Study on the microstructure and properties of a precipitation-strengthened heterostructured Fe-Ni-based superalloy fabricated by additive manufacturing with excellent mechanical and hydrogen resistance performance

A precipitation-strengthened Fe-Ni-based superalloy was fabricated by laser-directed energy deposition (LDED). Microstructure evolution, mechanical properties, strengthening mechanisms, and hydrogen charging response were investigated in as-deposited and heat-treated conditions. The as-deposited samples exhibit an ultimate tensile strength of 807.45 MPa and elongation of 35.6%. In contrast to the as-deposited state, the heat-treated samples undergo complete recrystallization of columnar grains into fine equiaxed grains. High-density nanoscale γ′ precipitates effectively pin dislocations via the Orowan bypass mechanism and core–shell precipitate structures, resulting in a multiscale heterogeneous microstructure. This enables an excellent synergy of strength and ductility at both room temperature and 650 °C. Microstructural characterisation combined with quantitative strengthening analysis reveals that precipitation strengthening and grain refinement are key contributors to the superior strength and high-temperature load-bearing capacity, while twin structures are beneficial for retaining exceptional ductility. Furthermore, the alloy exhibited remarkable resistance to hydrogen-induced degradation after high-temperature hydrogen charging. The absence of severe hydrogen embrittlement was primarily attributed to the synergistic effect of hydrogen-enhanced localised plasticity and multiscale hydrogen trapping mechanisms associated with twin boundaries, dislocations, precipitate interfaces, and the intrinsically low hydrogen diffusivity of the FCC matrix.