The presence and nature of vacancy type defects in nanometals detained by severe plastic deformation

Excessive concentrations of deformation-induced vacancy type defects have been found in nanometals processed by severe plastic deformation (SPD) but not in others which suggest them to be responsible for SPD-specific properties i.e. considerable ductility and particular phase existencies. Methods of annealing resistometry as well as of calorimetry proved well in determining the concentration of these defects, and their activation enthalpy being typical of both the defect type and the actual diffusion mechanism. For identification of complex vacancy defects, comparisons with results from X-ray line profile analysis for the dislocation density revealed to be indispensable. The vacancy concentrations measured in SPD-deformed metals markedly exceed that of conventionally deformed ones, because of the presence of enhanced hydrostatic pressure as being shown by systematic investigations with various pressures. While in SPD-processed Cu only vacancy agglomerates have been found, in SPD-processed Ni also single/double vacancies are present, and the total concentration of vacancy type defects is higher. These effects are suggested to arise from the higher stacking fault energy of Ni, and of its higher enthalpy for vacancy migration.