Three-dimensional maps of geometrically necessary dislocation densities in additively manufactured Ni-based superalloy IN718

Abstract

In this work, we present a method to characterize the subgranular three-dimensional distribution of geometrically necessary dislocations (GNDs) and apply it to an additively manufactured (AM) Ni-based superalloy IN718. A three-dimensional (3D) Electron Backscatter Diffraction (EBSD) data set was obtained using TriBeam tomography with resolutions of 1.5 μm in all three dimensions. We demonstrate that the method provides a 3D assessment of the subgranular GND distribution. For the various AM grains studied, the GNDs were arranged into long non-crystallographic subboundaries, lying within the interior of the grain and extending along the build direction and through the grain interior. The choice of 3D neighborhood in the calculation of GNDs is examined, finding that the subboundaries are sufficiently sharp such that considering only the nearest neighbors is sufficient. The total GND densities in the subboundaries are high, reaching in excess of 1013 m−2, whereas away from the subboundaries they are low, at around 1011 m−2 or below. The analysis reveals that GNDs are predominantly edge-oriented GNDs (>57% of total GND density) and are unequally distributed among the {111}-type planes, accumulating the least on the (111) plane. These findings reveal that AM grains contain well-organized patterns of dense GND subboundaries that would noticeably affect their mechanical performance.