Current Research Projects

In this project we study how Segregation Engineering can serve in the design of more robust and crack-free microstructures in Additive Manufacturing. More specific, we were able to reduce hot tearing in additive manufacturing of an Al x CoCrFeNi high-entropy alloy by grain boundary segregation engineering. [more]

Within this project we show that medium Mn steels can develop a pronounced discontinuous yielding when the austenite matrix fraction lies about 65 vol%. This phenomenon is investigated by a combination of multiple in situ characterization techniques covering the macroscopic down to the nanoscopic scale. [more]

In order to solve key challenges in lightweight transportation and safe infrastructures stronger steels with high ductility are urgently needed. In this work we introduce a new unique chemical boundary engineering (CBE) approach, which enables us to create a material with an ultrafine hierarchically heterogeneous microstructure even after heating to high temperatures. [more]

The call for high-performance lightweight materials is getting louder and louder in view of the urgently needed energy savings and emission reductions through weight reduction. By achieving the formation of simultaneously formed nanosized precipitations in our CCSs  materials, materials with ultrahigh specific tensile strength and excellent tensile elongation are designed.

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Steels are backbone materials of civilization since more than 3000 years. They retrieve their properties not from expensive chemical compositions but rather from complex nano- and microstructures. They cover a wider spectrum of properties than any other material. [more]

Stainless steels, invented more than 100 years ago, enable many sustainable applications: 1) they are among the most corrosion-resistant commodity alloys (corrosion being one of the biggest sustainability problems, destroying annually about 3.4% of the GDP (2.5 trillion Euros )).
2) they have with about 75% one of the highest end-of-life recycling rates of all mass-produced materials is use, averaged over all stainless steel grades.
However, there are 2 drawbacks: (a) high and expensive alloying content. (b) high weight.
We tackled both challenges: In a team effort we developed a new family of low-density stainless steels with ultra-high strength (> 1 GPa) and high ductility (> 35%). [more]

We are working on understanding the underlying mechanism of hydrogen embrittlement susceptibility in a Fe 28Mn 0.3C (Wt.%) alloy on the micro and nano scale by  exploring differentt hydrogen charging routes, for instance cathodic charging, gas charging and plasma charging. The defect behavior (dislocation density and arrangement, stacking faults, twins, ε-martensite, residual stresses) is investigated using deformation experiments coupled with electron channeling contrast imaging (ECCI) technique in both charged and uncharged conditions.Local residual stresses are measured with cross-correlation EBSD.In order to investigate the role of grain boundaries and stacking faults as hydrogen trapping sites, we also perform site-specific atom probe tomography (APT) studiesafter charging the samples with hydrogen/deuterium. [more]

 In this project, we reveal the subtle yet important interplay between the faceting of grain boundaries and their chemical decoration with solutes in an engineering Al-Zn-Mg-Cu alloy. Previously, the interplay of chemistry and faceting was revealed for specific grain boundaries in well-defined bicrystals, which are realistically not encountered in engineering alloys. [more]

In this project we investigate tensile fracture mechanisms of medium Mn steels with two typical types of microstructures. One group consists of ferrite (α) plus austenite (γ) and the other one of a layered structure with an austenite-ferrite constituent and δ-ferrite. [more]

In this project we show that medium Mn steels with an austenite matrix (austenite fraction ~65 vol%) can exhibit pronounced discontinuous yielding. A combination of multiple in situ characterization techniques from macroscopic (a few millimeters) down to nanoscopic scale (below 100 nm) is utilized to investigate this phenomenon. [more]

Since white etching crack (WEC) phenomena primarily occur in steels containing high amounts of carbon, we specifically elucidate the role of carbon in this failure mechanism in bearings. [more]

Severe plastic deformation leads to cementite decomposition in pearlitic and martensitic alloys, resulting in high-strength nanocrystalline ferrite. This effect can be employed to strengthen pearlitic wires but it can also be associated with material failure by white etching cracks (WECs) [more]

Within this project we aim to systematically understand carbide decomposition by applying different potential factors (deformation, heat, electric current, and hydrogen absorption) on a Fe-C binary pearlite. High resolution techniques, such as electron microscopy and atom probe tomography, are used for characterization of the underlying mechanisms. [more]

Well-functioning and failed CoCrMo implants have been shown to release different concentrations of Co, Cr and Ni into the blood.  The role of carbide phases present in CoCrMo alloys on the corrosion or dissolution mechanisms needs to elucidated in order to minimize the release of metal ions from an implant into the body [more]

The phase-field method is particularly well-suited to model coupled mechanical-thermal-chemical microstructure evolution and structure-property relations.
It has been successfully applied to model multiple thermo-chemo-mechanical processes including solidification, precipitation, fracture and dislocation motion. [more]

One purpose of metallurgical and materials science is the theory-guided tailoring of materials, including elasto-plastic mechanical response, chemical composition and microstructure, in order to obtain improved properties for a sustainable technological development. [more]

In this project, we successfully developed a crystal-glass high-entropy nanocomposite in CrFeCoNi-based system. The microstructure, composition and deformation mechanism of the novel crystal-glass high-entropy nanocomposite was comprehensively studied using probe-corrected scanning transmission electron microscope and atom probe tomography. This crystal-glass nanocomposite design provides a route to develop advanced structural materials with an outstanding combination of strength and ductility. [more]

To reach highest quality of microstructure and mechanical properties, adjustment of downstream processing parameters are often required along the process chain, dependent on exact chemical composition of the batch and the preceding casting, deformation and annealing processing steps. [more]

The focus of this project is the investigation of the kinetics of the deformation structure evolution and its influence on the strain hardening of a Fe-30.5Mn-2.1Al-1.2 (wt.%) steel. The observations are carried out during tensile deformation by transmission electron microscopy and electron channeling contrast imaging combined with electron backscatter diffraction. [more]

In this project we work on the corelative characterization of the atomic structure and composition of water-splitting catalysts. We aim to better understand reaction and degradation mechanisms of Ir-based catalysts for the oxygen evolution reaction (OER) by establishing structure-function relationships at the atomic scale. [more]

Thermoelectric materials can be used to generate electricity from a heat source through the Seebeck effect, whereby a temperature difference leads to a difference in voltage for power generation. The opposite effect, known as the Peltier effect, is exploited for heating and cooling for instance. The efficiency of the conversion can be increased by introducing defects that efficiently scatter phonons, i.e. the carriers of lattice vibrations and hence heat, but do not affect much the movement of electrons so as to maintain good electrical conductivity. [more]

This project targets to exploit or develop new methodologies to not only visualize the 3D morphology but also measure chemical distribution of as-synthesized nanostructures using atom probe tomography. [more]

This research focuses on studying the segregation behavior of solute atoms at defects like dislocations and grain boundaries (GBs). We aim at generating a connection between defect-related observations to mechanical properties. The outcome will provide input into the design of advanced alloys.
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By using the DAMASK simulation package we developed a new approach to predict the evolution of anisotropic yield functions by coupling large scale forming simulations directly with crystal plasticity-spectral based virtual experiments, realizing a multi-scale model for metal forming. [more]

A wide range of steels is nowadays used in Additive Manufacturing (AM). The different matrix microstructure components and phases such as austenite, ferrite, and martensite as well as the various precipitation phases such as intermetallic precipitates and carbides generally equip steels with a huge variability in microstructure and properties. [more]

New product development in the steel industry nowadays requires faster development of the new alloys with increased complexity. Moreover, for these complex new steel grades, it is more challenging to control their properties during the process chain. This leads to more experimental testing, more plant trials and also higher rejections due to unmatched requirements. Therefore, the steel companies wish to have a sophisticated offline through process model to capture the microstructure and engineering property evolution during manufacturing. [more]

Crystal Plasticity (CP) modeling [1] is a powerful and well established computational materials science tool to investigate mechanical structure–property relations in crystalline materials. It has been successfully applied to study diverse micromechanical phenomena ranging from strain hardening in single crystals to texture evolution in polycrystalline aggregates. [more]

Thermo-chemo-mechanical interactions due to thermally activated and/or mechanically induced processes govern the constitutive behaviour of metallic alloys during production and in service. Understanding these mechanisms and their influence on the material behaviour is of very high relevance for designing new alloys and corresponding thermomechanical processing routes. [more]

The Atom Probe Tomography group in the Microstructure Physics and Alloy Design department is developing integrated protocols for ultra-high vacuum cryogenic specimen transfer between platforms without exposure to atmospheric contamination. [more]

Advanced microscopy and spectroscopy offer unique opportunities to study the structure, composition, and bonding state of individual atoms from within complex, engineering materials. Such information can be collected at a spatial resolution of as small as 0.1 nm with the help of aberration correction. [more]

The project focuses on development and design of workflows, which enable advanced processing and analyses of various data obtained from different field ion emission microscope techniques such as field ion microscope (FIM), atom probe tomography (APT), electronic FIM (e-FIM) and time of flight enabled FIM (tof-FIM). [more]

Materials used in catalytic reactions are exposed to conditions that inevitably lead to microstructural and chemical changes in the bulk and on the surface. To understand their complex interplay and influence on the catalyst´s performance, one requires spatially resolved methods that encompass surface and bulk sensitivity on the nanoscale, ideally in-operando. [more]

In this project we try to expand the possibilities of using atom probe tomography (APT) to investigate proteins, their structures and binding to ligands. The project is funded by Volkswagenstiftung "Experiment" (Seeing atoms in biological materials - a new frontier for atomic-scale tomography) [more]

This project is part of Correlative atomic structural and compositional investigations on Co and CoNi-based superalloys as a part of SFB/Transregio 103 project “Superalloy Single Crystals”. This project deals with the identifying the local atomic diffusional mechanisms occurring during creep of new Co and Co/Ni based superalloys by correlative techniques. [more]

The objective of the project is to investigate grain boundary precipitation in comparison to bulk precipitation in a model Al-Zn-Mg-Cu alloy during aging. [more]

Understanding the deformation mechanisms observed in high performance materials, such as superalloys, allows us to design strategies for the development of materials exhibiting enhanced performance. In this project, we focus on the combination of structural information gained from electron microscopy and compositional measurements from atom probe tomography (APT). [more]

There is a high interest to understand the response of metallic (amorphous) glasses to rapid heating and plastic deformation. These two topics are addressed in this project using correlative electron microscopy and atom probe tomography. [more]

The interplay of mechanical loads and body fluids leads to local decomposition and surface alloying effects in the modular taper joints of hip implants.  We are investigating this engineering problem with state-of-the art correlative atom probe tomography and electron microscopy techniques. [more]

In this project we study the development of a maraging steel alloy consisting of Fe, Ni and Al, that shows pronounced response to the intrinsic heat treatment imposed during Laser Additive Manufacturing (LAM). Without any further heat treatment, it was possible to produce a maraging steel that is intrinsically precipitation strengthened by an extremely high number density of 1.2x10²⁵ m^-3 NiAl nanoparticles of 2‑4 nm size. The high number density is related to the low lattice mismatch between the martensitic matrix and the NiAl phase. [more]

In this project we work on the fabrication and the thermodynamic and metallurgical basics associated with the additive manufacturing of dense Mo-Si-B-based alloys. [more]

In this ongoing project, we investigate spinodal fluctuations at crystal defects such as grain boundaries and dislocations in Fe-Mn alloys using atom probe tomography, electron microscopy and thermodynamic modeling [1,2]. [more]

By characterizing the high N alloyed martensitic stainless bearing steel X30CrMoN15-1 in-depth, we rationalize the exceptional white etching crack resistance of this complex technical alloy in terms of the different grain boundary segregation behavior between nitrogen and carbon, the mechanical and thermodynamic stability of the precipitates, and the cleanliness of the steel. [more]

In this project, we aim to design novel NiCoCr-based medium entropy alloys (MEAs) and further enhance their mechanical properties by tuning the multiscale heterogeneous composite structures. This is being achieved by alloying of varying elements in the NiCoCr matrix and appropriate thermal-mechanical processing. [more]

The aim of the project is to elucidate the mechanism behind white etching crack (WEC) formation in bearing applications and to create materials that are resistant to this failure mechanism. The most prominent example for WEC failure are gear bearings of wind turbines. However, also many other applications from rails, over clutches to washing machines are concerned. [more]

This project studies the mechanical properties and microstructural evolution of a transformation-induced plasticity (TRIP)-assisted interstitial high-entropy alloy (iHEA) with a nominal composition of Fe_49.5Mn₃₀Co₁₀Cr₁₀C_0.5 (at. %) at cryogenic temperature (77 K). We aim to understand the hardening behavior of the iHEA at 77 K, and hence guide the future design of advanced HEA for cryogenic applications. [more]

In this project, we aim at significantly enhancing the strength-ductility combination of quinary high-entropy alloys (HEAs) with five principal elements by simultaneously introducing interstitial C/N and the transformation induced plasticity (TRIP) effect. Thus, a new class of alloys, namely, interstitially alloyed TRIP-assisted quinary (five-component) HEAs is being developed. [more]

In this project, we aim to understand the interstitial carbon effect on the recrystallization behavior of the equiatomic CoCrFeMnNi HEA and hence to tune the corresponding mechanical properties. [more]

In this project, we aim to enhance the mechanical properties of an equiatomic CoCrNi medium-entropy alloy (MEA) by interstitial alloying. Carbon and nitrogen with varying contents have been added into the face-centred cubic structured CoCrNi MEA. [more]

In this project, we aim to achieve an atomic scale understanding about the structure and phase transformation process in the dual-phase high-entropy alloys (HEAs) with transformation induced plasticity (TRIP) effect. Aberration-corrected scanning transmission electron microscopy (TEM) techniques are being applied ... [more]

In this project, a strategy of combining intermetallic phases and massive solid solutions is employed to design novel Refractory high-entropy alloys (RHEAs). [more]

In this project, we perform macro-/microscopic experiments and constitutive modelling to investigate the effects of stress amplitude and mean stress on the ratchetting strain and the overall cyclic behavior of interstitial high-entropy alloys (iHEAs)... [more]

In this project, we probe the invar effect in the high and medium entropy alloys over the huge unexplored compositional space. Combining experimental investigation (PPMS, EBSD, ECCI, APT and TEM) and theoretical calculation (DFT and Calphad)... [more]

In this project, we investigate the segregation behavior and complexions in the CoCrFeMnNi high-entropy alloys (HEAs). The structure and chemistry in the HEAs at varying conditions are being revealed systematically by combining multiple advanced techniques such as electron backscatter diffraction (EBSD) and atom probe tomography (APT). [more]

In this project, the electrochemical and corrosion behavior of high entropy alloys (HEAs) have been investigated by combining a micro-electrochemical scanning flow cell (SFC) and an inductively coupled plasma mass spectroscopy (ICP-MS) element analysis. [more]

In this project, the hydrogen embrittlement mechanisms in several types of high-entropy alloys (HEAs) have been investigated through combined techniques, e.g., low strain rate tensile testing under in-situ hydrogen charging, thermal desorption spectroscopy (TDS),... [more]

In collaboration with Prof. Dr. Helge-Otto Fabritius, Hamm-Lippstadt University of Applied Sciences, we investigate the potential of biomimetic substances such as hydroxyapatite for optimization and development of new formulations for prevention and treatment of a number of  prevalent clinical conditions in teeth. [more]

The unpredictable failure mechanism of White Etching Crack (WEC) formation in bearing steels urgently demands in-depth understanding of the underlying mechanisms in the microstructure. The first breakthrough was achieved by relating the formation of White Etching Areas (WEAs) to successive WEC movement. [more]

In a set of projects we study the field of strong and ductile non-equiatomic high-entropy alloys (HEAs). [more]