Structural anisotropy in equal-channel angular extruded nickel revealed by dilatometric study of excess volume

Structural anisotropy and excess volume in ultra-fine grained high-purity nickel prepared by equal-channel angular pressing (ECAP) is studied by means of dilatometry and compared with the processing route of high-pressure torsion. Both routes exhibit qualitatively similar three-stage behavior in length change upon defect annealing with a characteristic dependence on the measuring direction related to the deformation axes. Taking into account shape anisotropy of the crystallites, the length change in various directions can be quantitatively analyzed yielding direct access to the concentration of deformation-induced lattice vacancies, the vacancy relaxation, and the grain boundary expansion. The routes A12 and B(C)12 of ECAP are compared.