Importance of the size and proportion of microstructures

Metals are generally made up of a multitude of very small grains. Each grain occupies a polyhedral volume, and is in contact with its neighbors via a surface called the grain boundary. It consists of a single crystal whose orientation varies from grain to grain. If the structure and composition of all the grains are identical, the metal is said to be single-phase. If it is made up of grains of two different structures and/or compositions, it is said to be two-phase (figure 1 ), and in the case of three structures and/or compositions, three-phase. Beyond that, the metal is said to be polyphase.