The effect of environment and orientation on fatigue crack growth behaviour of CMSX-4 nickel base single crystal at 650 ○C

The fatigue behaviour of nickel base single crystal CMSX4 at 650 degreesC in air and vacuum has been investigated for two orientations with differing nominal crack propagation orientations but the same <001> tensile axis. The orientation containing a <110> crack growth direction shows better fatigue crack propagation resistance. This has been linked to differing orientations and amounts of interdendritic porosity (giving increased crack path roughness and shielding) increased stiffness along the crack growth direction (with implications for shear band decohesion) and lower resolved shear stresses along active slip systems, limiting faster Stage I crack growth. Faster crack growth rates are generally seen in vacuum compared with air, indicating the homogeneising effect of oxidation on slip, suppressing faster Stage I crack growth and possibly increasing oxidation-induced closure. The results of this study indicate that the effect of secondary orientation and environment in single crystal components will affect both failure mode and fatigue crack propagation rates. (C) 2003 Elsevier B.V. All rights reserved.