Unloading effects in the plastic properties of copper single crystals

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Abstract
Yield point phenomena occur on retesting copper single crystals during interrupted tensile deformation. The effect is observed only during the linear and parabolic regions of the stress-strain curve and the size of the yield drop is directly proportional to the reduction in stress on unloading. The ratio of the flow stress immediately before complete unloading to that on retesting is 0–986, and appears to be independent of temperature between −195°c and 100°c. The magnitude of the yield phenomenon at −195°c is markedly increased, however, by annealing at above −170°c, and this effect is independent of the amount of unloading. An explanation of these phenomena is advanced in which Lomer-Cottrell sessile dislocations are formed during unloading, and that it is these dislocations which prevent large scale reverse plasticity. The increased effect at −195°c on annealing is interpreted in terms of the thermally activated reversal of the apices of these sessile dislocations.

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