Pure metals, for all their useful properties, often fall short in practical applications. Pure iron rusts easily and is relatively soft. Pure gold is so malleable you can dent it with a fingernail. Pure aluminum is too weak for structural use. The solution humanity discovered thousands of years ago is alloying — mixing metals together (and sometimes adding nonmetals) to create materials that outperform their ingredients. Bronze, the alloy of copper and tin that gave an entire age its name, was harder and more durable than either pure metal. It was the Bronze Age innovation that allowed humans to make superior tools, weapons, and armor — changing the course of civilization.
How does mixing metals improve them? It comes down to disrupting the orderly crystal lattice. In a pure metal, layers of identically sized atoms slide over each other easily, making the metal soft. Add atoms of a different size, and they act like speed bumps, preventing layers from sliding — the alloy becomes harder and stronger. Steel, the most important alloy in the world, is simply iron with a small amount of carbon (0.2-2.1%). That tiny addition transforms soft iron into a material strong enough to build skyscrapers and bridges. Add chromium and nickel to steel, and you get stainless steel — resistant to rust because chromium forms a self-healing oxide layer on the surface.
The world of alloys is vast. Brass (copper + zinc) is used for musical instruments and plumbing fittings. Duralumin (aluminum + copper + magnesium + manganese) makes aircraft lightweight yet strong. Titanium alloys combine feathery lightness with steel-like strength for jet engines and surgical implants. Solder (tin + lead or tin + silver) melts at low temperatures to join electronic components. Nitinol (nickel + titanium) is a "shape memory" alloy — bend it, heat it, and it springs back to its original form, used in stents that expand inside arteries. Every smartphone, car, building, and aircraft is a testament to the transformative power of alloys.