Development of strong, oxidation and corrosion resistant nickel-based superalloys: critical review of challenges, progress and prospects

A comprehensive review of alloying effects in nickel-based single-crystal superalloys for turbine blades and vanes operating in a very aggressive environment of stress, oxidation and corrosion is presented. Exceptionally strong single-crystal superalloys have been developed containing increasing amounts of rhenium and decreasing amounts of chromium resulting in reduced environmental resistance. Interactions in a superalloy containing up to 15 alloying elements are complex and poorly understood. The superalloys can form brittle topologically closed-packed (TCP) phases unless the alloying additions are carefully selected. Development of superalloys with a required balance of strength and environmental resistance has been very challenging. Aluminium, chromium and tantalum are essential alloying elements for strength and environmental protection. Aluminium beyond an upper limit can lead to incipient melting during heat treatment necessary for achieving an optimum size and distribution of the γ′ phase. Rhenium and ruthenium additions contribute significantly to strength, while considerably degrading environmental resistance. Hafnium and yttrium singularly or in combinations improve oxidation and corrosion resistance. Progress in modelling based on thermodynamics, kinetics and regression analysis of prior data to simultaneously predict strength and environmental resistance has been limited since the strengthening and environmental degradation are distinctly separate mechanisms. The paper presents a critical review of alloying studies and provides an insight into future developments.