Browsing by Author "Rudhart, Christoph Paul"

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Open Access
Crack propagation in decagonal and icosahedral quasicrystals
(2004) Rudhart, Christoph Paul; Trebin, Hans-Rainer (Prof. Dr.)
In the pressent work the propagation of cracks in two-dimensional decagonal and three-dimensional icosahedral model quasicrystals under mode I loading is studied by means of molecular dynamics simulations. The dependence on temperature, applied load and underlying structure has been investigated in particular. The samples are endowed with an atomically sharp crack and loaded by linear scaling of the displacement field. Subsequently the response of the system is monitored by molecular dynamics simulations. In the decagonal systems three different regimes of propagation are discernable with increasing temperature: For low temperatures the crack velocity grows monotonically with the applied load. We observe that the crack follows the path of dislocations nucleated at its tip. For temperatures above 30% of the melting temperature the crack does not remain atomically sharp but is blunting spontaneously. In the temperature range between 70% and 80% of the melting temperature 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. These are also observed in amorphous solids. In conclusion at low temperatures the crack propagates along crystallographic planes just like in periodic crystals, whereas a glass-like behavior is dominant at high temperatures. In the icosahedral quasicrystals brittle fracture without any crack tip plasticity is observed irrespective of the orientation of the cleavage plane and the propagation direction. The fracture surfaces are not flat but show a pronounced roughness on the atomic scale, comparable to that observed experimentally. The magnitude of the roughness depends on both the cleavage plane and the propagation direction. The clusters inherent in the structure are not strictly circumvented but are cut to some extent by the moving crack. However, a detailed analysis of the fractured samples indicates that the roughness of the fracture surfaces can be assigned to the occurrence of the clusters.
Open Access
Zur numerischen Simulation des Bruchs von Quasikristallen
(1999) Rudhart, Christoph Paul
Quasikristalle zeigen neben dem von Kristallen bekannten phononischen einen weiteren, als phasonisch bezeichneten Freiheitsgrad. Mit dem phasonischen Freiheitsgrad verbunden sind Atomsprünge, die als phasonische Flips bezeichnet werden. Die atomaren Sprungprozesse spielen eine wichtige Rolle bei der Untersuchung der dynamischen und mechanischen Eigenschaften von Quasikristallen. Beispielsweise hinterlassen Versetzungsbewegungen in Quasikristallen Stapelfehlerebenen, die als Phasonenwände bezeichnet werden. Eine Phasonenwand besteht aus einer Ebene von phasonischen Defekten, die umgeordnete Atomkonfigurationen darstellen. Phasonenwände schwächen den Quasikristall in bezug auf die plastische Verformbarkeit. Mit geometrischen Methoden wurden die phasonischen Flips im binären ikosaedrischen Modell, einem Strukturmodell für die ikosaedrische Phase von AlZnMg, untersucht.