Superalloys are another enabling material for modern aircraft, where they are used extensively in the jet turbine engines. Some superalloys are capable of being used in load-bearing applications in excess of 80% of their incipient melting temperatures, while exhibiting high strength, good fatigue and creep resistance, good corrosion resistance, and the ability to operate at elevated temperatures for extended periods of time. As a general class of materials, superalloys include nickel, iron-nickel and cobalt based alloys that operate at temperatures exceeding 1000° F. In a jet engine, specific thrust is defined as thrust divided by weight (thrust/weight), and the most effective way of increasing thrust is through increasing the operating temperature of the engine. The remarkable role superalloy technology has played in allowing higher engine operating temperatures is illustrated in Fig. 1.5.
Due to the extremely high operating temperatures, about one third of the high pressure compressor, the combustor, and both the high and low pressure turbines require superalloys. Superalloys were originally developed using conventional ingot melting and wrought technology, but the highest temperature parts (i.e., turbine blades) are now castings. Originally, turbine blades
Single c r c c
' Cast ADS and SC
Hastelloy B PM AODS Ni-Base
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