Fatigue life of structures made of metastable austenitic steels, e.g., in civil engineering or chemical industry, depends strongly on the phenomenon of strain-induced phase transformation from g austenite to a 0 martensite. Due to the higher strength and the higher specific volume of the martensite phase, pronounced cyclic hardening throughout the fatigue life [1] and transformation-induced crack closure during fatigue crack propagation [2,3] can be observed.Contrary to stable austenitic steels, the Ni concentration in the metastable materials is rather low (approximately 8 wt%), and hence, cooling should result in g austenite, d ferrite, and a 0 martensite, according to the Schaeffler diagram, [4] as shown in Figure 1.From a thermodynamic point of view, cooling to room temperature is not sufficient to cause spontaneous transformation. Only when mechanical strain contributes with DG mech to the energy level of the parent austenite phase, the critical Gibbs energy difference DG as driving force for martensite formation is reached. This is schematically shown in Figure 2. By means of the following phenomenologic equations (cf. [5] , the concentration of alloying elements to be inserted in wt%) the martensite start temperature M s and the temperature M d30 where 30% plastic strain results in 50% martensite formation can be calculated:M d30 ¼ 413 À 462ð%C þ %NÞ À 9; 2ð%SiÞ À 8; 1ð%MnÞ À 13; 7ð%CrÞ À 20ð%NiÞ À 18; 5ð%MoÞAccording to earlier work, [6,7] a 0 martensite nucleates at hcp e marteniste band intersections. The e martensite bands are caused by an accumulation of stacking faults in the {111} slip planes of the fcc austenite phase, i.e., plasticity occurring in the g austenite leads to the formation of shear bands which transforms to e martensite with the {0001} basal planes being parallel to the {111} slip planes of the parent austenite phase. [7] During monotonic tension the transformation into a 0 martensite follows a sigmoidal relationship, as it is described COMMUNICATION [**] Acknowledgements, The financial support by Deutsche Forschungsgemeinschaft and the material supply by Deutsche Edelstahlwerke Südwestfalen are gratefully acknowledged.High cycle fatigue (HCF) life of metastable austenitic steels is governed by the ability of the parent austenite phase to transform into a 0 martensite via metastable e martensite. The mechanism of this strain-induced transformation is closely related to the grain size, the crystallographic orientation distribution, as well as to amplitude, and cyclic accumulation of plastic strain. Aim of the present study is to identify and to quantitatively describe the basic principles of strain-induced martensite formation by means of in situ cyclic deformation experiments in a scanning electron microscope (SEM) in combination with electron back-scattered diffraction (EBSD) and numerical modeling using a boundary element approach. It was shown that during HCF loading martensite formation is inhomogeneous and not directly linked with crack initiation. Only when the fatigue crack pro...