Phase relations in the U–Mo–Al ternary system

The phase relations in the U–Mo–Al system of quenched samples annealed at 800 °C for 2 weeks and at 400 °C for 2 months have been established using X-ray powder diffraction, scanning electron microscopy and energy dispersive spectroscopic analysis performed at room temperature. Two ternary Al-rich phases, UMo2−xAl20+x and U6Mo4+xAl43−x are found stable at 800 °C and 400 °C. They show significant homogeneity ranges resulting from Mo/Al substitution mechanism on various mixed crystallographic sites, as evidenced by single-crystal structure refinements. Substitution of up to 25 at.% of Al by Mo atoms is also observed for UAl2 (cubic MgCu2-type) giving a quite large extension (UAl2−xMox, 0 < x < 0.5) into the ternary system. Larger substitution (0.6 < x < 0.7 at T = 800 °C) stabilizes another ternary Laves phase, UAl2−xMox with the hexagonal MgZn2-type. There is no detectable solubility of Mo in UAl4, and it is of the order of 1 at.% in UAl3. The interaction layers between the γU–Mo alloys and the Al matrix in nuclear fuel plates can be successively estimated as composed of the two- and three-phase fields equilibrium indicated on the assessment of the phase relations drawn for samples heat-treated at 400 °C.