solid-solution strengthening

Dissolve a little salt in water and it is still liquid, just changed. Dissolve foreign atoms into a solid metal and you get a solid solution, the host lattice with stranger atoms scattered through it. Solid-solution strengthening is the fact that those strangers make the metal harder, because an atom that is too big or too small for its site pushes the surrounding lattice out of shape, and dislocations struggle to glide through that lumpy, strained terrain.

A substitutional solute sits in place of a host atom; if it is larger it squeezes its neighbours, if smaller it leaves them room to relax inward, and either way it plants a little knot of strain. An interstitial solute (a small atom like carbon in iron) wedges into the gaps between host atoms and strains the lattice even more strongly. A dislocation lowers its energy by sitting near these strain fields, so it gets pinned there and needs extra stress to tear free. More solute, and solute that mismatches the host size more, gives more strengthening; roughly, the added strength grows with the square root of the concentration.

This is why alloys are almost always stronger than the pure metals they are based on. Pure copper is soft; add zinc to make brass or tin to make bronze and it stiffens up markedly while staying workable. Interstitial carbon is what lets iron be hardened into steel at all. The trade is the familiar one, that more solute usually means somewhat less ductility, but solid-solution strengthening is gentle compared with the others and keeps metals formable, which is why it is the backbone of most engineering alloys.