Si-driven precipitation kinetics acceleration and its limits in Al-Sc-Zr alloys: Insights from SAXS, APT, and modelling

The industrial use of Sc-containing aluminium alloys will depend on understanding their sensitivity to impurities like silicon (Si), particularly as alloy recycling becomes more prevalent. This study investigates the impact of Si additions (0.01–0.5 wt.%) on precipitation kinetics in an Al–0.1Sc–0.1Zr alloy during isothermal ageing at 275 °C and 350 °C. Combining in-situ small-angle X-ray scattering (SAXS), atom probe tomography (APT), and Kampmann-Wagner numerical modelling, we capture the evolution of Al3Sc particle size, volume fraction, and chemistry. Our results confirm that Si accelerates Al3Sc formation and increases hardness at both temperatures, up to an optimal concentration, after which excessive Si appears to trap vacancies, slowing down further growth. APT reveals significant Si incorporation into early-stage precipitates, suggesting a cluster-to-precipitate transformation pathway. The model captures the strong temperature and composition dependent kinetics, highlighting the complex interplay between Si, Sc, and vacancy availability. These findings provide new mechanistic insights relevant to impurity-tolerant alloy design.