Discussion
For Al - 4.5 wt% Cu alloy, the correct sequence of precipitation during age hardening at room temperature is
Answer: A
The sequence is GP zone → θ″ → θ′ → θ.
In a supersaturated Al-4.5% Cu alloy, copper first clusters into Guinier-Preston zones, thin disc-shaped copper-rich layers fully coherent with the matrix. These give way to the coherent θ″ (sometimes called GP II), then to the semi-coherent θ′, and finally to the equilibrium, incoherent θ phase, CuAl2.
Each step lowers the free energy, and the coherent phases form first because their low interfacial energy makes them easiest to nucleate. The other options place θ′ before θ″ or the equilibrium θ before the transition phases, which reverses this order.
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