The effect of oxide films on dislocation-surface interactions in aluminum

Abstract

Aluminum single crystals were deformed in cyclic strain, both in air and in vacuum, and with both 50 and 100 Å thick oxide films on the surface. The results show that deformation in vacuum produces a layer of edge dislocation dipoles trapped just beneath the surface, as well as reducing the amount of surface slip. The effect of increasing the oxide coating thickness is to reduce the amount of slip observable on the surface of the oxide. As an aid in interpretation, the mechanical properties of thin, amorphous aluminum oxide films were measured in air and in vacuum. Young's modulus of aluminum oxide, E c, was found to increase by a factor of 4 under vacuum, and the fracture strength by a factor of 1.5. These increases are most likely due to the removal of absorbed water vapor. The increase in E c can account for the presence of the trapped dislocation dipoles under vacuum by using the theory of Head and of Conners, which predicts a net repulsive force on near surface dislocations when the modulus of the coating is greater than the modulus of the substrate. The reduction in surface slip observed under vacuum is due to the increase in both E c and the strength of the oxide coating.