The extent of fracture of anodic oxide films during tensile deformation of aluminum is shown to be controlled by the density of defects in the oxide. The density of defects was monitored by the "leakage" resistance of the film, while the extent of fracture of the oxide was obtained from measurements of reanodization current transients. Other experiments in a photoelectron microscope showed that these oxides are ruptured at emerging slip steps. This provided the basis of a simple model for oxide fracture, which requires the presence of a defect in the oxide at the site of an emerging slip step. A relationship between the leakage resistance and fracture susceptibility of the oxide was derived and confirmed by experiment, showing that both properties are controlled by the same type of defect. The density of these defects may be controlled by the surface preparation prior to anodization. If the density of defects is greatly reduced, even a thin (28 nm) oxide becomes strong enough to suppress slip step formation during tensile deformation.

Exoelectron emission from abraded metal surfaces in the darkIn this paper we show that photostimulated exoelectron emission resulting from the plastic deformation of a metal originates solely from the slip lines produced on the surface. Conclusive evidence is provided by micrographs formed by direct electron optical imaging of the exoelectrons. The exoelectrons are emitted only from bare metal revealed by a localized rupturing of the natural surface oxide during the formation of the slip step. This contrast mechanism is unique and offers a new form of electron microscopy for the study of plastic deformation of metals.

A Philips thermionic emission electron microscope has been converted into a photoemission electron microscope by the addition of a simple ultraviolet illumination system and an electron multiplier array for image intensification. Photoemission images are formed without heating or activation of the surface. One example includes the first electron optical image of exoelectron emission from an abraded metal surface.

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The development of photostimulated exoelectron emission (PSEE) was observed directly in a photoelectron microscope during plastic deformation of anodized aluminum. The PSEE defines the mode of fracture of the oxide as well as the tensile strain required to initiate fracture. Thick oxides (280 nm) develop cracks perpendicular to the stress direction which propagate independently of the underlying microstructure of the metal. Oxides ?70 nm thick fracture at the slip steps in the same manner as the very thin natural oxide. Contrary to the results of previous observers of PSEE, the tensile strain required for the onset of emission actually decreases slightly with increasing oxide thickness.