Kinetics of reactive diffusion in the (Au–Ni)/Sn system at solid-state temperatures

The kinetics of the reactive diffusion in the ternary (Au–Ni)/Sn system was experimentally studied at solid-state temperatures. Binary Au–Ni alloys containing 10 and 20 mass% of Ni were used to prepare sandwich Sn/(Au–Ni)/Sn diffusion couples by a diffusion bonding technique. The diffusion couples were isothermally annealed at temperatures of T = 433, 453 and 473 K for various times up to 1246 h. Due to annealing, Au1.5Ni0.5Sn8 and AuNi2Sn4 compound layers are formed along the (Au–Ni)/Sn interface in the Au–20Ni diffusion couple, but Au1.7Ni0.3Sn8, AuSn2 and Au6Ni4Sn15 compound layers dispersed with fine particles of AuNi2Sn4 are produced along the interface in the Au–10Ni diffusion couple. The total thickness l of the compound layers monotonically increases with increasing annealing time t according to the relationship l = k(t/t0)n, where t0 is unit time, 1 s. For both diffusion couples, the exponent n increases with decreasing annealing temperature. However, for the Au–20Ni diffusion couple, n is close to 0.5 at T = 453–473 K, and takes a value of 0.7 at T = 433 K. This means that volume diffusion is the rate-controlling process for the growth of the compound layers at higher annealing temperatures, but interface reaction contributes to the rate-controlling process at lower annealing temperatures. On the other hand, for the Au–10Ni diffusion couple, n is smaller than 0.5 at T = 453–473 K, but close to 0.5 at T = 433 K. The values n < 0.5 indicate that grain boundary diffusion as well as volume diffusion contributes to the rate-controlling process and grain growth occurs at certain rates in the compound layers. Consequently, for the reactive diffusion in the ternary (Au–Ni)/Sn system, the rate-controlling process considerably varies depending on the composition of the Au–Ni alloy and the annealing temperature.