*4.2. HCF Behavior*

For fatigue tests in the HCF regime, stress-controlled single step tests were performed. Typically, in order to obtain the limited number of cycles Nl in the range of 106–107 cycles, test frequencies in the range 5 Hz <sup>≤</sup> f <sup>≤</sup> 20 Hz are used for metallic materials. For Nl = 2 <sup>×</sup> 10<sup>6</sup> with a load frequency of f = 5 Hz, about 5 days, for Nl = 2 <sup>×</sup> 107 with a load frequency f = 20 Hz, about 12 days would be required. Usually, higher test frequencies can be used for HCF tests at ambient temperature (AT) of metallic materials but when investigating the fatigue behavior of metastable austenitic stainless steels, self-heating of the specimens has to be taken into account. Even if external cooling of the specimen is undertaken, significant increases in specimen temperature cannot be suppressed [70,71]. Hence, in this study, test frequencies for fatigue tests at nominally ambient temperature were f = 5 Hz (batch B) and for T = 300 ◦C f = 20 Hz (batch A).
