherency of the precipitates. The precipitate/matrix interface energy γ has been derived, independently of the diffusivities of solute atoms using a coarsening model developed by Kuehmann and Voorhees for ternary systems. The precipitates are coherent or semi-coherent with the matrix, the experimentally obtained value of γ is 0.2 J/m². This value lies between the reported values of γ  = 0.15 J/m² for Co precipitates and γ = 0.25 J/m² for γ-Fe precipitates.