Development of superalloys

The development of what are now termed superalloys is reviewed from the early days of high-temperature alloys in the light of the demands of engineers for materials to serve under differing conditions of stress, temperature, and environment. Before the introduction of the aircraft gas turbine the main interest lay in the field of low-alloy creep resisting steels for steam raising plant or in heat resistant alloys for relatively low-stressed furnace components. Piston aeroengines posed two problems – the exhaust valve and the turbo supercharger rotor – and special materials for these components were introduced, largely on the basis of high-temperature tensile strength. The gas turbine demanded high creep resistance and two main lines of alloy development were followed: carbide-hardened iron or cobalt-base alloys and γ′-hardened nickel-base alloys. The latter type has surpassed the former and now, either in wrought form or, predominantly for the most severe conditions, as castings, is the mainstay of superalloy applications. In recent years, progress has mainly been sought by the improvement of macrostructure: in castings by directional solidification to give columnar crystals or single crystals and in wrought products by powder technology. Severe hot corrosion has been combated by the application of protective surface coatings and the development of a series of wrought and cast high-chromium nickel-base alloys. It is anticipated that future progress in high-temperature materials technology will be sought in the application of a variety of metallurgical and physical techniques to specific engineering components in much the same manner as was done with the aeroengine exhaust valve of 50 years ago.MST/511