  Ageing mechanisms of the U-5\,\%wtNb system have been investigated on samples
exposed to temperatures of 150$\,^{\circ}$C for up to 5000\,hours. A variety of
surface and bulk analytic techniques have been used to investigate phase,
chemical and crystallographic changes. Characterisation of microstructural
evolution was carried out through secondary electron microscopy (SEM), energy
dispersive x-ray spectroscopy (EDS), electron backscatter diffraction (EBSD),
transmission electron microscopy (TEM) and x-ray diffraction (XRD). This
investigation suggests crystallographic defects such as twinning furthers the
martensitic tendencies with ageing. Resizing of the lattice and shuffling of
atoms results in a small progression from the $\alpha''$ towards the $\alpha'$
phase.
