Dynamic strain ageing and jerky flow in Al-Mg single crystals

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Abstract
Jerky flow behaviour, commonly referred to as the Portevin-Le Chatelier effect, has been studied in single-phase, fully annealed, single crystals of a high purity Al-Mg alloy. It has been found that the details of the jerky flow are different from those observed in specimens containing either grain or striation boundaries. At low temperatures the initiation of jerky flow was found to be accurately described by the Cottrell theory, but at high temperatures deviations from the predicted behaviour were observed. An expression has been derived which, by including the presence of thermal vacancies, predicts the observed high temperature curvature. From the analysis the magnesium-vacancy binding energy has been estimated to be 0.1 ± 0.1 ev. From the low temperature data values of the apparent vacancy migration energy have been found to be 0.4 ev in polycrystals and 0.6 ev in single crystals. Based on the concepts of dynamic strain ageing and on the mutual interaction of the specimen and the testing machine, a model has been proposed which can explain the multiplicity of the stress drops and their dependence on strain rate and temperature.