On the stress rupture behavior and substructural evolution of LPBF-fabricated IN738LC Ni-based superalloy under various heat treatments

This work deals with evolution of the microstructure and substructure of the IN738LC Ni-based superalloy fabricated by laser powder bed fusion following various post-processing heat treatments, before and after stress rupture testing. The microstructure, primarily composed of γ matrix, γ' precipitates, and carbides, remained stable even after creep rupture. Among the investigated conditions, hot isostatic pressing led to a marked improvement in creep resistance. Post-fracture analyses revealed w-type cracks and r-type defects concentrated along grain boundaries and near boundary precipitates, which were regions of localized micro strain. Several active strengthening mechanisms were identified during high-temperature loading, including dislocation pinning by carbides and γ', Orowan looping, Lomer–Cottrell lock formation, development of dislocation networks, and grain boundary serration. Considering the observation of creep damage at grain boundaries and dislocation activity within the grains, the combined mechanisms of grain boundary sliding and dislocation creep were identified as the governing creep mechanisms at 590 MPa and 760 ◦C test condition