[SY-H10] Symposium H-10

[SY-H10] Heterogeneous flow and internal friction in amorphous carbon

^○Richard Jana¹, Lars Pastewka¹, Daniele Savio², Volker L Dehringer³, Gábor Csányi³, Julian von Lautz², Michael Moseler², Peter Gumbsch^2,4 (1.Albert Ludwigs Universitaet Freiburg, Germany, 2.Fraunhofer-Institut für Werkstoffmechanik IWM, Germany, 3.University of Cambridge, UK, 4.Institue for Applied Materials, Karlsruhe Institute of Technology, Germany)

We use molecular dynamics simulations to probe the plastic response of representative bulk volumes of amorphous carbon at densities from 2.0 g cm^-3 to 3.3 g cm^-3 in simple and biaxial shear. We compare multiple interatomic potential expressions, in particular classical empirical bond-order potentials (screened Tersoff and REBO2), the modified embedded atom method (MEAM) and machine learning approaches, in particular the Gaussian approximation potential (GAP). After an initial elastic response, the samples yield without any strain hardening or softening. Individual plastic events are strikingly similar to those observed for bulk metallic glasses: Like in other amorphous materials, we find that plasticity is carried by fundamental rearrangements of regions of ~100 atoms, the shear transformation zone. We find that STZs coalesce to forms a shear band and that the relationship between stress and pressure during flow is well described by a Drucker-Prager law. Amorphous carbon is a prototypical single-component network material and its pair distribution function vanishes between first and second neighbor. This allows definition of an unambiguous nearest neighbor relationship and a mean coordination number. Stress correlates well with mean coordination, suggesting a simple constitutive model for this material. This relationship breaks down at low coordination numbers. We explain this with Thorpe’s constraint counting theory, which predicts that networks become floppy below a certain value of mean coordination.