The purpose of this study is to evaluate fatigue crack growth characteristics of cast Ni based super alloy, Alloy 246. Fatigue crack growth tests are conducted under stress intensity factor range, ΔK, increasing and ΔK decreasing tests to investigate difference of fracture surfaces. It is revealed that fracture surfaces are strongly affected by ΔK. Results of electron back scatter diffraction patterns (EBSD) show that fatigue crack propagates along slip direction. When AAT is increased gradually, fracture surfaces tend to remain mixed mode. When AAT is increased rapidly, fracture surfaces change from mixed mode to mode I. Fatigue crack growth rate of mode I fracture surface is faster than that of mixed mode fracture surface. Based on the evaluation of effective stress intensity factor range, ΔK_<eff>, obtained by displacement field around crack tip using digital image correlation (DIC), crack growth resistance of mixed mode fracture surface is higher than that of mode I fracture surface.
