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

Structural anisotropy and excess volume in ultra-finegrained high-purity nickel prepared by equal-channel angu-lar pressing (ECAP) is studied by means of dilatometry andcompared with the processing route of high-pressure tor-sion. Both routes exhibit qualitatively similar three-stagebehavior in length change upon defect annealing with acharacteristic dependence on the measuring direction re-lated to the deformation axes. Taking into account shapeanisotropy of the crystallites, the length change in variousdirections can be quantitatively analyzed yielding directaccess to the concentration of deformation-induced latticevacancies, the vacancy relaxation, and the grain boundaryexpansion. The routes A12 and BC12 of ECAP are com-pared.