The effect of thermal agitation on atomic arrangement in alloys

When two metals are allowed together in various proportions, a series of solid phases is formed. A characteristic phase diagram of a binary alloy system has regions of single phase, throughout which the alloy is homogeneous, alternating with regions in which two neighbouring phases coexist. The composition of a single phase can be varied continuously over a certain range. This feature of an alloy is in contrast to tire constant atomic ratio of a chemical compound, and is explained by the nature of the binding forces in an alloy which are predominantly those between the metal atoms of both kinds on the one hand and the common electronic system on the other hand, as opposed to the binding forces between atom and atom which predominate in other chemical compounds. Not only may the atomic ratio in a given phase be varied, but also an orderly space distribution of one kind of atom relative to the other, as found in topical chemical compounds, does not necessarily exist in an alloy. Although each phase is distinguished by possessing a characteristic crystalline structure which differs from that of other phases in the same alloy system, yet this structure may be merely an orderly arrangement of sites occupied by atoms. The manner in which the atoms are distributed amongst the sites of a given phase is often variable, and is, for instance, affected by the thermal treatment which the alloy has undergone.