Observation on Isothermal Reactive Diffusion between Solid Ni and Liquid Sn

Abstract

The kinetics of the reactive diffusion between solid Ni and liquid Sn was experimentally examined using Ni/Sn diffusion couples. The diffusion couples were prepared by an isothermal bonding technique and then immediately annealed in the temperature range of T = 533–623 K for various times up to t = 14.4 ks (4 h). During annealing, a compound layer of Ni₃Sn₄ is formed at the original Ni/Sn interface in the diffusion couple and grows mainly into the liquid Sn specimen. The mean thickness of the compound layer is proportional to a power function of the annealing time. The exponent n of the power function takes values between 0.31 and 0.43. Since there is no systematic dependence of n on T, we may consider that n is insensitive to T within experimental uncertainty. When growth of a compound layer with uniform thickness is controlled by volume diffusion, n is equivalent to 0.5. If boundary diffusion contributes to the layer growth and grain growth occurs in the compound layer, however, n becomes smaller than 0.5. Since grain growth practically takes place in the compound layer, it is concluded that the layer growth of Ni₃Sn₄ is mainly controlled by boundary diffusion at T = 533–623 K.