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o investigate 3D effects during the very high cycle fatigue behaviour of two-phase materials, tailored small-sized specimens of duplex (DSS) and super duplex stainless steel (SDSS) were tested by means of ultrasonic testing in ambient air, corrosive atmosphere and under vacuum conditions in situ within a scanning electron microscope. Selected experiments were carried out in combination with high-energy synchrotron diffraction). In general, fatigue damage manifests itself by preferential slip band formation in the softer fcc austenite phase as it was observed in situ by using a thermo-camera with microscopic resolution. Heat dissipation due to localized plasticity becomes visible as hot-spots offering the possibility for predict the onset of fatigue damage at an early state of VHCF life. Crack initiation is observed transgranular and intergranular at austenite/ferrite phase boundaries where slip band impingement results in local stress concentration. Slip band cracking within the austenite grains was observed in the case of the SDSS, while in the case of DSS, the crack starts in the bcc ferrite phase. A corrosive atmosphere promotes slip band cracking, leading to a strong tendency to micro cleavage and eventually to a drastic decrease in VHCF life. During crack propagation, the grain and phase boundaries act as microstructural obstacles, their strength is depending on the crystallographic misorientation relationship between adjacent grains. This is the key factor for limiting the fatigue life of duplex stainless steels and can be altered, e.g., by strengthening the fcc austenite by alloying with nitrogen or by spinodal decomposition of the bcc ferrite during 475°C treatment.