Research Projects

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Research Projects

<div style="text-align: justify;">The objective of this project is to understand the effect of strain rate sensitivity of non-equimolar high entropy alloy by nano-indentation. We want to study the effect of dislocation density on the strain rate sensitivity of non-equimolar high entropy alloy.</div>

Understanding the effect of strain rate sensitivity of non-equimolar high entropy alloy by nano-indentation

The objective of this project is to understand the effect of strain rate sensitivity of non-equimolar high entropy alloy by nano-indentation. We want to study the effect of dislocation density on the strain rate sensitivity of non-equimolar high entropy alloy.
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<div style="text-align: justify;">Global energy consumption to overcome friction is significant and minimization of this  consumption will allow monetary savings and a greener environment.</div>

Texture evolution during nanotribology in FCC metals

Global energy consumption to overcome friction is significant and minimization of this  consumption will allow monetary savings and a greener environment.
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<p style="text-align: justify;">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.</p>

Fracture initiation in FCC and BCC metals during tribology

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.

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<p style="text-align: justify;">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.</p>

Direct observation of dislocation plasticity in HEA by in-situ TEM

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.

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<div style="text-align: justify;">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.</div>

Development of Precipitation Strengthened CCAs in the AlCrFeNiTi system for High Temperature Structural Applications

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.
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<div style="text-align: justify;">The project in the scope of research activities of the Advanced Transmission Electron Microscopy group has two main objectives: (i) epitaxial thin film deposition and (ii) <em>in-situ</em> TEM tensile experiments.</div>

Epitaxial thin film growth for in-situ mechanical tensile experiments using advanced transmission electron microscopy

The project in the scope of research activities of the Advanced Transmission Electron Microscopy group has two main objectives: (i) epitaxial thin film deposition and (ii) in-situ TEM tensile experiments.
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<p style="text-align: justify;">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.</p>

Grain boundary segregation in bcc-metals: a case study in ferritic Iron

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.

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<div style="text-align: justify;">Fusion is one of the most promising safe, emissionless and limitless sources of energy. The extreme conditions in a fusion reactor, require the development of novel materials to withstand high temperature ion irradiation and at the same time provide sufficient mechanical stability.</div>

He Implantation

Fusion is one of the most promising safe, emissionless and limitless sources of energy. The extreme conditions in a fusion reactor, require the development of novel materials to withstand high temperature ion irradiation and at the same time provide sufficient mechanical stability.
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<div style="text-align: justify;">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.</div>

Hydrogen-microstructure interactions in Ferritic alloys at small scale

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.
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<div style="text-align: justify;">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.</div>

Combinatorial approach to tailor electrical and mechanical properties of alloyed thin film structures

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.
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<div style="text-align: justify;">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.</div>

TRR188_B03­-Damage initiation of DP800 at the micro scale

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.
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<div style="text-align: justify;">The elasto-plastic fracture mechanics is well established at the micron scale. However, can test protocols be easily downscaled to the micrometer length scale?</div>

Developing strategies for assessing elasto-plastic fracture mechanics

The elasto-plastic fracture mechanics is well established at the micron scale. However, can test protocols be easily downscaled to the micrometer length scale?
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<p style="text-align: justify;">The segregation of impurities to grain boundaries (GBs) has a significant influence on the cohesive properties, atomic arrangements and properties of such interfaces. The segregation strongly depends on the structural units of the GB as well as on the impurity atom itself. Aberration–corrected (S)TEM techniques in combination with atomistic simulations are applied to unravel the connection of grain boundary structure and chemistry at atomic resolution.</p>

Grain Boundary Segregation Phenomena

The segregation of impurities to grain boundaries (GBs) has a significant influence on the cohesive properties, atomic arrangements and properties of such interfaces. The segregation strongly depends on the structural units of the GB as well as on the impurity atom itself. Aberration–corrected (S)TEM techniques in combination with atomistic simulations are applied to unravel the connection of grain boundary structure and chemistry at atomic resolution.

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<p style="text-align: justify;">The fracture toughness of Au<sub>X</sub>Sn<sub>Y</sub> intermetallic compounds is measured as it is crucial for the reliability of electronic chips in industrial applications.</p>

Micro-fracture toughness of AuXSnY intermetallic compounds

The fracture toughness of AuXSnY intermetallic compounds is measured as it is crucial for the reliability of electronic chips in industrial applications.

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<div style="text-align: justify;">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.</div>

Setup of a microscale high temperature loading rig

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.
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<div style="text-align: justify;">Most materials are composed of microstructural constituents such as grains, phases and/or precipitates, and their resultant interfaces are critical for many material properties. </div>

Multi-scale modeling and simulation of interface-related mechanical properties

Most materials are composed of microstructural constituents such as grains, phases and/or precipitates, and their resultant interfaces are critical for many material properties.
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<div style="text-align: justify;">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.</div>

Can we design structural materials with switchable properties ?

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.
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<p style="text-align: justify;">Carbon(C)-containing martensitic steels are ideal candidates for high-strength applications, e.g. in automotive and aerospace applications, due to their excellent mechanical properties and low cost. Carbon can even redistribute at room temperature leading to the formation of nanoscale carbides that can significantly influence the mechanical properties.</p>

Carbon redistribution in C-supersaturated martensitic phase during room temperature aging

Carbon(C)-containing martensitic steels are ideal candidates for high-strength applications, e.g. in automotive and aerospace applications, due to their excellent mechanical properties and low cost. Carbon can even redistribute at room temperature leading to the formation of nanoscale carbides that can significantly influence the mechanical properties.

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<p style="text-align: justify;">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. </p>

Dislocation source activation at grain boundaries

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.

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<div style="text-align: justify;">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.</div>

Hydrogen embtittlement of steels: environment-assisted in-situ micro-mechanical tests

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.
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<p style="text-align: justify;">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.</p>

Influences of Ag segregation on grain boundary structures and dislocation-boundary interactions

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.

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<div style="text-align: justify;">The focus lies on the analysis of the mechanical behavior and their underlying deformation mechanisms in new ductile solid solution Mg alloys by performing micromechanical experiments with electron microscopy analyses.</div>

Resolving the deformation mechanisms of intrinsically ductile Mg alloys

The focus lies on the analysis of the mechanical behavior and their underlying deformation mechanisms in new ductile solid solution Mg alloys by performing micromechanical experiments with electron microscopy analyses.
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<div style="text-align: justify;">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 (&lt;1μm).</div>

The first stages of fatigue

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).
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<div style="text-align: justify;">The dislocation – grain boundary interactions are shown to depend strongly on the type of grain boundaries, as for instance shown in micropillar compression tests on bicrystalline copper [1,2]. 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. Within this project we aim for extending the aforementioned work and investigate the critical role of microstructure constraints.</div>

The role of microstructure constraints during slip transfer

The dislocation – grain boundary interactions are shown to depend strongly on the type of grain boundaries, as for instance shown in micropillar compression tests on bicrystalline copper [1,2]. 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. Within this project we aim for extending the aforementioned work and investigate the critical role of microstructure constraints.
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<div style="text-align: justify;">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:</div>

Micromechanics Facilities

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:
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<div style="text-align: justify;">By combining advanced characterization and mechanical testing of microsized, single-phase intermetallic samples through in situ micromechanical experiments inside an SEM or TEM, the mechanical response can be measured while simultaneously observing the microstructural changes. From these experiments, it is expected to get a much deeper insight in the complicated deformation mechanism of intermetallic phases, which is very much different from that in pure metals.</div>

In situ micromechanical studies of intermetallic phases

By combining advanced characterization and mechanical testing of microsized, single-phase intermetallic samples through in situ micromechanical experiments inside an SEM or TEM, the mechanical response can be measured while simultaneously observing the microstructural changes. From these experiments, it is expected to get a much deeper insight in the complicated deformation mechanism of intermetallic phases, which is very much different from that in pure metals.
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<div style="text-align: justify;">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.</div>

Fracture behavior of metallic glass thin films

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.
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<div style="text-align: justify;">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.</div>

Crystallization kinetics and microstructural evolution of metallic glasses

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.
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<div style="text-align: justify;">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.</div>

Corrosion and wear at the nanoscale

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.
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<div style="text-align: justify;">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.</div>

Nanotribology of iron-alloy microstructures

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.
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<div style="text-align: justify;">Even though most structural materials are polycrystalline, the dislocation grain boundary interaction is not thoroughly understood.</div>

Dislocation slip transfer in micron sized bi-crystals

Even though most structural materials are polycrystalline, the dislocation grain boundary interaction is not thoroughly understood.
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<div style="text-align: justify;">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.</div>

Size-dependent friction coefficient and surface evolution

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.
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<div style="text-align: justify;">Peritectoid transformations are a comparatively rare type of invariant reaction where in the solid state of a material, a phase <em>A</em> decomposes on heating into a mixture of two other phases <em>B</em> and <em>C</em></div>

Kinetics and microstructure evolution during peritectoid transformations

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
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