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

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The discrete nature of solids and the interatomic interactions strongly influence crack propagation. Lattice trapping results in stable cracks above and below the critical Griffith load. Local atomic arrangements near the crack front define fracture behaviour. The analysis of these processes on an atomic scale helps to understand principle mechanisms and their consequences, which also have to be incorporated in more coarse-grained descriptions to get reliable results. Largescale molecular dynamics simulations of fracture on the atomic level can supply information not accessible to experiment. But to simulate a specific material reasonable effective interatomic potentials are needed. In this paper, we report on the fitting and validation of potentials specifically generated for the fracture of C15 NbCr2. Results are compared to those derived with potentials for the elements from the literature. The comparison indicates that interactions fitted to elemental metals are not sufficient to determine alloy properties

Molecular dynamics simulations of crack propagation are performed for two extreme cases of complex metallic alloys (CMAs): 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 to the clusters in the quasicrystal. Although the latter are not rigid units, they have to be regarded as significant physical entities. arXiv:0704.1444v1 [cond-mat.mtrl-sci]

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-, two-and 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.