Metals make up the majority of the periodic table of elements. In their pure state, each metal has its own characteristic mass, melting point and physical properties. Mixing two or more of these metals into a blend with a new set of properties forms an alloy, a composite metal that can have strikingly different characteristics.
By definition, pure metals consist of a single element. Samples of these metals contain nothing but atoms of a single metallic substance. Alloys contain two or more elements or alloys melted and blended together, so their chemical formulas consist of more than one element. For example, the pure metal iron consists only of iron atoms. Steel, an alloy of iron and carbon, contains mostly iron atoms with isolated atoms of carbon that lend it strength. Adding the metals chromium or molybdenum to the steel produces yet another alloy: stainless steel.
Malleability and Ductility
One reason that manufacturers combine pure metals to form alloys is to change the physical properties of the metals. Pure metals may be too soft to hold up to regular use, but alloying them makes them tougher. As a pure metal, gold bends and stretches so easily that it would quickly pull out of shape if it were formed into a ring and worn on the finger. Jewelry manufacturers alloy pure gold with silver, copper or zinc to improve the metal's durability and rigidity. The gold contributes its color and resistance to corrosion; the other metals contribute their strength. The result is a 14-karat gold ring that withstands daily wear.
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In their natural elemental state, some pure metals react strongly with their surroundings, oxidizing and corroding until they become unusable. Blending these metals with less reactive metals changes their reactivity, extending the life of the alloyed item. Stainless steel takes its name from the fact that it does not readily rust or pit the way a pure iron tool would. Alloying metals is one means to make them less reactive and more suited to the needs of the manufacturer.
Light metals such as aluminum and titanium reduce the mass of pure metals with which they alloy. These lighter alloys play a vital role in the aerospace industry, as they allow manufacturers to design and build lighter craft. A lighter jet fighter can hold more fuel, equipment and ordnance than a heavy one. Aluminum alloy wheels lighten a vehicle's overall weight, contributing to better gas mileage and adding speed on the racetrack.
Thermal Tolerance and Melting Point
Alloying metals changes their thermal tolerance. As they consist of two or more pure metals, alloys have no single melting point, but instead melt over a range of temperatures. Their molecular structure can raise the overall melting range of the metal above that of any of its component metals. Raising the melting range of a metal has important implications for industrial and commercial use. The SR-71 Blackbird, one of the most technologically advanced reconnaissance aircraft of its time, relied on its lightweight titanium alloy frame to withstand the thermal stress of its supersonic flights.