Alloys and Metallic Bonding

🔩 Alloys and Metallic Bonding

An alloy is a mixture of a metal with one or more other elements — either other metals, or a non-metal such as carbon. Alloys are important because pure metals are often too soft or too weak for many practical applications.

Why alloys are harder than pure metals — the mechanism:

• In a pure metal, all atoms are the same size. They form regular layers that can slide easily over one another — making pure metals soft and easily deformed.
• When a different element is added, its atoms are a different size. These different-sized atoms sit within the metallic lattice, disrupting the regular arrangement of the layers.
• The disrupted layers cannot slide over each other as easily — any sliding is blocked by the different-sized atoms acting as obstacles.
• The result: the alloy is harder and stronger than the pure metal it is made from.

Link to metallic bonding: The "sea of electrons" model still applies in an alloy — the delocalised electrons still hold the positive ions together, explaining why alloys still conduct electricity and heat. What changes is the physical arrangement of ions in the layers, not the fundamental nature of the bonding.

Common alloys:

• Steel — iron + carbon: much harder than pure iron; used in construction and tools
• Stainless steel — iron + chromium (+ nickel): corrosion-resistant; used in cutlery and medical instruments
• Brass — copper + zinc: harder than pure copper; used in musical instruments and plumbing fittings
• Bronze — copper + tin: harder and stronger than pure copper; used historically for tools and coins