The previous chapters in this book confirm that, since the 1950s when the age of jetpowered civil aviation began, the superalloys have underpinned the improvements in performance of the modern gas turbine engine [1]. Today's designs are appreciably superior to the Ghost engine used for the first commercial flight of the Comet I; see Figure 6.1. The modern civil turbofans which power large two-engined aircraft such as the Boeing 777 at a take-off thrust approaching 100 000 lb – for example, Rolls-Royce's Trent 800 or General Electric's GE90 – have a fuel economy which has improved by a factor of about 2; the engine weight, when normalised against the thrust developed, is lower by about a factor of 4. Whilst a number of factors have contributed to this technological success, the development of the superalloys and insertion of components fabricated from them into the very hottest parts of the turbine have been absolutely critical to it. As the compositions of the alloys have evolved, the critical properties in creep and fatigue have improved markedly – and this has allowed the turbine entry temperature to be increased beyond 1700 K; see Figure 1.5. The development of new processes has played a key role too, particularly vacuum melting, directional solidification/investment casting and powder metallurgy techniques.

It is instructive to consider the technological, economic and societal pressures which have provided the incentive for these developments.