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Microstructure Physics and Alloy Design

Research in the department 'Microstructure Physics and Alloy Design' is on the relationships between synthesis, microstructure and mechanical as well as functional properties of nanostructured materials. Focus is placed on metallic alloys such as steels, high and medium entropy alloys, superalloys, magnesium, titanium, Heusler phases and aluminium alloys. Composites, semi-metallic thermoelectrics and solar cell semiconductors are considered as well. We study the microstructures and properties using theory and advanced characterization methods from the single atom level up to the macroscopic scale.

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Scientific Mission and Department Structure

We work on the relationships between synthesis, processing, microstructure and properties of compositionally and structurally complex metallic alloys including steels, magnesium, aluminium, superalloys, titanium and high entropy alloys. Focus is placed on phase transformations, design of metastable phases, micromechanics and complex defect substructures as well as their effects on the mechanical and functional properties.

We pursue these goals by developing and applying advanced characterization methods from the atomic level up to the macroscopic scale. Examples are chemically sensitive Field Ion Microscopy (FIM) which is based on the integration of atom probe tomography (APT), FIM and machine learning as well as correlative APT and scanning transmission electron microscopy (STEM) in concert with a reaction chamber and UHV-cryo transfer unit; Electron Channeling Contrast imaging under controlled diffraction conditions (ECCI); 3D electron backscatter diffraction (EBSD) and cross-correlation EBSD; in-situ micromechanical experiments correlated to local strain and hydrogen mapping; and standardized bulk high-throughput metallurgy and mechanical testing. Several of these techniques are developed and operated in collaboration with the groups of G. Dehm, C. Scheu, M. Rohwerder, R. Dunin-Borkowski (Ernst Ruska Centre in Jülich), J. Schneider (RWTH Aachen) and G. Eggeler (RU Bochum).

We design experiments based on theory-guidance and conduct them under well controlled boundary conditions: For example for better understanding, quantifying and improving our atomic scale APT and FIM probing methods we collaborate with the group of J. Neugebauer on the simulation of field evaporation and image gas ionization as well as on the use of machine learning for crystallographic pattern recognition in APT data sets. Regarding thermodynamics and structure–property relations we also collaborate with in-house ab initio experts for instance on phase equilibria for bulk and confined states and the thermodynamics of high entropy alloys. Concerning constitutive simulations we have developed further our in-house modular freeware simulation package DAMASK (Düsseldorf Advanced Material Simulation Kit). This is a hierarchically structured model of material point behaviour for the solution of elastoplastic boundary value problems along with damage and thermal effects.

As an example, one topic where many of these fields of interest overlap in the department, is the interplay of local chemical composition, phase metastability and transformations in confined regions, i.e. at decorated lattice defects. Correlative atomic-scale probing and thermodynamic theory show that in many alloys segregation to lattice imperfections is an ubiquitous phenomenon, yet, it is typically acquired through trapping of atoms to defects during tempering rather than being engineered in a purposeful and property-directed manner. This has motivated us to conduct systematic ‘Segregation Engineering’ experiments and develop from that corresponding alloy design and processing strategies where we utilize Gibbs and Fowler-Guggenheim – type decoration of lattice defects with the aim to turn these regions into chemo-structural entities that lead to beneficial mechanical and functional behaviour. This site specific manipulation of confined defect regions by chemistry has for example allowed us to tune lattice defects for improved local cohesion, confined phase transformation, scattering, fracture toughness and impurity trapping. Specific thermodynamic phenomena that we discovered in this context are composition-driven phase transformations of dislocation cores, complexion, confined hydrides, stacking faults, spinodals at grain boundaries and dislocations, and confined phase states and precursors phases preceding nucleation. With these approaches and topics we conduct materials engineering down to the atomic scale, Fig. 1.

Fig. 1: Research approach, interests and corresponding long-term method development in the Department for Microstructure Physics and Alloy Design.

Research Groups

The department is organized in scientific groups some of which are extramurally funded and non-permanent, Fig.2.

Fig. 2:Research groups in the Department for Microstructure Physics and Alloy Design. The left 5 groups are permanently funded. The middle and right groups are temporary initiatives which are funded by grants.

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