DSC STUDY OF PRECIPITATION PROCESSES IN Cu-Co-Si ALLOYS

Using differential scanning calorimetry (DSC) the precipitation processes of supersaturated solid solutions of three Cu-Co-Si alloys containing the same atomic cobalt content were investigated. Thermoanalytical and previous studies, reveal that the decomposition begins with cobalt clustering which initiates the precipitation of the Co2Si stoichiometric particles, which in turn dissolves after further heating. Volume fractions are unequivocally determined by the amount of cobalt present in these alloys. It is infered that surplus silicon atoms retained in the solution increase the reaction rate and dispersity of precipitate structure. Kinetic parameters were obtained by a convolution method based in the Mehl-Johnson-Avrami (MJA) formalism. The lower activation energy associated with cobalt clustering is attributed to the contribution of quenched-in vacancies. Superimposed to the MJA formalism and adaptative spherical diffusion model was used for Co2Si precipitation with particle size as a disposable parameter. This model further confirmed that as silicon content increases particle dispersity becomes more pronounced. Such results are also infered from a three dimensional diffusion dissolution model previously developed which adjusts quite well to such process in the present cases. Age hardening experiments are in line with all previous results obtained.