Interaction of oxygen with the stable Ti5Si3 surface

The atomic structure and surface energies of several low-index surfaces (0001), (1100) and (1120) of Ti5Si3 in dependence on their termination were calculated by the projector augmentedwave method within the density functional theory. It was revealed that the mixed TiSi-terminated (0001) surface is stable within the wide range of change in the Ti chemical potential. However, the Ti-terminated Ti5Si3(0001) surface is slightly lower in energy in the Ti-rich limit. The oxygen adsorption on the stable Ti5Si3(0001) surface with TiSi termination was also studied. It was shown that the three-fold coordinated F1 position in the center of the triangle formed by surface titanium atoms is the most preferred for oxygen adsorption on the surface. The appearance of silicon as neighbors of oxygen in other considered F-positions leads to a decrease in the adsorption energy. The factors responsible for the increase/decrease in the oxygen adsorption energy in the considered positions on the titanium silicide surface are discussed

Simulation of crack propagation in alumina with ab-initio based polarizable force field

We present an effective atomic interaction potential for crystalline alpha-Al2O3 generated by the program potfit. The Wolf direct, pairwise summation method with spherical truncation is used for electrostatic interactions. The polarizability of oxygen atoms is included by use of the Tangney-Scandolo interatomic force field approach. The potential is optimized to reproduce the forces, energies and stresses in relaxed and strained configurations as well as {0001}, {10-10} and {11-20} surfaces of Al2O3. Details of the force field generation are given, and its validation is demonstrated. We apply the developed potential to investigate crack propagation in alpha-Al2O3 single crystals.Comment: 8 pages, 5 figure

Molecular dynamics simulation of diffusion in decagonal quasicrystals with optimized interaction potentials

Die etwa 25 Jahre zurückliegende Entdeckung der Quasikristalle erweiterte das Gebiet der Festkörperphysik um einen neuen Strukturtyp. In der Erforschung dieser geordneten, jedoch nicht periodischen Strukturen wurden seither bedeutende Erfolge erzielt. Von verschiedenen Quasikristalltypen konnten zunehmend detaillierte Strukturmodelle erstellt werden. Zunächst wurden in der numerischen Simulation einfache binäre Modellsysteme untersucht, inzwischen sind realistische ternäre Strukturen Gegenstand der numerischen Forschung. Für die Molekulardynamiksimulation solcher Strukturen ist, neben einem geeigneten Strukturmodell, eine gute Modellierung der atomaren Wechselwirkungen erforderlich. Besonders für Simulationen nahe der Schmelztemperatur besteht Bedarf nach einem realistischen Wechselwirkungspotential. Da herkömmliche Potentiale üblicherweise an Parameter der Grundzustandsstruktur angepasst wurden, tendieren diese dazu, bei hohen Temperaturen zu versagen. Die numerische Exploration in diesem Temperaturbereich kann einen wichtigen Beitrag zum Verständnis der Quasikristalle liefern. Diffusionsprozesse, die bedeutend für die Entstehung von quasikristallinen Hochtemperaturphasen sind, können im Fall von Aluminium nur in der Simulation erforscht werden, weil für das übliche Messverfahren kein geeignetes Radioisotop existiert. Da Aluminium ein wesentlicher Bestandteil vieler Quasikristalle ist, ist das Verständnis der Aluminiumdiffusion von großer Bedeutung. Ziel dieser Arbeit ist die Untersuchung der Aluminiumdiffusion in den dekagonalen Quasikristallen AlNiCo und AlCoCu mittels Molekulardynamiksimulation. Dazu werden zunächst, durch Anpassung an Ab-initio-Daten, Wechselwirkungspotentiale für die Strukturen entwickelt. Mit diesen Potentialen werden Simulationen bei hohen Temperaturen durchgeführt. Die dabei auftretende Diffusion wird ausgewertet und mit Daten aus Ab-initio-Rechnungen verglichen. Die in der vorliegenden Arbeit konstruierten EAM-Potentiale für Hochtemperatursimulationen an dekagonalen Quasikristallen erwiesen sich als geeignet, um grundlegende Eigenschaften der Systeme richtig zu modellieren. Der Vergleich mit der Ab-initio-Rechnung lieferte für alle Atomsorten eine sehr gute Übereinstimmung in den aus der thermischen Bewegung resultierenden Atomaufenthaltswahrscheinlichkeiten der Strukturen. Auch einzelne mit den EAM-Potentialen untersuchten Al-Diffusionsprozesse konnten in der Ab-initio-MD-Simulation bestätigt werden. In den quasikristallinen Strukturen wurde in der klassischen Molekulardynamiksimulation weit reichende Aluminiumdiffusion beobachtet. Die durch Ab-initio-Rechnungen bestätigten Al-Diffusionsprozesse wurden detailliert untersucht. Dabei wurde deutlich, dass die Diffusion in der dekagonalen Ebene über quasikristallspezifische Mechanismen verläuft. Bedeutend ist hierbei die Besonderheit, dass Gebiete existieren, die zur Atomabgabe neigen, während andere ein zusätzliches Atom aufnehmen können. Es finden Kettenprozesse statt, an deren Start- und Endpositionen sich diese Regionen befinden. Charakterisch für diese Gebiete ist, dass die Al-Positionen nicht scharf lokalisiert sind, sondern innerhalb eines Bereichs eine kontinuierliche Al-Aufenthaltswahrscheinlichkeit besteht. In der periodischen Richtung existieren durchgängige Kanäle kontinuierlicher Al-Aufenthaltswahrscheinlichkeit. Durch diese Kanäle verläuft die Al-Diffusion in periodischer Richtung. Der Diffusionsmechanismus ist hierbei die direkte, synchron verlaufende, Bewegung innerhalb einer Reihe von Atomen. Dabei befinden sich immer drei Atome pro Schicht im Kanal. Bei mehr als drei Schichten findet meist eine Kopplung mit einem Prozess der dekagonalen Ebene statt, wobei ein Atom aus der dekagonalen Ebene in den Diffusionskanal springt, während ein anderes diesen verlässt. In diesem Fall diffundieren nur die sich dazwischen befindenden Atome entlang des Kanals. Zur Diffusion in der dekagonalen Ebene können deutlich mehr Al-Atome beitragen als zu jener in der periodischen Richtung, welche auf die Diffusionskanäle beschränkt ist. Da jedoch die Mobilität der Al-Atome in den Diffusionskanälen deutlich höher ist, liegen die Diffusionskoeffizienten für die periodische Richtung über jenen der dekagonalen Ebene. Die geringe Energiebarriere innerhalb der Diffusionskanäle wurde mit Ab-initio-Rechnungen bestätigt.With the discovery of quasicrystals in 1982, condensed matter physics was extended by a new kind of structure. Since then, the exploration of these ordered, but nonperiodic structures has lead to significant new results. There was an increase of detailed structure models of several quasicrystals. For a long time, in numerical simulations only binary model systems were studied, but real quasicrystals are mostly ternary or even quaternary. Nowadays these structures can be explored in numerical simulations. Molecular dynamics simulations of such realistic structures require, apart from an appropriate model of the structure, a well adjusted model of the atomic interactions. Especially for simulations at temperatures near the melting point, there is demand for realistic potentials. Since conventional potentials were usually constructed by adjustment to the ground state structure, these potentials tend to fail at high temperatures. Simulations at high temperatures provide an important contribution to the understandig of quasicrystals. Diffusion processes are fundamental in the formation of high temperature phases and the motion of defects. In the case of aluminium the diffusion cannot be measured experimentally, due to the lack of suitable radiotracers. Therefore numerical simulation is the sole possibilty to study the Al diffusion processes. Since aluminium is a basic component of many quasicrystals, the understanding of aluminum diffusion is of great importance. The goal of this thesis is the exploration of diffusion in the decagonal AlNiCo and AlCoCu quasicrystals via molecular dynamics simulation. For this purpose, atomic interaction potentials for these structures are developed by adjusting them to ab initio data. The newly developed EAM potentials for molecular dynamics simulations at high temperatures in AlNiCo and AlCoCu quasicrystals turned out to be well adapted for modelling the basic properties of these decagonal quasicrystals. The comparison with ab initio calculations shows very good agreement in the atom density maps of both structures. Similarly some specific diffusion processes, which were found in the simulations with EAM potentials, were validated in ab initio calculations. In the molecular dynamics simulation, long range Al diffusion was observed in both decagonal quasicrystal structures. Al diffusion processes, which were validated by ab initio calculations, were studied in detail. It was found that the diffusion in the decagonal plane proceeds via mechanisms which are specific to quasicrystals. Of great importance are sites which tend to emit atoms, whereas other sites can absorb atoms. Chain processes occur, where the initial and the final positions are at these sites. The important characteristic of these sites is that the Al positions are not localized, but there is a continuous Al density in these regions. In the periodic direction, channels of continuous Al density extend through the structure. The diffusion in this direction runs via such channels. The diffusion mechanism is a synchronous motion of atoms within a column. In each layer there are three atoms which are part of this column. With more than three layers there is usually a coupling with a jump process in the decagonal plane. An atom of the decagonal plane jumps into the diffusion channel, whereas another atom leaves the channel. In this case, only the atoms in-between diffuse along the channel. There are clearly more Al atoms which contribute to the diffusion in the decagonal plane than in the periodic direction, in which the diffusion is limited to the channels. However, since the mobility of Al atoms in the diffusion channels is significantly higher, the diffusion coefficients in the periodic direction are larger than those in the decagonal plane. The small energy barriers in the diffusion channels were validated in ab inito calculations

Molecular dynamics simulation of aluminium diffusion in decagonal quasicrystals

Aluminium diffusion in decagonal Al-Ni-Co and Al-Cu-Co quasicrystals is investigated by molecular dynamics simulations. Results obtained with newly developed EAM potentials are compared to previous work with effective pair potentials [Phys. Rev. Lett. 93, 075901 (2004)]. With both types of potential, strong aluminium diffusion is observed above two thirds of the melting temperature, and the general behaviour of the system is quite similar. The diffusion constant is measured as a function of temperature and pressure, and the activation enthalpies and activation volumes are determined from the resulting Arrhenius plot. For a number of important diffusion processes, the energy barriers are determined with molecular statics simulations. The qualitative behaviour of the dynamics is also confirmed by ab-initio simulations

Precipitation, planar defects and dislocations in alloys: Simulations on Ni

We present simulations of the formation of Ni3Si precipitates using a combination of molecular dynamics (MD) and the Metropolis Monte Carlo (MMC) method. Applying this technique to a Ni-Si solid solution in Cu matrix leads to Ni3Si precipitates with L12 structure as observed in experiments. Since L12 structured precipitates are most relevant for precipitation strengthening of several alloys, we focus on planar defects and dislocations in Ni3Si and Ni3Al. Ab initio calculations of the generalised stacking fault energies of Ni3Si presented in our previous work [S. Hocker, H. Lipp, E. Eisfeld, S. Schmauder, J. Roth, J. Chem. Phys. 149, 024701 (2018)] revealed that the complex stacking fault is not stable and the inflection point as well as the minimum corresponding to the antiphase boundary is shifted. In this study it is shown that this behaviour can be understood from the analysis of charge densities. Furthermore, the consequences on dislocations in Ni3Si and Ni3Al are discussed and interactions of edge dislocations with Ni3Si and Ni3Al precipitates are simulated

Role of intermediate metal and oxide layers in change of adhesion properties of TiAl/Al2O3 interface

A systematic study of the atomic and electronic structure of the γ-TiAl(111)/α-Al2O3(0001) interface with intermediate metal (Nb, Mo, Ni, Re) and oxide (Nb2O5, MoO3) layers has been performed by the projector augmented-wave method within density functional theory. The work of separation at the interfaces in dependence on the cleavage plane has been calculated. It is shown that a high adhesion energy obtained at the interface with the O-terminated α-Al2O3 is decreased at the γ-TiAl/Me interface but it remains enough high at the Me/α-Al2O3(0001)O interface due to a large ionic contribution to the chemical bonding. The influence of formation of intermediate impurity oxide layers on the adhesive properties of the alloy/oxide interface is discussed as well. The obtained results indicate that the fracture will occur inside the impurity oxide or its interface with the alloy

Ab initio investigation of Co/TaC interfaces

Co(0001)/TaC(01) interfaces with Re impurities are investigated using the plane-wave pseudopotential method within density functional theory. Two interface configurations and several interfacial sites for Re impurities are considered. The calculations reveal that Re doping can lead to more favourable interfaces with lower interface energies. It is shown that the decrease of interface energy can be understood from the electronic structure. Analyses of bond lengths, overlap populations, charge density distributions and densities of state demonstrate the importance of C-Co and C-Re bonds across the interface

Ab initio investigations of Fe(110)/graphene interfaces

Interfacial bonding of three different semi-coherent bcc-Fe(110)/graphene interfaces is investigated using the plane-wave pseudopotential method within density functional theory. The analysis of bond lengths, charge densities, charge transfer, magnetic moments and densities of states shows that interfacial adhesion can be understood from the electronic structure. Graphene is considered on Fe(110) surfaces as well as embedded in (110) planes of bulk Fe. Moreover, the influence of single vacancies in graphene is studied in case of graphene on the Fe(110) surfaces. It is found that a single vacancy in graphene leads to a strong increase of interfacial adhesion. The most important contribution to the adhesion is covalent bonding with hybridization of Fe states and C states which is most pronounced for neighboring atoms of the vacancy

IMD – the ITAP molecular dynamics simulation package

IMD is a computer simulation package designed for large-scale simulation studies in materials sciences. IMD can be run with a large number of effective many-body interactions which can be produced from ab-initio calculations. This report is intended to give an overview of the following topics: how to obtain IMD, the design of IMD, capabilities of IMD with special focus on laser ablation, and interaction of IMD with other codes