Continuum Dislocation Theory and Related Size Effects in Crystal Plasticity pp. 537-591
Authors: Dennis M. Kochmann, Khanh C. Le, Lehrstuhl fur Allgemeine Mechanik, Ruhr-Universitat Bochum, Bochum, Germany
Abstract: Macroscopically observable plastic deformation of single and polycrystals is produced essentially by the motion of a large number of certain two-dimensional lattice defects known as dislocations. On the other side, these newly formed dislocations in crystals pile up near various obstacles like grain or phase boundaries, or particulate inclusions, giving rise to size dependent hardening of the material. Dislocations appear in the deformed crystal lattice to reduce its energy. Motion of dislocations generates the dissipation of energy which, in turn, results in a resistance to dislocation motion. The understanding of nucleation mechanism and the motion of dislocations is therefore a cornerstone for describing plastic yielding, work hardening, and hysteresis effects in crystal plasticity. Furthermore, dislocations are not only a key microstructural defect for plastic slip but also the core ingredient for forming microstructural patterns and substructures. There are numerous examples. The first one is the formation of lamellar twin patterns in manganese steels and other TWIP-alloys, which has significant impact on the macroscopic stress-strain response. The formation of twins provides TWIP-alloys with excellent hardening behavior, allowing for higher stresses and larger strains than in common f.c.c. or b.c.c. metals (Allain et al., 2004). The other expample is the recrystallization produced by severe plastic deformation during equal channel angular extrusion (ECAE) which leads to almost dislocationfree grains of an average diameter of a few hundred nanometers, yielding materials with exceptional room-temperature strength (Segal, 1995; Iwahashi et al., 1996).