規則状態におけるFe-CoおよびβCuZn合金の動的挙動

Abstract

Kinetic behaviors of ordered Fe-Co and β CuZn alloys have been investigated with basic equations derived on the basis of the Bragg and Williams theory which is extended to a binary alloy with CsCl structure involving vacancies. It is then assumed that atom movements occur in two mechanisms, a direct interchange of atoms and an interchange through vacancies. The dependence of vacancy concentration upon long-range order is also considered to follow the method of Girifalco. Both of the mechanisms assumed here for the interchange of atoms play an important role in the ordering process. The interchange through vacancies is dominant particularly in the ordering process for β CuZn alloy. The numerical calculations have been carried out by using the appropriate values estimated from the data of diffusion and of other experiments. On heating of ordered Fe-Co alloys, the order parameters at room temperature are retained to about 500°C. Beyond 500°C they begin to vary rapidly with the maximum rate at about 550°C and finally reach their equilibrium values. This result is in agreement with the interpretation by Yokoyama, Takezawa and Higashida. On the other hand, the temperature dependence of the order parameter in the ordered β CuZn alloy exhibits only a negligible anomaly near room temperature.