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

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The study of interacting surfaces in relative motion - called tribology - is of great importance in modern life, particularly for metallic materials which are ubiquitously used in engineering components. From a materials scientist’s point of view, tribology is an extremely interesting field as phenomena like the evolution of the subsurface microstructure are only understood phenomenologically. This is especially true for high entropy alloys (HEAs), as a novel class of metallic alloys, which hold the promise of outstanding tribological properties. There is no comprehensive knowledge yet about the changes in the subsurface microstructure while sliding and the elementary mechanisms responsible for the microstructure’s evolution. Among the mechanisms that might be at work are dislocation slip, twinning, the development of shear bands, oxidation and mechanical mixing. This so far incomplete picture results in the lack of a structure-properties relation for HEAs in frictional contacts. Combining the complementary expertise of the Heilmaier group, focused on alloy development, and the Greiner group, with its track record in materials tribology, our research will tackle three central research questions: (A) Is there a defined correlation between the parameters of the initial microstructure of high entropy alloys and their friction and wear properties? (B) Are these elementary mechanisms correlated with fundamental deformation mechanisms obtained under uniaxial, monotonic load? (C) Can these properties and correlations be understood in light of characteristics unique to HEAs, like severe lattice distortion, sluggish diffusion or even the cocktail effect? Our approach to these questions is that of material scientists interested in the mechanics of the materials involved rather than that of a tribologist interested in an entire tribological system. Our investigation will begin with CoCrFeMnNi, one of the best characterized fcc high entropy alloys. Over the course of increasing sliding distances and by means of focused ion beam and scanning as well as transmission electron microscopy, we will reveal the elementary mechanisms governing microstructural changes as well as their sequential appearance. CoCrFeMnNi allows to correlate the elementary mechanisms identified during tribological loading to those found in the literature, e.g., for uniaxial tension or compression. The comparison with literature knowledge about the behavior of pure fcc metals and dilute solid solutions will provide insights to the effects which might be exclusive to HEAs. As the alloy’s crystal structure has a decisive impact on how the material behaves under a tribological load, we will start to investigate the bcc model HEA HfNbTaTiZr, during the last year of the requested funding period. It is anticipated that in the near future these results will allow to realize design guidelines for the composition and starting microstructure of HEAs with superior tribological properties.