arXiv subjects

Alexander Kauffmann

Publications and source records attributed to Alexander Kauffmann.

6 recordsLinked to original sources

arXiv2025

Revealing the tribological stress field by using deformation twins as probes

Microstructural evolution in metallic materials feedbacks with the loading conditions and influences the life time of parts and components. Therefore, the deformation mechanisms have to be fundamentally understood. Tribological loading causes a non-trivial, position-dependent, moving stress field. We present a systematic study on the influence of the complexity of the implemented material models on the calculated stress field. For the stress field validation, results of tribological experiments on single crystals with the activation of deformation twins are used. The resolved shear stresses calculated with the stress field models have to be highest on the experimentally identified twin systems. From this combination of modelling and experiment, it clearly follows that a stress field model considering plasticity is required. The widely used Hamilton stress field for tribological loading is limited due to only considering elastic strains. Here, the predictive quality of the stress field is sensitive to the assumed yield strength, work hardening and plastic anisotropy. Certain stress field models are close to the experimental data, but none completely replicate them. These results highlight that the model type and parameters have to be carefully determined in order to be able to predict how a metallic material deforms due to a sliding load.

cond-mat.mtrl-sci↗

arXiv2023

Deformation twins as a probe for tribologically induced stress states

Friction and wear of metals are critically influenced by the microstructures of the bodies constituting the tribological contact. Understanding the microstructural evolution taking place over the lifetime of a tribological system therefore is crucial for strategically designing tribological systems with tailored friction and wear properties. Here, we focus on single-crystalline CoCrFeMnNi that is prone to form twins at room temperature. Deformation twins feature a pronounced orientation dependence with a tension-compression anisotropy, a distinct strain release in an extended volume and robust onset stresses. This makes deformation twinning an ideal probe to experimentally investigate the complex stress fields occurring in a tribological contact. Our results clearly show a grain orientation dependence of twinning under tribological load. Unexpectedly, neither the crystal direction parallel to the sliding nor the normal direction are solely decisive for twinning. This experimental approach is ideal to experimentally validate tribological stress field models, as is demonstrates here.

cond-mat.mtrl-sci↗

arXiv2022

Temporal sequence of deformation twinning in CoCrNi under tribological load

Microstructural evolution under tribological load is known to change the friction and wear response of the entire tribological system. It is however not yet fully understood, how these changes take place on an elementary, mechanistic level. Revealing the temporal sequence of deformation mechanisms under a tribological load by experiments is scarce. The discrete strain release in the case of deformation twinning allows the identification of their temporal sequence. Therefore, a medium stacking fault energy material, namely CoCrNi, was investigated due to the significant contribution of deformation twinning to the overall deformation at room temperature. The investigated grain showed three activated twin systems. The evolution sequence of these is discussed critically based on several twin intersection mechanisms. The experimental analysis decodes the temporal sequence of the active twinning systems of highest resolved shear stresses for the first time. This approach gives experimental indications for the stress field under a tribological load.

cond-mat.mtrl-sci↗

arXiv2021

Dislocation-mediated and twinning-induced plasticity of CoCrFeMnNi in varying tribological loading scenarios

Coarse-grained, metallic materials undergo microstructure refinement during tribological loading. This in turn results in changing tribological properties, so understanding deformation under tribological load is mandatory when designing tribological systems. Single-trace experiments were conducted to understand the initiation of deformation mechanisms acting in various tribological systems. The main scope of this work was to investigate the influence of normal and friction forces as well as crystal orientations on the dominating deformation mechanism in a face-centred cubic concentrated solid solution. While varying the normal force is easily realised, varying friction forces were achieved by using several counter body materials paired against CoCrFeMnNi. The subsurface deformation layer was either mediated through dislocation slip or twinning, depending on the grain orientation and on the tribological system. A layer dominated by dislocation-based deformation is characterised by lattice rotation, the formation of a dislocation trace line or subgrain formation. Such behaviour is observed for tribological systems with a low friction coefficient. For systems dominated by deformation twinning, three types of twin appearance were observed: small twins interacting with the surface, large twins and grains with two active twin systems. Two different twinning mechanisms are discussed as responsible for these characteristics.

cond-mat.mtrl-sci↗

arXiv2013

Mesoscopic twin boundaries in epitaxial Ni-Mn-Ga films

Twin boundaries play an essential role in the use of magnetic shape memory alloy Ni-Mn-Ga as active material. Only if twin boundaries can be moved by an external magnetic field, high strain values of up to 10 % can be obtained. Therefore, understanding the observed twin microstructure of thin films is crucial for future application. We exemplarily present two different microstructural pattern, type X and type Y microstructure, using the example of two Ni-Mn-Ga films with similar film thickness and composition. The analysis of microstructure and structure of the thin films shows that both, type X and type Y pattern, are formed by mesoscopic 14M twin boundaries. The mesoscopic 14M twin boundaries are either tilted from the substrate surface in case of type X pattern or they are perpendicular to the surface for type Y pattern. Based on a recently proposed scenario for the nucleation of martensite in austenite, we can trace back the difference in twin boundary orientation to differently oriented martensitic nuclei.

cond-mat.mtrl-sci↗

arXiv2010

Jc Scaling and Anisotropies in Co-doped Ba-122 thin films

We have successfully grown epitaxial, superconducting films of Ba(Fe1-xCox)2As2 (Ba-122)with x ~ 0.1. The films grow without observable correlated defects parallel to the c-axis, as confirmed by TEM. This is also reflected in the absence of a caxis peak in Jc(θ). In contrast to cuprate high-Tc superconductors such as YBa2Cu3O7-d or even Bi2Sr2Ca2Cu3O10-d, the pnictides show a rather low anisotropic behavior in their Jc(θ) behavior as well as in their upper critical fields, Hc2. As a multiband superconductor, Ba-122 exhibits a temperature dependent electronic mass anisotropy.

cond-mat.supr-con↗