CRYSTAL STRUCTURES OF TRANSITION PHASES FORMED IN U/16.60 AT PERCENT NB/5.64 AT PERCENT ZR ALLOYS

Single-crystal X-ray diffraction studies of γ-quenched and aged alloys of uranium with 16.60 at% (7.5 wt%) niobium and 5.64 at% (2.5 wt%) zirconium reveal two metastable transition phases whose structures are derived from small atom displacements from some of the sites occupied in the A2 γ structure. The phases appear to be identical with those previously reported as having chemically ordered structures in γ-quenched uranium-molybdenum and uranium-niobium alloys. The first transition structure, γs, is formed on γ quenching; it is cubic, space group I4̄3m, with a unit cell parameter about twice that of the γ phase. It may be formally described as eight unit cells of the γ structure in which the body-centered atoms have moved in correlated 〈100〉 directions. On aging at 150 or 350 °C, these movements are further correlated along a particular [001] direction, producing the second transition structure, γ0; it is tetragonal, space group P4/nmm, with an atom arrangement similar to that reported for β-neptunium. No evidence was found to suggest significant chemical order in either transition structure. © 1969.