Effect of Si addition on the precipitation and mechanical/electrical properties of dilute Al–Zr-Sc alloys

Microstructural evolution and mechanical/electrical properties of aged Al-0.2 wt%Zr-0.05 wt%Sc alloys with different Si additions (0, 0.05, 0.10 and 0.15 wt%) were comparatively studied by using transmission electron microscopy, electrical conductivity measurements, and microhardness measurements. A significant Si addition effect was demonstrated that the Si addition accelerated the precipitation kinetics and enhanced the heterogeneous precipitation of Al3(Sc, Zr) nanoparticles. However, both the peak-aging temperature and peak microhardness during isochronal aging were insensitive to the Si addition. The underlying mechanism is related to a partitioning effect of Si among Sc, Zr and vacancy that produces a saturated concentration of Si-vacancy clusters to serve as heterogeneous nuclei. It was further manifested that dual-scale particles, i.e., Al3Zr dispersoids and Al3(Sc1-xZrx) nanoprecipitates in different length scales, could be produced by deliberately designing the heat treatment protocol. A coupling effect of the dual-scale particles led to an enhanced threshold stress in high-temperature creep testing, indicative of an improved creep resistance promoted by the minor Si addition. Moreover, the recrystallization resistance was seldom affected by Si addition, while slightly improved by introducing dual-scale particles.