Microstructure evolution and strengthening mechanism of γ′-strengthening superalloy prepared by laser powder bed fusion

Recently, γ′ phase has been introduced into the solid-solution strengthening Haynes 230 alloy to improve alloy strength for better satisfying the rigorous service environment. However, previous research shows that γ′ phase was not precipitated under the as-fabricated condition and harmful Laves phases with high-W content, which have never been reported, appeared in the novel alloy. Therefore, in this study, Laves phase dissolution mechanisms during solution heat treatments (SHT) and γ′ strengthening mechanism after subsequent aging heat treatment (AHT) of a novel γ′-strengthening superalloy prepared by laser powder bed fusion (LPBF) were first investigated. The results show that Laves phases continue to dissolve with the increment of SHT temperature and finally disappear after SHT-ed at 1200 °C. The higher dissolution difficulty of Laves phases is closely related to the lower diffusion coefficient of W element. The long-range diffusion of solute atoms mainly controls the dissolution of Laves phases in the early homogenization stage. As the holding time increases, the γ/Laves interface widths for the solute atoms diffusion decrease and thus the dissolution process is gradually governed by the interface reaction in the later stage. After AHT, numerous nanosized γ′ particles are precipitated. With increasing SHT temperature, the ultimate tensile strength (UTS) and yield strength (YS) of the AHT-ed samples decrease while the elongation (EL) increases, which is mainly caused by the Laves phases dissolution and grain recrystallization. The 1200+AHT sample presents an excellent combination of strength and ductility with UTS of 1467.7 MPa, YS of 1244.6 MPa and EL of 6.3%. The dominant strengthening contributions in the AHT-ed samples are γ′ strengthening and solution strengthening.