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

High-purity (HP) aluminum and commercial purity (CP) aluminum (major impurities: ∼250 at. ppm Si and ∼130 at. ppm Fe) are alloyed with ∼250 to ∼1100 at. ppm Sc and ∼50 at. 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.