A synergy of mechanical loading and oxidation damage in Ni-based superalloys: A critical review
Modern high temperature installations, such as aero-engines and other land-based gas-turbine engines, constitute elevated operation temperatures and lofty pressure ratios, in addition to rotational speeds exceeding 10,000 rpm, necessary for improved engine-efficiencies. Normally, elevated temperatures lead to excessive thermal stresses, while extreme rotational speeds tend to result in tempestuous mechanical stresses, in prohibitively corrosive exhaust gases. Furthermore, high temperatures generally lead to considerable deterioration in material mechanical properties, in addition to a tendency to promoting oxidation damage. Therefore, ideal materials for these applications should exhibit high mechanical-property stability against temperature changes, resistance to oxidation-, fatigue-, and creep-damage. Nickel-based superalloys, materials widely employed for this purpose, are a unique class of materials, designed for manufacture of components that form a backbone of gas turbine-engines. Mechanical behaviour is largely influenced by microstructural features and the synergy between characteristics and constituents of the operating conditions. This paper presents a critical review of the synergetic interaction between mechanical loading and oxidation damage in nickel-based superalloys’ behaviour. The review establishes that although oxygen-rich environments were widely cited for the observed enhanced oxidation damage, crack closure effect, reduced creep-load bearing capabilities and accelerated crack growth, to date, only limited work has been conducted to provide a clearer understanding of the mechanism by which interaction between mechanical loading and oxidation damage, lead to inferior mechanical behaviour. Oxidation appears to be a physical process driven by diffusion of oxygen and alloy elements, which tends to be aggravated by the application of a mechanical load. However, the mechanism by which this happens is still yet to be fully understood.