Alloys: Why Mixing Metals Makes Them Stronger

Pure iron is surprisingly soft — you can bend a thin rod of it by hand. Add a small amount of carbon and you get steel, strong enough to build bridges and skyscrapers. Pure gold is so soft that a ring made from it would scratch and bend with everyday wear; mix it with copper and silver and it becomes durable jewellery. Almost all the metals we use are alloys: mixtures of a metal with other elements. This article explains, at the level of atoms and bonds, why alloys behave so differently from pure metals.

What is an alloy?

An alloy is a mixture of a metal with one or more other elements — usually other metals, but sometimes non-metals such as carbon or silicon. The mixture keeps metallic bonding and metallic properties such as conductivity and lustre, but its other properties — especially hardness and strength — can be very different.

Alloys are mixtures, not compounds: their composition can vary, and the elements aren’t combined in fixed ratios. Most are made by melting the components together and letting them solidify.

Why pure metals are soft

In a pure metal, all the atoms are the same size and sit in neat, regular layers. When a force is applied, layers can slide over each other, because metallic bonding is non-directional — the sea of delocalised electrons keeps holding the ions together whatever their positions (see how metallic bonding explains conductivity, lustre and malleability).

In reality, layers don’t slide all at once. The sliding happens through tiny line defects in the crystal called dislocations, which travel through the lattice step by step, like moving a rug by pushing a ruck along it. Pure metals have regular lattices where dislocations move easily, so they deform easily.

How alloys resist deformation

When atoms of a different size are added, they distort the regular layers. Some regions are squeezed, others stretched. These distortions act like bumps on the rails: they make it much harder for dislocations to move, so layers can’t slide as easily. The alloy is harder and stronger, though usually a little less malleable and ductile.

This is called solid-solution strengthening, and it’s the main reason alloys are stronger than their pure components.

Two types of alloy

Substitutional alloys

The added atoms replace some of the main metal’s atoms in the lattice. This works when the atoms are similar in size (typically within about 15 % of each other) and have similar bonding.

Examples:

• Brass — copper and zinc. Harder than copper, easy to machine, corrosion-resistant, and a gold-like colour. Used for musical instruments, taps and door fittings.
• Bronze — copper and tin (typically around 12 % tin). Much harder than copper; so important historically that it named an era, the Bronze Age, beginning over 5,000 years ago.
• Sterling silver — 92.5 % silver and 7.5 % copper (usually), harder and more durable than pure silver.
• Gold alloys — measured in carats: 24-carat gold is pure; 18-carat is 18/24 = 75 % gold, with the rest usually silver and copper. The other metals harden the gold and change its colour (more copper gives “rose gold”).
• Cupronickel — copper and nickel, used in many “silver” coins.

Interstitial alloys

Much smaller atoms fit into the gaps (interstices) between the larger metal atoms, rather than replacing them. The small atoms lock the layers in place very effectively, so even small amounts cause a big increase in hardness.

The key example: steel. Carbon atoms (atomic radius about 70 pm) fit into gaps between iron atoms (about 126 pm).

Type of steel	Carbon content	Properties	Uses
Low-carbon (mild) steel	up to about 0.25 %	Soft, easily shaped	Car bodies, wire, construction beams
Medium-carbon steel	about 0.25–0.6 %	Stronger	Rails, gears, machinery
High-carbon steel	about 0.6–1.5 %	Very hard, but more brittle	Cutting tools, springs, blades
Cast iron	about 2–4 %	Hard, brittle, good for casting	Engine blocks, drain covers, cookware

Heat treatment — heating and then cooling steel at different rates — changes how the carbon is distributed and which crystal structures form, allowing smiths and engineers to fine-tune hardness and toughness. Quenching (rapid cooling in water or oil) makes steel very hard; tempering (gentle reheating) then reduces brittleness.

Alloys designed for specific properties

Alloying isn’t only about hardness. Adding different elements changes many properties:

• Stainless steel — iron with at least about 10.5 % chromium (often around 18 %) and usually nickel. The chromium forms an extremely thin, tough, self-healing layer of chromium(III) oxide on the surface, which stops the iron underneath from rusting. Used in cutlery, sinks, surgical instruments and food-processing equipment.
• Duralumin — aluminium with about 4 % copper and small amounts of magnesium and manganese. Much stronger than pure aluminium while staying light; historically used in aircraft structures.
• Solder — traditionally tin and lead, which melts at a lower temperature than either pure metal (about 183 °C for the 63 : 37 eutectic mixture). Because lead is toxic, modern electronics use lead-free solders, mainly tin with small amounts of silver and copper.
• Titanium alloys — titanium with aluminium and vanadium, strong, light and biocompatible, used in aircraft and joint replacements.
• Nitinol — a nickel–titanium alloy with shape memory: it can be bent at one temperature and returns to its original shape when warmed, because its crystal structure switches between two forms. It’s used in medical stents and orthodontic wires.
• Amalgams — alloys of mercury, such as the silver–tin–copper–mercury amalgams traditionally used in dental fillings.

Properties that change, and ones that don’t

Property	Pure metal	Alloy (typically)
Hardness and strength	Lower	Higher
Malleability and ductility	Higher	Lower
Electrical conductivity	Higher	Lower (distorted lattice scatters electrons)
Melting point	Sharp	Often lower, and may melt over a range
Corrosion resistance	Varies	Can be greatly improved (stainless steel)

The drop in electrical conductivity explains why pure copper is used for electrical wiring, while alloys such as nichrome (nickel–chromium) are used in heating elements, where high resistance is actually wanted.

A quick demonstration of the idea

A common classroom model uses a tray of identical marbles in neat rows to represent a pure metal: push one row and it slides easily. Add a few larger or smaller balls in among them, and the rows jam — pushing now needs much more force. It’s a simple way to see why alloys are harder.

Common misconceptions

• “Alloys are compounds.” They’re mixtures with variable composition.
• “Alloys have different bonding from metals.” They’re still held by metallic bonding.
• “Alloys are stronger because the bonds are stronger.” Mainly because irregular atom sizes stop layers (dislocations) moving.
• “Adding more carbon always improves steel.” Too much carbon makes steel brittle.

Key takeaways

• An alloy is a mixture of a metal with other elements, keeping metallic bonding.
• Pure metals are soft because identical atoms form regular layers that slide easily.
• Different-sized atoms distort the layers, making sliding harder → alloys are harder and stronger.
• Substitutional alloys (brass, bronze) replace atoms of similar size; interstitial alloys (steel) fit small atoms into gaps.
• Alloys can be designed for corrosion resistance (stainless steel), low melting point (solder), lightness (duralumin) or shape memory (nitinol).