What is Alloy?

Alternative Forms: Allay
Legal Definition
An alloy is a mixture of metals or a mixture of a metal and another element. Alloys are defined by a metallic bonding character. An alloy may be a solid solution of metal elements (a single phase) or a mixture of metallic phases (two or more solutions). Intermetallic compounds are alloys with a defined stoichiometry and crystal structure. Zintl phases are also sometimes considered alloys depending on bond types (see also: Van Arkel-Ketelaar triangle for information on classifying bonding in binary compounds).

Alloys are used in a wide variety of applications. In some cases, a combination of metals may reduce the overall cost of the material while preserving important properties. In other cases, the combination of metals imparts synergistic properties to the constituent metal elements such as corrosion resistance or mechanical strength. Examples of alloys are steel, solder, brass, pewter, duralumin, bronze and amalgams.

The alloy constituents are usually measured by mass. Alloys are usually classified as substitutional or interstitial alloys, depending on the atomic arrangement that forms the alloy. They can be further classified as homogeneous (consisting of a single phase), or heterogeneous (consisting of two or more phases) or intermetallic.
-- Wikipedia
Legal Definition
An inferior metal, used with gold. and silver in making coin or public money. Originally, it was one of the allowances known by the name of remedy for errors, in the weight and purity of coins. The practice of making such allowances continued in all European mints after the reasus, upon which they were originally founded, had, in a great measure, ceased. In the imperfection of the art of coining, the mixture of the metals used, and the striking of the coins, could not be effected with, perfect accuracy. There would be some variety in the mixture of metals made at different times, although intended to be in the same proportions, and in different pieces of coin, although struck by the same process and from the same die. But the art of coining metals has now so nearly attained perfection, that such allowances have become, if not altogether, in a great measure at least, unnecessary. The laws of the United States make no allowance for deficiencies of weight. See Report of the Secretary of State of the United States, to the Senate of the U. S., Feb. 22, 1821, pp. 63, 64.

2. The act of Congress of 2d of April, 1792, sect. 12, directs that the standard for all gold coins of the United States, shall be eleven parts fine to one part of alloy; and sect. 13, that the standard for all silver coins of the United States, shall be one thousand four hundred and eighty-five parts fine, to one hundred and seventy-nine parts alloy. 1 Story's L. U. S. 20. By the act of Congress, 18th Feb. 1831, 8, it is provided, that the standard for both gold and silver coin of the United States, shall be such, that of one thousand parts by weight, nine hundred shall be of pure metal, and one hundred of alloy; and the alloy of the silver coins shall be of copper, and the alloy of gold coins shall be of copper and silver, provided, that the silver do not exceed one-half of the whole alloy. See also, Smith's Wealth of Nations, vol. i., pp. 49, 50.
-- Bouviers Law Dictionary
Legal Definition
An inferior or cheaper metal mixed with gold or silver in manufacturing or coining. As respects coining, the amount of alloy is fixed by law, and is used to increase the hardness and durability of the coin.
-- Black's Law Dictionary