Abstract text (incl. figure legends and references)

Microstructural characterization of the high entropy alloy CoCrFeMnNi (HEA) after low cycle fatigue (LCF) at a temperature of 550°C was carried out by transmission electron microscopy (TEM). For the determination of the actual composition, high angle annular dark field (HAADF) scanning TEM (STEM) with an energy-dispersive X-ray (EDX) spectroscopy was used. For the crystallographic investigations, conventional TEM with selected area diffraction (SAD) was applied. The TEM samples after LCF test were prepared by electrochemical or mechanical thinning parallel to the stain direction.

We observe in the samples after LCF many additional defects in comparison to the as received sample. Fig. 1 shows inverted HAADF STEM images in large scale of the samples after LCF deformation under lowest and highest strain amplitudes of ± 0.2% (Fig. 1a) and ± 0.75% (Fig. 1b), where we can see a lot of defects. First of all, in both deformed samples, we observe thin twins (TW) and slip bands (SB). Similar twins were detected in the unreformed sample. Therefore, the kind of the twins, annealing or deforming, is not clear from these images. The broad of slip bands increases with strain amplitude. Moreover, in both samples there are two kinds of dislocations inside the grains: discrete dislocations (D1) and pileup dislocations along lines (D2). The density of dislocations D1 and D2 increases also with strain amplitude. In the sample deformed under strain amplitude ± 0.75 % (Fig. 2b), pileup of dislocations forms cells which were detected only in the sample deformed under high strain amplitudes from ± 0.5% to ± 0.75%. The density of such cells increases remarkably with strain amplitude. These cell substructures are formed because of dislocations cross slip and contribute to the near-steady state of the cyclic stress response.

The quantitative TEM EDX analysis of many grains shows an almost homogeneous distribution of alloying elements in the deformed sample with a concentration of (20 ± 3) at % for each element. However, additional secondary phases with an irregular morphology are observed close to grain boundaries with their sizes varying up to a few mm. One such case is presented in Fig. 2, where a HAADF STEM micrograph with EDX line scan (a) combined with a bright-field TEM image with SAD (b) of the same area are shown. Variation in the contrast along the inclusion in both, the STEM (Fig. 2a) and the TEM (Fig. 2b) images clearly indicate changes in chemical composition. The EDX line scan along the dashed line as well as area analysis in Fig. 2a shows that lower part of the inclusion is Mn- and Ni-rich (area 1, both elements having an atomic concentration of ca. 50 %) and the long upper part is Cr-rich (area 2). SAD from the latter part of the inclusion in Fig. 2b, which correspond to area 2, indicates a intermetalic σ phase. This phase consists of significantly more Cr and also some Mn, Fe and Co, but very little Ni.

Fig. 1. Inverted HAAD STEM images of the samples after deformation under strain amplitudes of ± 0.2% (a) and ± 0.75% (b). D1, D2 and cells indicate two kinds of dislocations. TW denotes twins, SB - slip bands. The scale bar in (b) is valid for (a).

Fig. 2. (a) HAADF STEM image with EDX line scan along the dashed arrow and elemental compositional measurements and (b) bright-field TEM image with SAD of circled areas of the same region of the sample tested under strain amplitude of ±0.3%.