Microstructure and Properties

The interplay of microstructure and properties is at the core of materials science and engineering and is key to design optimized – often multifunctional - materials. Fracture toughness, strength, ductility, thermal conductivity, thermal stability, corrosion resistance, electrical conductivity, magnetic coercivity, and magnetic hysteresis are prominent examples of material properties, which we tailor by the extrinsic and intrinsic “architecture” of materials. In contrast to ideal single crystals, advanced materials typically contain a complex microstructure. Examples of microstructure elements are stable or metastable phases (their alignment can be manipulated by synthesis and subsequent thermo-mechanical treatments), texture, stacking faults, interfaces (with and without enrichment of alloying additions), dislocations, and point defects; in addition, these “imperfections” contain themselves defects of lower dimensionality and can undergo phase transformations.

Our research deals with resolving the interplay of microstructure components and material properties and to establish quantitative relationships based on length-scale bridging experiments and simulations:

- Tuning stacking fault energy and/or electronic structure of materials to enhance strength and also toughness (steels, HEA/CCA alloys, metallic glasses)

- Phase transformations of grain boundaries and dislocations and their impact on transport properties (pure metals, alloys, intermetallic materials, phase diagrams and defect phase diagrams)

- Microstructure design for functional materials (thermoelectrics, photovoltaics, magnetic materials, …)

- Traps for hydrogen to prevent embrittlement and enable materials for hydrogen economy (steels, alloys, barrier coatings, hydrides)

- Experimental and computational tools to resolve microstructure details and properties with high spatial resolution

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
In this project we work on correlative atomic structural and compositional investigations on Co and CoNi-based superalloys as a part of SFB/Transregio 103 project “Superalloy Single Crystals”. The task is to image the boron segregation at grain boundaries in the Co-9Al-9W-0.005B alloy. 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
Despite the immanent advantages of metals and alloys processed by additive manufacturing (e.g. design freedom for complex geometry) and unexpected merits (e.g. superior mechanical performance) of AM processes, there are several remaining issues that need to be addressed in order to practically apply AM alloys to various industries. One of the most important issues is the mechanical behavior of AM alloys under hydrogen environments, since it is easily encountered in the industrial fields and has generally detrimental effects on metals and alloys. more
In AM, parts are built from layer by layer fusion of raw material (eg. wire, powder etc.). Such layer by layer application of heat results in a time-temperature profile which is fundamentally different from any of the contemporary heat treatments.  Previous work in the group has established that this unique thermal profile can be exploited for microstructural modifications (eg. clustering, precipitation) during manufacturing. The aim of this work is to develop a fundamental understanding of such a strongly non-linear, peak-like thermal history on the precipitation kinetics. more
Within this project we investigate chemical fluctuations at the nanometre scale in polycrystalline Cu(In,Ga)Se2 and CuInS2 thin-flims used as absorber material in solar cells. 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
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 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
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
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 Fe49.5Mn30Co10Cr10C0.5 (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 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, 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
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
TiAl-based alloys currently mature into application. Sufficient strength at high temperatures and ductility at ambient temperatures are crucial issues for these novel light-weight materials. By generation of two-phase lamellar TiAl + Ti3Al microstructures, these issues can be successfully solved. Because oxidation resistance at high temperatures is still a problem which could be improved by increasing the Al content, Al-rich TiAl alloys have recently come into focus. 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
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
The need to make energy generation and conversion more sustainable and to reduce the emission of harmful gases requires the development of novel high temperature stable materials.  An alternative alloy development strategy searches the central regions of multicomponent phase space for multi-principle-element alloys that have been previously unexplored. Several of the resulting compositionally complex alloys (CCAs) have been shown to possess novel property combinations and, in some cases, exceptional mechanical properties. 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
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
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
Deviations from the ideal, stoichiometric composition of tcp (tetrahedrally close-packed) intermetallic phases as, e.g., Laves phases can be partially compensated by point defects like antisite atoms or vacancies, but also planar defects may offer an opportunity to accommodate excess atoms. more
The Ni- and Co-based γ/γ’ superalloys are famous for their excellent high-temperature mechanical properties that result from their fine-scaled coherent microstructure of L12-ordered precipitates (γ’ phase) in an fcc solid solution matrix (γ phase). The only binary Co-based system showing this special type of microstructure is the Co-Ti system, where the Co solid solution is the γ phase and TiCo3 the L12-ordered γ’ phase. 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
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
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 fracture toughness of AuXSnY intermetallic compounds is measured as it is crucial for the reliability of electronic chips in industrial applications. 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
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
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
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
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
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
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