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Thermal stability and thermomechanical behavior of CrFeCoNi compositionally complex alloy thin films

The mechanical properties of bulk CrFeCoNi compositionally complex alloys (CCA) or high entropy alloys (HEA) are widely studied in literature [1]. Notably, these alloys show mechanical properties similar to the well studied quinary CrMnFeCoNi [2] . Nevertheless, little is known about the deformation mechanisms and the thermal behavior of these alloys in thin film form. The current project aims to investigate these properties within the framework of a joint  DFG/ANR project involving the collaboration of Prof. Alfred Ludwig (Ruhr-Universität Bochum, Germany), Dr. Dominique Chatain (CINaM, Marseille, France) and Dr. Natalie Bozzolo (CEMEF, Sophia Antipolis, France).

CrFeCoNi thin films are deposited by magnetron co-sputtering at RUB with a precise control of thickness and composition.  The thin films are investigated for the formation of secondary phases, microstructure evolution, thermal stability and thermomechanical properties. Our results show that films have a homogenous composition with predominantly fcc phase. However, the presence of a small fraction of Cr rich s phase has been detected which so far has been reported only for bulk counterparts after long annealing treatment or severe plastic deformation [3]. This has been attributed to a finer microstructural length scale (grain size <50 nm) of thin films. Further, we show that single phase fcc films could be formed by fine tuning the composition, while we report a large thermal stability up to ~800°C.

The films deposited on c-sapphire grew epitaxially with the orientations relationships similar to pure fcc metals. The hardness of the dense films ranges from 6.5 to 8 GPa in agreement with literature values for bulk nanocrystalline CCA. Future research directions aim to establish the mechanical behavior of single phase alloy thin films that remains scarcely studied compared to pure metal thin films.

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