Precipitation Mechanism of Secondary Phase in Inconel 617 Alloy

The precipitation of secondary phase is important for the strengthening effects of the bulk materials. In this work, SEM and TEM were used to study the precipitation mechanisms of M₂₃C₆ carbides and γ′-Ni3(Ti,Al), and their strengthening effects in Inconel 617 alloy aged at 700 ℃ for 0-1000 h. The results show that the hardness of the samples reaches the peak at 160 h during aging at 700 ℃. The increase of hardness is related to the precipitation of irregular M₂₃C₆ carbides at grain boundaries, the rodlike M₂₃C₆ carbides precipitated in the grains and the spherical γ′ phase. The strengthening effect of γ′ phase is more obvious than that of M₂₃C₆ carbides in this material. At the early stage of aging, M₂₃C₆ carbides consume Cr atoms, leaving Ti and Al atoms to form Al and Ti rich region around the carbides. And then, γ′ phase nucleates at this region easily. The two types of precipitates have coherent orientation relationship with each other and with the nearby matrix. The γ′ particles precipitate around the M₂₃C₆ carbides dependently, limiting the growth of the two types of precipitates and resulting in the irregular shape of γ′ particles and M₂₃C₆ carbides at the early stage of precipitation. The number of γ′ phases increases and grows to the interior of carbides, resulting in the gradual decomposition of carbides. After longer time of aging, the carbides only distribute on the grain boundaries, the carbides disappear in the interior of grains, and high number of spherical γ′ particles can be observed homogeneously in the matrix. Based on the experimental results, the precipitation and growth mechanisms of γ′ particles and M₂₃C₆ carbides were discussed.