Alloy Formation

An alloy is formed by intimately blending a base metal with one or more additional elements, which may themselves be metals, non-metals, or a mixture of both. In standard alloy terminology, the bulk (majority) metal present is termed the solvent, while the other element(s) present in smaller proportion are termed the solute(s) -- directly analogous to the solvent/solute terminology used for liquid solutions.

Whether two metals will readily form a genuine substitutional alloy (as opposed to remaining largely immiscible, or forming a distinct intermetallic compound instead) is governed by the empirical Hume-Rothery rules. For a true substitutional alloy to form, several conditions should be simultaneously satisfied: the difference between the atomic radii of the solvent and solute metals should be less than about 15%; both the solvent and the solute should share the same crystal structure; both should have similar valence; and the difference in electronegativity between the two metals should be close to zero (a large electronegativity difference instead favours formation of a distinct ionic or intermetallic compound, rather than a simple substitutional solid solution). …