Investigation of interdiffusion behavior in the Mo-Zr binary system via diffusion couple studies

Zirconium has recently garnered attention for use as a diffusion barrier between U-Mo metallic nuclear fuels and Al alloy cladding. In order to gain a fundamental understanding of the diffusional interactions, the interdiffusion behavior in the binary Mo-Zr system was investigated via solid-to-solid diffusion couples annealed in the temperature range of 750 to 1050 C. A combination of scanning electron microscopy, X-ray energy dispersive spectroscopy, and electron probe microanalysis were used to examine the microstructure and concentration profiles across the interdiffusion zone. A large β-Zr (cI2) solid solution layer and a thin (~ 1-2 μm) layer of Mo₂Zr (cF24) developed in all couples. Parabolic growth constants and concentration dependent interdiffusion coefficients were calculated for the Mo₂Zr and Zr solid solution phases, respectively. The pre-exponential factor and activation energy for growth of the Mo₂Zr phase were determined to be approximately 6.5 × 10^{- 15} m²/s and 90 kJ/mol, respectively. The interdiffusion coefficient in β-Zr solid solution decreased with an increase in Mo concentration. Both the pre-exponential factors (2 × 10^{- 8} m²/s at 2 at.% Mo to near 5 × 10^{- 8} m²/s at 9 at.% Mo) and activation energies (140 kJ/mol at 2 at.% Mo to approximately 155 kJ/mol at 9 at.% Mo) of interdiffusion coefficients were determined to increase with an increase in Mo concentration.