Influence of equal-channel angular pressing on grain structure and internal stresses of technically pure nickel

The structure, phase composition, defects, amplitude of internal stresses and their sources in ultrafine grained technically pure nickel, obtained by equal-channel angular pressing deformation, have been studied by the method of transmission electron microscopy. At equal-channel angular pressing the samples were subjected to shear deformation by compression along two intersecting channels of equal diameter at an angle of 120° at temperature T = 400 °C without intermediate annealing. Number of passes n = 4. The grain structure was studied. It was found that all grains were anisotropic. According to the dislocation structure, the grains were 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 was calculated. It was found that equal-channel angular pressing resulted in the formation of nanometer-sized particles of secondary phases in ultrafine grained nickel localized inside, on grain boundaries and at grain junctions. The sources of internal stresses are revealed and their amplitude is determined. Determination of the amplitude of internal stresses consisted in determining the amplitude of the curvature-torsion of the crystal lattice by bending extinction contours

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