High-entropy alloys are usually defined as single-phase alloys composed by at least five elements in the equiatomic composition stabilised by the configurational entropy [
1,
2]. However, also multiphase, non-equiatomic alloys consisting of only four elements are often referred to as HEA in the literature when some partial conditions are fulfilled [
1,
2,
3,
4]. A large variety of HEAs exists, but single-phase equiatomic five-element HEA Co-Cr-Fe-Mn-Ni, the so-called Cantor alloy, is one of the most studied [
5]. The single-phase solid solution high-entropy alloys have a potential for solid solution hardening and if the single-phase fcc lattice structure is reached, a large number of slip systems ensures good ductility of the material [
6,
7,
8,
9]. Nevertheless, the experimental data about mechanical behaviour of this alloy type are still not fully understood and in some cases are controversial. The preferential processing method of this type of HEA is some variant of casting usually followed by heat treatment [
5,
8,
9,
10,
11]. Also, powder metallurgy resulting in differences in the HEA microstructure as well as in the properties was used in [
7,
12,
13,
14]. The microstructure of the cast alloys is rather coarse with the grain sizes range from tens to hundreds of micrometres. On the contrary, when the powder metallurgy approach for the alloy preparation was applied, the ultra-fine-grained structure was reached. The precipitation of secondary phases (usually Cr rich phase) during the alloy preparation and processing was also described by some authors [
1,
6].
The tensile or compressive properties of CoCrFeMnNi alloy in the wide range of temperatures from cryogenic up to 1200 °C were studied, showing changes in the active deformation and damage mechanisms. The room temperature ultimate strength usually ranged from approx. 600 MPa [
11,
15,
16,
17] up to approx. 2 GPa [
18,
19] with the corresponding ductility in tenths of a percent, respectively, depending mostly on the grain size and processing route. The major active deformation mechanism at room temperature is the dislocation slip, while deformation-induced nanoscale twinning (nano twinning) is active at cryogenic temperatures, leading to a significant strain hardening [
6,
11,
16,
20]. In some cases, the (nano) twinning is connected with significant ductility of the HEA alloy at low temperatures. The creep resistance and deformation mechanisms acting at various elevated temperatures were investigated recently [
21]. The addition of stabilizing dispersion or addition of elements forming secondary phases into the microstructure enhanced the creep properties of HEA significantly [
22,
23]. The enhancement can be ascribed to the inhibited HEA grain growth controlled by the pinning effect of the second phase particles dispersion either on the grain boundaries or on the dislocations and twins. Also, the predominant deformation mechanism is changed from dislocation glide to the diffusional creep when dispersion is present in the HEA microstructure [
21,
22]. The HEA response to the dynamic loading was investigated only in a few works [
15,
24,
25,
26]. High cycle fatigue data obtained on a coarse-grained HEA (average grain size of 245 μm) suggested that the fatigue endurance limit is just below the alloy tensile yield strength. Cr–Mn base oxide inclusions of the size up to 5 μm present in the microstructure were determined as the fatigue crack initiation sites and deformation twins formed during cycling and their boundaries act as the fatigue crack propagation paths in this coarse-grained alloy [
15]. The effect of the grain size on the fatigue behaviour of an ultra-fine-grained CoCrFeMnNi HEA with fully recrystallized microstructure, processed by cold rolling of cast ingots and annealing was investigated by other authors in [
24]. Their findings indicate that the grain refinement has a positive effect on both the applicable stress amplitude and the fatigue endurance limit even though in the ultra-fine-grained structure the cracks were present due to the material processing. The pre-existing primary cracks act as the fatigue cracks initiation sites. Detailed analysis of the dislocation structure of the ultra-fine-grained fatigue tested material showed differences in the dislocation structure evolution at two different stress amplitudes used for testing. While very limited loose dislocations were detected in the interior of some grains (most of the grains were dislocation free) for low-stress amplitudes, dislocation activity in most grains was characteristic for high-stress amplitude. However, no dislocation arrangements such as cell structure or deformation twins were observed within the grains. Authors also indicated that no grain coarsening was observed during the material fatigue testing. The absence of the dislocation cell structure was explained by the small grain size [
27]. The absence of deformation twins was attributed to the raised twinning stress with grain refinement.
The present study is aimed at the characterization and identification of crack initiation mechanisms during cyclic loading of two mechanically alloyed and compacted by spark plasma sintering (SPS) CoCrFeNiMn HEAs. The SPS treatment performed at slightly different conditions resulted in different microstructure in terms of grain size influencing materials three-point bending static and fatigue properties. Fractographic analysis of the fatigue fracture surfaces revealed the same mechanism of the fatigue crack initiation for both materials, which was suggested by the observations performed by TEM on FIB foils extracted from the fatigue crack initiation sites.