Copper alloys are metal alloys that have copper as their principal component. They have high resistance against corrosion. Of the large number of different types, the best known traditional types are bronze, where tin is a significant addition, and brass, using zinc instead. Both of these are imprecise terms. Latten is a further term, mostly used for coins with a very high copper content. Today the term "copper alloy" tends to be substituted for all of these, especially by museums.
Copper deposits are abundant in most parts of the world (globally 70 parts per million), and it has therefore always been a relatively cheap metal. By contrast, tin is relatively rare (2 parts per million), and in Europe and the Mediterranean region, even in prehistoric times, it had to be traded considerable distances and was expensive, sometimes virtually unobtainable. Zinc is even more common at 75 parts per million but is harder to extract from its ores. Bronze with the ideal percentage of tin was therefore expensive, and the proportion of tin was often reduced to save cost. The discovery and exploitation of the Bolivian tin belt in the 19th century made tin far cheaper, although forecasts for future supplies are less positive.
There are as many as 400 different copper and copper alloy compositions loosely grouped into the categories: copper, high copper alloy, brasses, bronzes, cupronickel, copper–nickel–zinc (nickel silver), leaded copper, and special alloys.
The similarity in external appearance of the various alloys, along with the different combinations of elements used when making each alloy, can lead to confusion when categorizing the different compositions. The following table lists the principal alloying element for four of the more common types used in modern industry, along with the name for each type. Historical types, such as those that characterize the Bronze Age, are vaguer, as the mixtures were generally variable.
The following table outlines the chemical composition of various grades of copper alloys.
Brass is an alloy of copper with zinc. Brasses are usually yellow in color. The zinc content can vary between few % to about 40%; as long as it is kept under 15%, it does not markedly decrease the corrosion resistance of copper.
Brasses can be sensitive to selective leaching corrosion under certain conditions, when zinc is leached from the alloy (dezincification), leaving behind a spongy copper structure.
A bronze is an alloy of copper and other metals, most often tin, but also alumnium and silicon.
Copper is often alloyed with precious metals like gold (Au) and silver (Ag).
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Copper alloys that are resilient at high temperatures and maintain mechanical properties are used in many applications such as heat exchangers, castings, and rocket engines. Copper alloys typically have very high thermal conductivities compared to other structural alloys which give them an advantage when large heat fluxes are involved, as they are better at dissipating heat. But copperâÂÂs melting point is 1085 Celsius, which is lower than most structural alloys. Therefore, to make use of coppers excellent thermal properties at high temperatures, creep needs to be considered. Creep deformation occurs in materials at relatively high stresses and temperatures. It can dominate as a deformation mechanism in materials above ~0.35 ofàthe melting temperature, so designing against it is critical for high temperature applications. àThe working temperatures of high temperature copper alloys are up to 700 Celsius. Most of the leading high temperature copper alloys rely on oxide dispersion strengthening (ODS) or precipitation hardening (PH). Some alloys use different methods however, such as alloy, GRCop-84, which takes advantage of intermetallic compounds that form, in its microstructure. These precipitates pin the grains and inhibit grain boundary sliding. The advantage of ODS strengthening is that the oxides will not coarsen during temperature aging while PH alloys will, and the strengthening will be lost. In all cases, the goal of the strengthening mechanisms are to slow down creep deformation, and the various mechanisms that contribute to it such as dislocation glide, dislocation glide, and vacancy diffusion. Some examples of how these strengthening mechanisms work are by increasing the activation energy needed for lattice and grain boundary diffusion, introducing a threshold stress needed to climb or shear particles in matrix, or by pinning grains which inhibits grain boundary sliding. Other factors to be considered at high temperature are oxidation and thermomechanical fatigue which may contribute material degradation.