Mechanisms of fatigue hardening in copper single crystals

Abstract

Copper single crystals were cycled in push-pull at a constant total shear-strain amplitude, γ t , of ±0.0075, which gave a fatigue life, N f , of 42,500 cycles. Dislocation arrangements were determined as a function of the number of elapsed cycles and were correlated with the fatigue-hardening curve. The cyclic stress-strain curve was determined over the range γ t = ±0.0016–0.016. Results have been compared with other recent fatigue-hardening studies. Dislocation bundles in dipole and multipole configurations are formed by a mutual-trapping mechanism during rapid hardening. The bundles contribute to rapid hardening by providing effective barriers to continued dislocation motion on the primary and secondary slip systems. Dislocation structures in the earliest stages of rapid hardening were found to be analogous to unidirectional dislocation structures; the similarity rapidly fades, however, with continued cycling and the progressive development of a cell structure, until at saturation hardening (≈0.5 per cent N f ), a dislocation structure uniquely characteristic of cyclic deformation is developed. Strain accommodation during saturation hardening can be accomplished by the cooperative movement of dislocations within cell walls (or loop patches) and by the traffic of dislocations across cells (or between loop patches), with ∼50 per cent of the dissipated energy estimated to be available for point-defect production. The present results lead to a unified view of fatigue hardening for high- and low-strain amplitudes. The dislocation structures in low-amplitude loop patches and in high-amplitude cell walls appear to be identical, each being a product of the early mutual-trapping mechanism. A correlation is suggested between the occurrence of the Bauschinger effect and the development of simple dislocation bundles (Stage I type dislocation structure). It is postulated that the Bauschinger effect is a result of the relaxation of dislocations within these dislocation bundles.