<P> Viscous flow near grain boundaries, for example, can give rise to internal slip, creep and fatigue in metals . It can also contribute to significant changes in the microstructure like grain growth and localized densification due to the elimination of intergranular porosity . Screw dislocations may slip in the direction of any lattice plane containing the dislocation, while the principal driving force for "dislocation climb" is the movement or diffusion of vacancies through a crystal lattice . </P> <P> In addition, the nondirectional nature of metallic bonding is also thought to contribute significantly to the ductility of most metallic solids . When the planes of an ionic bond slide past one another, the resultant change in location shifts ions of the same charge into close proximity, resulting in the cleavage of the crystal; such shift is not observed in covalently bonded crystals where fracture and crystal fragmentation occurs . </P> <P> An alloy is a mixture of two or more elements in which the main component is a metal . Most pure metals are either too soft, brittle or chemically reactive for practical use . Combining different ratios of metals as alloys modifies the properties of pure metals to produce desirable characteristics . The aim of making alloys is generally to make them less brittle, harder, resistant to corrosion, or have a more desirable color and luster . Of all the metallic alloys in use today, the alloys of iron (steel, stainless steel, cast iron, tool steel, alloy steel) make up the largest proportion both by quantity and commercial value . Iron alloyed with various proportions of carbon gives low, mid and high carbon steels, with increasing carbon levels reducing ductility and toughness . The addition of silicon will produce cast irons, while the addition of chromium, nickel and molybdenum to carbon steels (more than 10%) results in stainless steels . </P> <P> Other significant metallic alloys are those of aluminium, titanium, copper and magnesium . Copper alloys have been known since prehistory--bronze gave the Bronze Age its name--and have many applications today, most importantly in electrical wiring . The alloys of the other three metals have been developed relatively recently; due to their chemical reactivity they require electrolytic extraction processes . The alloys of aluminium, titanium and magnesium are valued for their high strength - to - weight ratios; magnesium can also provide electromagnetic shielding . These materials are ideal for situations where high strength - to - weight ratio is more important than material cost, such as in aerospace and some automotive applications . </P>

What is the difference on the molecular level between the pure metal and the alloy
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