Phonon Driven BCC to Orthorhombic Transformation in U-Nb Alloys

The martensitic transformation in uranium alloys is of great strategic importance. We study the crystallography and model the well characterized BCC to orthorhombic phase tranformation in the shape memory alloy U-Nb for low Nb concentrations. Our predictions are consistent with the experimentally observed orientation relationship between the BCC and orthorhombic phases. We find that this temperature induced transformation is driven by a specific zone boundary phonon that couples to a particular shear mode. We also obtain a Landau free energy for this transformation. In addition, we compare our results with a similar shuffle based mechanism in a related martensitic alloy AuZn.