2012 Volume 53 Issue 7 Pages 1240-1246
The kinetics of the reactive diffusion between solid Fe and liquid Sn was experimentally examined using Fe/Sn diffusion couples. The diffusion couples were prepared by an isothermal bonding technique and then immediately annealed in the temperature range of T = 703–773 K for various times up to t = 90 ks (25 h). During annealing, a compound layer of FeSn2 is formed at the initial Fe/Sn interface in the diffusion couple and grows mainly into the liquid Sn specimen. At T = 703 K, the compound layer indicates a needle-like microstructure in the early stages but a columnar microstructure in the late stages. On the other hand, at T = 723–773 K, only the columnar microstructure is realized in the compound layer within the experimental annealing times. The mean thickness of the compound layer is proportional to a power function of the annealing time. The exponent n of the power function is mostly close to 0.6. In the early stages at T = 703 K, however, n is equal to unity within experimental uncertainty. If growth of a compound layer with a uniform thickness is controlled by volume diffusion, n is equivalent to 0.5. In contrast, n is equal to unity for longitudinal growth of a needle-like grain, even though volume diffusion is the rate-controlling process. This is the case for the layer growth in the early stages at T = 703 K. Since independent longitudinal growth of each columnar grain contributes to the layer growth governed by volume diffusion, n is slightly greater than 0.5 for the compound layer with the columnar microstructure. Consequently, the discontinuous variation of n corresponds to the transition from the needle-like microstructure to the columnar microstructure.