U.S.-France Dissertation Enhancement: Dislocation Induced Backstresses in Crystalline Materials

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Backstresses, or long-range internal stresses, have been suggested to exist in plastically deformed crystalline materials for decades. However, until recently, unambiguous evidence of this phenomenon has been absent. It is believed that these stresses are associated with dislocation heterogeneities in the deformed microstructures. The heterogeneities include cell and subgrain walls in monotonically deformed materials, and edge dislocation dipole bundles (veins) and edge dipole walls of persistent slip bands (PSBs) in cyclically deformed materials. Very recent (not yet all published) x-ray microbeam diffraction experiments performed using a synchrotron are able to determine the elastic strains within the dislocation cell interiors and cell walls. These are accomplished using, oriented, monotonically and cyclically (presaturation, i.e., no PSBs) deformed Cu single crystals. The results suggest that measurable long-range internal stresses (LRIS) are present in cell interiors and cell walls. These LRIS vary substantially from cell to cell as 0-50% the applied stress. Cell interiors have stresses of roughly 0.13-0.29 the applied stress and cell walls have stresses of opposite sign to interiors at roughly 0.07 the applied stress, although the latter value is preliminary (unpublished). Peter Geantil, a University of Southern California (USC) graduate student, will be performing detailed transmission electron microscopy on the dislocation substructure at USC as part of his dissertation research. Through collaboration with researchers at the Laboratoire d'Etude des Microstructures at CNRS-ONERA in France, this dislocation information will be substituted into the powerful dislocation dynamics code microMegas, in an effort to verify the experimental LRIS measurements. There is a two-fold benefit in performing this experiment. The dislocation dynamics simulations will verify the experimental data obtained, and dislocation modeling must model physical phenomena accurately, so the exercise will be a test of the model itself. Our understanding of plastic deformation will be enhanced. Additionally, the dislocation dynamics simulations will show analytically the origin of these stresses, and aid in developing the theory behind this phenomenon. This project involves collaboration between two leading research groups in an effort to explore important questions in the field of strength of materials. University of Southern California PhD student, Peter Geantil, will be sent to a major research institution, CNRS-ONERA in Paris, France. He will collaborate with some of the worlds leading experts in the area of modeling the mechanical behavior of materials to resolve important questions regarding details of the strength of materials. The project also involves collaboration with three US government laboratories: NIST, Oak Ridge National Laboratory, and the Advanced Photon Source at Argonne National Laboratory. This project will provide a deeper understanding of plastically deformed materials. More specifically, this work will lead to a deeper understanding of fatigue, the cause for most structural material failures. A better understanding of the details of fatigue will in turn lead to stronger, more reliable, materials. This collaboration consists of the best experimental effort that characterizes the variation in stress-states in materials microstructures, with the best theoretical / computational group in the same area.