Precipitates in Al–Cu alloys revisited: Atom-probe tomographic experiments and first-principles calculations of compositional evolution and interfacial segregation

Atom-probe tomography, transmission electron microscopy, X-ray diffraction and first-principles calculations are employed to study: (i) compositional evolution of GPII zones and θ′ precipitates; and (ii) solute segregation at α-Al/θ′ interfaces in Al–1.7 at.% Cu (Al–4 wt.% Cu) alloys. GPII zones are observed after aging at 438 K for 8 h, whereas higher aging temperatures, 463 K for 8 h and 533 K for 4 h, reveal only θ′ precipitates. Most GPII zones and θ′ precipitates are demonstrated to be Cu-deficient at the lower two aging temperatures; only the 533 K treatment resulted in θ′ stoichiometries consistent with the expected Al2Cu equilibrium composition. For alloys containing ∼200 at. ppm Si we find evidence of Si partitioning to GPII zones and θ′ precipitates. Significant Si segregation is observed at the coherent α-Al/θ′ interface for aging at 533 K, resulting in an interfacial Si concentration more than 11 times greater than in the α-Al matrix. Importantly, the Si interfacial concentration undergoes a transition from a non-equilibrium delocalized profile to an equilibrium localized profile as the aging temperature is increased from 463 to 533 K. Consistent with these measurements, first-principles calculations predict a strong thermodynamic driving force favoring Si partitioning to Cu sites in θ′. Silicon segregation at, and partitioning to, θ′ precipitates results in a decrease in interfacial free energy, and concomitantly an increase in the nucleation current. Our results suggest that Si catalyzes the early stages of precipitation in these alloys, consistent with the higher precipitate number densities observed in commercial Al–Cu–Si alloys.