The cyclic deformation and fatigue behaviour are studied in strain-controlled tests at room temperature and, in some cases, also at 130°C. Total strain Wöhler plots and cyclic stress-strain curves are derived in single-step tests. In parallel experiments, the mechanisms of cyclic strain localization, fatigue crack initiation and short crack growth and the microstructural stability during cyclic deformation are studied as quantitatively as possible with different microscopic techniques (LM, TEM, SEM/EBSP). Furthermore, defined two-stage fatigue tests till failure with two different successive strain amplitudes and accompanying microstructural studies are planned. The results are to be compared with the results of the single-step tests, and the deviations from the linear damage rule are to be assessed, and a microstructural fatigue life model is to be developed.