Influence of the A2-B2 Transition and High Pressure on Volume Diffusion in Fe₃Al

Self diffusion coefficients of Fe-59 in Fe3Al have been determined in the temperature range 847-1503 K, which includes the critical temperature of A2-B2 transition. The classical sectioning technique was used. Below the critical temperature of transition the Arrhenius plot presents a downwards curvature due to the B2 type of atomic ordering. The temperature dependence of the self diffusion coefficients is considered on the basis of Girifalco's model [1], modified for AB(3) composition. The vacancy migration energy is also estimated both in the A2 and B2 structures. It is concluded that the disordered A2 structure acts as any pure metal (the vacancy formation energy is larger than the vacancy migration energy), while with ordering this does not hold. In order to get additional information on the mechanism of diffusion the pressure dependence of Ni diffusion in the same alloy was examined. Experiments were performed in the pressure range 0-8.6 kbar at 1073K (A2 structure), and at 973 K (B2 structure). The evaluated activation volume refers to a vacancy like mechanism of diffusion in fair agreement with data available in literature [2].