Mechanisms of Cyclic Plastic Deformation in Metals

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This grant examines cyclic deformation, or fatigue, at a fundamental level. Metal fatigue is poorly understood, partly because the dislocation dynamics and internal stress-states during reversed deformation have not been characterized. The details of dislocation motion and interaction and the internal stress-state are critical to this process. In this research a detailed description of cyclic deformation, including the Bauschinger effect, the substantial reversible strains with reversal of the direction of deformation, and the eventual saturation of the flow stress, will be accomplished, using advanced experimental techniques. Conventional dark- field (DF) transmission electron microscopy (TEM) will be used on Cu single crystals to assess dislocation dipole spacings and distributions, which will allow a determination of the local stress-state during cyclic plasticity. Experiments will include convergent beam electron diffraction (CBED) in the TEM that can probe small (20-nm diameter beam) areas to assess changes in the lattice parameter in unloaded foils as well as in-situ, or under load. This will allow direct assessment of the local internal stress with relatively high accuracy. The dislocation dynamics will be studied by in-situ deformation in the high voltage transmission electron microscope (HVEM). The role of screw dislocations will be investigated, including the existence of pile-ups and cross-slip. The details of dislocations associated with anelasticity on unloading will also be studied. Specially oriented foils will complement earlier reversed deformation experiments in the HVEM to especially determine the nature of screw dislocation dynamics during fatigue. %%%In the past there has been only limited success with direct observation of dislocations during cyclic deformation, such as with in-situ cyclic or reversed plastic deformation tests such as in the transmission electron microscope. Also the internal stress-state has not been adequately determined. Internal stresses are widely suggested to exist in the vicinity of dislocation heterogeneities in cyclically (as well as monotonically) deformed microstructures. The heterogeneities include edge dislocation dipole bundles (veins) and the edge dipole walls of persistent slip bands (PSBs). The understanding developed from this project will aid in the design of thin film devices that undergo cyclic stresses.***