Recent observations of phase transformations in a U--Nb--Zr alloy

Dilatometry, x-ray diffraction, hardness, and optical microscopy were used to characterize polycrystalline U--7.5% Nb--2.5% Zr. Upon quenching this alloy from the bcc phase at 800 deg C, a metastable modification corresponding to a slight tetragonal distortion of the bcc lattice along with a short-range atom displacement ordering was revealed by x-ray diffraction. Minor compositional variations, differing quenching rates, and low temperature aging cause important alterations in this metastable structural state. Slight depletion of the alloying elements increases the degree of tetragonality. Aging the quenched alloy initiates transformations to other anisotropic structural states. Altogether four transformation stages have been detected by dilatometry. At temperatures as low as 150 deg C, two stages (one beginning after a few minutes and the otaer after a few hours) produce a volume expansion of the alloy and a dramatic increase in hardness. The first stage is attributed to solute clustering. The second stage corresponds to an isothermal martensitic shear transformation, resulting in a second phase which may be described as a monoclinic modification of the alpha -uranium orthorhombic structure. Cellular or discontinuous predipitation is observed at temperatures between 500 and 620 deg C. Light rolling deformation of the quenched alloy causes an immediate crystal reorientation to take place in certain grains. Metallographic and x-ray evidence hints that this reorientation is accomplished by mechanical twinning at low strains. (22 figures, 37 references) (auth