Materials

Metals achieve engineering importance because of their abundance, variety, and unique properties as conferred by metallic bonding. Twenty-four of the 26 most abundant elements in the Earth’s crust are metals, with only two nonmetallic elements, oxygen and silicon, exceeding metals in frequency. The two most abundant metallic elements, iron (5.0%) and aluminum (8.1%), are also the most commonly used structural metals. Iron is the most-used metal, in part because it can be extracted from its frequently occurring, enriched ores with considerably less energy penalty than aluminum, but also because of the very wide range of mechanical properties its alloys can provide (as will be seen below). The next 15 elements in frequency, found at least in parts per thousand, include most common engineering metals and alloys: calcium (3.6%), magnesium (2.1%), titanium (0.63%), manganese (0.10), chromium (0.037%), zirconium (0.026%), nickel (0.020%), vanadium (0.017%), copper (0.010%), uranium (0.008%), tungsten (0.005%), zinc (0.004%), lead (0.002%), cobalt (0.001%), and beryllium (0.001%). The cost of metals is strongly affected by strategic abundance as well as secondary factors such as extraction/processing cost and perceived value. Plain carbon steels and cast irons, iron alloys with carbon, are usually most cost-effective for ordinary mechanical applications. These alloys increase in cost with alloying additions.