Alloys – Why Mixing Metals Makes Them Stronger

Pure metals are, with few exceptions, disappointing materials. Pure iron is soft enough to bend by hand in thin sections. Pure aluminium is weak. Pure gold is so soft it deforms under ordinary handling. Almost every metal object you have ever used is an alloy, and the reason mixtures outperform their pure ingredients is a genuinely elegant piece of physics.

Metals are crystals

A metal is a regular three-dimensional lattice of atoms, arranged in repeating patterns. The electrons are not bound to individual atoms but shared across the whole structure, which is why metals conduct electricity and heat so well, and why they are shiny.

The regularity is also why pure metals are soft. When you bend a metal, the atomic planes slide over one another, and in a perfectly regular lattice they slide easily.

The defects that do the work

Real crystals are not perfect. They contain line defects called dislocations — places where a plane of atoms terminates partway through the structure.

Dislocations are how metals deform. Rather than an entire plane of atoms sliding at once, which would require enormous force, the dislocation moves through the crystal one atomic row at a time. It is the same principle as moving a heavy rug by pushing a ripple along it rather than dragging the whole thing.

This means the strength of a metal is not really about how strongly its atoms bond. It is about how easily dislocations can travel. Make that harder, and the metal gets stronger.

How alloying interferes

Adding a different element introduces atoms of a different size into the lattice. A larger or smaller atom distorts the surrounding structure, creating a local strain field that a dislocation has to push through. Enough of these and dislocation movement becomes difficult, which we observe as increased strength and hardness.

Carbon in iron is the classic case. Carbon atoms are small enough to sit in the gaps between iron atoms rather than replacing them, distorting the lattice considerably. A fraction of a per cent transforms soft iron into steel. More carbon gives greater hardness and less ductility, which is why the carbon content of steel is specified so carefully.

Other mechanisms do similar work. Precipitation hardening disperses tiny particles of a second phase through the metal, which dislocations must cut through or bend around — this is what makes aerospace aluminium alloys strong. Work hardening deforms the metal deliberately to generate so many dislocations that they obstruct each other. Grain refinement creates many small crystals rather than few large ones, so dislocations are repeatedly stopped at boundaries.

What you give up

Alloying almost always costs something. Stronger usually means less ductile and more brittle. Electrical conductivity falls, because the same lattice distortion that impedes dislocations also scatters electrons — copper wiring is nearly pure for exactly this reason. Corrosion behaviour changes, sometimes dramatically in either direction. Melting point shifts, and alloys generally melt across a range of temperature rather than at a single point.

Alloy design is therefore a negotiation. Stainless steel trades some strength and a great deal of cost for chromium’s protective oxide layer. Brass trades strength for machinability. Solder trades everything for a low melting point.

Where it gets difficult

All of this assumes a metal formed by casting or forging, where the structure develops under reasonably controlled and uniform conditions.

Metals built up layer by layer are a different situation. Each layer is melted and solidified in microseconds while sitting on material that has already cooled, producing very steep thermal gradients, grains that grow in the direction of heat flow rather than randomly, and residual stress locked into the part.

The result is a material whose properties depend on direction — often stronger along one axis than another — and which frequently needs heat treatment afterwards to relieve stress and normalise the structure. Anyone specifying parts produced by metal additive manufacturing is dealing with a genuinely different material from the same alloy in bar stock, even though the chemical composition is identical. The composition is only half the story; how the atoms were arranged on the way in is the other half.

An old technology

Humans were alloying copper with tin to make bronze more than five thousand years ago, entirely without knowing that dislocations existed. The Bronze Age is named after a materials science discovery made by trial and error, and it took until the twentieth century to explain why it worked.