Thermal faceting on the ductility-dip cracking fracture surfaces of nickel-based alloys - Occurrence, characterization, and implications for the cracking mechanism

Ductility-dip cracking (DDC) fracture surfaces containing thermal faceting (TF) are documented in this study. Research began with the discovery of TF on the DDC fracture surface in the weld metal of high-chromium nickel (Ni)-based filler metal 52M, a valuable alloy to the nuclear industry. Fracture surfaces were generated using strain-to-fracture (STF) and simulated strain ratcheting (SSR) testing on a Gleeble (R) thermomechanical simulator. Initial fractography performed by scanning electron microscopy (SEM) revealed faceting with preferred grain orientations and faceting interacting with void nucleation near grain boundary (GB) triple points. Internal voids were exposed via metallography, where TF was also observed on the free surface. A thin orientation- and sitespecific foil was extracted from the faceted fracture surface using focused ion beam (FIB) nanofabrication. Subsequent scanning transmission electron microscopy (STEM) characterization down to atomic-resolution revealed the facets have perfect crystallographic orientations parallel to close-packed directions. Sharp edges and transitions between individual facets on the scale of individual atomic columns and atomic planes were observed. An analysis of the deformation microstructure and the chemical composition underneath the faceted surface showed a low dislocation density and no evidence of localized segregation or diffusion, respectively. These findings have led to the development of a hypothesis related to the interdependence of TF and DDC and support initial claims regarding TF formation in DDC voids at elevated temperatures.