Finished Projects

The effect of surfaces and more specifically of surface reconstruction and rehybridization on the properties, composition, ordering and thermal stability of epitxailly grown alloys is investigated. [more]

III-Nitride alloys such as InN, GaN, and AlN dominate the optoelectronics industry with applications in light emitting devices (LED), laser diodes (LD), and power electronics and constitute one of the most important semiconducting materials nowadays. In this project the bulk thermodynamics of these alloys are investigated. [more]

Screw dislocation induced nanopipes are investigated by combining elasticity theory with density functional theory calculations. Based on these calculations a c-type screw dislocation phase diagram is constructed which describes the energetically most favorable core structures as function of the Ga, N and H chemical potentials. We find that nanopipes with diameters ranging from ≈1 to ≈2 nm are energetically favorable for high values of the H chemical potential and conditions that correspond to MOCVD and MOVPE growth. [more]

The synthesis of InGaN digital alloys in the form of short period InGaN/GaN superlttices is investigated by combining ab/initio and empirical potential calculations with PAMBE growth , Photoluminescence Spectroscopy, and HR(S)TEM characterization. [more]

ZnO is a wide band gap semiconductor which is of interest to such diverse areas of application as passivation layers on steel surfaces, catalysis, corrosion, adhesion, gas sensing, and micro- or optoelectronics. Understanding the surface structure and stoichiometry is of high practical interest and essential for any of the mentioned applications. Keeping in mind that the chemical environment interfacing with the surface plays a decisive role in the stabilisation and atomic structure of the surface reconstruction, we combine density functional theory (DFT) calculations with atomistic thermodynamics to investigate and understand the stability of polar Zn-terminated ZnO(0001) surfaces in dry and humid environment. [more]

Phase stabilities of multicomponent material systems are best simulated with CALPHAD. The underlying databases are mainly obtained from calorimetric measurements, but suffer often from the fact that for certain phases an experimental approach is not feasible or not conclusive. Here, ab initio calculations emerge as an alternative, which is carefully investigated for the Al-Mg-Si-Cu and Al-Sc system within this project. [more]

The computational design of high strength steels such as FeMn alloys often faces a combination of challenges: (1) the treatment of chemical complexity, (2) the treatment of magnetic disorder, in particular, in the paramagnetic state, and (3) the treatment of structural defects. Moreover, the interplay of these degrees of freedom also needs to be accounted for. In this project we particularly focus on this kind of coupling of different degrees of freedom, since we believe it is decisive to understand some of the phenomena observed in FeMn alloys. [more]

The aim of this project is to resolve the interplay of real space structure and electronic states in combination with magnetic disorder for iron-based superconductors.  We apply a combination of density-functional theory calculations and effective tight-binding models for the electronic energy dispersion. [more]

Combining concepts of semiconductor physics and corrosion science, we develop a novel approach that allows us to perform ab initio calculations under controlled potentiostat conditions for electrochemical systems. The proposed approach can be straightforwardly applied in standard density functional theory codes. [more]

The stability and concentration of extended defects is closely related to their atomic structure, which can already be complex for planar defects in pure elements or chemically ordered phases, such as Laves phases. In the case of multi-component alloys with off-stoichiometric compositions, the chemical degree of freedom adds another level of complexity. Particularly fascinating becomes the interplay of local chemistry and planar defects, however, if defect structures and/or compositions are created that would not be present otherwise. [more]

Solute-grain boundary interaction can have a strong impact on material properties, even at very dilute solute concentrations. Traditionally this interaction is represented with a single segregation energy; in this project we use empirical potentials to demonstrate that using a spectral representation of the interaction is important for accurately capturing temperature dependent behavior. Furthermore, we use empirical potentials in combination with machine learning to efficiently determine these spectra.  [more]

Hydrogen embrittlement is a persistent mode of failure in modern structural materials. The processes related to HE span various time and spatial scales. Thus we are establishing multiscale approaches that are based on the parameters and insights obtained by accurate ab initio calculations in order to simulate HE at the continuum level. [more]

The energetics as well as atomistic mechanisms underlying the segregation of impurities at Si grain boundaries (GB) and GB junctions have been investigated. [more]

Faceting of grain boundaries has a strong impact on the properties of structural, functional, and optoelectronic materials. In this project, we employ density-functional theory and modified embedded atom method calculations to investigate the energetics and thermodynamic stability of facets and line junctions in Silicon. We find that higher energy metastable GB phases can be stabilized by thermodynamics and not kinetics when constituting the facets at line junctions. This is in contrast to the common perception that the properties of faceting are merely driven by the anisotropic GB energies.  [more]

Local lattice distortion is one of the core effects in complex concentrated alloys (CCAs). It has been expected that the strength CCAs can be improved by inducing larger local lattice distortions. In collaboration with experimentalists, we demonstrated that VCoNi has larger local lattice distortions and indeed has much better strength than the prototypical CrCoNi CCA has. [more]

Medium and high-Mn steels constitute an important class in the development of advanced high strength steels (AHSS) for the automotive industry. It was observed that welding of Zn coated AHSS steel can induce liquid metal embrittlement (LME). In this project we will use ab initio methods to reveal the interplay between structure, chemistry, magnetism and decohesion in grain boundaries (GB). The primary focus will be to identifying key mechanisms for LME in order to systematically improve resistance of AHSS to this effect. [more]

It is very challenging to simulate within DFT extreme electric fields (a few 10¹⁰ V/m) at a surface, e.g. for studying field evaporation, the key mechanism in atom probe tomography (APT). We have developed a straight-forward scheme to incorporate an ideal plate counter-electrode in a nominally charged repeated-slab calculation by means of a generalized dipole correction of the standard electrostatic potential obtained from fully periodic FFT. [more]

Interstitial alloying can improve the mechanical properties of high-entropy alloys (HEAs). In some cases, the interstitial-alloying impact is very different from those in conventional alloys. We investigate the effect of interstitial alloying in fcc CrMnFeCoNi HEA as well as bcc refractory HEAs, particularly focusing on the solution energies and impact on, e.g., stacking fault energies, based on first-principles calculations. Our results clarity, e.g., that the interstitial solution energy in HEAs is no longer a single value but shows a substantial distribution due to the dependence on local chemical environments. [more]

At finite temperatures lattice vibrations and magnetic fluctuations are coexisting. To study potential coupling effects, a method is required, which considers both, the spin and the lattice degrees of freedom, simultaneously. We develop and implement such a method by combining atomistic spin dynamics with ab initio molecular dynamics. [more]

Magnetic properties of magnetocaloric materials is of utmost importance for their functional applications. In this project, we study the magnetic properties of different materials with the final goal to discover new magnetocaloric materials more suited for practical applications. [more]

The diffusion mechanisms in ordered binary alloys are more complicated than in materials with only one atom species. Several mechanisms, including, e.g., triple defect jump cycles, have been suggested in the literature. Within this project, we resolve which of them is energetically most favorable in FeAl and use the calculated barriers for large scale simulations. [more]

In order to explore the possibility of using high entropy alloys (HEAs) for functional applications such as magnetic refrigeration it is necessary to have an in-depth understanding of their magnetic properties. The main goal of this project is to understand and improve the magnetic properties (e.g., saturation magnetization, Curie temperature etc.) in different medium and HEAs. [more]

High-Mn-steels are excellent candidates for the next generation of high-strength materials. In such steels the prevailing plasticity mechanism is determined by stacking fault energy. In this study, we aim to develop a generalized first-principles framework that allows temperature- and composition-dependent atomic-scale description of the stacking fault properties, necessary to explore chemical trends, to deliver parameters for mesoscale models, and to identify new routes to optimize high-Mn-steels. [more]

The fundamental mechanisms of V-pits formation on epitaxially grown GaN polar surfaces are investigated combining state-of-the-art first-principles calculations and elasticity theory.
  [more]

Al-based alloys are promising structural materials owing to their mechanical properties and high thermal stability complemented by their light weight. Their performance and stability, however, is largely governed by the properties of inherent second phase particles. Computationally designing technologically-relevant alloys with desired properties therefore requires a comprehensive knowledge of the underlying atomistic processes and the interplay of strain and chemistry. The aim of this project is to integrate theory and experiments in understanding the thermodynamic and kinetics aspects of thermo-mechano-chemical coupling during the precipitate formation in Al based alloys taking the example of Al-Sc and Al-Sc-Zr systems. [more]

The supercell approach allows to model defects with efficient periodic boundary models. By making the supercell sufficiently large, in principle, the limit of an single defect can be recovered. In practice, defect-defect interactions are still relevant for affordable system sizes. We demonstrate that empirical extrapolation has its limitations if the underlying physics is not taken into account or not even known. [more]

In order to prepare raw data from scanning transmission electron microscopy for analysis, pattern detection algorithms are developed that allow to identify automatically higher-order feature such as crystalline grains, lattice defects, etc. from atomically resolved measurements. [more]

The aim of this project is the development of a complete first-principles based methodology to study the magneto-caloric effect (MCE) close to the technologically and scientifically relevant regime of first order magneto-structural transitions. [more]

Understanding hydrogen-assisted embrittlement of advanced high-strength steels is decisive for their application in automotive industry. Ab initio simulations have been employed in studying the hydrogen trapping of Cr/Mn containing iron carbides and the implication for hydrogen embrittlement. [more]

Eutectic Ti-Fe alloys exhibit a high strength (~1000 MPa), excellent ductility, and sufficient corrosion resistance making them promising candidates for numerous aerospace and automotive applications. Dual-phase Fe-Ti eutectics are composed of a rather brittle FeTi intermetallic phase with the B2 structure and a softer and more ductile β-Ti(Fe) alloy with varying Ti concentrations. [more]