Effect of Phosphorus on Microstructure and Mechanical Properties of IN718 Alloy after Hot Corrosion and Oxidation

Abstract

In this paper, the IN718 alloys added with various phosphorus concentration underwent hot corrosion in the mixed salts of 75%Na2SO4 and 25%NaCl at 650 °C and oxidation in air at 650 °C, and then the microstructure and properties of the alloys were characterized. No difference was found of the tensile properties of IN718 alloy after soaking at 100 h in air and in mixed salt. However, phosphorus exhibits an effect to inhibit the ingression of sulfur along the grain boundaries. The thermal exposure in air at 650 °C makes no differences on the tensile properties, but reduces the total term of the primary and secondary creep stages of IN718 alloy, while the alloy added with 0.019% phosphorus exhibits the same creep curves with and without thermal exposure. The interaction mechanism between phosphorus and hot corrosion and oxidation is discussed briefly in the paper. Introduction IN718 alloy, as one of the most important nickel-based superalloys, is widely used for high temperature parts of aircraft turbines and steam turbines. With various compositions, IN718 alloy is mainly strengthened by the precipitation of γ′′-Ni3Nb (DO22 structure) and γ′-Ni3(Al, Ti)(L12 structure) phases. Recently, it has been found that phosphorus greatly improved the stress rupture and creep properties of IN718 alloy [1-3]. The researches related to the mechanism of this phenomenon were mainly focused on the influence of phosphorus on the grain boundaries because it has a strong tendency to be highly concentrated there. It has also been proposed that when phosphorus segregates to the grain boundaries, it can inhibit the diffusion of oxygen atoms along the grain boundaries [4].