© G. Geelen, Max-Planck-Institut für Eisenforschung GmbH

Research Projects

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 explores the presence and consequences of grain boundary phase transitions (often termed “complexions” in literature) in pure and alloyed Cu and Al. If grain size gets smaller and smaller - like in nanocrystalline materials - the grain boundary (GB) volume can exceed several 10% of the total material volume and become a powerful lever to manipulate and set properties. The atomic coordination and chemistry of such GBs may undergo phase transitions, abrupt changes in structure and/or chemistry, which will impact the material behavior - like strength, thermal stability, electrical resistance – even for conventional materials. However, this interplay between GB phases and material properties is poorly understood. Experimentally, GBs are difficult to study - it needs atomic resolution and sensitivity with respect to chemistry to uncover their structure and possible complexions. In addition, it is unknown under which conditions phase transformations of GBs occur. A fundamental understanding requires atomistic modelling connected with smart experiments.
Nickel-based alloys are a particularly interesting class of materials due to their specific properties such as high-temperature strength, low-temperature ductility and toughness, oxidation resistance, hot-corrosion resistance, and weldability, becoming potential candidates for high-performance components that require corrosion resistance and good mechanical properties. This unparalleled combination of properties is achieved by adding alloying elements and changes in microstructure. This research project blended Ni-based metal welds produced by in situ alloying using the tandem GMAW process in a previous research project developed by the Welding Research and Technology Laboratory team at the Federal University of Ceará, in Brazil.
The goal of this project is to optimize the orientation mapping technique using four-dimensional scanning transmission electron microscopy (4D STEM) in conjunction with precession electron diffraction (PED). The development of complementary metal oxide semiconductor (CMOS)-based cameras has revolutionized the capabilities in data acquisition due to their high sensitivity and fast read out speeds. While scanning an almost parallel, nanometer sized electron probe across the sample, it is now possible to acquire high quality diffraction patterns at each beam position. This produces a complex 4D dataset, where local crystal symmetries, lattice strain and crystal orientation are encoded in the 2D diffraction patterns obtained for each point of the 2D raster grid. The high image quality of the diffraction patterns significantly improves the reliability to determine lattice symmetries and orientations and with this greatly enhances orientation mapping. [more]
In this project, we employ atomistic computer simulations to study grain boundaries. Primarily, molecular dynamics simulations are used to explore their energetics and mobility in Cu- and Al-based systems in close collaboration with experimental works in the GB-CORRELATE project.
This project deals with the phase quantification by nanoindentation and electron back scattered diffraction (EBSD), as well as a detailed analysis of the micromechanical compression behaviour, to understand deformation processes within an industrial produced complex bainitic microstructure. [more]
Copper is widely used in micro- and nanoelectronics devices as interconnects and conductive layers due to good electric and mechanical properties. But especially the mechanical properties degrade significantly at elevated temperatures during operating conditions due to segregation of contamination elements to the grain boundaries where they cause grain boundary embrittlement and promote mechanical failure, limiting the lifetime of devices. [more]
In this project, we explore the hydrogen-storage capabilities of quaternary refractory high-entropy alloys (RHEAs) showing transformation-induced plasticity (TRIP) by electro-chemical charging. The initially body-centered cubic (BCC) alloy can be partially transformed into the hexagonal close-packed (HCP) phase upon room temperature straining, allowing to adjust the HCP fraction by the pre-straining conditions. After electro-chemical charging a high fraction of compositionally complex face-centered cubic (FCC) hydrides form in the HCP phase. We employ atomic resolution imaging, electron energy loss spectroscopy (EELS) and in situ heating in the transmission electron microscope (TEM) to determine the hydride formation sequence, their stability and dissolution mechanisms. [more]
The aim of this project is to correlate the point defect structure of Fe1-xO to its mechanical, electrical and catalytic properties. Systematic stoichiometric variation of magnetron-sputtered Fe1-xO thin films are investigated regarding structural analysis by transition electron microscopy (TEM) and spectroscopy methods, which can reveal the defect point defect structure caused by chemical variation. Following this, the defect structure can be correlated to mechanical properties such as fracture toughness, electrical resistivity, and the catalytic properties for possible future water-splitting applications.
Materials degradation due to wear and corrosion is a major issue that can lead to efficiency loss or even failure. As wear may accelerate corrosion and corrosion may accelerate wear, this interaction is of increasing interest in the wind, hydroelectric, oil and gas energy domains and in the bio-medical field. [more]
This project aims to correlate the localised electrical properties of ceramic materials and the defects present within their microstructure. A systematic approach has been developed to create crack-free deformation in oxides through nanoindentation, while the localised defects are probed in-situ SEM to study the electronic properties. A coupling of dielectric spectroscopy is made with in-situ micro/nano-mechanical testing. The correlation between defects and electrical properties provides information about the local deformation-conductivity phenomena, improving electrical properties of material, and may enable predicting failure of materials. [more]
Metallic glasses are continuously prone to structural changes due to their metastable character. These structural modifications, such as segregation or crystallization, can be used to produce nanocomposite or nanocrystalline functional materials or they can represent a deterioration of the material properties. In either case, a fundamental understanding of the process kinetics and chemical/structural evolution is essential. [more]
  Scanning transmission electron microscopy (STEM) has become an increasingly versatile and sophisticated instrument for studying materials at the atomic scale, due to advancements in in situ capabilities, novel imaging and spectroscopy modalities and ultrafast detectors. The large multidimensional datasets that are produced are enormously rich in quantitative information about the sample, but they call for new approaches in terms of data and metadata management. At present, the multitude of proprietary data formats developed by instrument manufacturers hinder easy access to the raw data. Each format also has their own metadata representation. In light of FAIR (Findable, Accessible, Interoperable, Reusable) principles, it is becoming increasingly important to standardize (meta)data representation. The goal of this project is to develop universal, instrument and experiment independent TEM data and metadata formats.  [more]
This project will aim at developing MEMS based nanoforce sensors with capacitive sensing capabilities. The nanoforce sensors will be further incorporated with in situ SEM and TEM small scale testing systems, for allowing simultaneous visualization of the deformation process during mechanical tests [more]
Within this project, we will investigate the micromechanical properties of STO materials with low and higher content of dislocations at a wide range of strain rates (0.001/s-1000/s). Oxide ceramics have increasing importance as superconductors and their dislocation-based electrical functionalities that will affect these electrical properties. Hence it is fundamental to understand the deformation limits to introduce dislocations for both the fabrication process and in-use performance.
In this project, we develop two non-equiatomic FCC structued HEAs with different stacking fault energies (SFEs). [more]
Smaller is stronger” is well known in micromechanics, but the properties far from the quasi-static regime and the nominal temperatures remain unexplored. This research will bridge this gap on how materials behave under the extreme conditions of strain rate and temperature, to enhance fundamental understanding of their deformation mechanisms. The mechanical behavior of metals with different crystal topologies, i.e. FCC, BCC and HCP and alloy systems, will be investigated in a statistically relevant manner using dewetted microparticles as the test-beds. [more]
The structure of grain boundaries (GBs) is dependent on the crystallographic structure of the material, orientation of the neighbouring grains, composition of material and temperature. The abovementioned conditions set a specific structure of the GB which dictates several properties of the materials, e.g. mechanical behaviour, diffusion, and electrical properties. The latest properties are usually captured through macroscopic characterisation which limits investigation of individual GB types. This project aims to probe the electrical resistivity of individual GB segments having well-known structural and thermodynamic characters, and consequently interconnect their structural and electrical properties. [more]
We will investigate the electrothermomechanical response of individual metallic nanowires as a function of microstructural interfaces from the growth processes. This will be accomplished using in situ SEM 4-point probe-based electrical resistivity measurements and 2-point probe-based impedance measurements, as a function of mechanical strain and temperature. [more]
The goal of this project is to develop an environmental chamber for mechanical testing setups, which will enable mechanical metrology of different microarchitectures such as micropillars and microlattices, as a function of temperature, humidity and gaseous environment. [more]
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]
The formation of a face-centered cubic (FCC) titanium (Ti) phase is still one of the mysteries in this environmentally sensitive alloy family. We show in this project, how hydrides in Ti can be formed during sample preparation, reveal the underlying mechanisms and establish pathways to suppress or even eliminate the unexpected hydride formation. Hydride formation is mostly associated with the high diffusion rate and low solubility of hydrogen within the HCP matrix. Through plasmon loss electron energy loss spectroscopy (EELS) and atomic resolution imaging in combination with atom probe tomography (APT) we establish that predominantly TiHX (x~50 at.%) forms in commercially pure Ti and show that focused ion beam preparation at cryogenic temperatures can suppress hydride formation. [more]
The focus lies on the investigation of the fracture properties of materials down to the individual microstructural length scale, with special attention on grain/phase boundaries or material interfaces, while considering the role of crystallography, chemistry and non-ambient conditions such as high temperatures and cryogenic conditions on fracture. [more]
The current understanding of wear of metals shows that the crack initiation mechanism is related to surface fatigue which occurs as the metal experiences repeated loading cycles. However, it was revealed that cracks can form even in single stroke tracks and that the crystal orientation determines the crack patterns. [more]
This project (B06) is part of the SFB 1394 collaborative research centre (CRC), focused on structural and atomic complexity, defect phases and how they are related to material properties. The project started in January 2020 and has three important work packages: (i) fracture analysis of intermetallic phases, (ii) the relationship of fracture to temperature (BDTT) and composition, and (iii) interfacial shear strength analysis of Mg-Intermetallic thin films. [more]
The goal of this project is the investigation of interplay between the atomic-scale chemistry and the strain rate in affecting the deformation response of Zr-based BMGs. Of special interest are the shear transformation zone nucleation in the elastic regime and the shear band propagation in the plastic regime of BMGs. [more]
Understanding hydrogen-microstructure interactions in metallic alloys and composites is a key issue in the development of low-carbon-emission energy by e.g. fuel cells, or the prevention of detrimental phenomena such as hydrogen embrittlement. We develop and test infrastructure, through in-situ nanoindentation and related techniques, to study independently hydrogen absorption and further interaction with trap binding sites or defects and its effects on the mechanical behavior of metals. [more]
In this project the influence of hydrogen on the mechanical behaviour of iron-nickel alloys is investigated by in-situ nanoindentation during hydrogen load and atom probe tomography (APT) of the hydrogen influenced areas. [more]
Hydrogen embrittlement (HE) of steel is a great challenge in engineering applications. However, the HE mechanisms are not fully understood. Conventional studies of HE are mostly based on post mortem observations of the microstructure evolution and those results can be misleading due to intermediate H diffusion. Therefore, experiments with a simplified stress states and in-situ mechanical loading are required to better understand HE. [more]
With the support of DFG, in this project the interaction of H with mechanical, chemical and electrochemical properties in ferritic Fe-based alloys is investigated by the means of in-situ nanoindentation, which can characterize the mechanical behavior of independent features within a material upon the simultaneous charge of H. [more]
This project aims to investigate the influence of grain boundaries on mechanical behavior at ultra-high strain rates and low temperatures. [more]
This project studies the influence of grain boundary chemistry on mechanical behaviour using state-of-the-art micromechanical testing systems. For this purpose, we use Cu-Ag as a model system and compare the mechanical response/deformation behaviour of pure Cu bicrystals to that of Ag segregated Cu bicrystals. [more]
We plan to investigate the rate-dependent tensile properties of 2D materials such as HCP metal thin films and PbMoO4 (PMO) films by using a combination of a novel plan-view FIB based sample lift out method and a MEMS based in situ tensile testing platform inside a TEM. [more]
This project will aim at addressing the specific knowledge gap of experimental data on the mechanical behavior of microscale samples at ultra-short-time scales by the development of testing platforms capable of conducting quantitative micromechanical testing under extreme strain rates upto 10000/s and beyond. [more]
The full potential of energy materials can only be exploited if the interplay between mechanics and chemistry at the interfaces is well known. This leads to more sustainable and efficient energy solutions. [more]
Within this project, we will use a green laser beam source based selective melting to fabricate full dense copper architectures. The focus will be on identifying the process parameter-microstructure-mechanical property relationships in 3-dimensional copper lattice architectures, under both quasi-static and dynamic loading conditions. [more]
In this project, links are being established between local chemical variation and the mechanical response of laser-processed metallic alloys and advanced materials.

The precipitation of intermetallic phases from a supersaturated Co(Nb) solid solution is studied in a cooperation with the Hokkaido University of Science, Sapporo. [more]
Probing material properties at the micron scale requires dedicated machines and setups for sample manufacture, sample testing, and in-situ as well as post mortem defect analysis. Within the past years capabilities to produce and deform micron and submicron sized samples has been developed in the department for Structure and Nano-/ Micromechanics: [more]
Grain boundaries (GBs) affect many macroscopic properties of materials. In the case of metals grain growth, Hall–Petch hardening, diffusion, and electrical conductivity, for example, are influenced or caused by GBs. The goal of this project is to investigate the different GB phases (also called complexions) that can occur in tilt boundaries of fcc metals. We aim to investigate possible atomic structures and their thermodynamic description and connect them to mechanical properties.
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]
Biological materials in nature have a lot to teach us when in comes to creating tough bio-inspired designs. This project aims to explore the unknown impact mitigation mechanisms of the muskox head (ovibus moschatus) at several length scales and use this gained knowledge to develop a novel mesoscale (10 µm to 1000 µm) metamaterial that can mimic the high energy absorption characteristics of the muskox head without failure between strain rates of 0.001/s and 1000/s. [more]
Most materials are composed of microstructural constituents such as grains, phases and/or precipitates, and their resultant interfaces are critical for many material properties. [more]
The key to the design and construction of advanced materials with tailored mechanical properties is nano- and micro-scale plasticity. Significant influence also exists in shaping the mechanical behavior of materials on small length scales. [more]
Oxides find broad applications as catalysts or in electronic components, however are generally brittle materials where dislocations are difficult to activate in the covalent rigid lattice. Here, the link between plasticity and fracture is critical for wide-scale application of functional oxide materials. [more]
In this project, we aim to synthetize novel ZrCu thin film metallic glasses (TFMGs) with controlled composition and nanostructure, investigating the relationship with the mechanical behavior and focusing on the nanometre scale deformation mechanisms. Moreover, we aim to study the mechanical properties of films with complex architectures such as multilayers and amorphous-nanocrystalline composites. [more]
Grain boundaries (GB) are typically considered as 2-dimentional interfaces separating two differently oriented crystals inside a polycrystalline material. Understanding their structure and composition down to nano-scale regime is fundamental to explain their macroscopic properties. Discerning the contribution of GBs towards strength, corrosion resistance and high temperature properties is necessary to boost our efforts of making metals lighter, stronger and hence greener. Titanium (Ti) is one of the most attractive materials for aerospace and bio-medical industries where high strength to weight ratio and chemical inertness play a vital role. Ti also makes an interesting case owing to its allotropic transition from the hexagonal close packed (HCP) to the body-centred cubic (BCC) phase at 882 ºC. Despite of its widespread industrial use very little is known about its GB structure and their possible transitions. Hence, the aim of this project is to look deeply into the atomic structure of GBs in Ti and to resolve the impact of alloying additions on their structure through advanced transmission electron microscopy (TEM) techniques. A challenging, yet intriguing task is to obtain defined tilt GBs in Ti and we found that epitaxial thin films are excellent candidates for generating thin films containing pure tilt boundaries in them. [more]
Grain boundaries are one of the most prominent defects in engineering materials separating different crystallites, which determine their strength, corrosion resistance and failure. Typically, these interfaces are regarded as quasi two-dimensional defects and controlling their properties remains one of the most challenging tasks in materials engineering. However, although more than 50 years ago the concept that grain boundaries can undergo phase transformations was established by thermodynamic concepts, they have not been considered, since they could not be observed. Through a combination of atomic resolution scanning transmission electron microscopy (STEM) and advanced atomistic modelling we establish pathways to directly observe and explore grain boundary transitions in metallic alloys. [more]
The segregation of impurity elements to grain boundaries largely affects interfacial properties and is a key parameter in understanding grain boundary (GB) embrittlement. Furthermore, segregation mechanisms strongly depend on the underlying atomic structure of GBs and the type of alloying element. Here, we utilize aberration-corrected scanning transmission electron microscopy (STEM) in combination with atom probe tomography (APT) and first-principles density functional theory (DFT) calculations to explore the atomistic and thermodynamic origins of co-segregation of interstitial boron and carbon as well as substitutional aluminum in bcc-Fe. The impact on zinc segregation and its possible effect on liquid metal embrittlement are currently investigated by atomic scale microscopy. [more]
One of the still mysterious effects in high entropy alloys (HEAs) is how atoms in highly supersaturated solid solutions locally arrange in the given crystal lattice. Recent investigations indicate that chemical short range order (SRO) and local compositional fluctuations are characteristic for HEAs, which can significantly affect the mechanical properties. In this project, the characteristics of short range order and local compositional fluctuations in refractory high entropy alloys are revealed at atomic resolution and are correlated to micro- and macroscopic mechanical properties. [more]
Solid state dewetting (SSD), leading to break-up and agglomeration of the thin films, can occur in the films with thicknesses in the order of hundreds of nanometers below the melting temperature of the film. Understanding the underlying processes particularly matters where the stability of the film in applications such as micro-electronics is of concern. On the other hand, we might be able to produce nano/micro particle assemblies with specific size or composition through a well-controlled dewetting procedure and also manipulating the properties of the film such as its thickness and composition. In this project, we investigate the SSD behavior of CoxCu100-x alloy thin films. We specifically focus on the effect of the film composition on dewetting behavior. [more]
Statistical significance in materials science is a challenge that has been trying to overcome by miniaturization. However, this process is still limited to 4-5 tests per parameter variance, i.e. Size, orientation, grain size, composition, etc. as the process of fabricating pillars and testing has to be done one by one. With this project, we aim to fabricate arrays of well-defined and located particles that can be tested in an automated manner. With a statistically significant amount of samples tested per parameter variance, we expect to apply more complex statistical models and implement machine learning techniques to analyze this complex problem. [more]
The effect of Mo additions on the stability and crystal structure of the high-temperature phase Fe5Al8 (frequently called e phase) is investigated in a cooperation with the Los Alamos Neutron Science Center LANSCE. [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]
Global energy consumption to overcome friction is significant and minimization of this  consumption will allow monetary savings and a greener environment. [more]
In this project, we investigate a high angle grain boundary in elemental copper on the atomic scale which shows an alternating pattern of two different grain boundary phases. This work provides unprecedented views into the intrinsic mechanisms of GB phase transitions in simple elemental metals and opens entirely novel possibilities to kinetically engineer interfacial properties.
Extensive research has been focusing on face-centered cubic (FCC) high entropy alloys (HEAs) to establish the underlying mechanisms for their outstanding mechanical properties, for instance, an impressive combination of strength and ductility at cryogenic temperatures. One possibility suggested is that these new types of alloys show stronger Hall-Petch strengthening, where the grain size has a stronger impact on their yield strength. The origin of this grain boundary strengthening in HEAs seems to be originating from the different atomic radii of the supersaturated solid solution inducing high lattice strains. Hence, resolving the impact of compositional complexity on the atomic structure of grain boundaries in HEAs is crucial to understand their role in the strengthening mechanisms. [more]
The atomic arrangements in extended planar defects in different types of Laves phases is studied by high-resolution scanning transmission electron microscopy. To understand the role of such defect phases for hydrogen storage, their interaction with hydrogen will be investigated. [more]
To make electricity production more sustainable requires the development of novel high-temperature-stable materials capable of operating in harsh environments and not requiring large amounts of expensive and rare elements.  Conventional alloy development methodologies which specify one or two base elements and several alloying additions have been unable to introduce new alloys with the required combination of properties for these high temperature applications.  An alternative alloy development strategy searches the relatively unexplored central regions of multicomponent phase space for multi-principle-element alloys which can be optimised with a greater degree of control than possible using conventional alloying techniques. [more]
The goal of the study is to develop a quantitative description for microstructure evolution in pearlitic steel which consists of alternating layers of cementite and ferrite [more]
The aim of this project is to develop novel nanostructured Fe-Co-Ti-X (X = Si, Ge, Sn) compositionally complex alloys (CCAs) with adjustable magnetic properties by tailoring microstructure and phase constituents through compositional and process tuning. The key aspect of this work is to build a fundamental understanding of the correlation between microstructure and magnetic properties by length scale bridging characterization and property determination. The ultimate goal is to establish guidelines for designing alloys with high magnetization saturation (Ms) and low coercivity (Hc), to optimize the magnetic properties of CCAs for high frequency magnetic field applications. [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]
Adding 30 to 50 at.% aluminum to iron results in single-phase alloys with an ordered bcc-based crystal structure, so-called B2-ordered FeAl. Within the extended composition range of this intermetallic phase, the mechanical behavior varies in a very particular way. [more]
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