Thermodynamic study of the Np–Zr system

A thermodynamic model of the Np–Zr system is developed using the CALPHAD method, and a review of previous work performed on this system is presented here. In general, results obtained are in good agreement with those proposed from experimental observations. It is found that the nature of reactivity of Np with Zr, is different from that of U and Pu: an expected elevation of melting point of Np–Zr alloys was not seen and a miscibility gap existed between the high-temperature bcc phases of Np and Zr. Formation enthalpy of the bcc phase obtained from the model is compared with results from KKR–ASA–CPA calculations. Lattice stabilities of various phases in the system are compared to values obtained from first-principles LDA and GGA calculations. The δ-NpZr2 phase is modeled as a non-stoichiometric phase with a C32 structure, similar to what has been determined for the δ-phase in the U–Zr system. This phase is analogous to ω-phase in pure Zr, which is stabilized at high pressures. Two different possibilities for stability of the δ and ω phases have been proposed in the present work. Finally, calculated changes in enthalpy versus temperature are plotted for two alloys to guide future experimental work in resolving important issues in this system.