The crystallography of metastable phases in the uranium-niobium binary alloy system has been investigated by x-ray diffraction with an emphasis placed on the derivation of structural parameters such as atomic sites and atomic displacements. Application of an anisotropic microstrain broadening model to Rietveld refinements has permitted the investigation of microstructural sources of strain. In the α’’ phase alloys, the C1121/m space group has been shown to provide a good solution when supplied with anisotropic broadening capabilities, whereas in the γo alloys, it is suggested that there may be some additional nuances that are not fully encapsulated by the P4/nmm structure.
Subsequently, the behaviour of these alloys under low temperature ageing (< 200°C), known to significantly affect mechanical properties of these alloys, has been investigated. XRD showed structures of crystallographic phases to be largely unchanged under ageing but indicated subtle changes in strain, which may be pointing to a nano-scale rearrangement of the structure of mechanical twins, as has been suggested by other authors recently. Evidence for significant chemical segregation at low temperatures could not be found.
Clear phase changes have been observed at higher temperatures by in-situ synchrotron x-ray diffraction with a γo to α’’ reaction shown at medium temperatures (400°C) and complete phase separation by diffusion present at high temperatures (500°C). An intermediate cubic phase was also observed in some cases en-route to full phase separation.
Preparation of these alloys was also investigated. The routines used were assessed to produce surfaces from which mechanically induced stresses had been removed. Ultimately, this enabled collection of high quality EBSD patterns that enabled visualisation of the microstructure and reliable XRD pattern data.