Acoustic emission and dislocation avalanches

The experimental setup for a compression creep test on ice single crystal: Acoustic transducers are directly fixed on the ice. Experiments by Jerome Weiss in Grenoble

The AE energy distribution: The main figure shows the distribution of energy bursts for the different loading steps. In the inset we report a typical recorded acoustic signal displaying intermittent character.

Experimentally, the complex character of collective dislocation dynamics can be revealed by acoustic emission (AE) measurements The acoustic waves recorded in a piezoelectric transducer disclose the pulse-like changes of the local displacements undergoing in the material during plastic deformation, whereas a smooth plastic flow would not be detected. Thus this method is particularly useful to inspect possible fluctuations in the dislocation velocities and densities. We have chosen Ice single crystals as a model material to study glide dislocation dynamics due to the following reasons: (i) Transparency allows direct verifcation that AE activity is not related to microcracking. (ii) Within the range of temperature and stress corresponding to our experimental conditions, diffusional creep is not a signifcant mechanism of inelastic deformation which, in hexagonal ice single crystals, occurs essentially by dislocation glide on the basal planes along a preferred slip direction. (iii) An excellent coupling between sample and transducer can be obtained by fusion/freezing. Uniaxial compression creep experiments were performed on artificial ice single crystals, employing several steps of constant applied stress. We observed an intense acoustic activity, exhibiting a strong intermittent character (see the inset of Figure 3). In particular, we showed that the probability distribution of energy burst intensities exhibits a power law behavior spanning several decades.

For more information read the paper: M. C. Miguel, A. Vespignani, S. Zapperi, J. Weiss and J. R. Grasso, “Intermittent dislocation flow in viscoplastic deformation”, Nature 410, 667 (2001).

Dislocation dynamics

Snapshot of the total stress field and dislocations arrangement in a numerical simulation. Here one observes metastable structure formation (i.e. dipoles and walls) and the associated stress field which goes from light blue (low values) to dark blue (high values). The complex low-stress pathways joining dislocations are the result of the anisotropic elastic interactions.

We have considered the behavior of parallel straight edge dislocations moving in a single slip system under the action of constant stress. Dislocations interact with each other through the long-range elastic stress field they produce in the host material. The long-range and anisotropic character of this interaction is responsible for the remarkable features of plastic flow. In addition, we have included in the model the multiplication of dislocations and their annihilation. The model displays an intermittent steady-state characterized by dislocation avalanches with a power law distribution of energies that is in close agreement with the experiments. Due to their complicated mutual interactions, most of the time dislocations are jammed into metastable configurations, like walls and dipoles. The progressive jamming of dislocations into metastable structure also explains the power law creep relaxation observed in a vast variety of materials. The first observation of this phenomenon goes back to Andrade in 1910 and has been then repeatedly con�rmed during the last century. Our anal- ysis allows to reconduct the scaling found in Andrade creep to a second-order non-equilibrium critical phenomenon: a jamming transition of the dislocations.

For more information read the paper: M. C. Miguel, A. Vespignani, M. Zaiser and S. Zapperi, “Dislocation jamming and Andrade creep” Phys. Rev. Lett. 89, 165501 (2002)

Depinning of dislocations

We investigated the depinning transition occurring in dislocation assemblies. In particular, we considered the cases of regularly spaced pileups and low angle grain boundaries interacting with a disordered stress landscape provided by solute atoms, or by other immobile dislocations present in non-active slip systems. Using linear elasticity, we computed the stress originated by small deformations of these assemblies and the corresponding energy cost in two and three dimensions. Contrary to the case of isolated dislocation lines, which are usually approximated as elastic strings with an effective line tension, the deformations of a dislocation assembly cannot be described by local elastic interactions with a constant tension or stiffness. A nonlocal elastic kernel results as a consequence of long range interactions between dislocations. In light of this result, we revised statistical depinning theories and find novel results for Zener pinning in grain growth.

For more information read our papers:

S. Zapperi and M. Zaiser, “Depinning of a dislocation: effect of long range interactions”, Mat. Sci. and Eng. A 309-310, 348 (2001). P. Moretti, M.-C. Miguel, M. Zaiser, and S. Zapperi, “Depinning transition of dislocation assemblies: Pileups and low-angle grain boundaries” Phys. Rev. B 69, 214103 (2004)`

— StefanoZapperi – 08 Nov 2005