On the fatigue small crack behaviors of directionally solidified superalloy DZ4 by in situ SEM observations

Small fatigue crack behaviors in a nickel-based directionally solidified superalloy DZ4 were studied by in situ Scanning Electron Microscopy. The crack initiation and propagation manners were identified under different temperatures, i.e. 25 °C, 350 °C, 700 °C. Fatigue crack growth occurred preferentially along slip bands at 25 °C and 350 °C but by Mode-I type at 700 °C. The crack growth rate generally increased with temperature, especially between room temperature and 350 °C. The anomalous small crack growth was analyzed by in situ examining the effect of microstructure. The small cracks were found to be primarily microstructurally small and secondly mechanically small.