Development of Thermodynamic Model on Phase Diagram Database

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It is well known that magnetic properties such as the Curie temperature and the mean magnetic moment of ordered compounds have different values from those of disordered solutions. In consequence, the stability of the ordered phase is depressed (enhanced) due to the interaction between the chemical and magnetic ordering caused by the decrease (increase) of magnetic properties. In this project, a new thermodynamic model taking into account the interaction effect was developed by the collaboration with prof. Bo Sundman in the Royal Institute of Technology, Sweden. Model calculations to describe the phase stability of the b.c.c. and f.c.c. structures were performed taking into account the magnetic and the chemical ordering reactions and also the mutual interaction between them. Thermodynamic assessments of the Fe-X (X=A1, Ni and Co) systems were also carried out using two- or four-sublattice s-CEF, including the interaction effect. These calculations lead to the following conclusions.(1) The incomprehensible A2+B2 phase separation and tricritical point in the Fe-Al system originate in the depressed chemical and magnetic ordering energies caused by the mutual interaction.(2) The stability of both γ'-FeNi_3 (L1_2) and γ"-FeNi (L1_0) compounds is strongly enhanced by the additional magnetic energy due to the chemical ordering. The calculated phase diagram suggests that the γ"-FeNi (L1_0) phase is stable.(3) In the Fe-Co system, the stability of the B2 ordering arises mostly from the enhanced magnetic energy due to the chemical ordering.