Alloys
Enhancing strength by disrupting the lattice structure.
Definition
An alloy is a homogeneous mixture of a metal with other metals or non-metals, created to enhance properties like strength, hardness, or corrosion resistance.
The "Disrupted Lattice" Mechanism
Pure Metal
Atoms are identical in size and arranged in regular layers. These layers can easily slide over each other when a force is applied (malleable/ductile).
Alloy
An element with a different atomic radius disrupts the regular arrangement. Layers become "keyed" together, making it much harder for them to slide.
Common Examples
Steel
Iron + Carbon
Small C atoms fit in interstitial spaces, locking the Fe lattice. High tensile strength.
Brass
Copper + Zinc
Substitutional alloy. Corrosion resistant. Used in instruments.
Bronze
Copper + Tin
Harder than pure Cu. Used in historic weaponry/tools.
Think About It
Steel is made by adding just 0.2–2% carbon to iron. Why does such a tiny amount of carbon have such a dramatic effect on hardness?
Carbon atoms are much smaller than iron atoms. They fit into the interstitial spaces (gaps) between iron atoms in the lattice. Even a small number of carbon atoms is enough to disrupt the regular layered arrangement across thousands of layers, preventing them from sliding. This is an interstitial alloy — contrast with brass (substitutional), where similarly-sized Cu and Zn atoms swap positions.
Substitutional vs Interstitial Alloys
Alloying atoms replace host atoms. Requires similar atomic radii (within ~15%). Example: Brass (Cu + Zn).
Alloying atoms fit in gaps between host atoms. Requires much smaller atoms. Example: Steel (Fe + C).