When molten metal cools, it changes from a liquid to a solid state. At the molecular level, not one but many smaller crystals form, each with imperfections. Extra layers of atoms are squeezed in at some places, while other places may contain atoms of a different element or be missing atoms altogether. These imperfections become part of the metal and weaken the bonds between some of the layers.
The properties exhibited by alloys -- metals to which other metal or non-metal elements have been added -- can be quite different from those of the constituent parts. For instance, pure copper and silver are relatively soft metals that can be easily deformed by an external force. Combining the two in the right concentration, however, forms the alloy sterling silver, which is hard enough to use for cutlery and coins. The atoms of the solute (the metal in the smaller quantity of the ones being mixed) occupy the spaces between the atoms of the solvent (the metal in the larger quantity), making the structure of the alloy less easily deformed.
Because exposure to water, oxygen, and other materials in the atmosphere causes most metals to corrode, certain elements can be added to a metal to form a chemically stable film on its surface that helps the metal resist corrosion. Chromium and aluminum are two such elements with protective properties. When exposed to oxygen in air or water, these elements form a self-renewing, microscopically thin layer -- called an oxide film -- that protects the underlying metal from corrosion. The alloy stainless steel, known for its sleek, shiny surface and tremendous strength, contains a high concentration of chromium.
Most alloys, like steel, are manufactured by heating the constituent metals to their molten, or liquid, state and then allowing the mixture to cool and solidify. In some cases, however, when the molecular structures of certain elements do not tolerate melting, alloys are made by mixing the components in powdered form, heating the mixture short of melting, and then squeezing out the end product.
Even after an alloy is formed, its strength and hardness can be increased by a rolling and hammering process called work hardening. The applied stresses cause imperfections in the metal to become randomized. This eliminates the presence of large concentrations of weak points in the bonds between atoms.