Long Range Internal Stresses in Plastically Deformed Crystalline Materials

Title: Long Range Internal Stresses in Plastically Deformed Crystalline Materials

Speaker: Prof. Michael Kassner,  Choong Hoon Cho Chair and Professor of Chemical Engineering and Materials Science and Aerospace and Mechanical Engineering

Affiliation: University of Southern California 

ABSTRACT:  Long-range internal stresses (LRIS) are widely suggested to exist in materials as a result of dislocation heterogeneities in plastically deformed materials.  The dislocation heterogeneities include cell and subgrain walls in monotonically deformed materials and edge-dislocation dipole bundles (veins) and the edge dipole walls of persistent slip bands (PSBs) in cyclically deformed materials. Long-range internal stresses have long been suggested to be responsible for the Bauschinger effect in reversed and cyclic deformation.  Evidence for long-range internal stresses (LRIS) includes stress-dip tests, dislocation pinning of loaded materials, in-situ deformation experiments, and asymmetric x-ray line broadening analysis.  Other experiments, including recent dipole separation observations and convergent beam electron diffraction experiments, may be less supportive of LRIS. Most recently, long-range internal stress, was investigated by us using advanced x-ray microbeam diffraction experiments. These were accomplished using a synchrotron at the Advanced Photon Source that is able to determine the  elastic strains in very small volumes within the cell interiors, and very recently, within  the cell walls.  These were accomplished using oriented monotonically and cyclically deformed Cu single crystals  as well as severely deformed (SPD) Al by ECAP.  The results suggest that long-range internal stresses are present. The magnitude and variation of these stresses with position within the microstructure will be described. These results are placed in the context of earlier experiments and will show that the scientific community has generally overestimated LRIS. Additionally, new reversed deformation experiments with augmenting synchrotron diffraction provided new insights into the Bauschinger effect.