Introduction
Solid solutions are aggregates that are chemically homogeneous, resulting from the addition of one or more foreign elements (alloying elements) in a pure metal.
In certain cases, the metals present are miscible in any proportion. This is the case, for example, in gold (\(\ce{Au}\)) and silver (\(\ce{Ag}\)) which form a continuous solid solution. Most of the time, there is a limit to the concentration in solute atoms (solubility limit) above which the crystalline structure is modified. Within the concentration range limited by this concentration, the solid solution is termed primary or terminal (for example, \(\alpha\)brass). Beyond this concentration, the two constituents can form a solid solution with a different crystalline structure from that of the base metals: it is then a secondary or intermediate solid solution (for example, \(\beta\)).
Other solutions can be observed whose existence is only possible in a limited concentration range: the constituents are then said to form a stoichiometric compound, which will be described in the following part.
Alloying elements can incorporate into a solution with the base metal:
either by replacing its atoms, where it is known as a substitutional solid solution,
or by placing themselves at the interstices of its lattice, when it is then termed an interstitial solid solution.