Precipitation processes in Cu-Co-Si alloys

Abstract

The precipitation of cobalt and silicon atoms from supersaturated solid solutions of Cu-Co-Si alloys was studied during ageing between 400 and 600° C by electrical resistivity, thermoelectric power and calorimetric measurements, by mechanical testing and by transmission electron microscopy investigations. It has been found that the decomposition begins with the appearance of concentration fluctuations from which cobalt precipitates first. The clustering of cobalt atoms initiates the precipitation of silicon, and so particles with the stoichiometric Co₂Si composition are finally formed. The silicon that is in excess with respect to the stoichiometry of Co₂Si is retained in the solid solution but its increase in the initial supersaturated solid solution strongly enhances the nucleation of the particles and, therefore, results in a finer precipitate structure. On the basis of calorimetric measurements the effective activation energy of both the cobalt and silicon precipitation was determined to be around 1.5 eV.