Phases, Defects & Microstructure

Defects are crucial in determining materials properties and occur on a wide range of time and length scales, from point defects to extended ones. To simulate the rich world of phase stability, defects, and microstructure, from interstitial doping towards grain boundary mobility, requires advanced and diverse simulation techniques from electronic structure calculations, machine learning, thermodynamic integration and molecular dynamics simulations.

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]
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]
A high degree of configurational entropy is a key underlying assumption of many high entropy alloys (HEAs). However, for the vast majority of HEAs very little is known about the degree of short-range chemical order as well as potential decomposition. Recent studies for some prototypical face-centered cubic (fcc) HEAs such as CrCoNi showed that short-range order (SRO) can influence critical materials properties as, e.g., stacking fault energies. For refractory HEAs, due to slow diffusivity chemical ordering may hardly ever be achieved  under typical experimental conditions but could potentially influence creep properties long-term applications. In this project we therefore study the phase stability and short-range order of selected fcc as well as bcc refractory HEAs computationally. [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]
We simulate the ionization contrast in field ion microscopy arising from the electronic structure of the imaged surface. For this DFT calculations of the electrified surface are combined with the Tersoff-Hamann approximation to electron tunneling. The approach allows to explain the chemical contrast observed for NiRe alloys. [more]
Extremely strong (~10 V/nm) electric fields rupturing atomic bonds is a relatively well-studied concept in the field of molecular chemistry. When extended to crystalline systems, i.e. material surfaces, this concept is known as field evaporation and its exact mechanisms become more challenging to predict. Field evaporation is the central phenomenon that enables atom probe tomography (APT) and 3D field ion microscopy (3D-FIM), and obtaining atomically-accurate APT reconstructions will be impossible without an atomically-accurate understanding of how ions initially form and depart from the surface. By performing first-principles calculations on faceted surfaces under extreme fields, we search for such an understanding in pure metals and alloys. [more]
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