4.2.2. Elevated Temperature T = 300 ◦C

Figure 9 shows the development of plastic strain amplitude during fatigue loading of HCF specimens from batch A at 300 ◦C with a frequency f = 20 Hz. As these fatigue tests were performed at elevated temperature, a higher test frequency can be used to achieve a reasonable short testing time. At the beginning of the fatigue tests, cyclic softening was detected for all stress amplitudes followed by a saturation state for stress amplitudes 165 MPa ≤ σ<sup>a</sup> ≤ 200 MPa and finally crack growth up to specimen failure. Inversely, the stress amplitude of 160 MPa, after initial softening up to about 2 <sup>×</sup> <sup>10</sup><sup>4</sup> cycles, led to cyclic hardening, which reduced plastic strain amplitude down to zero. Consequently, macroscopic elastic cyclic loading occurred and at this stress amplitude the limit number of cycles Nl <sup>=</sup> <sup>2</sup> <sup>×</sup> <sup>10</sup><sup>7</sup> was achieved without failure. Ex situ magnetic Feritscope™ measurements quantified 0.13 FE% for this specimen and 0.00 FE% for all loadings > 160 MPa. That is, for only a small load amplitude at 300 ◦C, a change in the cyclic deformation behavior occurred from softening/saturation to cyclic hardening. This change in cyclic deformation character was also observed during total strain-controlled fatigue tests and detected in situ using electromagnetic induced ultrasonic signals in the specimen [46,47,63]. At 300 ◦C, the investigated batch A showed no α´-martensite formation during monotonic tensile loading (see Table 2) as well as in LCF [46,47,63] and HCF regimes at "higher" loading amplitudes (see Figures 7 and 9). As mentioned in the introduction, the cyclic deformation behavior of metastable austenitic steels depends on load parameters such as stress amplitude, frequency, and temperature, and results in different microstructural changes from planar dislocation slip, over wavy slip, until the development of stacking faults, twins, as well as ε and α´-martensite (see. Figure 1). As the volume fraction of α´-martensite measured by Feritscope™ is relatively low (ξ = 0.13 FE%) further microstructural changes have to play a role in the cyclic hardening of metastable austenitic steel at low stress amplitude in fatigue testing at 300 ◦C, which are still not entirely investigated and explained.

**Figure 9.** Development of plastic strain amplitude εa,p versus load cycles N in HCF tests at T = 300 ◦C with f = 20 Hz.
