Chapter 31 — Chapter 31 — Materials and Energy

A material is any substance or mixture that makes up a physical object of daily-life or industrial importance, e.g. plastics, metals, ceramics; it is understood by its composition, nature of chemical bonds and microstructure, and chosen for its physical, chemical, mechanical, electrical and thermal characteristics. Metals are hard, rigid, good conductors of heat and electricity, malleable and ductile: steel in bridges and buildings (strength), aluminium in aircraft bodies (light), copper in wires (conductivity). Less reactive metals (gold, platinum) occur in the free state; more reactive ones (Na, Al, Fe, Cu) occur combined as ores, e.g. hematite Fe₂O₃. Ores contain impurities and earthly materials called gangue. Alloying mixes two or more metals (or a metal with a non-metal) to alter the microstructure and improve strength, corrosion resistance, electrical or magnetic properties. Table 31.1: brass Cu 60–90% + Zn 10–40% (electrical fittings); bronze Cu 88% + Sn 12% (medals, statues); steel Fe 98–99% + C 0.1–2% (buildings, tools); duralumin Al 93–95% + Cu + Mg + Mn (aircraft body).