Microstructure and mechanical properties of a precipitation-strengthened Al-Zr-Sc-Er-Si alloy with a very small Sc content

Abstract

The precipitation hardening behavior of an Al-0.08Zr-0.014Sc-0.008Er-0.10Si (at.%) alloy was investigated utilizing microhardness, electrical conductivity, atom-probe tomography (APT), and compressive creep-measurements. This new composition, with a Sc:Zr atomic ratio of less than 1:5 represents a significant reduction of the alloy's cost when compared to the more usual Al-0.06Sc-0.02Zr based alloys with typical Sc:Zr atomic ratios of 3:1. To study the precipitation behavior of this low-Sc alloy, isothermal aging experiments between 350 and 425 °C for a duration of up to 6 months were performed. The low concentration of Sc, compensated by the high Zr concentration, permits the alloy to achieve a higher peak microhardness than the corresponding Sc-richer, Zr-leaner alloys. The low-Sc alloy also shows better over aging resistance, as anticipated from the smaller diffusivity of Zr when compared to Sc, leading to slower coarsening kinetics. Atom-probe tomography demonstrates that the high microhardness is due to the formation of a high number density of nano-precipitates, ∼1023 m−3 for peak aging conditions, with a mean radius of 1.9 nm, thus yielding a high volume fraction (0.35%) of nano-precipitates. Like alloys with much higher Sc and Er concentrations, the (Al,Si)3(Sc,Zr,Er) nano-precipitates still exhibit a core-shell structure with a concentration of Zr in the shell of up to 25 at.%, and a Sc- and Er-enriched core. Compressive creep experiments at 300 °C demonstrate that the new alloy, with only 0.014 at% Sc, is as creep resistant as a binary Al-0.08Sc at.% alloy, displaying a threshold stress of 17.5 ± 0.6 MPa at peak aged condition.