γ′/γ″ Interfacial Chemistry in an Inconel 718 Alloy after Service

摘要

Inconel 718 alloy is a precipitation-hardenable Ni-Fe base superalloy containing about 5 wt.% Nb along with lesser amounts of Al and Ti and also significant amounts of Cr and Mo. The alloy exhibits excellent creep-rupture strength at temperatures up to 700°C. It also combines good corrosion resistance and high strength with outstanding weldability. It is used widely in gas turbines, rocket motors, spacecraft, nuclear reactors, pumps and tooling. Its excellent mechanical properties are based on its specific microstructure and the corresponding thermal stability. The microstructure is composed of a disordered bcc γ matrix phase with mainly Ni, Cr and Fe; and three intermetallic precipitation phases (γ′ having a composition Ni{sub}3(Al,Ti), a cubic crystal structure and cubic or spherical particle shape (ii) γ″ having a composition Ni{sub}3Nb, a bct crystal structure and a lens-like disc shape (iii) δ having composition Ni{sub}3Nb, an orthorhombic crystal structure and forming as grain boundary particles and films as well as thin plates extending long distances into the grains). Depending on the (Al + Ti)/Nb ratios and heat treatment, the major strengthening phase may be γ′or γ″ or both. It was found that a peculiar precipitate morphology in which initially formed cubic γ′particles are coated with a shell of γ″ on all six {001} faces. This so-called "compact morphology" has shown to have a very slow rate of coarsening. Up to now, the specific morphology configuration of the precipitates and the fully coherent interfaces among the precipitates and the matrix are believed to contribute to the thermal stability. But the question arises that how the specific morphology keeps a very slow rate of coarsening, or how do the γ′/γ interfaces remain stable? The microstructure, in particular, the γ′/γ″ interfacial chemistry, of a blade made of Inconel 718 alloy after service in an airplane engine was investigated for understanding thermal stability of the nano-scale precipitates. The microstructure in the different regions of the blade foil and also the root region were investigated by TEM and 3DAP technique. The aim of the present investigation is to understand the microstructural stability in the blade after real service.