Solid solution with interstitially dissolved atoms.
Non-metal atoms having an atomic diameter d can be incorporated at interstitial sites of a host lattice consisting of metal atoms with a diameter D if the ratio of D/d is <0.41 (Fig. 1).As there are only few interstitial lattice sites which can be occupied, solubility is usually very low and less than 1%. Moreover, the ability to incorporate atoms is very limited due to lattice distortions and the associated stresses. For example, in the iron-carbon system, it is 0.10% (δ-solid solution), 2.06% (γ-solid solution) or 0.02% (α-solid solution) depending on the temperature. This means, for instance, that a maximum of 0.02% of carbon can be dissolved in an α-solid solution (at 723°C).
Small atom species which can induce the formation of interstitial solid solutions, such as hydrogen, nitrogen, carbon and boron, are the elements of the highest technical importance. Diffusibility usually decreases heavily, as the temperature falls. If the lattice modification is altered upon rapid quenching such that the lattice structure has lower solubility for the incorporated element (such as with iron), this results in heavy lattice distortions, i.e. high hardness values. These processes play an essential role in transformation hardening (see also Strengthening mechanisms).
Important examples of interstitial solid solutions include Fe-C, WC, Ti2H or Fe2N.
Substitutional solid solution