IV. SElECTING COPPER AllOYS FOR PHYSICAL PROPERTIES
Electrical Conductivity The International Annealed Copper Standard (lACS) is the recognized standard for metal conductivity. Its value in absolute tenns, 0.5800 Megmho/centimeter at 20 C (68 F), corresponds to a resistivity of exactly 17.241 nanohm-meter at that temperature. Highly refined, annealed, wrought coppers have lACS conductivities of 100% or slightly higher at 20 C (68 F), depending on purity. Less-pure coppers and cast copper alloys display conductivities ranging from 95% lACS down to between 5% and 10% lACS. By way of comparison, pure aluminum has a conductivity of about 60% lACS; 5052 aluminum alloy, 35%; carbon steel, 8.5%, and 18-8 stainless steel, about 2.3%,18 Electrical conductivity decreases
with increasing alloying content, or more precisely. with the amount of alloying element in solid solution. In a precipitation-hardenable alloy, heat treatment changes the amount of alloying element in solid solution, and therefore alters the alloy's conductivity. For example, the conductivity of chromium copper, C81500 (I % Cr), in the as-cast or solution-annealed state (tensile strength approximately 23-35 ksi, 172-241 MPa) is only 40%-50% lACS ; while in the fully hardened condition (tensile strength 51 ksi, 352 MPa) it rises to 80%-90% lACS. The lACS conductivities of some cast copper alloys are listed in Table 16, page 86." Conductivity nonnally falls with increasing temperature, a factor which must be taken into account in the design of electrical products. This temperature
dependence of electrical conductivity for a selection of cast copper alloys is shown in Figures IV -1, page 70.' When high strength is not an important design consideration, cast electrical connectors and other currentcarrying products can be made from copper C8 I 100. Applications requiring higher strength along with good electrical conductivity can utilize chromium copper, C81500, or one of the cast beryllium coppers, C8200(J...(:82800. Electrical conductivities of the beryllium coppers range between 82 % and 18% lACS. Their corresponding tensile strengths range from 45 ksi to 165 ksi (310 MPa to 1, 137 MPa) in the heattreated condition.
Thermal Conductivity The copper alloys are well known for their very favorable heat transfer properties. Table 17, page 87, ranks the copper alloys in order of their thennal conductivities at 20 C (68 F). Notice that unlike most other metals, the copper alloys' thennal conductivities increase with temperature. The phenomenon is illustrated in Figure IV-2, page 70. Designers can take advantage of this useful characteristic to improve the efficiency of copper alloy heat exchangers at elevated temperatures.
and aluminum bronzes, which contain up to a few percent iron precipitated as islands of an iron-rich phase, can, as a result, be slightly ferromagnetic . Magnetism in these alloys can be reduced several-fold by solutionannealing them at a high temperature, followed by rapid quenching. This retains the iron in solid solution, where it has little magnetic effect. Although it is not itself ferromagnetic, manganese can also impart ferromagnetic properties to copper alloys, as in the so-called Heusler alloys, which are based on 75% copper, 15% manganese and 10% aluminum. These alloys are ferromagnetic even though they contain none of the naturally ferromagnetic metals: iron, nickel and cobale
Thermal Expansion, The thermal expansion coefficients for copper and single-phase alpha alloys fall in a fairly narrow range between 9.4 - 10.0 X lO"/OF (16.9 - 18 x 10" / 'C), while those for beta and polyphase alloys (yellow brasses, high strength yellow brasses, silicon brass, etc.) are 10.0 - 12.0 X 10" / 'F (18.021.6 x 10路/ 0C).' Thennal expansion coefficients for copper casting alloys are given in Table 4, page 42.
Elastic Properties, Magnetic Properties. Copper is a diamagnetic metal, i.e., it has a negative magnetic susceptibility and is weakly repelled by magnetic fields. This property is shared by many copper alloys. On the other hand, high strength yellow brasses (manganese bronzes), copper-nickel alloys
Stress-strain curves for copper alloys have the rounded shape that signifies continuous yielding. Since there is no fixed yield point, yield strengths must be defined in terms of a given amount of engineering strain or extension under load. The strain values most often used are 0.2% offset and 0.5%
69