Prediction of crack initiation in single crystal Ni-base superalloys at high temperatures

Because of their excellent high temperature characteristics, Ni-based single-crystal alloys are used in applications where operating temperatures exceed 900°C. The initiation of cracks under these conditions is generally associated with micro-scale porosities (typically between 10 and 20 μm). These porosities or voids give rise to local intensification of the deformation, leading to microcracking and subsequent failure when adjacent microcracks coalesce and link with a free surface. In this work the initiation of a crack in the vicinity of a relatively blunt notch is examined. Two approaches are adopted - in one approach a void adjacent to the notch is modelled explicitly, in the other a damage variable is used to represent the growth and subsequent cracking of voids in the vicinity of the notch. A finite element approach is adopted in both studies, with a rate dependent slip-system based constitutive law used to represent the mechanical behaviour of the nickel alloy. The effect of oxidation on the predicted initiation times is also discussed.