Based upon nickel, but containing significant amounts of at least ten other elements including chromium and aluminium, the superalloys are high-temperature materials which display excellent resistance to mechanical and chemical degradation at temperatures close to their melting points. Since they first emerged in the 1950s, these alloys have had a unique impact. Consider the aeroengines which power the modern civil aircraft. The superalloys are employed in the very hottest sections of the turbines, under the heaviest of loads, with the utmost importance placed on assuring the integrity of the components fabricated from them. Indeed, the development of the superalloys has been intrinsically linked to the history of the jet engine for which they were designed; quite simply, a modern jet aeroplane could not fly without them. Further improvements in temperature capability are now being actively sought, for example for the engines to power the two-decked Airbus A380 and the Boeing 787 Dreamliner. Superalloys are being employed increasingly in the land-based turbine systems used for generating electricity, since fuel economy is improved and carbon emissions are reduced by the higher operating conditions so afforded. But new developments in superalloy metallurgy are required for the next generation of ultra-efficient power generation systems. Over the next 25 years, the world's installed power generation capacity is expected to double, due to the rapidly growing economies and populations of the developing countries, and because most of the current plant in the developed countries will need to be replaced.

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.

The gas turbine consists of many different pieces of turbomachinery, but the rows of turbine blading are of the greatest importance since many engine characteristics, for example the fuel economy and thrust, depend very strongly on the operating conditions which can be withstood by them. Thus very arduous temperatures and stresses are experienced by the materials employed, which are pushed near to the limits of their capability. This is particularly the case for the high-pressure blades, which are located nearest to the hot gases emerging from the combustion chamber. Their function is to extract work from the gas stream and to convert it to mechanical energy in the form of a rotating shaft, which drives the high-pressure compressor.

A consideration of the operating conditions experienced by the high-pressure turbine blades in a large civil turbofan engine, such as the Rolls-Royce Trent 800 or General Electric GE90, confirms this point. The temperature of the gas stream is about 1750 K, which is above the melting temperature of the superalloys from which the blades are made. The high-pressure shaft develops a power of about 50 MW – hence, with about 100 blades, each extracts about 500 kW, which is sufficient to satisfy the electricity requirement of about 500 homes. Each row of blades is expected to last at least 3 years, assuming they operate at 9 h/day. This is equivalent to about 5 million miles of flight, or ~500 circumferences of the world.

The turbine discs are amongst the most critical of components in the aeroengine. To put this into perspective, consider a typical modern civil turbofan such as the Trent 800 – the turbine discs represent about 20% of its total weight and their cost accounts for about 10% of the engine's value upon entry into service. For a military engine, such as the EJ200, the figures are nearer 5% and 25%. The primary function of the turbine discs is to provide fixturing for the turbine blades located in the gas stream, from which mechanical energy is extracted; the complete assembly of discs and blades is then capable of transmitting power to the fan and compressor sections, via the shafts which run along almost the complete length of the engine. During the design stages, their geometry must be optimised by balancing the competing demands of minimisation of weight, dimensional stability and mechanical integrity. Thus risk mitigation by an appropriate disc lifing strategy is vital, since a disc failure represents a potentially fatal hazard to the aircraft and its occupants. Metallurgical damage due to fatigue during service and its acceleration due to oxidation and/or corrosion must therefore be quantified and predicted, so that each disc is withdrawn from service after a prescribed number of take-off/landing cycles, known as the safe working life.

To convince oneself of the properties required of a turbine disc alloy, it is instructive to consider the operating conditions experienced by a typical high-pressure (HP) turbine disc.

As with any material, the superalloys suffer chemical and mechanical degradation when the operating temperatures are too high. Obviously the incipient melting temperature of a superalloy represents an upper limit on the temperature that can be withstood; this is usually no greater than about 1600 K. Despite this, the turbine entry temperature (TET) of the modern gas turbine continues to increase, with a take-off value of 1750 K being typical at the turn of this century; see Figure 1.5. Such extreme operating conditions have become possible only because action is taken to protect the components using surface engineering. In fact, the provision of such coatings and measures to ensure that they remain in place during service has become the most critical issue in the gas turbine field; in a state-of-the-art engine the components in the combustor and turbine sections would degrade very quickly were it not for the protection afforded by the coatings placed on them [1]. Thus, whilst the primary role of the superalloy substrate is to bear the mechanical stresses developed, an additional requirement is for mechanical and chemical compatibility with the coatings required to protect them.

Figure 5.1 summarises the different coating technologies which have become available [2,3] and ranks coating life and the temperature enhancement conferred by them in a relative way. It also serves as an introduction to the terminology used. The so-called diffusion coatings remain the most common form of surface protection.

Background: materials for high-temperature applications

Characteristics of high-temperature materials

Certain classes of material possess a remarkable ability to maintain their properties at elevated temperatures. These are the high-temperature materials. Their uses are many and varied, but good examples include the components for turbines, rockets and heat exchangers. For these applications, the performance characteristics are limited by the operating conditions which can be tolerated by the materials used. For example, the thrust and fuel economy displayed by the modern aeroengine is strongly dependent upon, and limited by, the high-temperature strength of the nickel-based superalloys used for its hottest sections.

What are the desirable characteristics of a high-temperature material? The first is an ability to withstand loading at an operating temperature close to its melting point. If the operating temperature is denoted Toper and the melting point Tm, a criterion based upon the homologous temperature τ defined as Toper/Tm is sensible; this should be greater than about 0.6. Thus, a superalloy operating at 1000°C in the vicinity of the melting temperature of nickel, 1455 °C, working at a τ of (1000 + 273)/(1455 + 273) ~ 0.75, is classified as a high-temperature material. But so is ice moving in a glacier field at –10 °C, since τ is 263/273 ~ 0.96, although its temperature is substantially lower. A second characteristic is a substantial resistance to mechanical degradation over extended periods of time.

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.

I am grateful to Cambridge University Press for allowing me to make some comments about Roger Reed's new textbook The Superalloys: Fundamentals and Applications. Nickel-based superalloys represent a very important class of engineering material, finding widespread application for example in critical components within the gas turbine engines used for jet propulsion and electricity generation. This is due to their superior mechanical properties that are maintained to elevated temperatures. Indeed, new classes of superalloy are continually being sought by gas turbine manufacturers around for the world for applications in the hottest parts of the engine. This is because higher temperatures result in improvements to the efficiency of the engine and therefore lower fuel burn. Engine performance is a major factor in any power plant competition, which helps to explain why all the engine manufacturers spend so much money developing future generations of superalloys.

The author has provided us with a textbook covering both the fundamentals and applications of superalloy technology. This is a significant and unique achievement, especially given the broad range of subject matter dealt with. In Chapter 1, the requirement for materials capable of operating at elevated temperatures is introduced along with the historical development of the nickel-based superalloys and their emergence as materials for high-temperature applications.

What this monograph is about

Certain crystalline materials can exist in more than one solid phase, where a phase is identified by a distinct crystal structure. Typically, one phase is preferred under certain conditions of stress and temperature, while another is favored under different conditions. As the stress or temperature varies, the material may therefore transform abruptly, from one phase to another, leading to a discontinuous change in the properties of the body. Examples of such materials include the shape-memory alloy NiTi, the ferroelectric alloy BaTiO3, the ferromagnetic alloy FeNi and the high-temperature superconducting ceramic alloy ErRh4B4. In each of these examples the transition occurs without diffusion and one speaks of the transformation as being martensitic (or displacive).

Alloys such as Au–47.5%Cd and Cu–15.3%Sn are known to have a cubic lattice at high temperatures and an orthorhombic lattice at low temperatures. Therefore, if such a material is subjected to thermal cycling, it will transform between these two phases. Similarly, alloys such as Ni–36%Al and Fe–7%Al–2%C transform between a high-temperature cubic phase and a low-temperature tetragonal phase, whereas near-equiatomic NiTi has a high-temperature cubic phase and low-temperature monoclinic phase.

If a stress-free single crystal of such a two-phase material is slowly cooled from a sufficiently high temperature, it starts out in the high-temperature phase and at first, merely undergoes a thermal contraction.

Introduction

We next turn to the dynamics of the two-phase nonlinearly elastic materials introduced in Chapter 2. As in the theory of mixed-phase equilibria and quasistatic processes for such materials set out in Chapter 3, the notion of driving force plays a central role in the analysis when inertial effects are taken into account. The indeterminacy exhibited in Chapter 3 by even the simplest static or quasistatic problems for two-phase materials manifests itself again in the present much richer dynamical context. Moreover, the continuum-mechanical interpretations of nucleation and kinetics again serve to restore the uniqueness of solutions to the dynamic problems to be considered here. As in the preceding chapters, thermal effects are omitted; they will be included in the more general settings of later chapters.

The main vehicle for our study of one-dimensional dynamics of two-phase materials is the impact problem. There is an enormous body of experimental literature pertaining to the response of solids to shock or impact loading, much of it motivated by questions concerning the behavior of materials at extremely high pressures, as occurs, for example, deep in the earth. The reader will find some guidance to the experimental literature in this field of the dynamic behavior of materials in the books by Graham [11] and Meyers [25].

Introduction

In the preceding chapter we determined the kinetics of a certain phase transformation using experiments that involved fast loading in which inertia was important. In the present chapter we determine the kinetics of a different transformation using data from quasistatic experiments. The transformation studied here is a twinning deformation, not a phase transformation, a twin boundary being an interface that separates two variants of martensite; see Example 1 in Section 12.2. The change in lattice orientation across a twin boundary makes it analogous, in certain ways, to a phase boundary, and in particular, the motion of a twin boundary is governed by a kinetic relation.

As we have seen, the simplest form of kinetic relation governing the isothermal motion of an interface relates the driving force on it to its normal velocity of propagation: Vn = Φ(f). Since the kinetic response function Φ here is a function of a single scalar independent variable, one set of experiments, say uniaxial tension tests, completely determines Φ; and the function Φ thus determined characterizes all motions of this interface such as, say, in biaxial conditions. If the deformation field is inhomogeneous, and the phase or twin boundary is curved, one would use this same kinetic relation locally, at each point along the interface, relating the driving force at that point to the normal velocity of propagation of that point.

Introduction

In this chapter, we assemble the basic field equations and jump conditions for a one-dimensional, purely mechanical theory of nonlinear elasticity; although thermal effects will be omitted, inertia will be taken into account. The theory presented here is general enough to describe nonlinearly elastic materials that, under suitable conditions of stress, are capable of existing in either of two phases. As we shall see, a key feature of this theory is that the potential energy of the material, as a function of strain at a fixed stress, has two local minima. The associated constitutive relation between stress and strain will then necessarily be nonmonotonic, possessing a maximum and a minimum connected by an unstable regime in which stress declines with increasing strain.

Experiments that provide the motivation for the theory about to be developed fall into two categories. The first of these involves slow tensile loading and unloading of slender bars or wires composed of materials such as shape-memory alloys. The model to be constructed to describe experiments of this kind is one of uniaxial stress in a one-dimensional nonlinearly elastic continuum, and the processes to be studied for this model are quasistatic. The stress-induced phase transitions in such experiments occur in tension, so the two minima in the potential energy density occur at positive – or extensional – values of strain, as do the extrema in the stress– strain relation.