Papers by Author: Jan Kratochvíl

Abstract: Within the framework of crystal plasticity the grain refinement observed in metals exposed to severe plastic deformation(SPD) is interpreted as an energy minimization process. For a crystalline solid it is less energetically costly to be fragmented in small regions(subgrains) deformed by a reduced number of slip systems and fitted together by local reorientations than to be deformed by homogeneous multislip.The energy minimization enhanced by hardening tends to reduce the size of the subgrains. On the other hand, the refinement increases the area of the boundaries between the subgrains, hence, it increases interface energy. Thus, the size of the subgrains is the result of a competition between these two tendencies:the internal and dissipative energy tend to decrease the refined structural size, whereas the interface energy opposes this tendency. Moreover, the formed substructure pattern follows the orientation of the active slip systems. The effective rotation of the active slip systems, which is a basic charac-teristic of SPD processing techniques, enhances the refinement process. It is proposed that the effective rotation causes a continuous reconstruction of the substructure pattern. The hindered destruction of the previous pattern leads to an increased hardening thus reducing the size of the subgrains. The proposed approach is demonstrated in the analysis of the refinement process induced by high pressure torsion and the results are correlated with the available observations.

Abstract: To estimate the fatigue endurance limit at the micro-scale two unlike dislocations gliding in a channel of a persistent slip bands are considered. The dislocations are modeled as moving planar flexible curves. The objective of the simulations is to determine the averaged stress in the channel needed for the dislocations to escape one another using statistics of encounters of such dislocation pairs. We employed an iso-strain approach, i.e. the assumption that the total strain everywhere in the channel is the same.