Diffusion in intermetallic phases of the Fe-Al system

Self-diffusion of Fe and impurity diffusion of In and Zn in three alloys of the Fe-Al system with the approximate compositions Fe3Al, Fe2Al and FeAl have been investigated by the radiotracer technique using the isotopes Fe-59, In-114m and Zn-65. Zn and In also were studied with the intention of 'simulating' Al diffusion for which no affordable radioactive isotope is obtainable. Diffusion of In and Zn is slightly faster than Fe diffusion in all cases. Most of the measurements took place in the B2 structure regions of the phase diagram, some in the A2 and few in the D0(3) structure region. In Fe3Al there are two ordering transitions, i.e., between A2 and B2 and between B2 and D0(3), both leading to an increase in the activation enthalpy of diffusion in the phase with higher degree of order. For increasing Al content the activation enthalpy of diffusion increases although the melting temperature decreases significantly. The results are discussed in the light of information on the defect structure in Fe-Al available from Mossbauer studies, positron annihilation studies, differential dilatometry, mechanical spectroscopy, resistivity annealing after quenching, and in the light of theoretical results from ab initio calculations of defect properties.