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Structure-properties relations in single phase fcc and bcc high entropy alloys under a tribological load

Subject Area Mechanical Properties of Metallic Materials and their Microstructural Origins
Term from 2019 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 409484397
 
Final Report Year 2024

Final Report Abstract

Tribological contacts describe two bodies in relative motion to each other. Such contacts occur basically everywhere in our everyday life, between shoes and floors, in joints and also in technical application e.g. in all kind of machinery. 23 % of the total energy is used to either overcome unwanted friction or to replace worn parts. This number has to be reduced. In this work, the aim is to understand fundamentally the microstructural evolution in the sub-surface area of metals and alloys under tribological loading as microstructural evolution can influence the friction coefficient as well as the wear particle formation. The end-goal is to be able to adjust these processes, in order to on the long run allow for tailored tribological behaviour. Shear stress-controlled deformation mechanisms determine the microstructural evolution. Therefore, it has to be known which crystallographic systems are activated. In material science, the activated crystallographic system is determined by calculating the resolved shear stress on each system and the one(s) with the highest resolved shear stresses are activated. The same should be done for tribological loading, but a reliable stress field model is missing to calculate the resolved shear stresses. The work performed through this grant added probes to the deformation layer to test various stress field models. As probes deformation twins has been used as these have two critical differences from dislocation motion: 1) twins are 3D extended defects with a distinct strain release; 2) twin formation is dependent on the crystallographic orientation, therefore exhibiting a pronounced tension-compression anisotropy. After some pre-tests on polycrystalline CoCrFeMnNi, experiments on single crystalline CoCrFeMnNi with careful chosen initial crystal orientation have been conducted. Different counter bodies have been used to change the friction coefficient. By doing so, data sets were created which are excellent to test various stress field models. The stress field models can be grouped into 1) single stress components, 2) analytical linear elastic model (Hamilton) 3) FE-simulations with varying material models. For the experiments resulting in high friction coefficients, the normal stress in sliding direction as well as the shear stress in sliding direction was identified to be the determining stress components for twin activation. The Hamilton stress field predicted twin formation on the experimentally identified twin systems with considering crystal rotation. For a low friction coefficient, the FE-simulations considering plasticity are required. Even as these model predictions are close to the experimental results, they do not fully agree. The results obtained through this grant are the very first to systematically investigate the stress field models. Even more importantly, the first experimental probe was developed that allows to systematically and unambiguous put stress field models for tribological loading to the test. This new capability will prove invaluable for the future development of materials tribology.

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