Highly parallel molecular dynamics / Monte Carlo coupling towards solutes segregation modelling

Diese Arbeit hat das Ziel die Reckalterung in Stahl mit niedrigem Kohlenstoffgehalt durch atomistische Simulation zu untersuchen. Die zugrundeliegenden Abläufe, z.B. Kohlenstoffsegregation und Versetzungsbewegung, spielen sich auf verschiedenen Zeitskalen ab. Aus diesem Grund wird eine zeitskalenüberbrückende Herangehensweise vorgeschlagen, um klassische Molekulardynamik (MD) und Metropolis-Monte-Carlo (MC) zu verbinden. Diese Verbindung wird durch die Verwendung "virtueller Atome" als Platzhalter für die Zwischengitterplätze erreicht. Hierfür wird eine Manager-Worker Parallelisierung unter Verwendung eines spekulativen Schemas entwickelt und implementiert. Das entworfene parallelisierte Konzept wird genutzt, um Kohlenstoffsegregation in verschiedenen Umgebungen (z.B. Einzelversetzung oder Versetzungsnetzwerk) erfolgreich zu simulieren. Es wird beobachtet, dass die lokale Kohlenstoffkonzentration in der Nähe von Versetzungen die Versetzungsbeweglichkeit signifikant behindert

Parallel hybrid Monte Carlo / Molecular Statics for Simulation of Solute Segregation in Solids

A parallel hybrid Monte Carlo/molecular statics method is presented for studying segregation of interstitial atoms in the solid state. The method is based on the efficient use of virtual atoms as placeholders to find energetically favorable sites for interstitials in a distorted environment. MC trial moves perform an exchange between a randomly chosen virtual atom with a carbon atom followed by a short energy minimization via MS to relax the lattice distortion. The proposed hybrid method is capable of modeling solute segregation in deformed crystalline metallic materials with a moderate MC efficiency. To improve sampling efficiency, the scheme is extended towards a biased MC approach, which takes into account the history of successful trial moves in the system. Parallelization of the hybrid MC/MS method is achieved by a Manager-Worker model which applies a speculative execution of trial moves, which are asynchronously executed on the cores. The technique is applied to an Fe-C system including a dislocation as a symmetry breaking perturbation in the system

Studying the effects of Nb on high-temperature deformation in TiAl alloys using atomistic simulations

Intermetallic γ(TiAl)-based alloys find their application as high-temperature materials for aero engine and automotive components. Microstructure optimization and microalloying play key roles in optimizing these alloys. Several pioneering experimental works showed improved mechanical properties of γ(TiAl)-based alloys containing Niobium (Nb). Despite Nb being a key alloying element, its contribution remains debated, if not least understood, due to the TiAl microstructure's complexity with hierarchical interfaces. This work examines the effects of Nb on the high-temperature deformation behavior of TiAl alloys using atomistic simulations. These revealed that Nb alloying retarded the stress-induced phase transformation of γ → α2, favoring a refined microstructure with the dislocation sources from microstructure boundaries and interfaces at high temperature and improving thus the ductility. Our microstructure-informed atomistic models reveal a comprehensive picture of the underlying nanomechanical events

Cluster Formation in Stochastic Disk Systems

The problem of randomly distributed disks is considered in the dilute regime in a two-dimensional domain. Disks are allowed to overlap and to form clusters which may be isolated or percolating. Depending on the number and size of the disks, distribution functions are obtained for dierent size and bond configurations of clusters. A statistical geometrical approach istaken to derive analytical probabilities for cluster formation in systems, where a maximum of four overlapping disks is considered. Monte Carlo computations are carried out to verify our theoretical approach which is shown to be in close agreement with numerical simulations