Browsing by Author "Roth, Johannes Werner"

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Atomistic simulation of shock waves : from simple crystals to complex quasicrystals
(2005) Roth, Johannes Werner; Trebin, Hans-Rainer (Prof. Dr.)
This habilitation thesis describes molecular dynamics simulations of solids. The impact of shock waves on a number of solids is studied: In the first part binary icosahedral quasicrystals and Laves crystals are treated, in the second part monatomic dodecagonal quasicrystals and body centered cubic crystals are dealt with. The third part contains studies of intermediate phases and solitons which show up in the body centered cubic crystals if shocked along a three-fold axis. In all cases three ranges of different behavior are observed: if the shock waves are weak, elastic deformations occur, in a medium range elastic and plastic waves or phase transitions are observed. If the shock waves are strong, the initial structures are completely destroyed. In this work we are concerned especially with the range of medium strong shock waves. For the binary crystal structure fragmentation occurs. The emerging crystallites are rotated with respect to each other and separated by boundary layers which are several atomic distances thick. The main difference between crystal and quasicrystal are phason-like defects which lead to a continuous transition between the range of weak and medium shock waves. For the monatomic crystal structures the Dzugutov potential has been applied to stabilize the structures. Here we find in the range of medium shock waves phase transitions from quasicrystals and approximants to the body centered cubic structure. Depending on the orientation and strength of the shock waves the transition takes places within a few atomic layers or spread out across many layers. In the quasicrystal and the approximants atomic flips are observed in the elastically compress region. Body centered cubic crystals possess an inherent instability along the three-fold axes. In many materials, this leads to a phase transition to the so-called omega-phase. In our case the omega-phase is stable only in a small range of compression, thus a forth- and back-transformation from body centered cubic to the omega-phase takes place, as long as the phase transition front moves slower than the speed of sound. If this is no longer the case, solitons shock up which contain in their interior. In summary several differences could be observed between crystals and quasicrystals. The results obtained for the Dzugutov potentials are comparable to the outcome of simulation of shock waves in iron with materials-specific interactions.