Internal Stresses and Their Sources in Nickel after Equal-Channel Angular Pressing

Abstract

The internal structure of grains, as well as the amplitude of internal stresses and their sources, in ultrafine grained technically pure nickel obtained by equal-channel angular pressing deformation was studied by the method of transmission electron microscopy. Under equal-channel angular pressing, the samples have been subjected to shear deformation by compression along two intersecting channels of equal diameter at an angle of 120° and temperature T = 400°C without intermediate annealing. Number of passes n = 4. The equal-channel angular pressing is found to lead to the formation of particles of secondary phases in ultrafine grained nickel with nanometer size and localized inside, at the boundaries and the joints of grains. The sources of internal stresses are revealed and their amplitude is determined. Determination of the amplitude of internal stresses is based on the determination of the curvature–torsion of the crystal lattice along bending extinction contours. It has been established that the sources of internal stresses are grain joints in which particles of secondary phases are present or absent; grain boundaries at which particles of secondary phases are present or absent; particles located on dislocations inside grains, and, finally, the dislocation structure in grains or parts of grains in which there are no particles of secondary phases. It has been found that internal stresses from all sources involve all grains regardless of their internal structure and are predominantly elastic in nature. This means that equal-channel angular pressing led mainly to the elastic distortion of nickel lattice.