13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Influence of Deformation-Induced alpha prime Martensite on the Crack Initiation Mechanism in a Metastable Austenitic Steel in the HCF and VHCF Regime Martina Zimmermann1,*, A. Grigorescu2, C. Müller-Bollenhagen3, H.-J. Christ2 1 Institut für Werkstoffwissenschaft, Technische Universität Dresden, Dresden 01062, Germany 2 Institut für Werkstofftechnik, Universität Siegen, Siegen 57068, Germany 3Continental Teves AG & Co. oHG, Hannover 30419, Germany * Corresponding author: martina.zimmermann@tu-dresden.de Abstract 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. Keywords Very high cycle fatigue, metastable austenitic steel, deformation induced martensite, fisheye fracture 1. Introduction In recent years, many investigations (e.g. [1,2]) focused on the diversity of damage mechanisms related to failure beyond the classical fatigue limit, i.e. at a number of loading cycles N > 107. At an early stage of research on the fatigue of materials in the very high cycle range, a distinction between single phase and defect free type I materials and multiphase type II materials with microstructural defects such as non-metallic inclusions or pores was proposed by Mughrabi [3,4]. The discussion about the fatigue behaviour of the two types of materials at very high number of cycles and the underlying damage mechanisms is often combined with hypotheses of the course of the S–N curve such as a duality of the curve related to the crack initiation position [5,6]. For type II materials a multistage S–N curve is defined by the change from surface crack initiation to a control of fatigue life by crack nucleation at internal defects. With regard to high strength steels, different models for damage mechanisms and the multistage S-N curve are currently under discussion. Murakami et al. [7] related the formation of a so-called fisheye fracture surface, which is commonly observed in these types of materials in the VHCF range, to a hydrogen embrittlement due to a hydrogen concentration in the vicinity of inclusions. According to Murakami and Endo, the VHCF behavior can be correlated to the size and hardness of nonmetallic inclusions [8]. A different explanation was suggested by Sakai et al. [9] stating that the fisheye formation by means of the formation of a fine granular layer around the inclusion is a consequence of a nucleation and coalescence of microdebondings. The formation of fisheye fractures due to multiple microcracks initiated by the decohesion of spherical carbides from the matrix around the dominating inclusion is given as the major damage mechanism model by Shiozawa et al. [10]. This model was confirmed
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