Nickel is the fifth most abundant element on earth. The atomic number is 28, and this places it in the first row of the d block of transition metals, alongside iron and cobalt. The atomic weight is 58.71, the weighted average of the five stable isotopes 58, 60, 61, 62 and 64, which are found with probabilities 67.7%, 26.2%, 1.25%, 3.66% and 1.16%, respectively. The crystal structure is face-centred cubic (FCC; see Figure 2.1), from ambient conditions to the melting point, 1455 °C, which represents an absolute limit for the temperature capability of the nickel-based superalloys. The density under ambient conditions is 8907 kg/m3. Thus, compared with other metals used for aerospace applications, for example, Ti (4508 kg/m3) and Al (2698 kg/m3), Ni is rather dense. This is a consequence of a small interatomic distance, arising from the strong cohesion provided by the outer d electrons – a characteristic of the transition metals.

In this chapter, some important aspects of the physical metallurgy of nickel and its alloys are considered. Section 2.1 is concerned with the compositions of the superalloys and the phases promoted by the presence of the alloying elements. Such composition– microstructure relationships have been established over many years, and considerable use of them is required when designing new grades of superalloy.