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

Abstract

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, Au 1.5Ni 0.5Sn 8 and AuNi 2Sn 4 compound layers are formed along the (Au–Ni)/Sn interface in the Au–20Ni diffusion couple, but Au 1.7Ni 0.3Sn 8, AuSn 2 and Au 6Ni 4Sn 15 compound layers dispersed with fine particles of AuNi 2Sn 4 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/ t 0) n , where t 0 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.