Long-Range Internal Stresses in Plastically Deformed Materials

University Of Southern California, Los Angeles CA

Investigators

Abstract

TECHNICAL SUMMARY This is a proposal to fund a domestic PhD candidate to definitively assess the existence of long-range internal stresses (LRIS) in plastically deformed materials. Backstresses or LRIS in the past have been suggested by many (but certainly not all) to exist in plastically deformed crystalline materials. Elevated stresses can be present in regions of elevated dislocation density or dislocation heterogeneities in the deformed microstructures. The heterogeneities include edge dislocation dipole bundles (veins) and the edge dipole walls of persistent slip bands (PSBs) in cyclically deformed materials and cell and subgrain walls in monotonically deformed materials. The existence of long-range internal stress is especially important for the understanding of cyclic deformation and also monotonic de-formation. Preliminary x-ray microbeam diffraction experiments that are able to deter-mine the elastic strains within the cell interiors were performed by the principal investi-gator using a synchrotron. These were accomplished using, oriented, monotonically and cyclically (presaturation, i.e., no PSBs) deformed Cu single crystals. The results suggest that small (17 to 29 % of the applied stress) long-range internal stresses may be present in cell interiors. These LRIS vary substantially from cell to cell as 0-50% the applied stress. This proposal includes an attempt to measure elastic strains in the cell walls which has been challenging due to the high defect density. This will help complete the definitive analysis of LRIS in deformed materials. Finally, detailed transmission electron micros-copy will be performed on the dislocation substructure (Burger?s vector analysis and pre-cise spatial positioning of wall dislocations), to assess through a dislocation dynamics code, a verification of the experimental LRIS. The understanding of plastic deformation, in general, is expected to be enhanced. We expect to be the first to definitively assess long-range internal stresses in plastically deformed materials. NON-TECHNICAL SUMMARY This is a project to uncover the secrets of the strength of materials using x-ray mi-crobeams. The use of intense, submicron, x-ray beams at the Advanced Photon Source in Illinois has led to the discovery that structural materials are under significant, variable internal stresses of opposite direction on submicron length scales corresponding to the dislocation substructure. This result has profound implications for understanding the me-chanical strength and behavior of materials. The presence of counterbalanced stresses within microscopic volumes (or cells) in deformed materials was predicted more than two decades ago and has been inferred from numerous indirect experiments. Yet, direct proof of their existence has been elusive, as spatially-resolved measurements of the stress mag-nitudes and distributions critical for testing theories and computer modeling were not possible before the development of high-resolution x-ray microbeams at synchrotron sources. The implications of this work are of direct relevance to important practical prob-lems such as sheet metal forming (for example, in automobile production) and metal fa-tigue, the latter being responsible for most structural materials failures.