Surface—subsurface grain structure relationships

Relationships between surface and subsurface grain structure are studied using a large three-dimensional (3D) microstructural dataset of a nickel-base superalloy Inconel 718. Comparative analyses of microstructural features were performed from both an orthographic, two-dimensional (2D) perspective and a full-3D one to underscore their distinctions on a grain-by-grain basis for a given surface microstructure. A positive correlation between grain size and the number of nearest neighbors of surface grains was observed; however, considerable variability exists per grain. We also present a detailed investigation of 3D surface grain structure and highlight the limitations of 2D surface grain characterizations. Several features of subsurface grain structure, including out-of-plane grain boundary inclination angle, surface-to-subsurface feature proximity, and subsurface grain boundary interface geometry are characterized. The analysis reveals a wide array of highly non-columnar subsurface grain structures with many points at the free surface being closer to grain boundaries and triple junctions beneath the surface than in the surface observation plane. Repeating these analyses for a synthetic dataset without twins reveals that our findings apply to other polycrystalline materials with near-equiaxed grains and limited texture.