Influence of Equal-Channel Angular Pressing on Grain Structure and Internal Stresses of Technically Pure Nickel

Using transmission electron microscopy, the structure, phase composition, defects, amplitude of internal stresses and their sources are studied in the ultrafine-grained technically pure nickel produced by the method of equal-channel angular pressing (ECAP). During ECAP, the samples are subjected to shear deformation by compression along two intersecting channels of equal diameters at an angle of 120° and a temperature of T = 400°C without intermediate annealing within four passes, n = 4. An examination of the grain structure demonstrates that all grains are anisotropic. According to the dislocation structure, the grains are classified into three types: 1) the smallest grains with no substructure (practically no dislocations) – dislocation-free grains, 2) larger grains containing chaotically distributed dislocations or a net substructure, and 3) the largest grains with a cellular or fragmented substructure. The average value of the scalar dislocation density in the grains of each type is calculated. It is found out that equal-channel angular pressing results in the formation of nanosized particles of secondary phases localized inside the grains, at grain boundaries and at grain junctions of ultrafine-grained nickel. The sources of internal stresses are revealed and their amplitude is determined. The amplitude of the internal stresses is calculated using the amplitude of the crystal lattice curvature-torsion from the bending extinction contours