Coarsening Behavior of Co Precipitates in Cu-Co Alloys
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IN a Cu alloy with less than 5 wt pct Co, spherical coherent Co precipitates are first formed in the Cu matrix by aging.[1] Livingston[2] have first reported that the average radius of Co precipitates in Cu alloys containing 0.7 to 3.2 wt pct Co increases with time, t, as t1/3 and the growth rate of the Co precipitates increases with increase in the Co content. Oriani[3] and Ardell and Nicholson[4] have estimated the Cu/Co interface energy, c, from the theory of Lifshitz and Slyozov[5] and Wagner[6] (LSW) using precipitate sizes reported by Livingston and available data for diffusivity, D, and equilibrium concentration, Ce, of Co in Cu. The estimates of c increase from about 0.30 to 1.0 J m-2 as the Co content of the alloys increases. The LSW theory is based on point precipitates, that is, it assumes a negligibly small volume fraction of precipitates. Therefore, it is necessary for coarsening of precipitates in actual alloys to take into account the shorter diffusion distance. Ardell[7] has tackled this problem and shown that the presence of a finite volume fraction of second-phase precipitates increases the coarsening rate of the precipitates. Furthermore, by considering the influence of volume fraction, f, of Co precipitates on the coarsening rate, values of c were recalculated from data obtained by Ardell and Nicholson.[4] Nevertheless, the resulting values of c increase from 0.18 to 0.32 J m-2 with increasing f. D. WATANABE, Graduate Student, and C. WATANABE, Assistant Professor, are with the Division of Innovative Technology and Science, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan. R. MONZEN, Professor, is with the Division of Innovative Technology and Science, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan. Contact e-mail: [email protected] Manuscript submitted October 5, 2007. Article published onlined February 14, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
By measuring independently both the coarsening rate of precipitates and the rate of decay of the matrix supersaturation during coarsening, independent reliable values of c and D can be determined from experimental data alone.[8,9] In this case, the influence of f on the coarsening rate of the precipitates does not have to be taken into consideration for the calculation of c. However, a correction for f is needed to determine a value of D. Moreover, a value of Ce in the matrix can be obtained solely from the kinetics describing the concentration changes. This approach has already been applied in several binary systems.[8–11] In a previous study,[12] we have examined the coarsening and shape of Co precipitates in a Cu1.0 wt pct Co alloy on aging at 873 to 998 K from measurements of the precipitate size by transmission electron microscopy (TEM) and the Co concentration in the matrix by electrical resistivity. The shape of Co precipitates changes from a sphere to an octahedron faceted on the matrix {111} via a cube with {001} as the precipitates grow. The average radius of Co precipitates increases with t as t1/3. For nons
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