Activation Area Measurements During Low Cycle Fatigue Deformation of the Equiatomic CrMnFeCoNi High Entropy Alloy

Initial evaluation of the hysteresis loops obtained from low cycle fatigue (LCF) tests of annealed CrMnFeCoNi at plastic strain amplitudes from 0.3% to 0.6% reveals a yield point in the first cycle at all amplitudes. In addition, unusually low work hardening rates are observed during early cycles. Plastic strain rate change experiments are used to assess thermal activation parameters that describe the length scale of dislocation interactions on the glide plane during LCF. Operational activation areas are calculated at both the evolving and the dynamic equilibrium microstructural states. Changes in operational activation area during the initial cycles (250 b 2 to 170 b 2 across all plastic strain amplitudes) reflect the dynamic nature of the microstructure, and the magnitudes are consistent with dislocation glide controlled by forest dislocations and the solution-strengthened matrix. Additionally, Haasen plots are used to differentiate the contributions of these rate-controlling obstacles to the total operational activation area at a microstructural state of dynamic equilibrium. Dislocation interactions with the solid solution matrix are characterized by an activation area of 285 b 2 . For forest dislocations, the activation area varies inversely with stress, reaching a finite value 365 b 2 at an offset flow stress of 150 MPa. Although cross slip contributes to microstructural development during cyclic deformation, its rate of initiation does not control the dislocation glide kinetics in the CrMnFeCoNi alloy, meaning that the types of obstacles controlling cyclic deformation in this complex alloy are the same as those observed in conventional solution strengthened alloys under similar deformation conditions. • Activation areas were determined from strain rate changes during LCF of CrMnFeCoNi • Forest dislocations and the solute-strengthened matrix control deformation rate • Cross slip does not control glide kinetics, but is active in cyclic deformation • Dislocation glide kinetics are similar in monotonic and cycle deformation • Small yield points and low work hardening rates were observed during early cycles