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Research Projects

In this project we study grain boundaries by using atomistic computer simulations. Using primarily, molecular dynamics simulations the energetics and mobility in the Cu and Al systems are examined in close collaboration with experimental works. In shear-coupled motion setups grain boundary phases of various grain boundary types are examined. [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]
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 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]
The goal of this project is to study the deformation mechanism, mechanical properties of silicon (Si) single crystal under nanotribological loading conditions by using ex situ scanning electron microscopy (SEM) and in situ transmission electron microscopy (TEM). The quantitative correlation between the mechanical properties linked with real time observations of deformation processes will provide a fundamental understanding of the tribological behavior of Si at the nanoscale. [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 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 effect on liquid metal embrittlement are currently investigated by atomic scale microscopy. [more]
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]
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]
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]
  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]
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]
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]
Here, we study strain und temperature induced phase transformation pathways in high entropy alloys (HEA) by aberration-corrected and in situ scanning transmission electron microscopy (S/TEM). The bidirectional phase transformation (face-centered cubic (FCC) → hexagonal close packed (HCP) → FCC) in a transformation-induced plasticity (TRIP)-assisted high-entropy alloy (HEA) is explored by a combination of atomic resolution imaging and in situ tensile straining. In a similar HEA, the temperature induced transformation from HCP to nanotwinned FCC and associated formation of nanocarbides at the nanotwin boundaries are investigated at atomic resolution by in situ heating. We aim to reveal the atomic scale origins of phase transformations to guide the design of advanced HEAs with a unique combination of strength, ductility and thermal stability. [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]
In this project, we develop two non-equiatomic FCC structued HEAs with different stacking fault energies (SFEs). [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]
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]
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]
This project aims at getting a deeper understanding of  dendrite-like precipitates formed by a solid state decomposition of the high-temperature phase. To investigate this phenomena systematic variation of cooling rates and annealing times and temperatures for different alloy compositions are carried out.

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]
Nano- and Micromechanical experiments are nowadays widely explored to investigate site specific mechanical properties of materials and material systems which were not previously accessible in bulk dimensions [1]. Currently, the testing protocols for materials at non-ambient conditions, like high temperature or chemical non-inert atmospheres, are developed worldwide for micro/nanoscale testing (e.g. [2-4]). [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]
This project aims for a detailed description of the micromechanical behavior of bainite in order to expand our understanding of the deformation processes within the complex bainitic microstructure. [more]
The thorough, mechanism-based, quantitative understanding of dislocation-grain boundary interactions is a central aim of the Nano- and Micromechanics group of the MPIE. For this purpose we isolate a defined grain boundary in a micron-sized sample. Subsequently, we measure and compare the mechanical properties with respect to single crystalline samples. [1-8] [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]
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]
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]
Because of their excellent corrosion resistance, high wear resistance and comparable low density, Fe–Al-based alloys are an interesting alternative for replacing stainless steels and possibly even Ni-base superalloys. Recent progress in increasing strength at high temperatures has evoked interest by industries to evaluate possibilities to employ Fe–Al-based alloys for various applications. These activities have matured to a point that industrial processing of parts is now investigated in more detail by considering economic aspects. [more]
The local accumulation of fatigue damage is not understood for micron sized materials possessing grain and phase boundaries. This is primarily due to the lack of a characterization technique measuring the decisive material parameters (e.g. local strains, dislocation densities, grain boundary character, etc.) non-destructively with high spatial resolution (<1μm). [more]
A novel design with independent tip and sample heating is developed to characterize materials at high temperatures. This design is realized by modifying a displacement controlled room temperature micro straining rig with addition of two miniature hot stages. [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]
The dislocation – grain boundary interactions are shown to depend strongly on the type of grain boundaries, for example in micropillar compression tests on bicrystalline copper [1]. The coherent Σ3/{111} twin is shown to be a weak obstacle for dislocation motion where perfect slip transfer can take place across the grain boundary [2]. However, a large number of CTBs in nanotwinned metals lead to increase in yield strength [3]. Within this project we aim for extending the work on micropillar compression of bicrystals with a single CTB to those with multiple CTBs to investigate the critical role of microstructure constraints on slip transfer. [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]
Within the EU project „ADVANCE - Sophisticated experiments and optimisation to advance an existing CALPHAD database for next generation TiAl alloys” MPIE is collaborating with Thermocalc-Software AB, Stockholm, Montanuniversität Leoben and Helmholtz-Zentrum Geesthacht. At MPIE the focus lies on the production and heat treatments of model alloys. By analysing them through metallography, X-ray diffraction, electron probe microanalysis and differential thermal analysis, the necessary data are obtained. Colleagues in Leoben perform atom probe tomography and transmission electron microscopy and in Geesthacht in situ synchrotron X-ray diffraction is carried out. All obtained data are optimised at the company Thermocalc and checked for consistency before they are implemented into the database. [more]
In this project, we aim to synthetize novel ZrCu thin film metallic glasses (TFMGs) with controlled thickness, composition and morphology, while investigating the relationship with the main mechanical properties and focusing on the nanometer scale deformation mechanisms. Moreover, we aim to investigate the thermal stability and the evolution of the atomic order performing dedicate annealing treatments.   [more]
This projects aims to correlate the electrical properties of ceramic materials and defects within their microstructure. A novel approach will be developed to this purpose coupling together in-situ dielectric spectroscopy with in-situ micro-  nano-mechanical testing enabling the formation of defect activated by plastic deformation. The correlation between defects and electrical properties will provide information about the local deformation phenomena, while enabling to predict failure of materials. [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 and diffusion. Recently it has been reported  of a phase transitions inside GBs opening the way to a new research field. This project aims to interconnect the electrical properties to the existing knowledge on GBs. [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 mechanical properties of bulk CrFeCoNi compositionally complex alloys (CCA) or high entropy alloys (HEA) are widely studied in literature [1]. Notably, these alloys show mechanical properties similar to the well studied quinary CrMnFeCoNi [2] . Nevertheless, little is known about the deformation mechanisms and the thermal behavior of these alloys in thin film form. The current project aims to investigate these properties within the framework of a joint  DFG/ANR project involving the collaboration of Prof. Alfred Ludwig (Ruhr-Universität Bochum, Germany), Dr. Dominique Chatain (CINaM, Marseille, France) and Dr. Natalie Bozzolo (CEMEF, Sophia Antipolis, France). [more]
Global energy consumption to overcome friction is significant and minimization of this  consumption will allow monetary savings and a greener environment. [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]
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]
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]
Copper is widely used in 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]
The TRR 188 is aiming for a thorough understanding and quantitative control of damage in advanced materials. As a subpart of TRR188, this project aims at microscopically studying the initiation of damage on dual phase steel DP800. [more]
The local accumulation of fatigue damage is not understood for materials possessing grain and phase boundaries. This is primarily due to the lack of a characterization technique measuring the decisive material parameters (e.g. local strains, dislocation densities, grain boundary character, etc.) nondestructively with high spatial resolution (<1μm). [more]
The elasto-plastic fracture mechanics is well established at the micron scale. However, can test protocols be easily downscaled to the micrometer length scale? [more]
The fracture toughness of AuXSnY intermetallic compounds is measured as it is crucial for the reliability of electronic chips in industrial applications. [more]
A novel design with independent tip and sample heating is developed to characterize materials at high temperatures. This design is realized by modifying a displacement controlled room temperature micro straining rig with addition of two miniature hot stages. [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]
Current engineering materials are designed to exhibit superior mechanical properties by carefully balancing their chemical composition and microstructure. However, once the material is produced, the material properties and behavior tend to remain same under the certain boundary conditions. [more]
While several methods are well-suited for studying dislocation transmission through grain boundaries, a quantitative approach understanding dislocation source activation in grain boundaries is currently lacking. [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]
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]
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 three years capabilities to produce and deform micron and submicron sized samples had been built up in the department Structure and Nano- Micromechanics: [more]
Focus: Microcantilever fracture tests were carried out on various metallic glass thin films systems to evaluate their fracture strength and fracture toughness as a function of Poisson’s ratio. [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]
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]
Nanotribology mechanisms, i.e. friction and wear, gain greater importance as the size of technological devices shrinks to the micro- and nanoscale. This project focuses on tribological experiments at the micro- and nanoscale of iron alloy microstructures. [more]
Wear and abrasion occur during sliding friction of metallic body and counter-body. Surface roughness is purposefully introduced into the metal to reduce wear and abrasion and to increase the lubricant flow. [more]
Peritectoid transformations are a comparatively rare type of invariant reaction where in the solid state of a material, a phase A decomposes on heating into a mixture of two other phases B and C [more]
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