# Finished Projects (All Topics)

### Alloying of Cementite

The thermodynamic stability of cementite can be controlled with chemical alloying: for instance, Si and Al are commonly used to suppress cementite formation, whereas Cr enhances it. In this project we employ a combination of parameter-free first-principles calculations and concepts of equilibrium thermodynamics to get insights into physical mechanisms which influence the thermodynamic stability of cementite upon alloying. [more]

### Dislocation pairing transition

Grain boundary structures can depend sensitively on the temperature. In this project we investigate via amplitude equations models a transition between paired and unpaired dislocations at low angle grain boundaries close to the melting point. [more]

### Elastic effects on heterogeneous nucleation

Heterogeneous nucleation and the growth of microstructures are often accompanied by elastic deformations which can significantly influence the thermodynamical properties and kinetics of these processes. Despite the obvious relevance of these phenomena, generally accepted theories which take into account the elastic long-range interactions in a consistent way are not yet available, especially under consideration of the atomic structures and ordering. In this project, we investigate the influence of these new effects on the dynamical processes of heterogeneous nucleation and microstructure development. [more]

### Electro-elastic model for phonon calculations

Phonon calculations in the DFT framework are very costly, in particular when phonons must be calculated throughout the Brillouin zone of a crystal.

However, a full DFT treatment of the phonons is rarely needed. It is sufficient to set up an empirical model for the interatomic interactions and calculate the phonons from the model, and use DFT calculations to determine the parameters in the model. The basic idea of the Coulomb-Hook model is to use charge-charge Coulomb interactions in addition to Hook's law for next-neighbor interactions. We have extended this approach to included anisotropic Born charges. [more]

However, a full DFT treatment of the phonons is rarely needed. It is sufficient to set up an empirical model for the interatomic interactions and calculate the phonons from the model, and use DFT calculations to determine the parameters in the model. The basic idea of the Coulomb-Hook model is to use charge-charge Coulomb interactions in addition to Hook's law for next-neighbor interactions. We have extended this approach to included anisotropic Born charges. [more]

### Eutectic Solidification

We study of the growth of a two-phase finger in eutectic systems. using a boundary-integral formulation and we complement our investigation by a phase-field validation of the stability of the pattern. This pattern was observed experimentally by Akamatsu and Faivre The deviations from the eutectic temperature and from the eutectic concentration provide two independent control parameters, leading to very different patterns depending on their relative importance. [more]

### Continuum modeling of fracture

Fracture is a fundamental mechanism of materials failure. Propagating cracks can exhibit a rich dynamical behavior controlled by a subtle interplay between microscopic failure processes in the crack tip region and macroscopic elasticity. Our modeling is mainly based on the phase field method, which is able to capture both the short-scale physics of failure and macroscopic linear elasticity within a self-consistent set of equations that can be simulated on experimentally relevant length and time scales.

[more]### Friction

Friction is important for many processes in nature and industry. It is a multiscale problem by the fact that the smallest asperities on the microscale can determine the sliding behavior and wear of metals as well as earthquake dynamics. Here we study in particular the motion of slow fronts between stick and slip regions.

[more]### III-Nitride surfaces

A prerequisite towards achieving biological and chemical sensing applications is to to investigate, understand and quantify the effect of the absorbants on the electronic structure of surfaces. Surface states at semiconductors influence the electronic properties of devices and heterostructures since they can induce Fermi level pinning and bending of the conduction and valence band at the surface and at the interfaces. [more]

### GPU computing

Modern graphics cards offer enormous computational capabilities for scientific computing with minimal costs. We are using this technology for phase field and amplitude equations simulations on a regular basis, with a gain of efficiency by up to a factor 250 in comparison to a single core CPU computation. This tremendous speedup is extremely valuable, as it reduces the time for individual simulations to finish drastically and therefore leads to much shorter development cycles also for new codes and applications to new problems. [more]

### Multiscale Simulations of Hydrogen embrittlement

Hydrogen induced embrittlement of metals is one of the long standing unresolved problems in Materials Science. A hierarchical multiscale approach is used to investigate the underlying atomistic mechanisms. [more]

### Hydrogen at cracks

Hydrogen at crack tips can embrittle steels and lead to catastrophic material failure. In this project we develop a continuum model for the formation of hydride zones in the tensile regions of a crack tip. It changes the fracture properties of static and propagating fractures. [more]

### Growth of InGaN surfaces

The growth of high In content InGaN alloys with sufficiently high crystal quality constitutes a puzzling situation where the complex interplay between surface morphologies, partial pressures and growth temperature plays a central role. We have investigated the bulk and surface thermodynamics of In

_{x}Ga_{1-x}N growth for the technologically relevant (0001) and (0001) growth planes by means of density functional theory calculations. [more]### Liquid metal embrittlement

Liquid Metal Embrittlement can be described as the penetration of a liquid metal with a low melting point along the grain boundaries of a material with a higher melting point, and a subsequent ductile-to-brittle transition, which leads to dramatic material failure. We investigate this phenomenon with continuum methods to describe the melting along grain boundaries. [more]

### Pattern formation in nanowires

Nanowires have a length to diameter ratio of about 1000 and a thickness of some nanometers and exhibit interesting electronic properties. Often, they grow defect-free in a catalytic reaction from a substrate, but recently interesting patterns have been observed in situations, where the wires grow in a controlled way with a screw dislocation inside the trunk. In this project we investigate the dislocation motion and morphology of the nanowire, as well as nanowire stability. [more]

### Peritectic Solidification

Peritectic transformations are essential for the production of steels. In this project we investigate solidification from the melt, using sharp interface and phase field methods.

[more]### Phase change materials

Phase change materials are a promising candidate for fast and nonvolatile data storage on computers. We develop models for the switching of the electrical resistivity in such materials, driven by the applications of an electrical current. [more]

### Phase field modeling of alloys

The modeling of alloy solidification strongly relies on the development of efficient phase field models. Quantitative predictions require a strict separation of the length scales in the problem, in particular of the phase field interface thickness from all the physical scales. Here we investigate in particular how models with so called thin interface asymptotics are related to deep physical symmetry relations. This understanding is central to true quantitative modeling. [more]

### Plastic effects on the kinetics of phase transformations

Many solid state phase transformations e.g. in steels or shape memory alloys are accompanied by severe mechanical deformations during the microstructure evolution. In contrast to elastic effects, which can nowadays be included e.g. in a phase field formulation of these processes, the proper incorporation of plastic deformation is not yet established. The reason is that different dissipative processes play a role here, which influence the motion of the interface. In this project we intend to develop novel sharp interface and phase field methods to simulate the microstructure evolution in the plastic regime. [more]

### Generation of optimal LCAO basis sets (QUAMOLs)

In this project, we explore the possibility to generate atomic orbital basis sets that optimally represent the electronic structure of a given material.

[more]### Semipolar surfaces

The majority of III-N are grown along the polar c-axis of the wurtzite structure, leading to an enhanced carrier separation due to polarization-induced electrostatic fields of the order of MV/cm. A promising approach preventing such effects is the growth along semipolar orientations, i.e. planes with a nonzero h, k, or i and a nonzero l Miller index. A fascinating feature of the semipolar planes is that depending on their orientation they may exhibit either Ga-polar or N-polar “flavor” and on the same time lye close to m- or a-plane non-polar character. However, it is still an open question whether the extensive experience gathered from the growth of polar and/or non-polar planes can be applied to the semipolar planes or whether novel atomistic mechanisms dominate the growth. A prerequisite to achieve smooth semipolar surface morphologies and high quality material is to gather a deep understanding of the atomistic mechanisms governing the growth of these surfaces. [more]

### Solid-melt gamma surfaces

Gamma surfaces are usually considered as an important concept to understand stacking faults, dislocation formation and plastic deformations. A crystal is cut into two halves, which are shifted against each other. Due to the mismatch of the lattice planes the energy therefore depends on the translation of the crystals. Here we investigate the crystal interactions at high temperatures, if they are separated by a thin melt layer. [more]

### Solidification in syntectic and monotectic alloys

We study steady-state solidification along the liquid-liquid interface in a syntectic system by means of a boundary-integral technique. We study the case of small asymmetry of the pattern and extract from the results the scaling relations in terms of the undercooling and the asymmetry parameter. We also investigate monotectic solidification using the phase-field method. [more]

### Triple junctions in phase field models

Phase field models are nowadays one of the most important methods to predict microstructure formation and the kinetics of phase transformations. In polycrystalline materials triple and higher order junctions naturally apear, which require an accurate treatment in phase field models.

[more]### Strain induced Cementite decomposition

Motivated by experimentally observed cemenetite decomposition in severely deformed pearlite, the thermodynamic driving force to increase carbon solubility of ferrite and the stability of C-vacancies in cementite under applied uniaxial strain are investigated. Applied true elastic strain of 0.03 gives rise to a factor of 100 increase in the C concentration in ferrite. [more]

### Crystal growth simulations of ternary group-III nitride alloys

Realization of the direct green light emitting diode (LED) by utilizing GaN based alloys is the next big challenge in solid-state LEDs. A major obstacle is the rather poor material quality of In-rich (exceeding x = 0.3) In

[more]_{x}Ga_{1-x}N epilayers. To this end, we have developed an effective crystal growth modeling technique in order to elucidate the growth processes of InGaN epilayers on the atomic scale. Our growth simulations reveal a hitherto unexpected √3×√3 pattern in In_{1/3}Ga_{2/3}N epilayer structures.### Non-radiative recombination at point defects in GaN

Non-radiative recombination limits the efficiency of GaN based light-emitting diodes. Our project aims to evaluate the non-radiative recombination mechanism of multiphonon emission centers in GaN by using first-principles calculation.

[more]### Grain boundary premelting

Grain boundaries strongly influence the mechanical behavior and other materials properties. At high homologous temperature, grain boundaries can display pronounced disorder, manifested in the most extreme case by the formation of nanometer-scale intergranular films with liquid-like properties. The formation of those films below the bulk melting point, typically referred to as grain boundary premelting, can dramatically reduce shear resistance and lead to catastrophic materials failure.

[more]### Bainitic press hardening

Press hardening is an important processing technology for the production of safety relevant components for automotive applications. This technology allows forming of complex shaped parts made of ultrahigh strength steels. Depending on quenching conditions martensitic or bainitic microstructures form, with a strong coupling between mechanical load, internal stresses, chemical composition and phase transformation kinetics. [more]

### Oxide scale growth

We develop phase field models for the description of oxide scale growth. Our aim is to provide a computational framework which is e.g. able to capture the processes which lead to the formation of protective oxides as well as diffusion through membranes driven by differences in gas partial pressures and electrochemical potential differences.

[more]### Extended defects in III-Nitrides

One of the controversial issues in the field of III-Nitrides is the effect dislocations have on the optoelectronic properties of the devices.

[more]