Deformation and damage mechanisms of a bimodal 12Cr-ODS steel under high-temperature cyclic loading

• Disparity in cyclic deformation behavior due to strain amplitude and temperature was studied in a bimodal ferritic 12Cr-ODS steel.
• Steel exhibits Masing behavior at 550 °C and minor but a progressive deviation at 650 °C.
• Dislocation arrangements varied as a consequence of different grains sizes, orientations, and oxide particle distributions.
• A propensity towards cross-slip and hence cell structure formation increased with increase in strain amplitude and temperature.
• Stages of crack path include stage I and stage II fatigue damage, duration of which is a function of testing conditions.

The present study reports on the high-temperature low-cycle fatigue (LCF) behavior of a ferritic bimodal 12Cr oxide dispersion strengthened steel. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The measured cyclic stress response showed four distinct stages which include instant initial cyclic softening followed by gradual cyclic hardening thereafter continuous linear cyclic softening and finally crack initiation and growth stage. The rate and amount of these stages depend on both strain amplitude and testing temperature. The cyclic stress-strain and strain-life relationships were obtained through the test results, and related LCF parameters were calculated. Microstructural investigations, using electron microscopy, were carried out to shed light on the deformation mechanisms. At 550 °C, no appreciable microstructural evolution occurred, which is consistent with the manifestation of Masing behavior. A propensity towards cross-slip and hence the formation of three-dimensional dislocation structures increased with increase in loading amplitude and testing temperature, which assisted in accumulating higher inelastic strain and led to slightly higher cyclic hardening at these conditions. Furthermore, W-enriched Laves phase particles precipitated at grain boundaries. The evolution of cell structures with strain amplitude at 650 °C resulted in a small deviation from Masing behavior. The damage studies revealed multiple surface initiated transgranular cracks under higher strain amplitudes and a single surface initiated transgranular crack under lower strain amplitudes. The different stages of the crack path, whose durations depend on the testing conditions, revealed distinct fatigue fracture features.