<P> Although the elements of an alloy usually must 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 consisting of identical crystals, called a phase . If as the mixture cools the constituents become insoluble, they may 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 phase has . However, in other alloys, the insoluble elements may not separate until after crystallization occurs . If cooled very quickly, they first crystallize as a homogeneous phase, but they are supersaturated with the secondary constituents . As time passes, the atoms of these supersaturated alloys can separate from the crystal lattice, becoming more stable, and form a second phase that serve to reinforce the crystals internally . Some alloys, such as electrum which is an alloy consisting of silver and gold, occur naturally . Meteorites are sometimes made of naturally occurring alloys of iron and nickel, but are not native to the 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 to the ancients, because it is much stronger and harder than either of its components . Steel was another common alloy . However, in ancient times, it could only be created as an accidental byproduct from the heating of iron ore in fires (smelting) during the manufacture of iron . Other ancient alloys include pewter, brass and pig iron . In the modern age, steel can be created in many forms . Carbon steel can be made by varying only the carbon content, producing soft alloys like mild steel or hard alloys like 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 usually alloyed with most modern steels because of its ability to remove unwanted impurities, like phosphorus, sulfur and oxygen, which can have detrimental effects on the alloy . However, most alloys were not created until the 1900s, such as various aluminium, titanium, nickel, and magnesium alloys . Some modern superalloys, such as incoloy, inconel, and hastelloy, may consist of a multitude of different elements . </P> <P> As a noun, the term alloy is used to describe a mixture of atoms in which the primary constituent is a metal . When used as a verb, the term refers to the act of mixing a metal with other elements . The primary metal is called the base, the matrix, or the solvent . The secondary constituents are often called solutes . If there is a mixture of only two types of atoms (not counting impurities) such as a copper - nickel alloy, then it is called a binary alloy . If there are three types of atoms forming the mixture, such as iron, nickel and chromium, then it is called a ternary alloy . An alloy with four constituents is a quaternary alloy, while a five - part alloy is termed a quinary alloy . Because the percentage of each constituent can be varied, with any mixture the entire range of possible variations is called a system . In this respect, all of the various forms of an alloy containing only two constituents, like iron and carbon, is called a binary system, while all of the alloy combinations possible with a ternary alloy, such as alloys of iron, carbon and chromium, is called a ternary system . </P> <P> Although an alloy is technically an impure metal, when referring to alloys, the term "impurities" usually denotes those elements which are not desired . Such impurities are introduced from the base metals and alloying elements, but are removed during processing . For instance, sulfur is a common impurity in steel . Sulfur combines readily with iron to form iron sulfide, which is very brittle, creating weak spots in the steel . Lithium, sodium and calcium are common impurities in aluminium alloys, which can have adverse effects on the structural integrity of castings . Conversely, otherwise pure - metals that simply contain unwanted impurities are often called "impure metals" and are not usually referred to as alloys . Oxygen, present in the air, readily combines with most metals to form metal oxides; especially at higher temperatures encountered during alloying . Great care is often taken during the alloying process to remove excess impurities, using fluxes, chemical additives, or other methods of extractive metallurgy . </P> <P> In practice, some alloys are used so predominantly with respect to their base metals that the name of the primary constituent is also used as the name of the alloy . For example, 14 karat gold is an alloy of gold with other elements . Similarly, the silver used in jewelry and the aluminium used as a structural building material are also alloys . </P>

Why metals are often used in the form of alloys