Segregation at ! - Al / " ! - precipitate interfaces in Al - Cu alloys : Atom - probe tomographic experiments and first - principles calculations

Atom-probe tomography, transmission electron-microscopy, x-ray diffraction, and firstprinciples calculations were employed to study the (i) compositional evolution of GPII-zones and "!-precipitates, and (ii) solute segregation at !-Al/"!-interfaces in Al-4 wt. % Cu alloys. GPII zones are observed for aging at 438 K for 8 h, whereas higher aging temperatures, 463 K for 8 h and 533 K for 4 h, reveal only "!-precipitates. Most GPII-zones and "!-precipitates were found to be Cu-deficient at the lower aging temperatures; only the high temperature treatment (533 K) resulted in "! stoichiometries consistent with the expected Al2Cu equilibrium composition. For alloys containing ca. 200 at. ppm Si we find evidence of Si partitioning to GPII-zones and "!precipitates. Significant Si segregation is observed at the coherent !-Al/"!-interface, with aging at 533 K resulting in an interfacial Si concentration more than 11 times greater than in the Al matrix. Importantly, the Si interfacial concentration profile undergoes a transition from nonmonotonic to confined as the aging temperature is raised from 463 K to 533 K. Consistent with these measurements, first-principles calculations predict a strong thermodynamic driving force favoring Si partitioning at Cu sites in "!. Silicon segregation and partitioning to "!precipitates results in a decrease in interfacial free energy and concomitantly an increase in the rate of precipitate nucleation. 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.