The bain and orthorhombic paths of the bcc–fcc transformation in a bcc metal

Abstract

The energy of the Bain and orthorhombic paths in niobium and the instability of phonons during uniaxial deformation along <001> are studied with the application of the ab initio method. The orthorhombic transformation path is refined with regard to its symmetry. The calculation of the phonon spectrum in the entire irreducible Brillouin zone depending on deformation makes it possible to find the softest branches of the phonon spectrum responsible for the loss of stability of the structure. The nature of the loss of stability is revealed, and the strain at which stability is lost both in tension and compression is evaluated. Possible mechanisms determining the stability of the structure and theoretical strength of niobium are discussed. The results obtained can relate to experimental situations when small defect-free regions are deformed, for example, in nanostructured materials, when surface layers are modified by modern methods of plastic deformation, and during nanoindentation.