EBSD-AFM Hybrid Analysis on Early Fatigue Damage in Austenitic Stainless Steel under Cyclic Torsional Loading

Abstract

Both the electron backscatter diffraction (EBSD) and atomic force microscopy (AFM) methods were used to investigate the active slip systems and crack initiation behavior in an austenitic stainless steel, SUS316NG, fatigued to one-fourth of the fatigue life under cyclic torsional loading. Most of the active slip systems under torsional loading took place on the primary planes with the largest Schmid factor. Fatigue cracks observed on the surface were classified into four types : GB (grain boundary), TB (twin boundary), SB (slip band) and TC (transcrystalline) cracks. The density distribution of TC cracks was maximum, followed by those of GB, SB, and TB cracks. The development of the surface topography due to fatigue was studied by AFM examination of replicated surfaces of the specimen. On the basis of EBSD and AFM observations, the following four parameters were measured : h, the depth of intrusion vertical to the surface ; S, the slip displacement ; SA, the component of slip displacement parallel to the surface ; SB, the component of the slip displacement perpendicular to the surface trace. Both h and SB values showed a sharp increase at the onset of crack initiation. The critical values of h and SB at crack initiation were 160nm and 170nm, respectively.