Structural changes during crystallization and vitrification of dilute FCC-based binary alloys

Structural mechanisms causing local coordination changes during crystallization and vitrification in dilute face-centered-cubic (FCC) alloys was investigated using model Al-Sm, which also serves as prototype for lightweight Al-rare earth (RE) structural alloys. Molecular dynamics simulations were performed to study the solidification behavior of Al-1at.%Sm and Al-5at.%Sm at 10(10), 10(11) and 10(12) K/s cooling rates. Two structural features were identified from these simulations, which are related to the molten state and the glass transition (T-g). In case of Al-1at.%Sm, we learn that, near the melting point, liquid phase manifested pockets of unique transitional structures comprising triangular arrangements in near-parallel layers that encapsulated a FCC-HCP coordinated core. We defined such a structure as the pre-critical nucleus, which is contained within an otherwise predominantly uncoordinated amorphous liquid phase. However, within the range of cooling rates employed, Al-5at.%Sm manifested only amorphous structure after solidification. The liquid structure in the transitional state contained temperature dependent icosahedron clusters that manifested as double-peak in the radial distribution function. Near the T-g Al-5at.%Sm achieves additional local ordering via the formation of inter-penetrating icosahedral frameworks. Combined, the structural mechanisms identified in Al-1at.%Sm and Al-5at.%Sm opens new alloy design pathways by (i) stabilizing pockets of pre-critical nucleus to form refined solidification microstructure, and (H) tuning T-g in the solid amorphous phase, respectively.