9614562 Kassner The metal fatigue phenomenon is poorly understood, in part because the dislocation dynamics of reversed deformation is not well characterized. The direct observation of dislocations during cyclic deformation (such as with in-situ cyclic or reversed plastic deformation tests in the high voltage transmission electron microscope) is hindered by the buckling of thin foils when the applied tensile or shear stress is reversed. Recent experiments show that buckling of the foil can be avoided and dislocation movement can be reversed by tensile stressing in alternating perpendicular directions. A new specimen stage was designed to facilitate this loading and tested in the high voltage transmission electron microscope under an NSF small grant for exploratory research (SGER). A series of reversed deformation and fatigue experiments were successfully performed in the microscope on aluminum single crystals pre-cyclically deformed to presaturation, forming dense edge-dislocation dipole bundles with screw dislocations that span the channel. It was found that screw dislocations move early during reversal although edges and screws contribute equally to the strain amplitude. Dislocation loop expansion from the dipole veins appears to be a major source of plastic strain amplitude. Dipole flipping does not appear to contribute significantly to reversible strain. In this project these preliminary conclusions are examined using the new techniques. These experiments are extended by improving the quality of the microscopy images. The important case where persistent slip bands form is included in the approach because the slip bands create extrusions and intrusions which may be sites for fatigue crack nucleation. %%% Fatigue failure is a major failure mechanism in metal alloy components used in applications that see alternating loads. This project provides insights into the fatigue mechanism in reversed loading situations. *** _
