title: Free volumes in bulk nanocrystalline metals studied by the complementary techniques of positron annihilation and dilatometry.
abstract: Free-volume type defects, such as vacancies, vacancy-agglomerates, dislocations, and grain boundaries represent a key parameter in the properties of ultrafine-grained and nanocrystalline materials. Such free-volume type defects are introduced in high excess concentration during the processes of structural refinement by severe plastic deformation. The direct method of time-differential dilatometry is applied in the present work to determine the total amount and the kinetics of free volume by measuring the irreversible length change upon annealing of bulk nanocrystalline metals (Fe, Cu, Ni) prepared by high-pressure torsion (HPT). In the case of HPT-deformed Ni and Cu, distinct substages of the length change upon linear heating occur due to the loss of grain boundaries in the wake of crystallite growth. The data on dilatometric length change can be directly related to the fast annealing of free-volume type defects studied byin situDoppler broadening measurements performed at the high-intensity positron beam of the FRM II (Garching, Munich, Germany).
authors.affiliation: Institute of Materials Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria.
authors.forename: Roland
authors.initials: R
authors.lastname: Würschum
authors.forename: Bernd
authors.initials: B
authors.lastname: Oberdorfer
authors.forename: Eva-Maria
authors.initials: EM
authors.lastname: Steyskal
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Sprengel
authors.forename: Werner
authors.initials: W
authors.lastname: Puff
authors.forename: Philip
authors.initials: P
authors.lastname: Pikart
authors.forename: Christoph
authors.initials: C
authors.lastname: Hugenschmidt
authors.forename: Reinhard
authors.initials: R
authors.lastname: Pippan
keywords: Dilatometry; Grain boundaries; Nanocrystalline materials; Positron annihilation