Heavy plastic Deformation in Uniaxial und Multiaxial Stress States

Ultrafine-grained and nanocrystalline materials have exceptional physical (especially mechanical and magnetic) properties. In metals and alloys such structures can be produced in the bulk by severe plastic deformation (SPD). This seems to open a road to commercial application. Our joint projects were aimed at fragmentation, i.e., the progress of local misorientation by the interaction of lattice defects. For this purpose, tools for SPD had to be implemented. Besides the construction of an Equal Channel Angular extrusion, own developments have been performed such as High Pressure torsion and Cyclic Channel Die compression, where the latter could be even patented. The materials produced in these ways were studied - in addition to conventional methods - by two new techniques which had to be adapted to SPD-specimens: Electron Back Scattering Patterns (EBSP) and Multireflection X-Ray Bragg Profile Analysis (HXPA). It was possible to separate the effects of accumulated strain, variations of strain path and of hydrostatic pressure. The accumulated strain is the principal reason for fragmentation. The strain path has influence on the onset strains for the hardening stages III, IV and V. The influence of hydrostatic pressure is twofold: a) it permits to reach extremely large strains far beyond those reached by conventional methods and b) it leads to additional hardening which can be related to additionally stored deformation defects as compared to conventional deformation. Preliminary theoretical studies and model calculations suggest that this is due to a suppression of the diffusion of point defects and, thus, of dynamic recovery. Our joint projects were well timed to ensure a good position in the present upswing of research in this field. This is documented by new cooperations with other well known international partners, by joint research projects with industry and by the mandates to organise a COST workshop (action 523 ot the EU) as well as one of the important international conferences ("NanoSPD2") on this topic. So far, the joint projects have led to 35 published papers as well as 79 public presentations, 32 of them invited. We plan to submit a joint project in continuation. It will be directed towards alloys, i.e. the influence of the solute atoms and of precipitations on the alloys` mechanical properties after SPD and their thermal stability. Emphasis will be also given to both experimental as well as theoretical analyses of the mechanisms of deformation induced grain formation.