Experimental investigation of the U-rich region and thermodynamic re-assessment of the U–Ti system

The U–Ti binary system was investigated via experimental measurements and thermodynamic modeling. In order to clarify the disputed phase equilibria of the U–Ti system, 11 alloys with different contents of Ti were prepared by arc melting. The investigated Ti content covers the U-rich portion within the composition range from 1 at.% Ti to 33.3 at.% Ti. All samples were annealed at the temperatures of 640, 720 and 730 °C, and then analyzed by optical microscopy, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and differential scanning calorimeter. The phase equilibria associated with γ(U,Ti), αU, βU, and δ-U2Ti were experimentally determined in detail. The reaction temperature and composition of the invariant reaction γ(U,Ti) → βU + δ-U2Ti were precisely determined, and the invariant reaction temperature of the reaction βU→ αU + δ-U2Ti was updated. The solid solubility of Ti in αU at low temperatures was also revised by experiments. The temperature and the enthalpy of transition δ-U2Ti to γ(U,Ti) were newly measured by the differential scanning calorimeter. Finally, a set of self-consistent thermodynamic parameters were obtained by taking into account the present experimental findings and reliable literature data using CALPHAD (Calculation of phase diagrams) technique, and all calculated results are in good agreement with the experimental ones.