The properties of ultrafine-grained metals prepared by severe plastic deformation are closely related to their high amount of excess volume localized in defects, such as grain boundaries, dislocations, and vacancies. A powerful tool to study free volume-type defects on an absolute concentration scale has turned out to be difference dilatometry . This method allows the precise measurement of the volume change associated with the annealing out of these defects upon time-linear heating.
The present work focuses on a comparative dilatometric study of the two processing techniques of high-pressure torsion (HPT) and equal-channel angular pressing (ECAP) using nickel as a model system. Both routes yield an anisotropic shape of the submicrometer-sized crystallites which manifests in an orientation-dependent dilatometric length change owing to the anisotropic annealing out of deformation-induced lattice vacancies. For both HPT- and ECAP-processed Ni a pronounced recrystallization stage is monitored by dilatometry from which the amount of grain boundary excess volume can be deduced. Specific differences observed for HPT- and ECAP-Ni in the annealing behaviour with respect to amplitude and temperature are associated with the different degree of deformation and grain refinement achievable with both routes.
Financial support by the FWF Austrian Science Fund is appreciated (project P25628-N20).
 W. Sprengel, B. Oberdorfer, E.-M. Steyskal, R. Würschum: Dilatometry: a powerful tool for the study of defects in ultrafine-grained metals, J. Mater. Sci. 47 (2012) 7921-7925