High speed investigation of spatio-temporal localization of plastic deformation and fracture of notched Al-Mg specimens exhibiting intermittent plasticity

Intermittent plasticity, known as the Portevin-Le Chatelier (PLC) effect and the yield point phenomenon, is a striking example of unstable mechanical behavior of metals and alloys caused by localization of plastic deformation within the PLC and Lüders bands. In present work dynamics and morphology these bands in notched specimens of an AlMg6 (AA5059) commercial alloy under stress-rate controlled tensile tests was investigated. The strain and force responses to the formation and propagation of deformation bands were measured synchronously with high-speed video recording of the specimen surface with a time resolution of 0.2 ms. The results show that the notch is an attractor of deformation bands from the Lüders band to the neck. The notch reduces the effective size of the gauge part of the specimen to a value comparable to the width of the specimen and causes premature sudden failure, reducing the resource of strength and ductility of the alloy. The deformation bands generated by the notch tip cause strain jumps, i.e. steps in the stress-strain curve and stress drops in the complex structure of the force response. It was established that the local rate of plastic deformation in the Lüders band and the PLC band exceeds the average strain rate of the specimen by 3 and 3.5 orders of magnitude, respectively. The spatial statistical distribution of the bands has a sharp maximum in the section along which the main crack will pass. It is a shear crack (type II) that propagates viscously at a velocity of several m/s along the PLC band in the neck structure. Before the rupture the moments of PLC band nucleation self-organize into time sequence that obeys an exponential law. The role of Lüders and subsequent PLC bands in the mechanism of neck formation and failure of a notched specimen is discussed.