Nano-phase transformation of composite precipitates in multicomponent Al-Mg-Si(-Sc) alloys

Abstract

• Sc-atoms may distribute at the β"-precipitate/Al-matrix interface in an Al-Mg-Si-Sc alloy. • Sc-atoms can enter the B'-substructures and/or locate at the precipitate/Al interfaces. • Sc-atoms can greatly improve the stability of transitional β"/B'/β′ composite precipitates. • Sc-addition exhibits a much better thermal stability than that without Sc. Sc-addition can significantly enhance the performance of the micro-alloyed Al-Mg-Si-Sc alloys. However, the mechanisms by which the Sc element modifies the microstructure of the alloys are still unknown in many cases. Here, using atomic-scale transmission electron microscopy and atomic-resolution spectroscopy, we have revealed the microstructural differences between two age-hardened Al-0.5Mg-0.4Si (wt.%) alloys with and without Sc-addition. The first significant effect of Sc-addition on the precipitation microstructure of the Al-Mg-Si-Sc alloy is that Sc-atoms may distribute at the β"-precipitate/Al-matrix interface and therefore accelerate aging kinetics at the initial stage of hardening. The second significant effect of Sc-addition is that in the transition from the β"-hardened peak-age stage to the β′-hardened late stage, Sc-atoms can greatly improve the stability of transitional β"/B'/β′ composite precipitates by entering the B'-substructures and/or locating at the precipitate/Al interfaces. As such Sc-atoms effectively suppress β" to β' transformation and cross-sectional coarsening of both β" and composite precipitates, leading to much finer precipitate needles with smaller diameter but much larger length, as compared with those precipitate needles formed in the alloy without Sc-addition. Hence, the alloy with Sc-addition exhibits a much better thermal stability than that without Sc.