MICROSCOPIC AND MACROSCOPIC ASPECTS OF SHEAR BAND FORMATION IN INTERNALLY NITRIDED SINGLE-CRYSTALS OF FE-TI-MN ALLOYS

The formation of localized shear bands in single crystals of internally nitrided alloys of Fe-Ti-Mn was studied experimentally and theoretically. The experimental studies included observation of the dislocation substructures that are formed at and near shear band/matrix interfaces along with documentation of the crystallography of the localized shearing process. Computational studies of tensile deformation of the crystals using the finite element method, that incorporated a large strain, strain rate dependent constitutive theory for crystalline slip, were carried out and the results were compared to the experimental observations. Mechanical tests, and in situ metallographic observations of shear band formation, showed that localized shearing occurs while the materials are continuously hardening and before 'damage,' through microfracture, begins. Electron diffraction and imaging of the deformation substructures near shear band interfaces showed that an important part of the localization mechanism is nonuniform lattice reorientations that cause a 'geometrical softening' of the lattice.