Amorphous Solids
Amorphous materials are those whose structure has no repetitive arrangement of the atoms of which it is comprised. In a sense, they have no “structure.” Although gases and liquids are amorphous materials, the only important amorphous solids are the glasses, and they are frequently considered simply as supercooled liquids.
Glass behaves as a typical liquid at high temperatures. The atoms are very mobile and do not vibrate in a fixed location in space. A given mass of hot glass, like any liquid, takes the shape of the container in which it is placed.
As a hot glass cools, its atoms vibrate at lower amplitudes and come closer together, resulting in an overall thermal contraction or decrease in specific volume. This decrease in specific volume of a liquid as temperature decreases is approximately linear and occurs with all liquids, including liquid metals. This is illustrated in Fig. 7.1.
When any unalloyed liquid metal (a pure metallic element) or chemical compound is cooled to its freezing (or melting) temperature Tm, the atoms come much closer together and become relatively immobile with respect to one another. They form a crystalline structure with very efficient packing, and thus there is a very marked decrease in specific volume at this temperature, as shown in Fig. 7.1. When an alloyed liquid metal freezes to form a solid solution, the transition from liquid to solid takes place in the range of temperatures between the liquidus and the solidus. Further cooling of both solid metals results in a further decrease in specific volume, also linear but of lower slope than in the liquid state.