Non-planar Dislocation Cores: A Ubiquitous Phenomenon Controlling Mechanical Properties of Materials

Vaclav Vitek, University of Pennsylvania

Dislocation characteristics and behavior in close-packed crystals, in particular FCC, have been habitually regarded as the paradigm of dislocation behavior in all crystalline materials. An inherent assumption, often not explicitly stated, is that dislocation cores are for any orientation of the dislocation line spread into the slip planes. In FCC crystals the reason is dissociation into Shockley partials. However, we will argue in this presentation that this type of core spreading, and ensuing dislocation behavior, is a very special case while it is considerably more common that for some orientations of the dislocation line the cores extend into several non-parallel crystal planes. These dislocations then usually control the plastic properties of such materials which results in unexpected deformation modes, strong and unusual dependence of the flow and yield stress on temperature and crystal orientation, inapplicability of the Schmid law and, generally, phenomena not expected within the simple prototype of dislocation behavior. The most widely recognized example is the screw dislocation in BCC metals. Hence, we present first results of extensive computer modeling of dislocations in transition BCC metals that reveal features such as dependence of the Peierls barrier on the applied stress tensor, leading to the significant influence of shear stresses perpendicular to the glide direction upon the plastic flow. Since the principal source of information about the dislocation cores is computer modeling, we summarize results of such studies in hexagonal metals and several intermetallic compounds, including Ni3Al, NiAl and TiAl. They all reveal non-planar dislocation cores which lead to dislocation behavior that can explain a variety of deformation properties of these materials. The general finding is that non-planar cores are more significant the more complex and open is the crystal structure. Hence, non-planar dislocation cores are by no means limited to metallic materials but play an important role in covalently bonded crystals, including molecular crystals, such as, for example, the monoclinic anthracene.

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