THERMODYNAMIC ANALYSIS OF ZR-U AND ZR-NP ALLOYS IN VIEW OF F-D INTERACTION

Mixing of the f-states of an actinide metal with the d-states of a tra nsition metal (X) is dependent on both metals and composition. In the Zr-U system, there is an intermediate phase, delta, which could be reg arded as an omega-phase solid solution: the omega-structure is stabili zed against the alpha-Zr hcp structure by the addition of U. This sugg ests that there is considerable mixing between the U valence shell and Zr-d-shell, which increases the Zr-d-band occupancy. There is also a delta-like intermediate phase in the Zr-Np system. The mutual solubili ty of the bcc phases of beta-Zr and gamma-Np, however, appears to be l imited, although beta-Zr and gamma-U are completely miscible. Thermody namic analysis of the Np-Zr phase diagram has shown that the miscibili ty gap in the bcc phase region can intersect the solidus if the intera ction between Zr and Np is comparable to that between Zr and U. A pert inent discussion of systematic trends in alloying U with 4d transition metals is also provided.