FWF- Fr. Vol. i.m.kr.Met - Free volumes in nanocrystalline bulk metals

In this project lattice defects in bulk nanocrystalline metals could be identified, quantitatively analyzed, and characterized with respect to stability by means of a combination of macroscopic meas-urements – high precision length measurements, so-called dilatometry – and atomistic spectroscopic techniques (positron-electron annihilation). Part of the studies were performed at the high-intensity positron beam of the research neutron source FRM II which enabled in-situ studies of the defect ki-netics on the same time scale as in the dilatometric measurements. In this project, among others, for the first time, the excess volume of grain boundaries – a fundamental structural material parameter – could be determined experimentally by the direct and absolute technique of dilatometry.

Due to the ultrafine crystallite size, nanocrystalline metals may exhibit novel and improved properties in comparison to their coarse-grained counterparts. In this project, bulk nanocrystalline metals were prepared by severe plastic deformation, so-called high-pressure torsion (HPT). The macroscopic sample dimensions (on the length scale of cm) which can be achieved by HPT not only provide the base for technical applications but also allowed the novel access to material defects by the highly specific technique of dilatometry in the present project. Metals with body-centred cubic (Fe, Ta) and face-centred cubic structure (Cu, Ni) were prepared and studied. For all studied samples high concen-trations of free volumes of the order of several 10-3 (thousandth part) could be detected. Correlating the results with positron annihilation spectroscopy and crystallite sizes as determined from scanning electron microscopy, the free volume could be attributed to the various types of lattice defects (lattice vacancies, dislocations, grain boundaries). In addition, the kinetics of defect annealing was analyzed quantitatively. The information and data derived from the present project may contribute to a more profound understanding of the processes of structural refinement during HPT as well as of the partic-ular mechanical properties and the fast atomic diffusion in these ultrafine grained materials.

The project was performed in close cooperation with groups of the Erich-Schmidt Institute (OeAD, Leoben), of the physics faculty of the university Vienna, and of the research neutron source Heinz Maier-Leibnitz (Munich-Garching). Results of the project were in part published in two papers in Physical Review Letters (see also FWF press release:
http://www.fwf.ac.at/en/public_relations/press/pv201011-en.html).

In this project lattice defects in bulk nanocrystalline metals could be identified, quantitatively ana-lyzed, and characterized with respect to stability by means of a combination of macroscopic meas-urements high precision length measurements, so-called dilatometry and atomistic spectroscopic techniques (positron-electron annihilation). Part of the studies were performed at the high-intensity positron beam of the research neutron source FRM II which enabled in-situ studies of the defect ki-netics on the same time scale as in the dilatometric measurements. In this project, among others, for the first time, the excess volume of grain boundaries a fundamental structural material parameter could be determined experimentally by the direct and absolute technique of dilatometry. Due to the ultrafine crystallite size, nanocrystalline metals may exhibit novel and improved properties in comparison to their coarse-grained counterparts. In this project, bulk nanocrystalline metals were prepared by severe plastic deformation, so-called high-pressure torsion (HPT). The macroscopic sample dimensions (on the length scale of cm) which can be achieved by HPT not only provide the base for technical applications but also allowed the novel access to material defects by the highly specific technique of dilatometry in the present project. Metals with body-centred cubic (Fe, Ta) and face-centred cubic structure (Cu, Ni) were prepared and studied. For all studied samples high concen-trations of free volumes of the order of several 10-3 (thousandth part) could be detected. Correlating the results with positron annihilation spectroscopy and crystallite sizes as determined from scanning electron microscopy, the free volume could be attributed to the various types of lattice defects (lattice vacancies, dislocations, grain boundaries). In addition, the kinetics of defect annealing was analyzed quantitatively. The information and data derived from the present project may contribute to a more profound understanding of the processes of structural refinement during HPT as well as of the partic-ular mechanical properties and the fast atomic diffusion in these ultrafine grained materials. The project was performed in close cooperation with groups of the Erich-Schmidt Institute (OeAD, Leoben), of the physics faculty of the university Vienna, and of the research neutron source Heinz Maier-Leibnitz (Munich-Garching). Results of the project were in part published in two papers in Physical Review Letters (see also FWF press release: http://www.fwf.ac.at/en/public_relations/press/pv201011-en.html).