INVESTIGATION OF MICROSTRUCTURAL CHANGES WITHIN WHITE ETCHING AREA ('BUTTERFLY') UNDER ROLLING CONTACT FATIGUE (RCF) USING TEM AND 3D CRACK RECONSTRUCTION BY FOCUSED ION BEAM (FIB)

摘要

Rolling contact fatigue (RCF) is an extreme high-cycle fatigue process (10~9 load cycles or more), involving very small-scale plastic deformation in the steel matrix adjacent to stress-raising nonmetallic inclusions, leading to the formation of so-called butterfly cracks. This paper presents TEM and Dual Beam (combination of SEM and Focused Ion Beam - FIB), investigations performed on the White Etching Area (WEA) (the "butterfly wings") surrounding the butterfly cracks. The material investigated is a hardened and tempered model steel with a matrix composition similar to the commercial steel grade, SAE 52100 that is widely used for bearing applications, containing artificially introduced spherical Al_2O_3 inclusions. The TEM samples, containing the cracks, were selected from specific locations using precise FIB preparation process, allowing the TEM analyses of the large microstructural changes in the butterfly wing between the butterfly crack and the steel matrix around it. The results show a formation of the nano-crystalline ferrite that is not uniformly distributed inside the butterfly wing, while the steel matrix is still unchanged (tempered martensitic structure). This paper presents a 3D reconstruction of the butterfly cracks caused by RCF. Series of cross sections were produced along the crack by Dual Beam, using the Ion beam "Slice and View" method. It allowed a complete 3D crack reconstruction.