<p>We investigated the effects of grain boundary serration on the creep properties of the Ni-based superalloy Nimonic 80A (hereinafter, 80A). A slow-cooling step was introduced to the standard heat treatment of 80A to promote the precipitation of intergranular carbides and the rearrangement of grain boundaries to low-index planes. For the modified heat treatment, two solutioning temperatures—1080 and 1200&#xa0;°C—were selected to vary the initial microstructure of 80A before the slow cooling. The degree of boundary serration, which was evaluated using the fast Fourier transform, was positively correlated with the solutioning temperature. However, a higher solutioning temperature caused the complete dissolution of primary carbides and the precipitation of high-density and faceted intergranular M₂₃C₆ carbides, which resulted in the intergranular cracking with sharp grain-boundary facets. Serrated 80A with a lower solutioning temperature exhibited a 23% increase in the creep-rupture life compared with non-serrated 80A. This improvement is attributed to the enhanced resistance to intergranular cracking, which was confirmed by investigating the fracture morphology and cavity distribution. These results suggest that the formation of boundary serration and the degree of carbide precipitation are important factors in the microstructural design of 80A for improved creep properties.</p> Graphical Abstract <p></p>