2023 年 109 巻 1 号 p. 76-85
A cell structure development and a crack initiation during a fatigue of an Fe-3 mass%Si alloy was investigated through electron channeling contrast imaging in a scanning electron microscope and electron back-scatter diffraction analysis. The crystal rotation regions (CRRs), deformation bands (DBs), and cell bands (CBs) together formed a hierarchy in the dislocation structures. In the early stage of fatigue, deformation is constrained near grain boundaries; this impedes further dislocation propagation. This restriction is attributed the formation of CRRs with a width of several hundred micrometers. Further, DBs that were several microns wide were developed inside the CRRs, and CBs with a width of several hundred nanometers were formed inside the DBs. Meanwhile, a crack was initiated from a CRR near a grain boundary. At the crack tip, a DB penetrating the CRR was formed parallel to the crack-propagation direction. It was elucidated that the cell boundary in the DB had a high misorientation angle of approximately 10 degrees, which greatly affected to the crack initiation. In addition, the penetrating DB was composed of elongated cells and cell bands. Prior to crack initiation, the boundaries of the cell bands evolved in proportion to the increasing dislocation density during fatigue. The elongated direction of the cell boundary, which was almost parallel to the {110} plane with a tilt boundary feature, dominated the crack-propagation direction. The formation plane and the cell boundary development process can be explained by analyzing the geometrical relationship of the activated slip systems between adjacent cells.
