Microstructure instability of additively manufactured alloy 718 fabricated by laser powder bed fusion during thermal exposure at 600–1000

Microstructure and precipitation behavior of alloy 718 produced by laser powder bed fusion (L-PBF) and its recrystallized counterpart were evaluated after thermal exposure at a temperature range of 600–1000 ℃ for up to 1000 h. The as-built 718 featured a microstructure of columnar grains, and dislocation cell structures (DCSs) with chemical segregations (rich in Nb, Ti, Mo and depleted in Cr, Fe) and precipitates (Laves phase and carbonitride). The DCSs remained thermally stable for up to 1000 h at 600 ℃ or 100 h at 700 ℃, but only 10 h at 800 ℃. With the temperature increasing to 900 ℃ and 1000 ℃ for 1 h duration, the DCSs tended to partially disassemble, and the chemical segregations were removed. The chemical segregations at cell boundaries of the as-built 718 promoted the formation of γ′, γ″ and δ phases during aging, leading to a spatially heterogeneous distribution of γ′ and γ″ phases between the cell boundaries and interiors during short exposure time or at lower temperatures. Meanwhile, the coarsening of γ′ and γ″ phases and the phase transformation of γ″ to δ at 700–800 ℃ were facilitated in the as-built 718 compared to the recrystallized ones. A unique precipitation distribution was observed at 700 ℃ that γ′ and γ″ phases precipitated both at the cell boundaries and within cell interiors, while only γ′ formed in the vicinity of cell boundaries. Time-temperature-transformation curves for additively manufactured and recrystallized 718 were constructed based on the microstructure analyses. This study indicates that conventional heat treatment procedures may be suboptimized for additively manufactured 718 in high temperature applications. •The thermal stability of dislocation cells, segregations, and Laves phases was systematically evaluated in as-built 718.•A time-temperature-transformation curve was developed for as-built 718 and compared with its recrystallized counterpart.•γ″-free zones near cell boundaries and rapid γ″ to δ transitions were observed in aged as-built 718.•Conventional heat treatments may be suboptized for additively manufactured 718.