Effect of coherency on coarsening of second-phase precipitates in Cu-base alloys

The effects of matrix/precipitate interface states on coarsening of Co and c-Fe precipitates in a Cu– 4 wt.%Co and a Cu–2 wt.%Fe alloy aged at 500 and 700 C have been examined by transmission electron microscopy (TEM) observations, electrical resistivity measurements, and length-change measurements. Analyses of TEM images show that the average radius for coherent/ semi-coherent transition is 6–12 nm for the Co precipitates and 10–20 nm for the c-Fe precipitates. The coarsening rates of the Co and c-Fe precipitates are unchanged by the transitions in coherency of the precipitates. The interface energies c of coherent Co and c-Fe precipitates are estimated from data on coarsening alone as 0.15 and 0.27 J m-2. From length-change measurements of the Cu– Co and Cu–Fe alloys during aging, the estimates of the isotropic misfit strains of Co and c-Fe precipitates are -0.018 and -0.016 for the coherent interfaces and -0.013 and -0.012 for the semi-coherent interfaces. Free energy analyses for the coarsening of Co and c-Fe precipitates reveal that the values of c of semi-coherent Cu/Co and Cu/c-Fe interfaces are 0.24 and 0.34 J m-2.