A New Method for Fracture Surface Studies: Combined HEDM Orientation Mapping and Absorption Tomography Applied to a Nickel Superalloy
Crystallographic directions normal to the fracture surface and orientation distributions are important statistics that give valuable insights into the phenomenon of crack initiation and propagation in metallic polycrystalline materials. In this thesis, we present a new method to extract crystallographic direction information at and around fracture surfaces. This method combines near field High Energy X-ray Difraction Microscopy (HEDM) and high energy X-ray tomography. This new method gives much better accuracy than the current state-of-the-art techniques based on scanning electron microscopy because it is inherently three-dimensional. In this thesis, not only the new method is presented, but it is also applied to the study of two halves of a sample of Rene88DT that was tested in fatigue to complete fracture. A pin suitable for HEDM was cut from each half so as to surround the initiation point. Volumetric data is collected at beamline 1-ID at the Advanced Photon Source (APS). A volumetric registration scheme is developed to align the data from the two pins so as to reconstruct the crystallography across the fracture. The fracture surface is characterized and the distribution of crystallographic directions normal to the fracture surface is studied in detail. Finally, the initiation site of the fracture is located and compared with a prediction from a simulation based on a viscoplastic deformation model, and qualitative agreement is found.