The flow stress gradient between the surface and centre of deformed copper single crystals

Abstract

The flow stress distribution below the surface of previously deformed large copper single crystals is obtained when these are sliced into smaller component crystals. It is found that a considerable flow stress gradient extends for a distance of about 2 mm below the surface, measured along the slip direction. Electron microscope studies show that there is an increase in the cell size of dislocation structures near the surface and a decrease in dislocation density. On the basis of flow stress distribution measurements the stress–strain curves of single crystals of the same orientation but different sizes are shown to be consistent with the complete absence of a stage II in an infinitely large crystal. It is further shown experimentally that for very large crystals the value of τ₁₁₁ is reduced considerably. This leads to the conclusion that τ₁₁₁ should not be related to a fundamental physical phenomenon such as the stacking-fault energy of a metal.