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When copper and aluminum are alloyed, the resulting material is a metal alloy, a mixture that lacks a fixed chemical formula. Instead, alloys are defined by the weight percentages of their constituent metals.
At 550 °C (1,022°F), liquid copper dissolves into molten aluminum, forming a homogeneous solid solution. This solution can contain up to 5.6 % copper by weight—the maximum solubility before saturation. Cooling reduces copper’s solubility, driving the solution into a supersaturated state.
During cooling, copper atoms diffuse through the aluminum matrix and precipitate as the intermetallic compound CuAl2. This phase has a fixed stoichiometry—two aluminum atoms for every copper atom—and represents 49.5 % aluminum by weight, giving it a definitive chemical formula.
The crystallization of CuAl2 disrupts slip planes in the aluminum lattice, a mechanism known as precipitation hardening. By carefully controlling the temperature‑time profile during processing, manufacturers can maximize this hardening effect, resulting in stronger copper‑aluminum alloys.
In addition to CuAl2, the system can produce the intermetallics CuAl and Cu9Al4 over extended time periods and under specific thermal conditions. Their appearance depends on the exact cooling rate, residence time, and local composition within the alloy.
