The metals that we’re most familiar with—metals such as iron, gold, silver, aluminum and copper—are elements, basic substances that consist of only one type of atom. Pure aluminum, for example, is just aluminum, not a mixture of different chemical substances. Most metals in their pure forms, however, are not very useful for practical applications. They may be too soft or too brittle or too weak to hold up to everyday use.
The solution to this problem is a metal alloy, a man-made mixture of a pure metal with other substances—often other metals, but sometimes non-metallic substances—that have qualities that make them more useful than pure metals. An alloy is made by melting the base metal and then introducing one or more other substances into the molten metal. The other substances dissolve into the base metal, and when the mixture cools and crystallizes, the resulting substance usually has markedly different structural qualities from the pure base metal.
The alloy with which we’re probably most familiar is an alloy of iron, carbon and a few other elements. We call this alloy steel, and it is significantly harder and stronger than pure iron. Steel can be made with varying concentrations of alloying elements, all of which produce different mechanical qualities, and the introduction of other metals in the alloy—nickel, titanium, chromium or molybdenum, for example—produces different characteristics as well.
Stainless steel is an alloy with a relatively high concentration of chromium. The chromium in the steel forms a layer of chromium oxide on the metal’s surface, and that layer protects the steel from oxidation, a process that produces iron oxide—or, as we more often call it, rust. Stainless steel is resistant to surface corrosion, and any corrosion that does occur is unlikely to spread to the interior structure of the metal.
We like to think of the ornamental metals that we use for jewelry and decoration as pure, but like most metals, gold and silver have some inconvenient characteristics in their pure forms. Pure gold and silver are very soft metals, and everyday objects made from them are easily damaged and not very durable. Consequently, gold and silver used in jewelry are usually alloys that are harder and stronger than the pure metals.
Copper is the most common alloying element in jewelry-grade gold and silver. A silver alloy consisting of at least 92.5% silver and 7.5% other metals, such as copper, is called sterling silver. Gold alloys are classified on a scale in which pure gold is called 24-karat gold; a gold alloy that contains 18 parts gold to 6 parts other metal is called 18-karat gold, an alloy with 14 parts gold is 14-karat gold, and so on.
Aluminum is a strong, lightweight metal, and it is relatively easy to work. It isn’t magnetic like iron, and it is resistant to corrosion, making it an attractive alternative to steel in many applications. Aluminum alloys are generally stronger and harder than pure aluminum, however, and as is the case with other metals, aluminum is almost always alloyed—most commonly with copper, zinc, manganese, magnesium or silicon—in practical applications.
Titanium is attractive for structural uses because it is strong, lightweight and resistant to corrosion; pure titanium is as strong as steel but about half its weight. Titanium can be alloyed with other metals—aluminum, molybdenum and vanadium among them—to produce practical titanium alloys, and it can be added to iron to make an especially strong, light steel.