Calorimetric evaluation of precipitation in Cu rich, Cu-CoSi

Abstract

The precipitation process of cobalt and silicon atoms from supersaturated solid solutions of Cu-Co-Si for two compositions was studied by differential scanning calorimetry (DSC). Calorimetric traces analysis showed the presence of two overlapping exothermic reactions (stages 1 and 2), which can are attributed to two precipitation processes. First stage correspond to the preceding formation of a cobalt precipitate, while the second stage correspond to the formation of stoichiometric CO₂Si composition which takes place by silicon diffusion to the first precipitate. Heat contents during the stages 1 and 2 are proportional to precipitates volume fractions. Activation energies of both precipitates, calculated from the Kissinger method, are consistent with those corresponding to diffussion of Co in Cu and Si in Cu. Both processes can be decribed by the Johnson-Mehl Avrami (JMA) equation. Values of n are compatible with precipitate nucleation from the solid solution (stage 1) and with growth of paticles from preexisting Co precipitates. Furthermore,, the kinetic of the concentration decay of Co and Si in the matrix was estimated as function of the transformated fraction for each thermal event and from their respective volume fractions.