Anomalous growth of Al₈Mo₃ phase during interdiffusion and reaction between Al and Mo

Molybdenum (Mo) has been reported as a potential candidate for the diffusion barrier between U-10Mo fuel and AA6061 cladding in monolithic fuel plates. Interdiffusion and reaction between Al and Mo was investigated using solid-to-solid diffusion couples annealed in the temperature range from 425° to 625 °C. Thermo-kinetic parameters, i.e. growth rate and integrated diffusion coefficients, were determined using Wagner's approach. At low temperatures (T ≤ 450 °C), negligible diffusion interaction was observed between Al and Mo. At intermediate temperatures (T = 475° - 600 °C), diffusion couples were observed to mainly develop a layer of Al₈Mo₃ intermetallic phase, along with a thin, discontinuous layer of Al₂₂Mo₅ intermetallic phase between the terminal Al and Al₈Mo₃ layer. At high temperature (T = 625 °C), continuous layers of both of Al₈Mo₃ and Al₂₂Mo₅ intermetallic phases developed in the diffusion couple. The Al₈Mo₃ phase grew with a needle-like, non-planar morphology at intermediate temperatures, contrary to the planar growth of the intermediate phase in binary diffusion couple as per Gibbs phase rule. In general, the degree of non-planarity of needle-like Al₈Mo₃ phase increased with an increase in annealing time at 500 °C, but decreased with an increase in annealing temperature. This non-planar growth was simulated based on three-dimensional anisotropic diffusion using a quadric representation of the diffusivity tensor.