DFT calculations are an important ingredient in designing tailored materials. However, DFT calculations are typically limited to several hundreds of atoms and in many relevant cases (dislocations, grain boundaries) strong finite size effects must be expected. For such systems it is desirable to have a method available which allows to treat significantly larger systems.
A complete description of the thermodynamics of metallic alloys with strong local magnetic moments such as steels, crucially relies on an accurate determination of magnetic excitations. At high temperatures anharmonic atomic vibrations might become likewise important. Here, we investigate the coupling of both mechanisms.
The description of random alloys poses a significant challenge for atomistic modelling. A number of appraoches, including the cluster-expansion approach, the coherent potential approximation, as well as special quasirandom structures have been developed to this end. In this study we investigate the accuracy of Special quasirandom structures for the description of the elastic properties of random alloys.
Large-scale coarse-graining and O(N) algorithms typically require localized atomic basis sets. The systematic improvement of such basis sets is not trivial. We present an approach to construct an atomic-orbital basis set from a plane-wave DFT calculation by a nearly lossless compression.
GaN based nanowires (NWs) have recently emerged as potential candidates for nanodevice applications. The majority of the reported GaN NWs have their axial direction along the c-axis, while the facets are assumed to consist of non-polar surfaces.
The project aims to study strain-induced interaction in dilute solid solutions by means of microscopic elasticity theory, using parameters from atomistic modeling employing EAM-potentials and ab-initio calculations.
In this project, we use a space separation based on our quantitatively optimized atomic orbitals (Quamols) to define a local atomic energy. With this quantity we analyze defects, surfaces, and disordered systems to get insights, which are hidden in a total energy treatment.
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.
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.
The modeling of alloy solidification strongly depends 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 a so called thin interface asymptotics are related to deep physical symmetry relations. This understanding is central to true quantitative modeling.
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.
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.
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.
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.