Formation of quasicrystals in molecular dynamics simulations

Abstract

Quasicrystals are ordered crystalline structures without any translational periodicity in three dimensions. They offer interesting physical applications due to their structure, especially because of their electronic properties. The mechanism of quasicrystal formation - particularly in the context of mechanical alloying (MA) - remains an open problem. This thesis investigates the crystallographic changes during simulations of MA, aiming to find conditions that lead to a quasi-crystalline structure. Additionally, the frictional behavior of the system is investigated, since also changes in this behavior can indicate quasi-crystalline ordering.For this, different crystal systems and their potentials were investigated to select the most promising candidate for quasicrystal creation. Molecular dynamics (MD) simulations were then performed with LAMMPS using the modified embedded atom method (MEAM) to investigate MA during the sliding of an Fe slab over an Al3Cu slab. The normal load applied to the Fe slab was gradually varied to investigate the systems response to this. The computed load and the friction force were used to examine the frictional behavior of the system, determine the coefficient of friction, and to derive the form of the friction law for this ternary system. Structural analysis using the radial distribution function (RDF) as well as stoichiometric and Ackland-Jones analyses were performed to quantify the structural changes occurring in the system on an atomic level. The data obtained was visualized combining OVITO, Python-scripts, and Gnuplot.The results show that, although no dominant quasi-crystalline structure is produced, interesting structural changes of the system take place when increasing the load. For low applied loads, the friction-law has the Amontons-Coulomb form with a Derjaguin offset. This linear dependency is lost with the structural changes in the system happening for high loads. Further analysis led to finding of quasi-crystalline ordering as in experimental setups.Although the simulations show that there is a change in the crystallographic order of the system, quasicrystals in significant quantities were not found. It is assumed that this is caused by the modest size of the systems considered for finishing simulations in a reasonable time. Therefore, it can be supposed that other setups might lead to more MA and also to a higher probability of creating quasicrystals in a significant amount.