Experimental study of phase relations in the U–Ti–Al ternary system

• Isothermal sections of the U–Ti–Al system were investigated for 923 K and 1123 K.
• The crystallographic properties of the equilibrium phases were checked.
• The microstructure of quenched alloys was discussed.
• The existence and the identification of any intermediate phases were carried out.
• The formation reaction of UTi2Al20 was determined.

The phase relations in the ternary U–Ti–Al system were established for the whole concentration range for two temperatures, 923 K and 1123 K. They were derived from quenched samples annealed at 1123 K for about 500 h and at 923 K for about 720 h using X-ray powder diffraction, scanning electron microscopy and energy dispersive spectroscopic analysis. Only one ternary phase, the Al-rich compound, UTi2Al20, was found. It forms by a peritectic reaction at 1365(5) K, and remains stable at least down to 923 K. Based on single-crystal structure refinements, it was confirmed that it adopts the CeCr2Al20 structure type (cubic, Fm3¯d, n°227) with lattice parameter at room temperature, a = 14.634(1) Å. Unlike most of the isotypic aluminides, UTi2Al20 should be regarded as a line compound. At 1123 K, the isothermal section is characterized by the extended homogeneity ranges due to mutual exchange between U and Ti and between Ti and Al in the unary phases, whereas at 923 K, the mutual solubility between U and Ti was found null. Regarding the binary phases, only in the Laves phase, UAl2 (cubic-C15, MgCu2-type) such solubilities were observed with limits of about 2 at% of U by Ti and 5.5 at% of Al by Ti, at 1123 K. For the other binary compounds, the solubility of the third component was found negligible at both temperatures.