First-principles density functional theory study of phase transformations in NbCr2 and TaCr2

Hexagonal to cubic phase transformations in the intermetallic Laves-phase compounds NbCr2 and TaCr2 have been studied systematically using the synchroshear deformation mechanism and first-principles density functional theory (DFT). The DFT results provide a quantitative description of the activation energy profiles during the deformation process. By means of these profiles we discuss several transformation possibilities and propose explanations for recent experimental data. In particular it is shown that the synchroshear deformation mechanism should prevail in the two Laves phases studied. Furthermore, the activation energy barriers for TaCr2 are about 17% larger than those for NbCr2. We discuss how this may lead to the experimentally observed, significantly slower transformation kinetics of TaCr2, compared to NbCr2. Both the observation of fine-scale twins and the origin for the twin formation can also be understood from the activation energy profiles.