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Research Projects

Conventional alloy development methodologies which specify a single base element and several alloying elements have been unable to introduce new alloys at an acceptable rate for the increasingly specialised application requirements of modern technologies. An alternative alloy development strategy searches the previously unexplored central regions of multi-component phase space for alloys whose properties can be tuned with a greater degree of control than previously achievable. The targeted exploration of composition spaces containing five or more elements presents a significant challenge due to the vast number of possible alloy combinations. Novel approaches are required to efficiently map the boundaries of unique phase and morphology formation domains over large regions of multi-principle-element composition space. more
A part of this project is to investigate the relationship between GB misorientation and atomic structure with GB migration by ex situ and in situ heating experiments. Furthermore, different pure tilt GBs will be investigated by aberration-corrected (S)TEM. more
This project with the acronym GB-CORRELATE is supported by an Advanced Grant for Gerhard Dehm by the European Research Council (ERC) and started in August 2018.
The project GB-CORRELATE targets on (i) predicting and resolving GB phase transitions, (ii) establishing guidelines for GB phase transitions and GB phase diagrams, (iii) correlating GB phase transitions with property changes, (iv) providing compositional-structural design criteria for GB engineering, (v) which will be tested by demonstrators with tailored GB strength and GB mobility. GB-CORRELATE focusses on Cu and Al alloys in form of thin films as this allows to implement a hierarchical strategy expanding from individual special GB to GB networks and a transfer of the GB concepts to thin film applications. more
Grain boundaries are one of the most important constituents of a polycrystalline material and play a crucial role in dictating the properties of a bulk material in service or under processing conditions. Bulk properties of a material like fatigue strength, corrosion, liquid metal embrittlement, and others strongly depend on grain boundary properties such as cohesive strength, energy, mobility, etc. These boundary properties in turn are governed by the structure and chemistry of a grain boundary. Furthermore, it has recently been realized that grain boundaries themselves can be described as interface-stabilized phases. We are just at the advent to utilize the phase character of grain boundaries as a material design element. more
In conventional metallic materials, the increase of strength by dislocation hardening generally sacrifices ductility. In recent years, a novel alloy design concept has drawn great attention, where multi-principal elements are mixed at equimolar or near equimolar concentrations to form highly concentrated solid solutions, termed high-entropy alloys (HEAs). To promote the wide use of HEAs as structural materials, it is highly desirable to improve the strength of HEAs while maintaining good ductility. In this project, we demonstrate an approach to improve the strength and ductility simultaneously by tuning the stacking fault energy and deformation mechanism in single-phase face-center cubic (FCC) high-entropy alloys (HEA) as shown in Fig. 1. more
The objectives of this project is to understand the strengthening mechanisms of high entropy alloys (HEAs) from a dislocation plasticity point of view. The effects of microstructure and local composition, down to the atomic scale, on the plastic deformation are also investigated to establish a fundamental structure-property relationship of HEAs. more
Segregation of specific elements to grain boundaries (GB) alters their structure and with this the mechanical and physical properties of the material. The fundamental atomic-scale processes depend on the GB structure, chemistry as well as thermodynamic parameters. Aberration-corrected high resolution (S)TEM techniques are applied to α-Iron bicrystals to explore the atomistic origins of segregation in bcc-metals. more
The mostly unknown influence of Ag as solute segregate at copper grain boundaries on mechanical properties is studied by aberration-corrected STEM from an atomistic structural point of view and by in-situ TEM nanocompression experiments to visualize dislocation-grain boundary interactions. more

Closed Projects

The project in the scope of research activities of the Advanced Transmission Electron Microscopy group has two main objectives: (i) epitaxial thin film deposition and (ii) in-situ TEM tensile experiments. more
Fusion is one of the most promising safe, emissionless and limitless sources of energy. The extreme conditions in a fusion reactor, require the development of novel materials to withstand high temperature ion irradiation and at the same time provide sufficient mechanical stability. more
The segregation of impurities to grain boundaries (GBs) has a significant influence on the cohesive properties, atomic arrangements and properties of such interfaces. The segregation strongly depends on the structural units of the GB as well as on the impurity atom itself. Aberration–corrected (S)TEM techniques in combination with atomistic simulations are applied to unravel the connection of grain boundary structure and chemistry at atomic resolution. more
Carbon(C)-containing martensitic steels are ideal candidates for high-strength applications, e.g. in automotive and aerospace applications, due to their excellent mechanical properties and low cost. Carbon can even redistribute at room temperature leading to the formation of nanoscale carbides that can significantly influence the mechanical properties. more
The development of nanostructured metals and alloys with superior mechanical properties is of paramount importance for both, a fundamental scientific understanding of the structure property relationship of materials and future technological applications in modern micro- and nanotechnologies.
A structural hierarchy due to chemical ordering, dimensionality and spatial arrangement of the constituent phases was obtained in a precipitation strengthened ferritic alloy. Nearest-neighbor ordered B2-NiAl precipitates were coherently embedded in the disordered bcc-Fe matrix. Throughout the solid-state aging heat treatment a coherent substructure of the next-nearest-neighbor ordered L21-Ni2TiAl phase formed only within the primary B2-NiAl precipitates. more
Ferritic superalloys are an attractive alternative to Cr-rich martensitic steels or Ni-based superalloys for high-temperature applications in thermal power plants due to their excellent mechanical properties, oxidation resistance and low density. Strengthening of the Fe-matrix by coherent B2-NiAl precipitates leads to an increase in creep resistance up to temperatures of 700 ºC and stresses of 100 MPa. more
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