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, Au1.5M0.5Sn8 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 I of the compound layers monotonically increases with increasing annealing time t according to the relationship 1 = k(t/t(0))(n), where to 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-control ling 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. (c) 2006 Elsevier B.V. All rights reserved.