How Alloying Elements Affect the Properties of Copper
Trace elements are often added to metals to improve certain characteristics of the metal. Alloying can increase the strength, hardness, electrical and thermal conductivity, corrosion resistance, or change the color of a metal. But 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.
Solid solution strengthening of copper is a common strengthening method. 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. Copper by itself is relatively soft compared with 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. 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 aging process 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 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.
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.
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