An interpretation of the Haasen–Kelly effect

Abstract

In 1957, P. Haasen and A. Kelly presented an account of experiments that showed that, in Al and Ni single crystals, the mere act of unloading during a tensile test caused a small but reproducible transient increase in flow stress, a ‘yield point’. The increase is proportional to the current work-hardened stress level. It occurs at the lowest temperatures, where diffusion is not possible, but it is enhanced at higher temperatures. The effect was interpreted as arising from dislocations that ‘anchor themselves during unloading and that this process of anchoring, which prevents reverse plastic flow, is also responsible for the yield-point effect on reloading’. Recent ideas emphasising the role of dislocation dipoles as sources for slip provide a natural explanation for these effects.

Keywords:

• dislocations
• Haasen–Kelly effect
• work-hardening
• yield

Acknowledgements

It is a pleasure to acknowledge the many years of discussions and encouragement from Tony Kelly. I am grateful to a referee for helpful criticism, and to the Cavendish Laboratory and to Robinson College for support.