Frohmut Rösch

Dynamic fracture of icosahedral model quasicrystals: A molecular dynamics study

Ebert et al. [Phys. Rev. Lett. 77, 3827 (1996)] have fractured icosahedral Al-Mn-Pd single crystals in ultrahigh vacuum and have investigated the cleavage planes in-situ by scanning tunneling microscopy (STM). Globular patterns in the STM-images were interpreted as clusters of atoms. These are significant structural units of quasicrystals. The experiments of Ebert et al. imply that they are also stable physical entities, a property controversially discussed currently. For a clarification we performed the first large scale fracture simulations on three-dimensional complex binary systems. We studied the propagation of mode I cracks in an icosahedral model quasicrystal by molecular dynamics techniques at low temperature. In particular we examined how the shape of the cleavage plane is influenced by the clusters inherent in the model and how it depends on the plane structure. Brittle fracture with no indication of dislocation activity is observed. The crack surfaces are rough on the scale of the clusters, but exhibit constant average heights for orientations perpendicular to high symmetry axes. From detailed analyses of the fractured samples we conclude that both, the plane structure and the clusters, strongly influence dynamic fracture in quasicrystals and that the clusters therefore have to be regarded as physical entities.

Fracture of complex metallic alloys : an atomistic study of model systems

Molecular dynamics simulations of crack propagation are performed for two extreme cases of complex metallic alloys. In a model quasicrystal the structure is determined by clusters of atoms, whereas the model C15 Laves phase is a simple periodic stacking of a unit cell. The simulations reveal that the basic building units of the structures also govern their fracture behaviour. Atoms in the Laves phase play a comparable role with the clusters in the quasicrystal. Although the latter are not rigid units, they have to be regarded as significant physical entities.

Brittle fracture in a complex metallic compound from an atomistic viewpoint: NbCr2, a case study

Material-specific atomistic aspects of brittle fracture are studied for the first time for a complex metallic compound with realistic embedded-atom-method potentials. Crack propagation occurs on an atomic level by a successive rupture of cohesive bonds. In many theoretical models of fracture, however, a coarse-grained approach is applied and the explicit influence of the discrete nature of matter is not taken into account. In this paper, numerical experiments on the complex metallic compound NbCr2 are presented to illustrate why it is necessary to perform atomistic simulations to understand the details of fracture behaviour: the number, strength and orientation of bonds approached by a crack determine whether, where and how it propagates.

Crack front propagation by kink formation

The fracture of a three-dimensional brittle solid generates two-dimensional surfaces, which are formed behind a one-dimensional crack front. For quasi-static cracks on a (111) cleavage plane in silicon front propagation by kink-pair formation was proposed and proven by a reaction pathway analysis with Stillinger-Weber potentials. Here, we demonstrate that the kink-pair mechanism is much more general: we also observe it in molecular-dynamics simulations of a complex metallic alloy, the C15 NbCr2 Friauf-Laves phase, where we applied carefully selected embedded-atom-method potentials. The numerical experiments highlight that kink formation is essential for crack propagation in any brittle material.

Are clusters physical entities

Abstract Complex metallic alloys frequently exhibit atomic clusters as structural units. In quasicrystals, they are densely arranged retaining an overall long-range quasiperiodic translational order. Thus, clusters are geometrical entities. The often posed question is, whether these are also responsible for physical properties. Fracture experiments and simulations strongly point towards a positive answer. However, a recent analysis of experimental fracture surfaces questions a signature of the clusters. From the self-affine behavior of a model system it is shown that the findings cannot vitiate the role of the clusters in the fracture process.

Crack Propagation in Icosahedral Model Quasicrystals

Propagation of mode-I cracks in a three-dimensional model quasicrystal is studied by molecular dynamics simulations. The samples are endowed with an atomically sharp crack and subsequently loaded by linear scaling of the displacement field. The response of the system is then monitored during the simulation. In particular, the crack surface morphology is investigated in dependence of the orientation of the fracture plane. For this purpose, fracture surfaces perpendicular to two- and fivefold axes are compared. For both directions, brittle fracture with rough fracture surfaces is observed.

Cleavage Planes of Icosahedral Quasicrystals: A Molecular Dynamics Study

The propagation of mode I cracks in a three-dimensional icosahedral model quasicrystal has been studied by molecular dynamics techniques. In particular, the dependence on the plane structure and the influence of clusters have been investigated. Crack propagation was simulated in planes perpendicular to five-, twoand pseudo-twofold axes of the binary icosahedral model. Brittle fracture without any crack tip plasticity is observed. The fracture surfaces turn out to be rough on the scale of the clusters. These are not strictly circumvented, but to some extent cut by the dynamic crack. However, compared to the flat seed cracks the clusters are intersected less frequently. Thus the roughness of the crack surfaces can be attributed to the clusters, whereas the constant average heights of the fracture surfaces reflect the plane structure of the quasicrystal. Furthermore a distinct anisotropy with respect to the in-plane propagation direction is found.