ABSTRACT
The nickel-and cobalt-based superalloys are no doubt capable of developing higher strength at higher temperatures than the iron-based alloys, but such high strength is achieved only by reduction of their chromium content. The lower chromium content is required not only to maintain microstructural stability but to allow additions of alloying elements such as aluminum, titanium, and niobium for optimum creep properties. The influence of chromium content on the oxidation resistance and creep rupture strength of some of the currently used superalloys is shown in Fig. 6.42a and b, respectively [65]. Gradually, iron-, nickel-, and cobalt-based superalloys found applications in most of the high-temperature components of the aircraft jet engine, such as turbine blades, disks, combustion systems, tail pipes, and so on, and provided a major step forward in the development of shaft power gas turbines for stationary and transport purposes (particularly for naval ships). There is no doubt that the development of aircraft gas turbine engines has been spectacular over the past five decades; the thrust-toweight ratio of about 2:1 for the earliest engine has been increased to about 7: 1 in a modem military aircraft. Such gain has been achieved predominantly by enhanced thermodynamic efficiency as a consequence of the increased temperature of the inlet gas to the tubing. However, the major credit for such advances must go to the design and development engineers engaged in superior materials development, for whom the permissible temperature of operation for a given lifetime under specified mechanical stresses could be substantially increased. However, such significant developments in materials are reaching an asymptote because these alloys can be used at a maximum metal temperature of 1323 K as their melting points are of the order of 1523-1573 K. Another significant development is that despite the fact that use of the superalloys has allowed the gas turbine operating temperature to increase by 423 K since the 1960s, developments in blade cooling technology over the same period have allowed an additional increase in temperature of 573 K, as illustrated in Fig. 6.43 [65].
