When an Al-Cu alloy is solution treated, quenched and aged, the precipitates change roughly in the sequence GP zones → θ'' → θ' → θ (Al₂Cu). Which statement about GP zones is correct?

Answer: A. They are disk-shaped clusters of Cu atoms one to a few atomic layers thick on the {100} planes of the matrix, fully coherent with it and able to form even during aging at room temperature

Cu atoms made supersaturated by quenching first gather on the {100} planes of the matrix to form disk-shaped GP zones one to a few atomic layers thick and a few nanometers across. They keep the crystal structure of the matrix and are fully coherent, and with their small interface energy the nucleation barrier is low, so with the help of excess vacancies retained by quenching they form even at room temperature and are responsible for the hardening of natural aging. As aging proceeds, the alloy passes through the coherent and semicoherent intermediate phases θ'' and θ' to the incoherent, coarse equilibrium phase θ and softens on overaging. The band near grain boundaries where Cu and vacancies are depleted and no precipitates form is the precipitate-free zone (PFZ), a different thing from GP zones. Coarse θ is not a GP zone but the equilibrium phase of the final stage of aging.