Influence of Deformation-Induced alpha prime Martensite on the Crack Initiation Mechanism in a Metastable Austenitic Steel in the HCF and VHCF Regime

摘要

Fatigue failure in the VHCF regime of high strength steels is dominated by subsurface crack initiation around non-metallic inclusions with fisheye fracture morphology.However,this unique failure mechanism cannot be thoroughly confirmed for metastable austenitic steel 1.4301 (AISI 304),for its crack origin is heavily dependant on fatigue life and its amount of deformation-induced alpha prime martensite.In the HCF regime cracks originate at the specimen surface from inclusions exceeding 0.02 mm,irrespective of the volume fraction of deformation-induced alpha prime martensite.For a volume fraction beyond VM = 30％ the higher notch sensitivity of the martensite phase can lead to crack initiation at micro flaws at the surface.In the VHCF regime the influence of the martensite content is even more dominant,as for VM = 54％ the crack origin shifts to subsurface inclusions.While the formation of a fine granular area is observed for all cases with VM > 50％,the occurrence of a typical fisheye morphology depends on the inclusion's relative position to the specimen's surface.The diversity of crack initiation mechanisms can be explained by the interaction of the austenite and martensite phases and its different ductilities with the inclusions as shown by TEM micrographs and Murakami's model of hydrogen-induced VHCF failure.