Man's use of materials, both as a craft and more recently as a science, depends on his ability to produce a particular microstructure with desirable properties in the material when it has been fabricated into a useful object. Such a microstructure occurs, for example, in a steel crankshaft heat-treated for maximum strength, a glass lens heat-treated for fracture resistance or a small crystal of silicon containing non-uniform distributions of solute acting as a complex electronic circuit. Such microstructures are almost always thermodynamically unstable. This situation arises since for any alloy there is only one completely stable structure and there is an infinite number of possible unstable microstructures. The one with the best properties is therefore almost always one of the unstable ones. The desired structure is usually produced by some combination of heat-treatment, solute diffusion and deformation, in the course of which the transformation is arrested, normally by cooling to room temperature, at the right time to obtain the optimum structure. The success of these processes, many of which were derived from craft skills, to give materials with good strength, toughness, electrical properties, etc., is an essential part of current technology. There is, however, a price to be paid in that all these structures are potentially unstable, so that the structures can, and frequently do, transform with time into less desirable forms, especially if used at elevated temperatures.

Introduction

Materials science in general and metallurgy in particular are concerned with understanding both the structure of useful materials, and also the relationship between that structure and the properties of the material. On the basis of this understanding, together with a large element of empirical development, considerable improvements in useful properties have been achieved, mainly by changes in the microstructure of the material. The term microstructure as normally used covers structural features in the size range from atoms (0.3 nm) up to the external shape of the specimen at a size of millimetres or metres. These structural features include the composition, the crystal structure, the grain size, the size and distribution of additional phases and so on, all of which are controlled by the normal methods of alloying, fabrication and heat treatment.

The materials scientist, having achieved some sort of optimum microstructure for a particular property or application, has not completed the task. The important area of the stability of the microstructure remains to be considered. This concern arises since almost none of the useful structures in materials science are thermodynarnically stable: changes that will increase the total entropy or decrease the material's free energy are almost always possible. So if the original structure was an optimum one then such changes will degrade the material's structure and properties.

Introduction

The three main interfaces which are important in metallic systems are the solid–gas interface (the external surface), the interface between two crystals of the same phase which differ only in orientation (the grain boundary) and the interface between two different phases (the interphase boundary). The interphase boundary provides an almost infinite range of possibilities since in addition to the possible difference of orientation of the two crystals, the crystals can also differ in crystal structure, lattice parameter and in composition. In this almost infinite array of possible structures and thus properties two limiting conditions can be recognised. In one case where the interface is formed, as it often is in metallic systems, by precipitation of a second phase within a primary crystal structure then a particular orientation relationship develops between the phases. This produces an interface with a close atomic fit which minimises the interfacial energy (see §2.2.2 and Doherty (1982)). This, in turn, can introduce difficulties for interfacial mobility in that growth ledges may be needed, see §1.3. The opposite extreme occurs when the two phases have no orientation relationship with each other. As a result their interface will be a high energy, incoherent one that usually provides no particular crystallographic barrier to mobility. Examples of this type of incoherent interface arise when the two phases come in contact by growth processes rather than by nucleation.

Almost all metallurgical materials are metastable in one way or another. Manipulating the metastability in alloy microstructures has proved to be essential in order to obtain the wide range of properties needed for different kinds of manufactured component. The conventional metallurgical processing methods of casting, deformation and heat treatment are used to control microstructural features such as chemical homogeneity, grain size, extent of precipitation and dislocation substructure. These are associated with relatively slight deviations from equilibrium, and are discussed in the other chapters of this book. In recent years a variety of processing methods have been developed to manufacture alloys with highly metastable microstructures, that is, with greater deviations from equilibrium. These highly metastable alloys are the subject of the present chapter.

The different methods of manufacturing highly metastable alloys all depend upon manoeuvring the material into a condition far from equilibrium, and simultaneously removing its thermal energy to freeze it into a metastable state. The microstructures that can then develop depend upon both thermodynamic and kinetic factors. Thermodynamic conditions define a set of possible alloy microstructures with lower free energy than the starting state. Kinetic behaviour determines which of these microstructures actually develops during manufacture. The main kinds of metastable material that can be manufactured are microcrystalline and nanocrystalline alloys with ultra-fine grain sizes, segregation-free highly supersaturated solid solutions, new metastable crystalline alloy compounds, amorphous alloys with non-crystalline disordered atomic structures, and quasi-crystalline alloys with ordered but non-periodic atomic structures.

For the second edition, the objectives and the approach previously used have been maintained. In many areas the science base of the subject has shown little change since the first edition and here the text has only been modified by improved examples where available. In other areas the subject has advanced significantly and the text has been updated with the insights. Topics previously covered incompletely, notably the highly unstable microstructure produced initially by rapid solidification but subsequently by other processing routes, have been greatly expanded. Other significant developments that have taken place include the detailed experimental studies of homogeneous nucleation, the growth of Widmanstätten precipitates and precipitate coarsening, and the new insights into the nucleation of recrystallisation, and grain growth and its inhibition by second-phase particles. In other areas, despite the importance of the subject, progress has been disappointingly slow. As in the first edition, we have tried to indicate where there are unsolved problems. The first edition provided the authors with a rich supply of fruitful research topics and we hope that this was also true for our readers and will be equally true for the second edition. Microstructural stability of metallic (and other industrially important) materials remains a field of research with many scientific and potential engineering applications.

The authors are again grateful to Professor Robert Cahn FRS for his efforts to get this volume completed and his much-appreciated enthusiasm.

Instability due to non-uniform solute distribution

The simplest instability in a metallic microstructure is that produced by a nonuniform distribution of solute in an otherwise stable single phase. Such a distribution always raises the free energy of the alloy and so it will decay to a uniform distribution at a rate determined by the thermodynamics and kinetics of diffusion. The kinetics of diffusion and its relationship to the concentration and mobility of point defects is one of the best-established topics in materials science, see, for example, Shewmon (1989), and so these topics need not be repeated here. However, the thermodynamics of diffusion is less widely discussed and is described below. Some new ideas on diffusion in alloys showing different rates of atomic motion in binary alloys as indicated by the Kirkendall effect are also described.

Thermodynamics of diffusion

Fig. 2.1 shows the free energy-composition diagram for a binary alloy. In stable regions of the system, where the second differential of the free energy with composition, ∂2G/∂C2, is less than zero, the free energy of composition C3, is increased from G3 to G′3 if it exists as a mixture of and rather than as a single uniform composition. The rise in G caused by any non-uniform solute distribution in stable regions of any phase provides the driving ‘force’ for the diffusion that homogenises the distribution.

Introduction

The earlier chapters of this book have dealt with the major causes of instability in the microstructure of metals and alloys, but there remains a series of other influences which can also modify the structure. Plastic deformation and irradiation can totally alter the defect structure in metal crystals, and corrosion can completely destroy not only the metallic microstructure but the metal itself. These subjects are, however, too extensive in scope and too important to materials science to be dealt with in a brief chapter. In addition they do not fall within a reasonable definition of the stability of microstructure. However, there have been some interesting and important investigations of the stability of dispersed second-phase precipitates under conditions of plastic deformation, and to a smaller extent, of their stability under irradiation. These instabilities will be discussed here. Other types of external influences include: temperature gradients; gravitational, electrical and magnetic fields; and, finally, annealing under imposed elastic stresses. All of these influences have been found to cause changes in precipitate morphology, and will form the subject of this chapter.

Many of the most informative experiments in this area have been carried out on transparent non-metallic materials such as ice and potassium chloride. The results of these investigations appear to be of direct application to metals and so, for the purposes of this chapter, any material subjected to a relevant and interesting experiment will be considered metallic.