Defects and structural anisotropy in ultrafine-crystalline metals

The present project aims at a detailed comparative study and comprehensive understanding of free volume-type defects in ultrafine crystalline metals which are produced by equal channel angular pressing (ECAP) and high- pressure torsion (HPT). Ultrafine crystalline metals prepared by severe plastic deformation (SPD) have become a focus of materials research over the last years owing to their outstanding mechanical properties such as high strength in combination with good ductility. The atomistic processes, which occur during SPD-induced structural refinement and which give rise to the particular mechanical behaviour, are intimately related to structural defects available in these materials in high abundant concentrations, such as grain boundaries, dislocations, or lattice vacancies and their agglomerates. Therefore, a study of these defects is of pivotal importance. The defect-specific method to be used will be differential dilatometry which the author and his co-workers could recently successfully establish for studying the absolute concentration of free-volume type defects. A focus of the project will be on the correlation between structural anisotropy and defect characteristics. This is motivated, on the one hand, by the fact that structural anisotropy turned out to be a highly relevant issue for the properties of these materials and, on the other hand, by the fact that structural anisotropy can be controlled to a great extent by means of the strain path, i.e., by the method of deformation or the combination of different methods (HPT, ECAP, additional rolling) as well as by means of the deformation conditions (monotonic/cyclic, temperature, etc.). An important goal will be to find out how the defect characteristics of these metals are controlled by the strain path. For this purpose systematic dilatometric studies in dependence of strain path will be performed using the different deformation routes or combinations thereof. This shall clarify in how far distinct differences of the properties of differently SPD-processed metals are related to variations in their defect characteristics. But these studies are, on the other hand, of great relevance in a much wider scope of the physics of condensed matter since they yield unprecedented direct access to fundamental defect parameters. In fact, not only the excess volume of grain boundaries can be measured directly by dilatometry but possibly also the volume of relaxed lattice vacancies by making use of the anisotropy of defect annealing which may prevail in these SPD-processed structurally anisotropic metals. Accompanying structural characterization, particularly by scanning electron microscopy (SEM), and sample preparation by EACP and HPT will be performed by making use of collaborations with expert groups of the Erich Schmid Institute (Leoben, Austria), of the Institute of Materials Physics of the University of Münster (Germany) and Institute of Electron Microscopy of TU Graz.

In this project the correlation between the anisotropy of the microstructure and the characteristics of defect annealing in ultrafine crystalline metals could be identified and quantitatively analyzed using the specific experimental technique of high precision length measurements (dilatometry) in combination with structural characterization by neutron diffraction and scanning electron microscopy. Among others, the volume of the relaxed lattice vacancy - a fundamental structural parameter in condensed matter physics - could be determined experimentally. The access to anisotropic, i.e., orientation-dependent processes revealed to be the outstanding advantage of dilatometry compared to other techniques of thermal analysis. The ultrafine crystalline metals with grain sizes in the regime of 100 nanometers (i.e., one tenth of a micrometer) were produced by techniques of severe plastic deformation (SPD), so-called equal channel angular pressing and high-pressure torsion. The processes of structural refinement by severe plastic deformation as well as the particular mechanical behaviour of this attractive class of materials are intimately related to their structural defects which are available in highly abundant concentrations and which were in the focus of this project. A model could be proposed and successfully tested for directly determining the volume of lattice vacancies by means of dilatometric measurements of the orientation-dependent length change. Furthermore, kinetic diffusion models were developed quantitatively describing the length change due to the annealing out of deformation-induced free-volume type defects. These model tools are considered to be of more general interest going beyond the scope of SPD-metals. Neutron diffraction and difference dilatometry clearly indicated that the observed anisotropy in length change upon defect annealing is solely attributable to the microstructure rather than to internal stresses. The examined close correlation between structural anisotropy and the anisotropic annealing of excess volume yielded - apart from the vacancy volume - also direct access to the grain boundary excess volume, a further key parameter in materials science. The project was performed in close cooperation with groups of the Erich-Schmid Institute (OeAD and University of Leoben), the research neutron source Heinz Maier-Leibnitz (TU Munich, Garching, Germany), the Austrian Institute of Technolgy (AIT, Wiener Neustadt), the Institute of Materials Physics of the University of Münster (Germany), and the Institute of Electron Microscopy and Nanoanalytics of TU Graz.