The activity of cobalt and silicon in the Co-Si system with special focus on the alpha-Co solid solution

The activities of cobalt and silicon at 1463 K have been determined across the whole composition range in the Co - Si system, including the alpha - Co solid solution, for which no activity data were previously available. Simple and reactive Knudsen effusion mass spectroscopy employed in this work were shown to successfully overcome problems normally encountered in studying high-temperature solid binary systems, such as slow equilibration and low partial pressures of the components. The composition dependence of the cobalt activities within the alpha - Co solid solution phase was used to calculate the self-interaction coefficients of silicon in alpha - Co: In gamma(Si)(infinity) = -10.4 +/- 0.2 and epsilon(Si)((Si)) = 18.6 +/- 0.8. The regular solution model was shown to be a fairly good description of the alpha - Co solid solution, with an energy parameter Z(CoSi) of -120 +/- 5 kJ.mol(-1). The results compare well with literature data on similar systems, such as Fe - Si and Ni - Si.