Diffusion in gold-aluminium

A combination of reflectivity, adhesion and resistance measurements has been used to measure the rates of phase boundary movement in thin-film couples during the interdiffusion of gold and aluminium. The various measurements have been compared and correlated to show that diffusion occurs in two distinct stages. The first stage leads only to formation of Au2Al which is initially in a partially disordered state and the diffusion is followed by an ordering process. In the presence of excess aluminium, this is followed by the second stage, which is much slower, where the Au2Al already formed is gradually transformed into AuAl2. The rate constant D′ for broadening of the AuAl2 phase was found to be 1·44 × 10−15 cm2/sec at 130°c assuming x 2 = D′t and the diffusion coefficient in this phase was calculated to be 2·2 × 10−14cm2/sec at 130°c with a corresponding activation energy of 21·6 kcal/mole giving a value of 0·011 cm2/sec for D 0.The corresponding rate constant for the first stage was 2·79 × 10−15 cm2/sec at 50°c with an activation energy of 23·5 kcal/mole but in the absence of accurate supplementary data the diffusion coefficient in Au2Al could not be calculated accurately and the value of 3 × 10−14 cm2/sec at 50°c is correct only in order of magnitude. It suggests a D 0 value of the order of 200 cm2/sec.