Evolution of Defect Substructure in the Ni_3Al Alloy in the Course of Severe plastic Deformation by Torsion under Pressure

摘要

Transmission electron microscopy was used to study the evolution of defect substructure in a Ni_3Al-based alloy (Ni-18 at. percent Al-8 at. percent Cr-1 at. percent Zr-0.15 at. percent B) in the process of severe plastic deformation by torsion under pressure. The crystal-lattice reorientation in the process of formation of an ultrafine-grained structural state was established to develop against the background of a preliminarily formed highly imperfect substructure with a high crystal-lattice curvature and to be accompanied by the formation of discrete misorientation boundaries with a high effective density of partial disclinations. The high anisotropy of the displacement fields in the process of torsional deformation results in a significant structural (with respect to the sizes of submicron grains) and crystallographic (with respect to the directions of the vectors of discrete and continuous misorientations) anisotropy of the ultrafine-grained states that are developed. Based on experimental data, a scheme of submicron-sized fragmentation on several structural levels is constructed. Possible dislocation-disclination mechanisms of the fragmentation are discussed.