EBSD法による極低炭素鋼の疲労き裂発生過程における結晶方位変化の評価

Abstract

The crystallographic orientation changes during fatigue crack initiation process in ultra-low carbon steel were evaluated by the electron backscattered diffraction (EBSD). EBSD measurement and fatigue test were alternately carried out using a small specimen, and the change of crystallographic orientation at the identical area was successfully evaluated. Based on the measurements, three crystallographic orientation parameters, Image Quality (IQ), Kernel Average Misorientation (KAM), Grain Reference Orientation Deviation (GROD) were calculated. IQ, which represents diffraction pattern quality and is lowered by increase of crystalline defects, exhibited irregular variation. KAM, which is the misorientation between neighboring measuring points in identical grain, increased until fatigue crack initiation and then decreased. However, KAM was strongly dependent on the step size of scanning electron beam. GROD, which is misorientation between each measuring points and the datum orientation (in this study, it is defined as the orientation at the measuring point with the minimum KAM in each grains), increased until fatigue crack initiation and then remained constant. These results indicated that GROD is the most useful parameter among those three parameters for evaluation of the fatigue damage dominating fatigue crack initiation.