純鉄多結晶における疲労破面近傍のEBSD法を用いた結晶方位の変化についての解析

抄録

To investigate the effect of free surface on lattice rotation near fatigue crack, fatigue crack propagation test was performed on a compact tension (CT) specimen of pure iron polycrystal. After a fatigue crack propagation test on the CT specimen, the free surface was repolished to measure the lattice rotation at top surface by electron backscatter diffraction (EBSD) technique. We analyzed change in misorientation angle from a reference matrix along a line perpendicular to fracture surface. It is confirmed that the misorientation angle from a reference matrix orientation increasing with decreasing distance from fracture surface at grains neighboring fracture surface. This local lattice rotation can be caused by dislocations emitted from a tip of a propagating fatigue crack. In addition to the misorientation analysis, we estimated the direction of a rotation axis. In the grains faced to free surface, the lattice rotations were often occurred not around the free surface normal but around the axis lying in the free surface plane. This rotation axis can be explained by screw dislocation emission from Mode-III type crack. By contrast, in grains locating at through-thickness center, the lattices rotated around various axes including the free surface normal. It is suggested that edge dislocation emission from Mode-I crack became relatively dominant at the though-thickness center.