Aluminum and silicon diffusion in Fe-Cr-Al alloys

Interdiffusion of Al sub 86.4 Si sub 13.6 and Fe sub 79.03 Cr sub 19.03 Al sub 1.14 Si sub 0.8 alloys (Crofer 18) was studied by means of electron microprobe analysis, scanning electron and optical microscopy and x-ray analysis. Formation of intermetallic phases during interdiffusion was only observed for c sub Al > 30 at.%. Effects of deviations from the Boltzmann-Matano and the thin-film geometry on the resulting diffusivities were assessed numerically. The results are of general interest for diffusion measurements involving samples of given geometry as in many industrial applications. The tracer diffusivities of Al, Si, in Fe sub 79.03 Cr19.03 Al sub 1.14 Si sub 0.8 turned out to be quite similar and close to the Fe diffusivity. They are somewhat smaller than in pure Fe. The intrinsic diffusion coefficient of Al exhibits a small increase with increasing Al concnetration up to c sub Al < 20 at.% mainly due to a corresponding increase in the thermodynamic factor. Interdiffusion at high Al concentrations proved to be much faster and is hence not rate limiting.