Crystallographic and kinetic origins of acicular and banded microstructures in U–Nb alloys

The transition from the acicular to banded structure in dilute U–Nb alloys (∼5 at.%Nb) has been investigated. The acicular morphology consists of large, single orientation laths with high densities of dislocations and deformation twins, while the banded morphology is comprised of fine transformation twins, often with complex, multiple twin orientation relationships. Detailed Transmission Electron Microscopy (TEM) studies of the crystallographic relationships and deformation structures of these two microstructures are presented. In addition, controlled thermal histories and Continuous Cooling Transformation (CCT) curves have been evaluated to characterize the transformation pathways associated with the change in morphology. While the change from acicular to banded morphology is associated with increasing Nb concentrations, the competition between a temperature invariant reaction (at higher cooling rates) and a thermally activated transformation is investigated.