From Crystalline to Glassy: Crack Propagation Modes in Decagonal Quasicrystals
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From Crystalline to Glassy: Crack Propagation Modes in Decagonal Quasicrystals Christoph Rudhart1, Peter Gumbsch2,3, and Hans-Rainer Trebin1 1 Institut für Theoretische und Angewandte Physik, Universität Stuttgart, D-70550 Stuttgart, Germany 2 Institut für Zuverlässigkeit von Bauteilen und Systemen, Universität Karlsruhe, D-76131 Karlsruhe, Germany 3 Fraunhofer Institut für Werkstoffmechanik, D-79194 Freiburg, Germany ABSTRACT The propagation of mode-I cracks in a two-dimensional decagonal model quasicrystal is studied by molecular dynamics simulations. The samples are endowed with an atomically sharp seed crack and a temperature gradient. Subsequently the crack is loaded by linear scaling of the displacement field. The response of the crack running into regions of increasing temperature is monitored. For low temperatures below 30% of the melting temperature Tm the model-quasicrystal fails by brittle fracture. We observe that the crack follows the path of dislocations nucleated at its tip. The crack propagates along well defined planes and circumvents tightly bound clusters. In the medium temperature regime from 30% to 70% Tm the crack is blunting spontaneously by dislocation emission. In the range of 70%-80% Tm the quasicrystal fails by nucleation, growth and coalescence of micro-voids. This gradual, dislocation-free crack extension is caused by plastic deformation which is mediated by localized rearrangements comparable to so-called shear transformation zones in amorphous solids.
INTRODUCTION In experiments, quasicrystals show a pronounced brittle to ductile transition (BDT) that occurs at high homologous temperatures of about 80% of the melting temperature Tm. While different models exist for the BDT in crystalline materials (see [1,2] and references therein ), the BDT is less well understood in amorphous or quasicrystalline materials where first models have appeared recently [3,4]. On the macroscopic scale the BDT is a complicated phenomenon that depends not only on the material under consideration and the temperature but also on the loading rate and the microstructure of the solid. Obviously a phenomenon where processes on many different length scales are involved cannot be understood merely by atomistic models. However, like in crystalline materials, the macroscopic changes should be a consequence of processes that occur essentially on the atomic scale. Thus we use simple model quasicrystals to study the elementary atomic processes that may be important for the understanding of the BDT in real quasicrystals. Earlier simulations of the fracture of quasicrystals [5,6] and most of the experiments (e.g. [7,8]) have been performed at low temperatures, where quasicrystals fail by brittle fracture. Fracture at higher temperatures and the BDT have not been systematically studied yet. Here we apply molecular dynamics simulations to study the dynamic crack propagation in a wide range of temperatures.
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Figure 1: Sketch of the strip geometry for the fracture simulations with temperature gradient. The s
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