Which is the correct sequence for precipitation hardening an Al-Cu alloy (duralumin type)?

Answer: A. Solution treatment → quench → aging

First, a solution treatment holds the alloy above the solvus to dissolve the Cu, and quenching carries the supersaturated solid solution down to room temperature. Holding it at room temperature (natural aging) or at about 100–200 °C (artificial aging) then forms fine precipitates that are coherent or semicoherent with the matrix, such as GP zones, θ″ and θ′, which obstruct dislocation motion and raise the strength. Aging for too long coarsens the stable θ phase (Al₂Cu) and lowers the strength, which is overaging. Slow cooling after solution treatment precipitates coarse equilibrium phases that do not help strengthening, so the quench is essential.