Effect of serrated grain boundaries on mechanical anisotropy in L-PBF fabricated GH4099 alloy

Nickel-based superalloy GH4099 fabricated via laser powder bed fusion (L-PBF) inevitably develops pronounced microstructural anisotropy and columnar grains aligned with the build direction due to rapid solidification and steep thermal gradients, which compromises structural stability and reliability in service. To address this, we propose and validate a reproducible heat-treatment window (slow cooling followed by aging) that induces stable serrated grain boundaries (SGB) in L-PBF-built GH4099. Compared with the as-deposited (AD) and conventional solution treated and aged (STA) states, the SGB condition yields equiaxed grains with pronounced boundary undulations and uniformly dispersed M₂₃C₆ along boundaries, while preserving a fine and consistent intragranular γ′ population (∼20–21 nm). Texture intensity decreased from 4.60 to 3.06, suggesting a reduction in plastic anisotropy. Quantitatively, boundary curvature increases from 0.099 μm⁻¹ to 0.128 μm⁻¹ (vertical direction), and the SGB-H attains a superior strength–ductility balance with YS of 797.1 MPa, UTS of 1227.1 MPa, and elongation of 44.8 %, compared with AD-H (YS 588 MPa, UTS 989 MPa, elongation 47.8 %). Cyclic loading–unloading analysis shows that SGB-H presents a smaller hysteresis loop area at cycle 9, dropping from 9.114 to 7.337 kJ/m³ compared with STA-H. The back stress is reduced by roughly 60–73 MPa and the friction stress by about 100 MPa, resulting in a σ_b/σ_flow ratio of 0.48–0.53 rather than above 0.56. These data indicate SGB effectively disperses GND pile-ups and lowers local interfacial friction, improving deformation compatibility and cyclic stability.