Numerical simulation of dislocation motion in icosahedral by Schaaf G.

By Schaaf G.

The 1st part is a quick creation to quasicrystals. Their constitution and a new release procedure are defined in addition to the version quasicrystal utilized in the simulations. the subsequent part offers with the idea of dislocations and plasticity. a few uncomplicated definitions and homes are defined and prolonged to the quasicrystalline case, including result of experiments and simulations. Molecular dynamics (MD) is the subject of the 3rd part. We convey how a shear deformation may be modeled. A dialogue of the visualization method follows. A bankruptcy with the result of the simulations and a precis finish this thesis.

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Quasicrystalline dislocations can be observed in TEM micrographs because their strain field produces a diffraction contrast. In crystals, the direction of the Burgers vector b is determined by the extinction condition g · b = 0, where g is a diffraction vector used for imaging [74]. In quasicrystals, this condition must be evaluated in the hyperlattice and, hence, splits into a phonon and a phason part G · B = g · b + g ⊥ · b⊥ = 0 . 35) Eq. 35) can be fulfilled in two different ways [75, 76]. 36) is equal to the crystalline case.

From measurements of large activation enthalpies of 7 eV12 or 80 kB T a diffusive nature of the deformation mechanism could be excluded13 . More important, the activation volumes between 40b3 and 245b3 [88, 89] are too large for a Peierls mechanism. Feuerbacher et al. ’s structure model [82]) would act as rate-controlling obstacles. Their extraordinary stability has been shown in STM investigations of cleavage surfaces [90]. The clusters were preserved because the cracks had circumvented them. Messerschmidt et al.

This is reminiscent of observations in tetrahedrally coordinated covalent crystals and bcc metals, both of which are controlled by a Peierls mechanism. The nature of a Peierls mechanism in a quasicrystal has been discussed by Takeuchi et al. [66]. They proposed a Peierls potential like that of the upper curve in the left part of fig. 12. Similar to periodic lattices, the energy of a straight dislocation varies as a function of its position. However, this variation is quasiperiodic, with Peierls hills of various heights.

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