Growth behavior of Fe2Al5 during reactive diffusion between Fe and Al at solid-state temperatures

The solid-state reactive diffusion between Fe and Al was experimentally observed using Al/Fe/Al diffusion couples prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed in the temperature range between T = 823 and 913 K for various times up to 120 h. Due to annealing, Fe2Al5 is produced as a layer at the Fe/Al interface in the diffusion couple. Although FeAl, FeAl2 and FeAl3 are also stable intermetallic compounds in this temperature range, these compounds were not recognized in the diffusion couple within the experimental annealing times. The mean thickness of the Fe2Al5 layer is proportional to the square root of the annealing time, and grain growth occurs in the Fe2Al5 layer. This means that the growth of the Fe2Al5 layer is controlled by the volume diffusion of the constituent elements in each phase. On the basis of the observation for the growth rate, we may estimate that the interdiffusion coefficient is about one order of magnitude smaller for Fe2Al5 than for Al but more than three orders of magnitude greater for Fe2Al5 than for Fe. If nucleation occurs sufficiently fast for all the compounds, the interdiffusion coefficient should be more than two orders of magnitude smaller for FeAl, FeAl2 and FeAl3 than for Fe2Al5.