<p>Austenitic stainless steels used in high-temperature service environments, such as nuclear reactor applications, are susceptible to corrosion, with grain boundaries acting as preferential degradation sites. The production of single-crystal castings may offer a potential pathway to mitigate grain boundary-mediated corrosion. In this study, Nb-stabilized single-crystal austenitic stainless steels were produced using the Bridgman directional solidification process, and the resulting solidification structures were characterized using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The as-cast microstructure consists primarily of austenite with minor retained δ-ferrite and niobium carbide (NbC). The δ-ferrite is attributed to incomplete δ→γ transformation during cooling, while skeletal NbC is inferred to form during the final stages of solidification through eutectic or near-eutectic reactions. In addition, nanoscale NbC precipitates were observed within the retained δ-ferrite phase, with interfacial dislocation structures accommodating lattice misfit. Crystallographic analysis suggests a specific orientation relationship between NbC and δ-ferrite. Hardness measurements performed on selected crystallographic planes ((100) and (114)) indicate lower hardness relative to polycrystalline counterparts, which may reflect the absence of grain boundary strengthening. These results provide insight into process–structure relationships governing phase formation and defect structures in directionally solidified Nb-stabilized stainless steels and may inform the design of single-crystal cast alloys for service in aggressive environments.</p>