EBSD-AFM hybrid analysis of fatigue slip system and crack initiation in polycrystalline metal under cyclic torsional loading

In the present study, we utilize both EBSD and AFM methods with replica technique to investigate the active slip systems and fatigue crack initiation behavior in face-centered cubic polycrystalline metal, austenitic stainless steel, SUS3I6NG, under cyclic torsional loading. The surface of a fatigued specimen was investigated with the hybrid EBSD-AFM method. The analysis on the active slip systems of those grains based on Schmid´ plastic theory, shows that in polycrystalline metal most grains possess the plane with the largest Schmid factor value as their primary slip plane. Fatigue cracks on the free surface were observed by scanning electron microscopy (SEM) and were classified into 4 types: GB (grain boundary), TB (twin boundary), SB (slip band) and TC (transcrystalline). The lengths and angles between the cracks and axis of those 4 types of crack were also measured respectively. The results of crack density distribution in comparison with the earlier research on tensional fatigue show that under low loading level TC cracks dominate the initiation stage regardless of the loading mode. By using the EBSD-AFM hybrid method, the condition of initiation of SB cracks was discussed with 4 parameters: h, depth of intrusion vertical to the surface; S, slip displacement; S_A, the element of slip displacement parallel to the surface; S_B, the element of slip displacement vertical to the surface trace. The result indicates that S_B of accumulated slip deformation takes a constant value at crack initiation.