"The yield strength of a metal is increased by anything that impedes the movement of dislocations through the structure. Dislocations are defects in the crystal structure accumulated from the original crystallization of the metal or created under various stresses. When enough of them pile up in one place, plastic deformation occurs. The movement of dislocations is hindered by anything that interrupts the crystal structure, like the presence of the stress field around a substitutional or interstitial atom trapped in the lattice, or grain boundaries, or even getting tangled up with other dislocations that are in different orientations. So if grain boundaries are a pinning point, we want to make more of them by thermal treatment to produce smaller grain size. If substitutional and interstitial atoms in the lattice pin dislocations, then we can add alloying elements to pure metals, for example carbon in iron, or tin in copper to make bronze. The last thing I mentioned, entanglement of dislocations, is produced by work-hardening - rolling or otherwise deforming the original crystal structure at cold (or anyway, not hot) temperatures to create these tangles. There are other ways to increase yield strength, but these are three of them."
