How Alloys Change the Properties of Metals

In a pure solid metal, atoms are arranged in a repeating structure. The outer electrons are not tied to one atom in the same way as electrons in many covalent substances. These mobile electrons help explain why metals conduct electricity and heat.

Layers of metal atoms can sometimes slide past one another when a force is applied. This ability contributes to properties such as malleability, which lets a metal be hammered or rolled into shape, and ductility, which lets it be drawn into wires.

For some uses, easy movement between layers is helpful. For others, a pure metal may be too soft. Engineers can change the structure by adding atoms of different elements, making movement through the lattice more difficult.

An alloy contains a metal mixed with one or more additional elements. If the added atoms have different sizes from the main metal atoms, they distort the regular arrangement. This can make it harder for layers of atoms to slide past each other.

As a result, an alloy may be harder or stronger than the pure metal. Steel is mainly iron mixed with carbon, and different carbon levels and additional elements produce steels with different properties. Stainless steel contains elements such as chromium that help it resist corrosion.

Bronze is mainly copper mixed with tin, while brass is mainly copper mixed with zinc. Both behave differently from pure copper. The exact composition matters, so the word alloy describes a family of materials rather than one fixed recipe.

Making one property better can make another less useful. A very hard alloy may become more brittle, while an alloy designed for high temperature may be expensive or difficult to shape. Materials scientists therefore choose compositions by balancing several requirements.

An aircraft component, cooking pan, bridge cable and musical instrument may all need different combinations of strength, mass, corrosion resistance, conductivity and cost. There is no universally best alloy.

When comparing materials, ask:

• •Which elements are present in the alloy?
• •How does the structure differ from the pure metal?
• •Which properties improve?
• •Which useful properties may be reduced?
• •What job is the material designed to perform?

Gold jewellery offers a clear worked example. Pure gold is described as 24 carat, and it is soft enough to scratch and bend easily. An 18 carat alloy is 18 parts gold out of 24, which is 75 per cent gold. The remaining quarter is usually a mixture of metals such as silver and copper, which make the alloy harder and more durable. The added metals also change colour. More copper gives a rose tint, while white gold often includes palladium or nickel and is commonly plated with rhodium for a brighter finish.

Alloys can also change how a metal melts. A pure metal melts at a single, sharp temperature, but many alloys soften over a range of temperatures, and some melt at a lower temperature than any of their ingredients alone. Solder uses this behaviour. Traditional solder mixed tin with lead so it would melt easily and join electrical components without damaging them. Because lead is toxic, most modern electronics now use lead-free solders based mainly on tin with small amounts of other metals, such as silver and copper.

Two pieces of steel with exactly the same composition can behave very differently depending on how they were heated and cooled. If a blacksmith heats a high-carbon steel until it glows and then plunges it into water or oil, a process called quenching, the carbon atoms are trapped in a strained crystal structure. The steel becomes very hard, which is useful for a cutting edge, but also brittle, so a quenched blade could snap if dropped or bent.

To fix this, the steel is tempered by reheating it to a much lower temperature for a while. Some of the internal strain relaxes, so the metal loses a little hardness but gains toughness, meaning it can absorb a knock without breaking. Before thermometers were common, smiths judged tempering by the thin layer of oxide that forms on polished steel, which changes colour from pale straw through brown and purple to blue as the temperature rises. Tools that need a hard edge are tempered less, while springs are tempered more.

Stainless steel contains at least about 10.5 per cent chromium, and that chromium protects the iron in a surprising way. Ordinary iron rusts because its oxide forms a loose, flaky layer that crumbles away and exposes fresh metal underneath. Chromium instead reacts with oxygen in the air to form an extremely thin, tightly attached oxide film, far too thin to see. This film seals the surface and blocks further attack. If the steel is scratched, the exposed chromium reacts with oxygen again and the film repairs itself, which is why a scratched sink does not start rusting along the mark.

The name still promises more than the metal can always deliver. Chloride ions, found in salt water, sweat and some cleaning products, can break through the film at tiny points and cause pitting, small holes that grow beneath the surface. Steel cut off from oxygen, such as under a deposit or in a tight crevice, may also struggle to repair its film. Grades used on boats and coasts often include molybdenum for extra resistance to chlorides. Even in a kitchen, rust spots can appear when particles of ordinary steel from wire wool or other tools are left lying on a stainless surface.

Some alloys can be bent out of shape and then return to their original form when warmed. The best known is nitinol, made from roughly equal numbers of nickel and titanium atoms. Its properties were discovered in the early 1960s at the United States Naval Ordnance Laboratory, which gave the alloy its name: nickel, titanium and the laboratory's initials. The trick lies in two crystal structures. At lower temperatures the atoms take up an arrangement that can be deformed by shifting whole blocks of the crystal without breaking bonds. On heating, the alloy switches back to its higher temperature structure and its original shape.

With the right composition, nitinol shows a related behaviour called superelasticity at body temperature. It can be bent or squashed by a surprising amount and then spring straight back as soon as the force is removed. Orthodontists use nitinol wires that keep pushing gently on teeth as they move, and some spectacle frames can be twisted without breaking. In medicine, small mesh tubes called stents can be compressed into a thin catheter, threaded into a narrowed blood vessel and released, where they open out to hold the vessel wide. All of this comes from how the atoms rearrange themselves, not from any added spring.

Alloys change metal properties by mixing different atoms into the metallic structure. The altered arrangement can make sliding more difficult and can also change corrosion resistance, conductivity and other behaviours. Instead of asking whether an alloy is simply better, ask which set of properties makes it suitable for a particular task.