Constitution

Abstract

Materials, when examined microscopically, may be observed to be single phase, or to contain more than one phase. A phase may be defined as a portion of matter that is homogeneous. Mechanical sub-division of a phase will produce small portions indistinguishable from one another. A polycrystalline pure metal is, therefore, a single-phase material as all the individual crystal grains are alike in composition, crystal structure and state. A material does not have to contain only one constituent to be classed as single phase. An unsaturated solution of common salt in water is a single phase. But if the amount of salt is increased beyond the saturation limit the system will be two phase, with a saturated solution of salt in water in equilibrium with excess solid salt.