Fatigue crack growth of Ni3Al(CrB) single crystals at ambient and elevated temperatures

Fatigue crack propagation behavior of compact tension specimens of Ni3Al(CrB) single crystals has been examined at room temperature (RT) and elevated temperature. The results indicate that crystallographic cracking occurs on two or more {111} planes simultaneously at RT. With increasing temperature, the cube slip plane becomes active and plays an important role in determining the cracking path. Orientation and temperature dependence of the fatigue crack initiation threshold from the notch root exists. Similar to the flow stress, the crack initiation threshold increases with temperature, and the threshold for ((1) over bar 00)[010] orientation is higher than that for (110)[1(1) over bar0$]. Both orientation and temperature affect the Fatigue crack growth rates (FCGRs). The FCGRs of ((1) over bar 00)[010] are higher than that of (110)[1(1) over bar0$] in the Paris regime. A crystallographic relationship between the primary crack plane and the secondary slip plane ahead of the crack rip is suggested to explain the dependence of FCGRs on orientation. The oxidation embrittlement at the crack tip and the activated cube slip should be responsible for the acceleration of FCGRs in the Paris regime and the oxide-induced crack closure has a significant effect on the deceleration of FCGRs in the near-threshold regime at elevated temperatures. Both the relationship of orientations between octahedral slip planes and the applied load axis and the increasing activity of the cube slip plane with temperature determine the significant variation of fracture path with temperature. (C) 1997 Acta Metallurgica Inc.