Chemistry – Metals - Alloys and their properties | e-Consult

Strengthening a metal by adding a small amount of another element is achieved by introducing obstacles to dislocation movement within the metal's crystal lattice. Dislocations are linear defects in the crystal structure that allow the metal to deform under stress. When a metal is deformed, dislocations move through the lattice. The presence of an alloying element disrupts this movement.

A common example is steel, which is iron strengthened by adding carbon. Carbon atoms are much smaller than iron atoms. When carbon atoms substitute for iron atoms in the iron lattice, they create distortions and strain fields around themselves. These distortions make it more difficult for dislocations to move, thus increasing the strength and hardness of the steel. The more carbon added (up to a certain limit, typically around 2%), the stronger and harder the steel becomes. However, excessive carbon can reduce ductility and make the steel brittle.

Therefore, the addition of a small amount of a different element (the alloying element) creates obstacles to dislocation movement, effectively strengthening the metal. This is a fundamental principle behind the strengthening of many alloys.