Intermittent plasticity associated with the spatio-temporal dynamics of deformation bands during creep tests in an AlMg polycrystal

Many structural Al-based alloys, and AlMg alloys, in particular, exhibit an intermittent plasticity on the macroscopic scale also known as the Portevin-Le Chatelier (PLC) effect which is associated with repetitive stress drops related with the formation of the bands of localized plastic deformation in tests with a constant imposed strain rate. In the current work we present a comprehensive investigation of a discontinuous creep in commercial AlMg6 alloy used in the aerospace industry. This phenomenon manifests itself as strain bursts during creep tensile tests, where the creep curves contain macroscopic strain jumps in the magnitudes of several percent. The experimental investigation employs the measurements of the load and strain sensors synchronized with a high-speed video camera to capture the deformation band formation and propagation. Unexpectedly, an individual strain burst is found to generate a complex jerky load response which consists of a number of stress drops (similar to the PLC effect) due to inertia of the specimen - creep machine mechanical system. Each stress drop is associated with a fast evolution stage of a single deformation band which takes the shape of a widening neck in a flat specimen. During the widening of the band its “center of gravity” remains stationary. We have established that the main mechanism responsible for the development of the strain burst is the multiplication of the deformation bands, when the boundary of each band generates a secondary band and so on. As a result of the relay-race transfer of the deformation from band to band, a macrolocalized plastic deformation propagates along the specimen. Complementary statistical, spectral, fractal and dynamical analyses are performed on the stress-time series recorded during a single strain burst evolution. It is shown that when the initial specimen length increases the intermittent load response exhibits a tendency to the state of self-organized criticality. Furthermore, transitions between distinct dynamical PLC-like regimes of type A and B during strain burst are explored. We found that the time-periodic stress oscillations in form of a limit cycle can be spontaneously generated in short-time range during dynamical regime of type A. Nonlinear aspects of crystal plasticity conformably to discontinuous creep are discussed.