Roles of impurities on precipitation kinetics of dilute Al-Sc alloys

High-purity (HP) aluminum and commercial purity (CP) aluminum (major impurities: ∼250at.ppm Si and ∼130at.ppm Fe) are alloyed with ∼250 to ∼1100at.ppm Sc and ∼50at.ppm RE (RE=La, Ce, Pr, or Nd). The alloys are homogenized at 640°C and aged at 300°C. The precipitation kinetics, basic mechanical properties, and microstructure are studied using AC electrical conductivity, microhardness measurements, scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy, and atom-probe tomography, respectively. The Fe and RE elements form micrometer-scale diameter Al_∼3(Fe,RE) primary precipitates, which have no effect on the mechanical properties. Silicon accelerates the precipitation kinetics of nanometer-scale diameter Al₃Sc precipitates, increasing their number density, thereby resulting in higher microhardness values for CP aluminum than the HP aluminum having the same Sc concentration. Additionally, the Sc equilibrium solubility in the α-Al matrix is estimated and Orowan's strengthening mechanism is confirmed for the Al₃Sc precipitates.