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In the present study, the influence of the deformation-induced non-diffusional phase transformation from austenite into martensite of a metastable stainless steel on the short crack growth behaviour in the high cycle fatigue regime was investigated. For this purpose in situ as well as ex situ cyclic deformation experiments in combination with scanning electron microscopy and supplementing imaging and analytical techniques (i.e., electron backscatter diffraction for orientation and phase analysis) were employed. Cyclic loading of the AISI304L stainless steel investigated at stress amplitudes slightly above the classical fatigue limit was found to lead to a local transformation from the face centred cubic (fcc) austenite into the body centred cubic (bcc) α′-martensite. The nucleation takes place in selected areas of the microstructure in form of lamellae even already after few initial cycles near active slip systems of high Schmid factors of almost 0.5. In the majority of cases cracks initiate at twin boundaries in the absence of any martensite formation. After a first stage of shear-stress-controlled propagation along the respective twin boundary, the cracks change growth mode and direction and continue to grow in a transcrystalline manner perpendicular to the loading axis. In this stage, an alternating growth on two adjacent {111} slip systems results in low-indexed crack planes such as {100} or {110}. As a consequence of the plastic deformation at the tip of the propagating short crack a transformation into α′-martensite occurs. This martensite formation results in an increase of the specific volume and gives rise to compressive stresses leading to premature contact of the crack surfaces and hence to a retardation of the short crack growth

Crack initiation and short crack growth in metastable austenitic stainless steel in the high cycle fatigue regime