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The precipitation and evolution of intersected plate-like γ′ (γ′ P) phase within interdendritic β during long-term aging and its relevant influence on the creep behavior of a multiphase Ni 3 Al-based intermetallic alloy are investigated. Results show that the precipitation and growth of the γ′ P precipitates during 800 °C aging of original as-cast alloy are significantly rapid, which can be fully precipitated in 60 min to form the intersected morphology and continue to coarsen and grow during the extended aging and subsequent 800 °C creep processes. The γ′ P precipitates have the Bain orientation relationship (OR) of (100)γ′ P //(100) β and [0-11]γ′ P //[001] β with the interdendritic β-matrix. The precipitation of intersected γ′ P phase is sensitive to the elemental fluctuations in β-matrix, and the preceding high-temperature annealing treatment can significantly reduce the elemental inhomogenization and inhibit its generation within the interdendritic β. Precipitation of adequate intersected γ′ P phase can effectively inhibit the dislocation proliferation and crack initiation in the brittle interdendritic β during creep, resulted in the improved creep properties. Slightly rafted γ′ after long-term aging at 800 °C transitioned into R-type γ′-raft structure during creep at 800 °C/220 MPa, and can induce larger creep resistance than N-type γ′-raft structure. [ABSTRACT FROM AUTHOR]

In present work, the unique envelope structure (about 1.1 μm in width, and ~3 vol%) in an as-cast multiphase Ni 3 Al-based intermetallic alloy was characterized in detail, its formation mechanism during solidification and widening behavior during long-term aging process were analyzed, and relevant effects on creep behaviors of this Ni 3 Al-based intermetallic alloy under 800 oC/220 MPa were evaluated. Results show that the envelope structure located between the γ'+γ dendrite and interdendritic β phase is γ′-Ni 3 Al phase with ordered face-centered L1 2 structure, which plays a vital role in adjoining the heteroid γ'+γ dendrite/interdendritic β phases. The γ′-envelope possesses a coherent orientation relationship of [101] γ'-envelope //[01‾1] dendritic-γ with the dendritic-γ phase (face-centered A1 structure) due to their similar face-centered lattice type, while there exists a semi-coherent transition region between the γ′-envelope (face-centered L1 2 structure) and interdendritic β phase (body-centered B2 structure) to accommodate their relative lattice mismatching. The formation of γ′-envelope during solidification process is induced by nickel enrichment in the residual liquidoid after interdendritic β phase, and its widening during long-term aging at 800 oC is mainly controlled by the diffusion of Ni, Al, Hf and Mo atoms at the interfaces between γ'+γ dendrite/γ′-envelope/interdendritic β phase under thermal activations, involving the relevant phase transformation of β (NiAl) + γ (Ni) → γ' (Ni 3 Al). The widening of γ′-envelope resulted in a decrease of interdendritic β phase and an increase of γ′ phase in volume fraction, benefited in the improved creep rupture life and reduced steady-state creep rate. However, the γ′-envelope widening also plays a positive role in crack initiation during creep deformation, since besides the GBs and the semi-coherent interfaces of the γ′-envelope/interdendritic β, cracks are also found to initiate near the Cr 23 C 6 carbides surrounded by the widened γ′-envelope. [ABSTRACT FROM AUTHOR]