Influence of multiple-pass equal-channel angular pressing and heat treatment on the fatigue threshold of the medium-entropy alloy CrCoNi

• Δ K th increases with increasing grain size in CrCoNi following an exponential trend • This trend was not observed for the ECAP-processed and heat treated conditions • Partial recrystallization reduces Δ K th to the range of the ECAP-processed condition • The low Δ K th is due to the heterogeneous deformation behavior The medium-entropy alloy CrCoNi is well-known for its outstanding combination of fracture toughness and strength at room and cryogenic temperatures. However, if a metal or an alloy reaches its fracture toughness, cracks propagate unstably, leading to the fracture of the material. Therefore, it is crucial to tailor the microstructure at the beginning of the fatigue-crack propagation curve, where the fatigue threshold already covers 90–95% of the material’s lifetime and is most sensitive to microstructural features. In this study, the influence of the grain size as well as the influence of a partial recrystallization on the fatigue threshold are studied in detail. Therefore, the well-known severe plastic deformation process equal-channel angular pressing (ECAP) in combination with a subsequent heat treatment is performed to obtain different grain sizes (0.25 µm, 3 µm, 28 µm, 32 µm) and different degrees of recrystallization (0%, 40%, 70%, 99%). Fatigue-crack propagation tests are carried out under three-point bending conditions using a resonant testing machine and a load ratio of R = 0.1. The fatigue threshold increases with increasing grain size with an exponential trend. Surprisingly, the ECAP-processed and heat treated conditions do not follow this exponential trend and show deviations. In addition, a partial recrystallization reduces the fatigue threshold to the range of the ECAP-processed condition. After in-depth scanning electron microscopy analysis (SEM, EBSD), these low fatigue thresholds of the partially recrystallized conditions are attributed to the heterogeneous deformation behavior, the large internal multiaxial stresses, and the thus reduced slip reversibility at the crack tip.