Study of Grain Boundary and Dislocation Transmission based on a Finite-Deformation Framework with an Application to Description of Nanoindentation Tests

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Grain boundaries (GBs) play an important role in the plastic deformation of metals carried out by the motion of dislocations. Indentation tests close to grain boundaries have also shown the occurrence of dislocation pile-up and then transmission of dislocations through GBs in a so called GB pop-in event. This proposal will address the complexity of interactions between dislocations and grain boundaries as well as the effect of grain boundaries on the dislocation motions based on a finite-deformation gradient crystal-plasticity framework. The thermodynamically consistent model includes the energetic and dissipative governing components and is first accompanied with a yield function controlling microtractions associated with the densities of dislocations accumulated at grain boundaries and then extended to incorporate automatically the effect of the relative misorientation of adjacent grains at the grain boundary. The model is developed based on a novel approach and implemented in the finite-element software ABAQUS. The numerical work is supported by nanoindentation and high resolution electron microscopy on reference bcc materials. The grain boundary pop-in behavior is studied for different grain boundary misorientations using nanoindentations with Berkovich and spherical indenters. The nanoindentation experiments will compromise a wide range of rate and size dependent studies, quantifying the GB pop-in effect in dependence of the indentation distance to GBs and the indenter orientation. Furthermore, sequential polishing and high resolution electron back scatter diffraction (HR-EBSD) is used to assess the local orientation gradients around the residual impressions for indentations within grain interior as well as indentations exhibiting the GB pop-in. This research work will provide a comprehensive description of the dislocation grain boundary interaction, based on a combined theoretical and experimental approach.