The technological developments wrought since the early twentieth century have required materials that resist higher and higher temperatures. Applications of these developments lie mainly in the following areas:
Gas turbines (stationary and on aircraft), whose blades operate at temperatures of 800–950 K. The burner and afterburner sections operate at even higher temperatures, viz. 1,300–1,400 K.
Nuclear reactors, where pressure vessels and piping operate at 650–750 K. Reactor skirts operate at 850–950 K.
Chemical and petrochemical industries.
All of these temperatures are in the range (0.4–0.65) Tm, where Tm is the melting point of the material in kelvin.
The degradation undergone by materials in these extreme conditions can be classified into two groups:
Mechanical degradation. In spite of initially resisting the applied loads, the material undergoes anelastic deformation; its dimensions change with time.
Chemical degradation. This is due to the reaction of the material with the chemical environment and to the diffusion of external elements into the materials. Chlorination (which affects the properties of superalloys used in jet turbines) and internal oxidation are examples of chemical degradation.
This chapter deals exclusively with mechanical degradation. The time-dependent deformation of a material is known as creep. A great number of high-temperature failures can be attributed either to creep or to a combination of creep and fatigue. Creep is characterized by a slow flow of the material, which behaves as if it were viscous.