Equilibrium-phase precipitation behaviors of typical nickel-base alloys for 700℃ advanced ultra-supercritical boiler tubes

Thermodynamic equilibrium phase diagrams of three candidate materials, Inconel740, Inconel617, and GH2984, for 700℃ advanced ultra-supercritical boiler tubes were calculated by Thermo-Calc software and nickel-base alloy databases. Precipitation behaviors of phases in the three alloys were investigated by comparison. There are similar equilibrium phases in the three alloys, including γ, γ', carbides, σ, η, δ, μ, and α-Cr. Serious segregations of Mo, Ti and Nb may occur during solidification based on the calculation, which would decrease the initial melting points of the three alloys; therefore the consequent homogenization treatment is quite important. On the other hand, the amount of γ' precipitated in Inconel740 is higher than that in the other two alloys at 750℃. Al and Ti contents can strongly effect the precipitation of γ' and η phases. The calculation of carbides indicates that primary carbides in Inconel617 are different from those in the other two alloys, and the precipitation temperature range of secondary carbides in Inconel617 is larger. The presence of σ in GH2984 alloy is harmful to the mechanical properties due to a higher Fe content.