Steel incorporates small carbon atoms that sit in the gaps between much larger iron atoms within the iron crystal lattice. How should steel be classified?

Core Concept

Two alloy geometries exist, distinguished by where the solute atoms reside. Substitutional alloys exchange similar-sized atoms at normal lattice sites, while interstitial alloys lodge far smaller atoms in the voids of the host lattice. Carbon's radius is much less than iron's, so carbon fits into octahedral holes between iron atoms rather than replacing them. These embedded atoms distort the lattice and hinder plane-to-plane sliding, making steel harder than pure iron.