Evolution of Slip Through the Thickness of a Single-Crystal Nickel-Base Superalloy Notched Specimen

Abstract

Deformation mechanisms and failure modes of FCC (face centered cubic) single crystal components subjected to triaxial states of static and fatigue stress are very complicated to predict, because plasticity precedes fracture in regions of stress concentration, and the evolution of plasticity on the surface and through the thickness is influenced by elastic and plastic anisotropy. The triaxial stress state at regions of stress concentration results in the activation of many slip systems that otherwise would not be activated during uniaxial testing. We recently presented [1] results from a numerical and experimental investigation of evolution of slip systems at the surface of notched FCC single crystal specimens, as a function of secondary crystallographic orientation. Results showed that the slip sector boundaries have complex curved shapes with several slip systems active simultaneously near the notch. We extend our work on slip at the surface to investigating the evolution of slip or plastic deformation through the thickness of the specimen. A single crystal double-edge-notched rectangular specimen of a Ni-base superalloy, under the tensile loading ([001] load orientation and [110] notch direction) is considered. A three dimensional (3-D) finite element model (FEM) including elastic anisotropy is used for the numerical investigation. Results indicate that the stress distribution and slip fields are a strong function of axial location through the thickness. Numerical results are verified by comparing them with experimentally observed slip fields. We demonstrate that inclusion of three dimensional analysis and elastic anisotropy is important for predicting evolution of slip at the surface and through the specimen thickness. The resolved shear stresses (RSS) on the dominant slip systems and the normal stress on the dominant planes are shown to vary significantly from the surface to the midplane of the specimen. Based on the consideration of RSS, normal stress and the number of activated slip systems at each thickness level, it is concluded that fatigue cracks most likely start in the midplane, for the orientation reported here.