ABSTRACT
Many construction materials, particularly metals and some ceramics, consist of small crystals or grains within which the atoms are packed in regular, repeating, three-dimensional patterns giving a long-range order. It is also impossible to produce a completely pure material and some foreign atoms will also be present, thus producing a solid solution. A substitutional impurity occurs when the foreign atoms take the place of the parent atoms, resulting in a substitutional solid solution. The impurities may occur by chance during manufacture, but nearly all metals used in construction are in fact alloys in which controlled quantities of carefully selected impurities have been deliberately added to enhance one or more properties. The strong directional covalent bonds give rise to the brittle nature of most ceramics, with failure often initiated at a defect in the structure. The polymer chains can pack together either in a 'random-walk' disordered manner, giving an amorphous structure, or in regular repeating patterns, giving a crystalline structure.
