Tensile failure of 4130 steel having different ultrafine grained structures

For the potential use of ultrafine grained materials for high performance kinetic energy penetrators of caliber projectile, the shear failure of 4130 steel having two different ultrafine grained (UFG) structures was examined under the tensile mode. 4130 steel with a lamellar UFG structure and with an equiaxed UFG structure was fabricated by equal channel angular pressing with routes A and Bc, respectively. Yield strength of UFG steel was enhanced almost twice compared to that of coarse grained counterpart while the former exhibited drastic ductility loss due to a lack of strain hardenability. Tensile failure of the steel having a lamellar UFG structure is manifested in sequence by formation of few sharp shear bands inclined by 45° to the stress axis, propagation of only one shear band of them, broad conjugate shear band formation and propagation, and failure in the shear mode along the first-formed shear band. In addition, large cracks parallel to the tensile stress were simultaneously developed due to its lamellar nature. The steel with an equiaxed UFG structure undergoing tensile rupture is characterized by a series of sequential processes of formation of one broad shear band inclined by 45° to the stress axis, its propagation, another broad conjugate shear band formation and propagation, and failure in the opening mode with the zigzag pattern. These characteristics were discussed with the aid of the models suggested for enhanced shear formability of nanocrystalline and/or UFG materials.