REACTION-DIFFUSION IN THE AU-IN SYSTEM

Phase growth in the Au-In system has been studied by means of diffusio n couples for temperatures between 50 and 150-degrees-C. On couples in volving pure Au and In the following observations have been made: Only four of the seven phases predicted by the phase diagram have been obs erved: AuIn2, AuIn, Au7In3 and a fourth phase that should be Au4In. Th e AuIn2 layer represents the major part of the reaction zone. The AuIn 2 and Au7In3 layers have respectively an equiaxial and a columnar grai n structure; this reveals a difference in their growth process: AuIn2 growing by bulk diffusion of In and Au7In3 probably by intergranular d iffusion of Au. Growth kinetics of both the AuIn2 layer and the reacti on zone are linear in the first stage of reaction. Thus an interfacial reaction is the rate-limiting step at the beginning of growth. Growth of the AuIn2 COMPound has been studied between its two adjacent phase s, i.e. AuIn and In. Even in this couple with equilibrium concentratio n at the interfaces, the first stage of growth is controlled by an int erfacial reaction. Kinetic laws taking into account both interfacial r eaction and diffusion have shown to fit with experimental points.