Florian Schäfer

Universität des Saarlandes | UKS · Department of Materials Science and Materials Engineering

Micro-bending beams, machined by focused ion beam technique in the f.c.c. nickel based superalloy CMSX-4, are used to study the effect of the stress gradient during fatigue bending tests. Avoiding a dislocation interaction, a transcrystalline stage-I fatigue crack has been initiated at a notch placed on a slip plane in a single-slip geometry. da/dN...

Stage-I fatigue cracks are commonly described by the model of Bilby, Cottrell and Swinden (BCS model). However, since several experimental investigations have shown a dislocation-free zone (DFZ) in front of crack-tips, it is necessary to validate the new DFZ model and to examine the deviations to the BCS model. Therefore, the dislocation density di...

What determines if a grain boundary acts as a crack initiation site during fatigue? The grain boundary defines a mismatch of slip systems, which strongly depends on the three-dimensional positions of the boundary plane. In the case of a coherent Σ3 twin boundary, the boundary plane, and thus the mismatch between the slip systems, is known for all p...

Crack tip strain field measurements contribute eminently to the evaluation of analytical crack models and simulations to predict crack growth. Digital Image Correlation (DIC) in combination with scanning electron microscopy using carefully adjusted acquisition parameters is a useful tool for the investigation of local strain fields. The usual assum...

The effect of hydrogen on the fatigue behaviour of materials has been studied extensively during the last hundred years, but is just poorly understood due to the complex interplay between hydrogen and deformation processes. In this context, hydrogen damage of metals is becoming one of the major challenges of decarbonisation. While most work focuses...

In composites, outstanding properties of two materials can be combined. In particular, Metal‐Matrix Composites (MMCs) can combine the properties of a high‐strength ductile metallic matrix with special properties of embedded ceramic particles. This hybrid can be used to create a functional material. However, during consolidation, the thermal load of...

In the present work, the sub-surface microstructure of a heat treated and worn 26 wt% Cr white cast iron was investigated to gain better insight into the tribological behaviour of the material. The samples were destabilized at 980 °C for 0 (Q_0), 30 (Q_30) and 90 (Q_90) minutes followed by air cooling, and later subjected to dry-sliding linear reci...

It has long been known that plastic deformation and fatigue damage lead to a heat generation im metals. In the past two decades, thermographic approaches have been improved by thermography and widely adapted to obtain an estimate for the fatigue strength from the change in temperature or the amount of heat generation. In this study, based on a 40-y...

The plasticity of metals leads to an internal heat generation during fatigue testing. Therefore, qualitative and quantitative thermography are widely accepted for assessing fatigue damage. The onset of significant heat generation and temperature rise is usually attributed to the fatigue limit of a material. Now, a 40-year-old but just rarely used m...

Nickel(Ni)/aluminium(Al) hybrid foams are Al base foams coated with Ni by electrodeposition. Hybrid foams offer an enhanced energy absorption capacity. To ensure a good adhering Ni coating, necessary for a shear resistant interface, the influence of a chemical pre-treatment of the base foam was investigated by a combination of an interface morpholo...

In the present work, interactions of nanoindentation-induced dislocations (NIDs) with a low-angle grain boundary (LAGB) are investigated in a single-crystalline CrCoNi medium-entropy alloy (MEA). Microstructural evolutions before and after nanoindentation were examined using accurate electron channeling contrast imaging. In the as-grown state, the...

The FeSe compound is the simplest high-temperature superconductor (HTSc) possible, and relatively cheap, not containing any rare-earth material. Although the transition temperature, Tc, is just below 10 K, the upper critical fields are comparable with other HTSc. Preparing FeSe using solid-state sintering yields samples exhibiting strong ferromagne...

The process of short fatigue crack growth plays a significant role for the lifetime of materials in the high and very high cycle regimes. Fatigue crack growth is strongly influenced by interactions with microstructural obstacles, such as grain boundaries or phase boundaries, requiring a better understanding of these interactions to enhance the life...

The development of the sintering process for the synthetization of superconducting Iron-selenide (FeSe) polycrystal enables a quick and cheap fabrication of superconducting bulks for trapped field applications without the use of Rare Earths or toxic elements. A relevant number of papers were published in the literature in order to determine the bes...

To understand the interaction of dislocations with microstructural obstacles it is necessary to find test methods which are able to resolve the interaction of only a single or few defects with obstacles like interfaces. Therefore, the investigation of micro specimens has been established over the years as a suitable method to test the influence of...

Ni/Al hybrid foams are a new class of innovative cellular composite materials consisting of open-cell aluminium (Al) foams electrochemically coated with nanocrystalline nickel (Ni). They may be used for lightweight construction elements or as crash energy absorbers. The Ni coating strengthens the Al foam achieving an up to ten times higher energy a...

The dominant retardation mechanism of fatigue cracks after an overload is still under discussion for modeling variable amplitude loading. Is it plasticity induced crack closure in the wake of the crack or the residual stress field in the K-dominated region in front of the crack? Over the thickness, fracture mechanics changes from plane stress to pl...

Conducting fracture mechanics at small scales and extrapolating the behavior to larger length scales is a powerful tool to analyze materials and components with limited volumes of investigation. A critical requirement for this technique is the validity of the test methods at these scales on one hand, but also that the measured fracture mechanical q...

Metal foams are cellular materials, offering a strong structure-property relationship. Their global properties are strongly connected to the local micromechanical properties of the struts and to the microstructure of the foams. The mechanical properties of individual struts differ widely from the bulk matrix material as a result of differences in s...

Crack initiation at grain boundaries due to blocked slip transfer of dislocations is a main failure mechanism during the fatigue of metals. A quantification of the resistance effect of a grain boundary is needed to assess a textured or texture-free microstructure for fatigue strength. Geometric approaches based on the misalignment of slip systems i...

Stage-I-fatigue-cracks are used as highly localized dislocation sources with well-known Burger’s vectors to study the interaction between dislocations and grain boundaries. This interaction in the plastic zone is of particular interest to understand the fluctuating crack growth in the very short crack regime. In the case of a blocked slip band the...

Micro-bending beams of the f.c.c. nickel based superalloy CMSX-4, machined by the focused ion beam technique (FIB), are used to study the effect of a stress gradient associated with the bending configuration on the fatigue crack growth behavior in small dimensions. To avoid an impact of dislocation reactions on the crack growth behavior, a trans-cr...

In recent studies, many groups have investigated the interaction of dislocations and grain boundaries by bi-crystals and micro-specimen experiments. Partially, these experiments were combined with supplementary simulations by discrete dislocation dynamics, but quantitative data for the grain boundary resistance against slip transfer is still missin...

Micro-bending beams, machined by focused ion beam technique in the f.c.c. nickel based superalloy CMSX-4, are used to study size effects like stress gradients during fatigue bending tests or strain gradient plasticity effects during micro-tension. One focus is the influence of the typically used notching of micro-specimens instead of having a stand...

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Stage I fatigue cracks are commonly described by the model of Bilby, Cottrell and Swinden (BCS-model). However, since several experimental investigations have shown a dislocation-free zone (DFZ) in front of crack tips, it is necessary to validate the new DFZ-model and to examine the deviations to the BCS-model. Therefore the dislocation density dis...

Stage-I-fatigue- cracks are used as highly localized dislocation sources with well-known Burger’s vectors to study the interaction between dislocations and grain boundaries. The interaction of a plastic zone in front of a crack tip with a grain boundary is of particular interest to understand crack growth in the very short crack regime. The chosen...

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Das Wachstumsverhalten kurzer Ermüdungsrisse bestimmt maßgeblich die Lebensdauer von Bauteilen im HCF- und VHCF-Regime. Kurze Ermüdungsrisse wachsen zunächst im Stadium-I durch die Emission von Versetzungen auf einem Gleitsystem. Die Wechselwirkung der Versetzungen in der sich ausbildenden plastischen Zone mit der lokalen Mikrostruktur ist wachstum...

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