Microdiffraction Experiments and Modeling for Analyzing Multiscale Dislocation Ensembles in Materials
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Microdiffraction Experiments and Modeling for Analyzing Multiscale Dislocation Ensembles in Materials G.E. Ice, R.I. Barabash and J. Pang Metals & Ceramics Divisions, Oak Ridge National Laboratory, Oak Ridge TN 37831 ABSTRACT The intensity distribution of Laue diffraction is analyzed as a function of local misorientation. We show how unpaired dislocations alter the white beam Laue patterns for isolated dislocations, for dislocation walls, and for a combination of both. We consider the effect of different statistically and geometrically necessary dislocation densities on the intensity distribution along and perpendicular to the Laue streak. A 3D x-ray crystal microscope is used to analyze the complicated plastic-elastic field in a grain of a Ni polycrystalline sample during in-situ uniaxial pulling. A change of dislocation activity with depth is demonstrated. The dislocation slip systems and their densities are determined at various depths. The model parameters are used to simulate the whole Laue pattern including details about the contours for specific Laue spots; good agreement is found between simulated and experimental contours. INTRODUCTION The recent developments of strain gradient plasticity represent an effort to bridge the gap between classical plasticity and dislocation theory1-4. Plastic strain gradients appear either because of the geometry of loading or because the material is plastically inhomogeneous. Several length scales are introduced in the framework of strain gradient plasticity. Considering different models of geometrically necessary dislocations, GNDs, Gao3 et al. related effective strain gradient η to the density of GNDs, n+. Polychromatic x-ray microdiffraction offers a fundamentally new approach to the study of mesoscale dynamics in single crystals and polycrystalline materials5,6. With white-beam microdiffraction it is possible to quantitatively analyze a sample at different structure levels. In particular, it is now possible to analyze details of structure corresponding to nano (cells) - and mesoscopic (fragments) levels. Different slip systems cause distinctly different streaking in their Laue patterns. Examination of streaked patterns enables the detection of statistically stored dislocations and GNDs, and the quantitative determination of dislocation patterning parameters. We use a multiscale hierarchical framework similar to a cell-wall model7-8, o extend the dislocation/disclination description for the length scales introduced in strain gradient plasticity theory. We show that diffraction experiments give natural criteria for the aforementioned length scales. EXPERIMENTAL DETAILS The 3D x-ray crystal microscope uses a modified Laue diffraction method based on polychromatic radiation5,6. This approach allows for true 3D mapping of crystalline phase, orientation, elastic strain and plastic deformation with unprecedented spatial resolution. As the polychromatic beam penetrates the sample, it produces a Laue pattern in each subgrain that it intercepts. The overlapping Laue patterns
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