Morphology of Compounds Formed by Isothermal Reactive Diffusion between Solid Fe and Liquid Al

The morphology of the compounds formed by the reactive diffusion between solid Fe and liquid Al was experimentally observed using Fe/Al diffusion couples. The diffusion couples were prepared by an isothermal bonding technique and then immediately annealed at temperatures of T=973, 1023 and 1073 K for various times up to t=2.4×103 s. At these temperatures, Fe is solid, but Al is liquid. During annealing, a compound region consisting of Fe2Al5 and FeAl3 is formed at the Fe/Al interface in the diffusion couple and grows towards the Fe solid specimen. However, FeAl2 and FeAl were not detected clearly. The thickness is much smaller for FeAl3 than for Fe2Al5, and thus the compound region is mainly composed of Fe2Al5. At T=973–1073 K, FeAl3 is produced as a rather uniform thin layer. On the other hand, the Fe2Al5 region shows the irregular tongue-like morphology at T=973–1023 K but the uniform layer morphology at T=1073 K. The irregularity of the Fe2Al5 region is attributed to the anisotropy for the interdiffusion coefficient of Fe2Al5. The temperature dependence of the irregularity implies that the anisotropy is large at T=973–1023 K but small at T=1073 K. The mean thickness of the compound region is proportional to a power function of the annealing time. Although the activation enthalpy of the proportionality coefficient is evaluated with an Arrhenius equation, the morphology of the Fe2Al5 region varies depending on the annealing temperature. In such a case, the rate-controlling process for the growth of the compound region cannot be readily estimated from the activation enthalpy.