Solid-state reactive diffusion between Fe and Al was observed experimentally by using Al/Fe/Al diffusion couples which were prepared by using a diffusion bonding technique. The diffusion couples were isothermally annealed at 823 to 913K, for times of up to 120h. Due to annealing, a Fe2Al5 layer was produced at the Fe/Al interface in the diffusion couple. Although FeAl, FeAl2 and FeAl3 were also stable within this temperature range, these compounds were not seen in the diffusion couple within the experimental annealing times. The mean thickness of the Fe2Al5 layer was proportional to the square root of the annealing time, and grain growth occurred in the Fe2Al5 layer. This showed that the growth of the Fe2Al5 layer was controlled by volume diffusion of the constituent elements in each phase. On the basis of the observed growth rate, it was estimated that the interdiffusion coefficient was about an order of magnitude smaller for Fe2Al5 than for Al, but was more than 3 orders of magnitude greater for Fe2Al5 than for Fe. If nucleation occurred sufficiently fast for all the compounds, the interdiffusion coefficient was deduced to be more than 2 orders of magnitude smaller for FeAl, FeAl2 and FeAl3, than for Fe2Al5.

Growth Behavior of Fe2Al5 during Reactive Diffusion between Fe and Al at Solid-State Temperatures. D.Naoi, M.Kajihara: Materials Science and Engineering A, 2007, 459[1-2], 375-82

Figure 2

Meyer-Neldel Plot for Various Multilayers