Diffusion in intermetallic phases of the Fe-Al and Fe-Si systems

Self-diffusion of Fe and impurity diffusion of In in Fe-aluminides of 26.5, 34 and 50 at.% aluminium have been investigated over wide temperature ranges. For Fe diffusion in Fe3Al an influence of A2-B2 and B2-DO, transitions has been observed. The activation enthalpy increases with increasing order. Diffusion in alloys with higher Al content shows linear Arrhenius behaviour. Impurity diffusion of In is faster than Fe diffusion in Fe66Al34 and Fe50Al50 by about a factor of two. In Fe3Al the ratio between the diffusion coefficients of In and Fe varies between two and ten. Indium is homologous to Al and considered as substitute of Al, for which an affordable radiotracer is not available. Self-diffusion of Fe (and Si) and impurity diffusion of Ge in DO3-type Fe-Si alloys (24, 21 and 18 at.% Si) published in detail elsewhere are summarized. Diffusion of the majority component Fe is very fast. It occurs via nearest neighbour jumps into vacant sites on the Fe sublattice, which are available in high thermal concentrations. Fe diffusion is fastest for the stoichiometric alloy (highest Si content) and decreases with increasing Fe content. Ge (and Si) diffusion is slower than Fe diffusion by orders of magnitude. Some preliminary results on Fe diffusion in the B20 structured compound FeSi are also reported. Diffusion is by orders of magnitude slower than in Fe3Si indicating a strong influence of structure and/or decreasing metallicity on diffusion in this silicide. The results will also be discussed in connection with available studies of Mo ss bauer spectroscopy, positron annihilation experiments and ab-initio electron theory calculations of defect properties. (C) 1997 Elsevier Science S.A.