Forschergruppe
Nonlinear response to probe vitrification

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Project Samwer (Göttingen)

Nonlinear effects induced by mechanical stresses in glass forming systems far from equilibrium

Summary

The elastic-plastic transition in disordered materials often is introduced via thermal activation (glass transition) or strong mechanical deformation close to the rupture point of many oxide or metallic systems. Little is known about the cooperative interaction of atoms/molecules and groups of atoms in the anelastic and nonlinear regime of the response functions due to external stress. If one generally accepts that disordered systems exhibit various modes of structural relaxations (so-called α, β and fast processes) the frequency dependence of these nonlinear effects is of great interest. It would provide information whether local deformations proceed via α-, β-relaxation modes coupled by the extended stress field surrounding an excitation, or via fast processes and their local change of configuration. We would like to use mechanical spectroscopy to probe the universality of the nonlinear effects in disordered material, particularly in metallic glasses but also in oxide systems and orbital liquids inside crystalline materials. The latter is of prime interest as a rather new class of disordered material and very sensitive to small strain changes.

P1 Konrad Samwer, Universität Göttingen

Nonlinear effects induced by mechanical stresses in glass-forming systems far from equilibrium

Summary
This project is dedicated to nonlinear effects in the strain response of bulk metallic glasses
under applied tensile mechanical stresses. It focusses on (i) the strain response to a periodic
excitation under additionally applied strong mechanical load, exhibiting higher harmonics as a
consequence of nonlinear effects. Further, the time dependent lengthening under constantly
applied tensile stress is studied. It shows creep-like characteristics which indicate an anelasticto-
plastic phase transition in dependence of stress and temperature (ii). Finally, the analysis
of such creep curves–and, in particular, their statistical properties–gives important insights into
the of avalanche-like nature of the rearrangement events (iii). The macroscopic experimental
approach is mainly based on different modes of the dynamical mechanical analysis (DMA)
method. Furthermore, we will make a step forward from global to local measurements in the
upcoming funding period, too. Therefore we will perform complex force microscopy eperiments
under periodic excitation (atomic force acoustic microscopy, AFAM) and nanoindentation measurements
using a diamond tip in an AFM for intermittent effects in the so-called elastic regime.