X-ray diffraction analyses of aged U–Nb alloys

Abstract

An aging study of compositionally homogeneous U–5.6Nb and U–7.7Nb (wt.%) was undertaken to improve the understanding of the decomposition of metastable martensitic phases in the U–Nb system. Artificially aging these depleted ( 238U) uranium alloys at 373, 473, 523 and 573 K for times up to 70 days resulted in significant age hardening. However, the microstructural changes giving rise to this hardening are subtle and elude standard light optical and electron microscopy techniques; this subtlety motivated the present study of aging-related changes by X-ray diffraction. A sealed tube X-ray diffractometer was used to record powder diffraction patterns from these aged polycrystalline U–Nb samples along with a CeO 2 standard. Data were analyzed using GSAS for full-pattern Rietveld refinements. Planar defects as well as strain broadening appear to have caused unusual peak profiles, which makes data interpretation challenging. Lattice parameter and unit cell volume trends are presented. U–5.6Nb experienced a reversal in the b lattice parameter and unit cell volume during aging, which has both similarities and differences to aging in dilute U–Ti and U–Mo alloys. The longest-time aging unit cell volume trends of U–5.6Nb indicate Nb depletion of the matrix phase, while the corresponding behavior at all aging times in U–7.7Nb is indicative of Nb enrichment in its matrix phase.