Deformation and damage mechanisms of ODS steels under high-temperature cyclic loading

The aggressive operating conditions of future nuclear power plants including generation IV fission and fusion reactors will be beyond those experienced in current nuclear power plants. Hence, the high irradiation resistance, the high creep resistance as well as the high fatigue strength are the main material properties that will be required to build future reactors with enhanced efficiency and safety. Due to their good resistance to swelling under irradiation and their improved mechanical properties, oxide dispersion strengthened (ODS) steels are promising structural material candidates. Nevertheless, a clear understanding of their deformation and damage mechanisms under various loading conditions (especially cyclic loading) are still lacking. In this scope, this work has been performed to obtain a better description of the deformation and damage mechanisms of a tempered martensitic Fe-9%Cr based ODS steel. This includes understanding of its monotonic behavior by testing under tensile loading, pure-fatigue/continuous cycling (PF/CC) response by examining within low-cycle fatigue (LCF) regime and creep-fatigue (CF) behavior by introducing tensile hold-time in PF/CC waveform.
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Prior to testing, comprehensive description of the microstructure in undeformed state is necessary, which was uncovered by means of electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Thereafter, tensile tests were performed within the temperature range varying from room temperature (RT) to 800 °C at the nominal strain rates of 10-3 s-1 and 10-4 s-1. The obtained results were analyzed to visualize the influence of temperature, strain rate and applied heat treatment. While the resulting microstructural evolution was characterized via TEM, the fracture surface investigations were carried by using scanning electron microscope (SEM). In addition, various active strengthening mechanism's contributions to measured RT yield stress were estimated and compared. Comparisons were also made with respect to the strength and ductility of the similar non-ODS as well as other ODS steels reported in literature.
High-temperature PF/CC behavior was delineated by performing fully reversed strain-controlled LCF tests (using nominal strain rate of 10-3 s-1 and triangular waveform with R = -1) in air at 550 °C and 650 °C for different strain amplitude values ranging from ± 0.4% to ± 0.9%. The cyclic stress-strain and strain-life relationships were obtained through the test results, and related LCF parameters were calculated. Postmortem microstructural investigations were carried out using both EBSD and TEM to shed light on the active deformation mechanisms. To explore damage mechanisms, fatigue crack initiation/propagation as well as fracture characteristics were examined via SEM. Lastly, thorough comparison of the measured cyclic stress response and lifetime were made with that of the similar non-ODS as well as other ODS steels tested or taken from literature.

Finally, CF interaction was studied at 650 °C by introducing hold-time of up to 30 min at peak tensile strain of 0.7%. The observed cyclic stress response and lifetime were then compared with that obtained under PF/CC waveform. The additional deformation and damage modifications brought by introducing creep into the PF/CC waveform were scrutinized. Here, EBSD and TEM were used to compare microstructural evolution. Whereas, damage modifications in terms of important observations from the fatigue-cracked specimen surfaces, cross-sections and fracture surfaces were examined via SEM.