Van der Waals-Zeeman Colloquium

Thursday, 28 Apr 2009, 16:00h

Statistical properties of material yielding and plastic deformation mediated by dislocations

Prof. Carmen Miguel
Departament de Física Fonamental, Facultat de Física, Universitat de Barcelona

Dislocation assemblies in crystalline substrates represent a new example of the broad class of systems that exhibit a jamming (or yielding) transition. Experimental observations on this broad class of systems ranging from granular media to foams show that their dynamics becomes increasingly heterogeneous when approaching the jamming threshold. Likewise, experiments on crystal plasticity at various scales have recently emphasized the heterogeneous character of plastic flow. Plastic avalanches and highly fluctuating stress strain relations have been reported for materials as diverse as ice or several conventional metals. In this talk, we will discuss the statistical behavior of dislocations in simple models of plastic deformation. Jamming characteristics such as slow dynamics, intermittency and scaling features near the jamming threshold are also observed in these model studies. Qualitatively, this can be understood to result from a combination of kinetic constraints on dislocation motion due to the underlying crystal structure and long range anisotropic interactions between dislocations, which together may induce metastable jammed dislocation configurations. We will explore the analogy between these simple dislocation ensembles and other systems exhibiting jamming. At microscopic scales, where only a limited number of dislocations mediate the deformation process, heterogeneous behavior in the mechanical properties is even more dramatic and technologically relevant. Plastic slip seems to be broadly distributed, both in experiments and numerical simulations, but the interpretation of these findings within the framework of collective dislocation phenomena and the yielding transition may not be appropriate. All of these results confirm that plasticity is an excellent playground for statistical mechanics methods and ideas.