Small amounts of alloying elements are often added to metals to improve certain characteristics of the metal.
Alloying can increase or reduce the strength, hardness, electrical and thermal conductivity, corrosion resistance, or
change the color of a metal. The addition of a substance to improve one property may have unintended effects on
other properties. This page describes the effects of various alloying elements on copper and copper alloys such
as brass and bronze.
-
Strength
Solid solution strengthening of copper is a common procedure. Small
amounts of an alloying element added to molten copper will completely dissolve and form a homogeneous
microstructure (a single phase). At some point, additional amounts of the alloying element will not
dissolve; the exact amount is dependent on the solid solubility of the particular element in copper.
When that solid solubility limit is exceeded, two distinct microstructures form with different compositions
and hardnesses. Unalloyed copper is relatively soft compared to common structural metals. An
alloy with tin added to copper is known as bronze; the resulting alloy is stronger and harder than either
of the pure metals. The same is true when zinc is added to copper to form alloys known as brass. It should be noted that neither 'brass' nor 'bronze' is a concrete, technical term.
Tin is more effective in strengthening copper than zinc, but is also more expensive and has a greater detrimental
effect on the electrical and thermal conductivities than zinc. Aluminum (forming alloys known as aluminum bronzes),
Manganese, Nickel, and Silicon can also be added to strengthen copper.
Another copper strengthening method is precipitation hardening. The process involves quenching a
supersaturated solid solution from an elevated temperature, then reheating to a lower temperature (aging)
to allow the excess solute to precipitate out and form a second phase. This process is often used for
copper alloys containing beryllium, chromium, nickel, or zirconium. Precipitation hardening offers
distinct advantages. Fabrication is relatively easy using the soft solution-annealed form of the
quenched metal. The subsequent aging process of the fabricated part can be performed using relatively
inexpensive and unsophisticated furnaces. Often the heat treatment can be performed in air, at moderate
furnace temperatures, and with little or no controlled cooling. Many combinations of ductility,
impact resistance, hardness, conductivity, and strength can be obtained by varying the heat treatment
times and temperatures.
-
Electrical and Thermal Conductivity
Pure copper is a very good conductor of both electricity and heat.
The International Annealed Copper Standard (IACS; a high purity copper with a resistivity of 0.0000017 Ohm-cm) is
still sometimes used as an electrical conductivity standard for metals. The best way to increase the electrical and
thermal conductivity of copper is to decrease the impurity levels. The existence of impurities and all common
alloying elements, except for silver, will decrease the electrical and thermal conductivity of copper. As the
amount of the second element increases, the electrical conductivity of the alloy decreases. Cadmium has the
smallest effect on resulting alloy's electrical conductivity, followed by increasing effects from zinc, tin, nickel,
aluminum, manganese, silicon, then phosphorus. Although different mechanisms are involved in thermal conductivity,
the addition of increasing amounts of elements or impurities also produces a drop in thermal conductivity. Zinc has
very minor effect on the thermal conductivity of copper, followed by increasing effects from nickel, tin, manganese,
silicon, and serious effects from phosphorus. Phosphorus is often used to deoxidize copper, which can increase the
hardness and strength, but severely affect the conductivity. Silicon can be used instead of phosphorus to deoxidize
copper when conductivity is important.
-
Color
Pure copper has a reddish gold color which quickly oxides to a dull green. Since copper often
contains natural impurities or is alloyed with more than one element, it is difficult to state the specific effect each
alloying element has on the resulting alloy's color. Electrolytic tough pitch copper contains silver and often
trace amount of iron and sulfur and has a soft pink color. Gilding copper is a reddish brown color and contains
zinc, iron, and lead. Brass is often used as an ornamental metal, since it has an appearance very similar to that
of gold and is much less expensive. Brasses contain varying amounts of zinc, iron, and lead and can vary from
reddish to greenish to brownish gold. Nickel silver, which contains nickel, zinc, iron, lead, and manganese, can
have a grayish-white to silver appearance.
For more general information about over 2500 specific copper, brass, and bronze alloy data sheets, choose one of the following links, or use one of our search techniques to reach specific products. MatWeb has a complete list of mechanical, electrical, and thermal properties and composition specifications for copper-base alloys.
