Since metals constitute a class of solids it might well be asked what aspects of Physical Metallurgy do not already fall within the scope of solid state physics. If Kittel's book [1.1] is taken as a guide there is at least one basic metallurgical concept which is unfamiliar to solid state physicists. Metallurgy relates the properties of metals and metallic ‘mixtures’ or alloys to their microstructure. Whereas solid state physics is based on the crystal structure of a single crystal in which all the atoms occupy sites in a three-dimensional lattice, metallurgy takes into account that the perfect regularity of the arrangement is often restricted to microscopic regions and differs from that in neighbouring regions. In other words, superimposed on a macroscopic piece of metal there is another pattern known as the microstructure, much coarser than the crystal structure which forms the foundation of solid state physics.
It happens that many of the properties of metals, especially those that are technologically important, are determined by the microstructure. The most important property from the point of view of mechanical engineering is the strength. This is strongly influenced by the microstructure and is thus one, but by no means the only, microstructure sensitive property. In order to define ‘strength’ and give it some physical meaning (chapters 12 and 14) we must first (chapters 3, 4, 11, 13, 15, etc.) examine the microstructure of metals and describe it quantitatively. The experimental techniques used to characterize the metallic microstructure are described in chapter 2.