Role of silicon in accelerating the nucleation of Al ₃(Sc,Zr) precipitates in dilute Al-Sc-Zr alloys

The effects of adding 0.02 or 0.06 at.% Si to Al-0.06Sc-0.06Zr (at.%) are studied to determine the impact of Si on accelerating Al ₃(Sc,Zr) precipitation kinetics in dilute Al-Sc-based alloys. Precipitation in the 0.06 at.% Si alloy, measured by microhardness and atom-probe tomography (APT), is accelerated for aging times <4 h at 275 and 300 °C, compared with the 0.02 at.% Si alloy. Experimental partial radial distribution functions of the α-Al matrix of the high-Si alloy reveal considerable Si-Sc clustering, which is attributed to attractive Si-Sc binding energies at the first and second nearest-neighbor distances, as confirmed by first-principles calculations. Calculations also indicate that Si-Sc binding decreases both the vacancy formation energy near Sc and the Sc migration energy in Al. APT further demonstrates that Si partitions preferentially to the Sc-enriched core rather than the Zr-enriched shell in the core/shell Al ₃(Sc,Zr) (L1 ₂) precipitates in the high-Si alloy subjected to double aging (8 h/300 °C for Sc precipitation and 32 days/400 °C for Zr precipitation). Calculations of the driving force for Si partitioning confirm that: (i) Si partitions preferentially to the Al ₃(Sc,Zr) (L1 ₂) precipitates, occupying the Al sublattice site; (ii) Si increases the driving force for the precipitation of Al ₃Sc; and (iii) Si partitions preferentially to Al ₃Sc (L1 ₂) rather than Al ₃Zr (L1 ₂).