In Situ SEM Observation and Analysis of Martensitic Transformation During Short Fatigue Crack Propagation in Metastable Austenitic Steel

High cycle fatigue (HCF) life of metastable austenitic steels is governed by the ability of the parent austenite phase to transform into alpha' martensite via metastable epsilon 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 (S EM) 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 propagates by operating multiple slip systems, the cyclic plastic zone exhibits martensitic transformation.