The U–Ti system: Strengths and weaknesses of the CALPHAD method

Input from Density Functional Theory (DFT) calculations is used to understand phase equilibria in a binary metallic alloy fuel system: U–Ti. The CALPHAD approach is employed to calculate a U–Ti phase diagram that is consistent not only with experimental data but also—more importantly—with thermodynamic data from DFT calculations: heat of formation of γ(bcc)-U–Ti alloys as a function of composition, and formation enthalpy of the δ-U2Ti compound. Three DFT-based electronic structure methods are utilized: SR-KKR-ASA-CPA, SR-EMTO-CPA, and FPLMTO-SQS, and the use of derived ab initio data avoids the manifestation of unreasonable or inaccurate phase stabilities that result from an otherwise unconstrained Gibbs energy minimization within the CALPHAD approach. We also investigate phase formation of the δ-U2Ti phase in the U–Ti system, that stabilizes in the same C32 structure as other binary metallic fuel alloys such as U–Zr and Np–Zr.

► Phase equilibria and diagram of the binary U–Ti alloy system is investigated.
► A CALPHAD thermodynamic model is proposed.
► Experimental data and alloy DFT calculations are used to develop the model.
► It is shown that DFT data is essential to prevent unphysical phase diagrams.
► Stability of the δ-phase is rationalized through electronic structure calculations.