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Chromium copper alloys are high copper alloys, containing 0.6 to 1.2% Cr. The chromium copper alloys are used for their high strength, corrosion resistance and electrical conductivity. The chromium copper alloys are age hardenable, which, in this case, means that a change in properties occurs at elevated temperature due to the precipitation of chromium out of the solid solution. The strength of fully aged chromium copper is nearly twice that of pure copper and its conductivity remains high at 85% IACS, or 85% that of pure copper.
These high strength alloys retain their strength at elevated temperatures. The corrosion resistance of chromium copper alloys is better than that of pure copper because chromium improves the chemical properties of the protective oxide film. Chromium copper has excellent cold formability and good hot workability. It is used in applications such as resistance welding electrodes, seam welding wheels, switch gears, cable connectors, circuit breaker parts, molds, spot welding tips, and electrical and thermal conductors that require strength. Chromium copper alloys are designated as UNS C18050 through C18600, the cast alloys are C81400 through C81540.
A fast cool prevents the chromium from precipitating out of the solid solution, so the resulting cast structure consists of a single phase alpha copper structure. The first material to solidify is pure copper, followed by a eutectic mixture of alpha and chromium. The alpha and chromium eutectic material forms a lamellar structure in the interdendritic regions. The microstructure of the wrought alloy consists of equiaxed, twinned grains of alpha copper solid solution.
The chromium precipitates, or hardening precipitates, can be very fine and may not be visible at low magnifications.
| Element | Content (%) |
|---|---|
| Cu | 99.1 |
| Cr | 0.9 |
| Properties | Metric | Imperial |
|---|---|---|
| Density | 8.89 g/cm3 | 0.321 lb/in3 |
| Melting point | 1070°C | 1958°F |
| Properties | Metric | Imperial |
|---|---|---|
| Hardness, Rockwell B (50.8 - 76.2 mm/2.00 - 3.00 in) | 65 | 65 |
| Hardness, Rockwell B (25.4 - 50.8 mm/1.00 - 2.00 in) | 70 | 70 |
| Hardness, Rockwell B (≤25.4 mm/1.00 in, heat treated) | 75.0 | 75.0 |
| Tensile strength at break (any thickness) | 379 MPa | 55000 psi |
| Yield strength (0.500, any thickness) | 379 MPa | 55000 psi |
| Modulus of elasticity | 117 GPa | 17000 ksi |
| Poissons ratio | 0.181 | 0.181 |
| Shear modulus | 49.6 GPa | 7200 ksi |
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