Temperature and ΔK dependence of grain boundary effect on fatigue crack propagation in a two-dimensional polycrystalline Ni-base superalloy

Effects of grain boundaries (GBs) on fatigue crack propagation (FCP) in a cast Ni-base superalloy were experimentally investigated using “two-dimensional” polycrystalline specimens extracted from a directionally solidified material. FCP tests were conducted at room temperature (RT), 700 °C and 900 °C under the stress intensity factor range, Δ K , increasing conditions, and distinctive FCP behavior both in grain interiors and around the GBs was observed in-situ. At RT, the crack propagated in the shearing mode along slip planes. At 700 °C, the cracking mode transitioned from the opening mode to the shearing mode with the increasing Δ K. At 900 °C, only the opening mode cracking occurred. The shearing cracks at RT and in the high Δ K region at 700 °C were retarded near the GBs, whereas the opening mode crack in the low Δ K region at 700 °C was not affected by the GBs. The retardation was attributed to a transition from the shearing mode to the opening mode induced by the slip plane incompatibility at the GBs, and the degree of the retardation showed a good correlation with the size of the shearing crack plane. At 900 °C, the crack propagated along high angle GBs with higher FCP rates than those in grain interiors. The acceleration was greater at the GBs with larger misorientation, which was attributed to higher GB energy. In high Δ K regions, secondary cracks were nucleated along high angle GBs ahead of main crack tips possibly because of external stress normal to the GBs and/or the strain incompatibility at the GBs.