Precipitation behavior and anisotropic creep deformation behavior of laser powder bed fusion processed Hastelloy X at 1173 K

Hastelloy X was fabricated using Laser Powder Bed Fusion (LPBF). Creep tests of the as-built samples were conducted at 1173 K under conditions in which the build direction was parallel or perpendicular to the stress axis. The creep rupture time for samples with the stress axis parallel to the build direction was comparable to that of conventionally manufactured Hastelloy X. However, for samples with the stress axis perpendicular to the build direction, the creep rupture time was shorter by one order of magnitude. To elucidate the cause of this anisotropy, the microstructure of the creep-ruptured samples was analyzed using scanning electron microscopy-electron backscatter diffraction (SEM-EBSD). Precipitates were classified using EBSD pattern matching, successfully and more efficiently revealing the precipitation sites of four types of precipitates compared to traditional methods such as SEM-energy dispersive spectroscopy (SEM-EDS) and transmission electron microscopy-selected area diffraction pattern (TEM-SADP) analysis. The specific precipitates did not influence the formation of creep voids and cracks. Crack propagation occurred at high-angle grain boundaries, which were preferentially aligned parallel to the build direction and exhibited a significant deviation in the Taylor factor between adjacent grains. The anisotropy in the creep strength was attributed to the distribution of high-angle grain boundaries formed during the LPBF process, as these boundaries facilitated crack propagation when the stress axis was perpendicular to the build direction.