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Dynamics of the deformation band formation during creep in an Al-Mg alloy was investigated by simultaneous high-speed video recording of deformation bands with an extremely high time resolution of 40 µs and measurement of the concomitant acoustic emission. The results are consistent with the mechanism where the trigger of a strain step on the creep curve is the nucleation and rapid widening of a primary deformation band, generating an acoustic emission signal within one millisecond. Correlations between various stages of the deformation band formation, acoustic emission, and stress response are explored in detail. Notably, the stress drop rate and the amplitude of the acoustic emission signal are found to be linearly related. The mechanism of the acoustic emission associated with the initial rapid stage of the band development is discussed.

The dynamics of macrolocalized deformation bands generated by the impact of a Vickers indenter on the surface of an AlMg6 aluminum-magnesium alloy deformed under creep conditions was studied by high-speed video recording and acoustic emission methods. It has been established that these bands are the trigger for the development of a macroscopic jump in plastic deformation on the creep curve. It is shown that in alloys exhibiting the Portevin-Le Chatelier effect it is necessary to take into account the initiation and propagation of deformation macrobands during surface erosion damage and to estimate the volume and morphology of the plastic zone during impact microindentation of a deformable alloy. Keywords: intermittent deformation, deformation band, impact, indenter, aluminum-magnesium alloy.

Formation of bands of macroscopic strain localization during staircase creep in an AlMg polycrystal is studied by the acoustic emission (AE) technique and high-speed video-recording with an image acquisition rate up to 50 000 frames per second. The simultaneous measurements by two methods allow to distinguish different types of embryo deformation bands and concomitant AE signals and establish correlations between the band evolution and the acoustic response. It is found that the fastest stages of band formation, associated with its emergence to the surface and subsequent accelerated expansion, generate complex AE bursts in the frequency band ~0.05-1 MHz. The correlations hidden in the complex structure of an individual acoustic burst are investigated by methods of statistical and fractal analysis. On the other hand, relationships between average parameters of various physical responses to discontinuous creep are assessed. Particularly, a close correspondence is found between the envelope of the acoustic burst and the rate of stress change during formation of a single deformation band. Evolution of dynamical behavior of embryo bands with increasing creep stress is discussed. Notably, a qualitative change in the AE waveform observed on approaching the ultimate stress is considered from the viewpoint of anticipation of the oncoming fracture.

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