Vacancy-tuned precipitation pathways in Al-1.7 Cu-0.025In-0.025Sb (at.%) alloy

Microalloying is a routine method to optimize precipitation and mechanical properties in light metals. Here we study how In and Sb (0.025 at.%) additions in an Al-1.7 at.% Cu alloy benefit precipitation during ageing and investigate the underlying mechanism using scanning transmission electron microscopy and density functional theory (DFT) calculations. The combined additions accelerate precipitation kinetics and increase peak hardness through two different ways. In samples aged directly at low temperatures (≤200 °C) after solid solution treatment, cubic close-packed InSb nanocrystals form first. The truncated {002} surfaces of InSb particles induce the preferential nucleation of Guinier-Preston (GP) zones, θ″ and θ′ successively. However, in samples aged at 250 °C, precipitation of θ′ precedes that of InSb particles. Supersaturated vacancies trapped by In and Sb solute atoms/clusters play a critical role in switching the precipitation sequence. By tuning how strong the binding between a vacancy and solute atoms is, we successfully invert the temperature dependence of the precipitation sequence. These findings will contribute to understanding precipitation mechanisms and optimizing precipitate distribution in aluminium precipitation hardenable alloys.