Elastic Deformation Induced Non-equilibrium Segregation of P in IN718 Alloy

IN718 alloy is a precipitation hardened, nickel-based superalloy that is used extensively in the aircraft engine industry. The excellent cyclic fatigue resistance, high tensile strength, fracture toughness and oxidation resistance allow the alloy to be safely used at maximum useful service temperature of about 650oC. Many researchers improve the alloy's high-temperature strength through adding of P. However, segregation of P at grain boundary can be induced the grain boundary embrittlement, decreased the tension reduction in area. In this paper, intermediate-temperature embrittlement (ITE) for a IN718 alloy has been experimentally studied by elevated-temperature tension tests. International standard ISO 6892-2 (Metallic Materials Tensile Testing) indicates that the variations in temperature of this measurement system have been found to have a larger potential effect on test results, which is a kind of uncertainty contribution not related to test equipment. International standard ISO 6892-2 maintains that strain rate variations can induce the measurement uncertainty of mechanical properties in tensile testing, which will imperil the reliability of tension tests. The measurement uncertainty of reduction in area relative to strain rate, called as strain rate embrittlement (SRE), is first described experimentally for IN718 alloy in this paper. Then the measurement uncertainty is clarified based on microscopic theory of elastic deformation in metals. It is shown that the elastic deformation of tension test induced the segregation of impurities to grain boundaries and the relevant embrittlement which produces the measurement uncertainty of reduction in area.