Understanding the formation of (Al,Si)3Sc and V-phase (AlSc2Si2) in Al-Si-Sc alloys via ex situ heat treatments and in situ transmission electron microscopy studies

Highlights: • Effect of Sc additions to Al-Si (6xxx series Al-alloys) was shown to be profound. • (Al,Si)₃Sc nanoprecipitates in Al-Si-Sc alloys result in significant hardening. • Rod-shaped V-phase (AlSc₂Si₂) formed was deleterious to mechanical properties. • Multiple orientation relationships of V-phase with aluminium were determined. • In situ TEM heating used to study precipitation mechanisms of Sc-containing phases. -- Abstract: Scandium (Sc) containing Al-Si model alloys were heat-treated at various temperatures to determine the interaction between Si and Sc. The (Al,Si)₃Sc phase was formed at ~350 °C and V-phase (AlSc₂Si₂) was observed to form at above ~450 °C, as determined from x-ray diffraction and selected area electron diffraction. Hardening of the alloys was observed due to the formation of coherent, nanoscale (Al,Si)₃Sc precipitates; whereas significant softening was observed upon the formation of coarse V-phase. Herein, the V-phase is studied in detail and has for the first time been revealed to have a rod morphology. The Transmission electron microscopy (TEM) allowed for the determination of six new orientation relationships (ORs) with the α-Al matrix for the V-phase. Additionally, in situ TEM heating experiments at 550 °C revealed that the formation of V-phase occurs due to a nucleation and growth mechanism following the dissolution of Si particles and nanoscale (Al,Si)₃Sc precipitates.