Ordering and phase separation of the BCC phase in the Fe-Cu-Al system

Phase equilibria at 1073 K, 1173 K, and 1273 K in the Fe-Cu portion of the Fe-Cu-Al system were determined by a diffusion couple technique, metallogra phy, and transmission electron microscopy (TEM). It is shown that there is a miscibility gap of the bcc phase which is divided into A2 + A2, A2 + B2 a nd B2 + B2 two-phase regions. Based on the experimental data of phase equil ibria of the present as well as of previous studies, a thermodynamic assess ment of phase equilibria was made, in which the order-disorder transition o f the bcc phase is taken into account. The thermodynamic parameters of fee, bcc, and liquid phases were evaluated to describe the Gibbs energy of each phase leading to a better fit between calculations and experimental findin gs above 1073 K.