THERMODYNAMIC ANALYSIS OF SOLUBILITY AND MISCIBILITY GAP IN FERROMAGNETIC ALPHA-IRON ALLOYS

The free energy of αFe-X solid solution is resolved into their magnetic and non-magnetic components. The non-magnetic component is described in terms of the regular solution model, and the magnetic component is approximated by a modification of that of pure αFe, as has been proposed by Zener [3] and Hillert et al. [8]. It is shown that the chemical potential of an alloying element is extraordinarily high in the ferro-magnetic state, if it lowers markedly the Curie temperature. This causes the solubility of the element to be decreased peculiarly in the ferromagnetic region. It is also demonstrated that the miscibility gap and its spinodal in αFe-X solid solution are not simple parabolic but are of abnormal shape, expanding towards the Fe side, and having a sharp horn or a broad bump at the Curie temperature. It is further shown that the addition of cobalt raises excessively the chemical potential of the alloying element in the ferromagnetic state and enlarges remarkably the magnetic anomalies in the solubility as well as the miscibility gap. © 1979.