Influence of microstructure on fatigue crack nucleation and microstructurally short crack growth of an austenitic stainless steel

In this study, the effect of microstructure on crack nucleation and microstructurally short fatigue crack growth is investigated for a metastable austenitic stainless steel. Fatigue tests were conducted at an intermediate fatigue life regime between 10(4) and 10(6) cycles such that martensitic phase transformation occurs given sufficient localized deformation. Through the use of scanning electron microscopy, along with electron backscatter diffraction, several micro-cracks were analyzed and compared. The influence of microstructural features such as twin boundaries, slip band intrusions/extrusions, grain boundaries, inclusions, and martensitic transformed areas on the crack initiation life is discussed. The initiation stages of crack nucleation and the subsequent microstructurally short crack growth for each microstructural feature are compared revealing that twin boundaries and slip bands are the most dominant initiation features. However, the initiation mechanism governing crack nucleation for each feature was different. Additionally, the phase transformation behavior showed only minor effects on the microstructurally short crack growth leading up to an engineering crack. It was found that while the cracks that propagated more quickly had larger transformed martensitic zones around the crack tip, this was due mostly to the size of the crack. Interestingly, the initiation life in the transitional fatigue regime was observed to be more sensitive to crack initiation feature than the martensitic transformation.