An alloy is a mixture of chemical elements, which forms an impure substance (mixture) that retains the characteristics of a metal. It differs from an impure metal in that, with an alloy, the added elements are well controlled to produce desirable properties, while impure metals (such as wrought iron) are less controlled, although they are often considered useful. Alloys are obtained by mixing two or more elements, at least one of which is a metal. It is generally called the primary metal or base metal, and the name of this metal may also be the name of the alloy. The other constituents may or may not be metals, but, when mixed with the molten base, they will be soluble and will therefore dissolve in the mixture.
The mechanical properties of alloys are usually quite different from those of their individual constituents. A metal that is normally very soft (malleable), such as aluminum, can be altered by alloying it with another soft metal, such as copper. Although both metals are very soft and ductile, the resulting aluminum alloy will have much higher strength. Adding a small amount of non-metallic carbon to iron increases its ductility and gives it greater strength by forming an alloy known as steel. Due to its very high strength, but still substantial toughness, and its ability to be greatly altered by heat treatment, steel is one of the most useful and common alloys in the modern world. By adding chromium to steel, its corrosion resistance can be improved, creating stainless steel, while adding silicon will alter its electrical characteristics, producing electrical steel.
Like oil and water, a molten metal may not always mix with another element. For example, pure iron is almost completely immiscible with copper. Even when constituents are soluble, each will normally have a saturation point, beyond which a greater proportion of the constituent cannot be added. Iron, for example, can contain a maximum of 6.67% carbon. Although the elements in an alloy should normally be soluble in the liquid state, they may not always be soluble in the solid state. If the metals remain soluble when solid, the alloy forms a solid solution, becoming a homogeneous structure made up of identical crystals, called the "Phase (matter)" phase. If as the mixture cools the components become insoluble, they can separate to form two or more different types of crystals, creating a heterogeneous microstructure of different phases, some with more of one constituent than the other.
However, in other alloys, insoluble elements may not separate until crystallization occurs. If cooled very quickly, they first crystallize as a homogeneous phase, but are supersaturated with secondary components. As time passes, the atoms of these supersaturated alloys can separate from the crystalline network, becoming more stable, and forming a second phase that serves to strengthen the crystals internally.
Some alloys, such as electro "Electro (alloy)"), an alloy of silver and gold, occur naturally. Meteorites are sometimes formed from natural alloys of iron and nickel, but they are not native to Earth. One of the first alloys made by humans was bronze, which is a mixture of the metals tin and copper. Bronze was an extremely useful alloy for the ancients, because it is much stronger and harder than any of its components. Steel was another common alloy. However, in ancient times, it could only be created as an accidental byproduct of heating iron ore in the smelting processes "Casting (metallurgy)") during iron manufacturing. Other ancient alloys include pewter, brass and cast iron. In the modern era, steel can be produced in many ways: varying only the carbon content, producing soft alloys such as carbon steel or hard alloys such as spring steel. Alloy steels can be made by adding other elements, such as chromium, molybdenum, vanadium or nickel, resulting in alloys such as high speed steel or tool steel. Small amounts of manganese are typically alloyed with most modern steels due to its ability to remove unwanted impurities, such as phosphorus, sulfur, and oxygen, which can have detrimental effects on the alloy. However, most alloys were not created until the 19th century, such as various types of aluminum, titanium, nickel, and magnesium compounds. Some modern superalloys, such as incoloy), inconel and hastelloy, can consist of a multitude of different elements.
An alloy is technically an impure metal, but when referring to alloys, the term impurities generally denotes undesirable elements. These impurities come from the base metals and alloying elements, but are removed during the production process. For example, sulfur is a common impurity in steel, which combines easily with iron to form iron sulfide, which is very brittle and creates weak points in the metal.[21] Lithium, sodium and calcium are common impurities in aluminum alloys, which can have adverse effects on the structural failure of castings. In contrast, pure metals that contain unwanted impurities are often called impure metals and are not usually called alloys.[22].