Microstructural evolution and mechanical instability of Mar-M509 superalloy fabricated by laser powder bed fusion under short-term thermal exposure

The thermal stability of microstructure and mechanical performance is crucial for the industrial application of laser powder bed fusion (LPBF) superalloy components in gas turbines and jet engines. This work investigated the microstructural evolution and strengthening mechanism of LPBF Mar-M509 cobalt-based superalloy before and after thermal exposure at 1200 °C using multi-scale microstructural characterization and in situ neutron diffraction tensile testing. The as-built Mar-M509 superalloy exhibited a heterogeneous microstructural features with coarse columnar and fine equiaxed grains, both containing dendritic and cellular substructures enriched with nanoscale carbides and high-density dislocations. The ultra high strength (1504 ± 4 MPa) of the as-built sample was primarily attributed to dislocation-precipitation synergistic strengthening. After thermal exposure at 1200 °C for 4 h, the dendritic and cellular substructures disappeared and the dislocation density decreased significantly. Meanwhile, Cr, Ta, and other solute elements segregated along fast diffusion channels such as dislocations, stacking faults, and grain boundaries, and combined with the liberated carbon to re-precipitate as coarse M₂₃C₆ and MC carbides. The precipitation of microscale M₂₃C₆ carbides at the grain boundaries not only weakened the Orowan strengthening mechanism but also reduced the grain boundary strength, resulting in a decrease in the tensile strength to 1192 ± 8 MPa. High-temperature stress rupture testing further revealed microstructural instability in the as-built sample, which adversely affected its stress rupture life. Although the elongation of the sample after thermal exposure decreased from 25.9 % to 10.64 %, its stress rupture life increased from 119:01 h:min to 331 h. This study reveals the microstructural evolution and instability of LPBF Mar-M509 superalloy under high-temperature exposure and the impacts on mechanical properties, which provides critical support for the development of cobalt-based superalloys in high-temperature application fields. © 2025 Elsevier B.V.