A new perspective on the precipitation sequence in a high-purity Al-1.74 at.% Cu alloy by employing positron annihilation spectroscopy: experiment and theory

We study Al-1.74 at.% Cu as a typical binary model alloy to obtain new information on the precipitation sequence. The alloy has been solution heat treated, rapidly quenched, and then isochronally annealed up to 540 °C. We reveal new effects on the evolution of the precipitation sequence by combining positron annihilation lifetime and coincidence Doppler broadening spectroscopies. Positron parameters do very sensitively respond to changes in the type of precipitates present. We find that in the as-quenched state after exposing the alloy just a few minutes to room temperature GP-I zones containing vacancies inside their copper platelet dominate. They represent about 80% of all detected GP-I zones. At 200 °C GP-II (Θ″) is the dominating type of precipitate. However, the GP-II zones have obviously expelled all structural vacancies on growing from larger GP-I zones. Also the transition from GP-II zones dissolving while precipitates of the Θ′ phase do form at about 250 °C can be clearly observed. The signals support that Θ′ precipitates contain vacancies on their Cu sublattice, since they have to grow in a copper-poor environment. Finally, our signals indicate coarsening of Θ′-precipitates for T > 400 °C and their dissolution until 450 °C. All our experimental results agree well with ab initio theoretical calculations of positron parameters. However, the formation of the equilibrium Θ-phase cannot be observed since these have a too low number density due to their large size.