Growth behaviour of intermetallic compounds during reactive diffusion between aluminum and magnesium at 573-673 K

A potential new research reactor fuel design proposes to use U-Mo fuel in a Mg matrix that is clad in Al. Interdiffusion between the Mg containing fuel core and Al cladding can result in the formation of intermetallic compounds that can be detrimental to fuel element performance, particularly during abnormal operation at elevated temperatures. This interaction must be understood and mitigated in order to maximize the safe residence time of a fuel element in a reactor at elevated temperatures. The kinetics of the reactive diffusion in the binary Al-Mg system were experimentally studied using Al/Mg cylindrical diffusion couples representative of fuel elements (no U-Mo present). The diffusion couples were isothermally annealed at temperatures of 573 K, 623 K, and 673 K for various times. The microstructures developed during annealing were observed with optical microscopy and scanning electron microscopy (SEM). The Al/Mg concentration profiles were analyzed with SEM- energy-dispersive X-ray spectroscopy (EDS). Layers of the intermetallic compounds, β (Al₃Mg₂) and γ (Al₁₂Mg₁₇) phases, were formed between the Al and Mg during annealing. The β layer was observed to grow faster than the γ phase in the diffusion couples. The thickness of each layer can be expressed by a power function of the annealing time with the exponent n close to 0.5 for the β phase and less than 0.5 for the γ phase. The results suggest that the growth of β phase is controlled by lattice diffusion and that of the γ phase by grain boundary and lattice diffusion. Metallographic examination showed the Al diffusion along Mg grain boundaries in the columnar growth of γ phase during annealing. Based on the reactive diffusion equation developed in this work, in the absence of irradiation effects, a fuel element could safely reside at elevated temperatures for over 110 hours. (author)