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

Abstract

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 (~1–2μm) 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 6.5×10−15m2/s and 90kJ/mol, respectively. The interdiffusion coefficient in β-Zr solid solution decreased with an increase in Mo concentration. Both the pre-exponential factors (2×10−8m2/s at 2at.% Mo to near 5×10−8m2/s at 9at.% Mo) and activation energies (140kJ/mol at 2at.% Mo to approximately 155kJ/mol at 9at.% Mo) of interdiffusion coefficients were determined to increase with an increase in Mo concentration.