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

Many important phenomena occurring in polycrystalline materials under large plastic strain, like microstructure, deformation localization and in-grain texture evolution can be predicted by high-resolution modeling of crystals. Unfortunately, the simulation mesh gets distorted during the deformation because of the heterogeneity of the plastic deformation in polycrystals. After reaching high local strain levels, it is no longer possible to continue the simulation, because the mesh distortion reduces the accuracy of the results. In this project we introduce two different adaptive remeshing approaches for simulating large deformation of 3D polycrystals with high resolution under periodic boundary conditions.
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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

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

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

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

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

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

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

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 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

In this project, we aim to synthetise novel ZrCu thin film metallic glasses (TFMGs) with controlled thickness, composition and microstructure, while investigating the relationship with the mechanical behaviour focusing on the nanometre scale deformation mechanisms. Moreover, we aim to study the mechanical properties of film with complex architectures such as multilayers and amorphous-nanocrystalline composites. 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 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

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

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

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

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