Atomistic-scale study on the role of CNT as a nucleating agent of α-Al in Al-CNT composite

Ex situ reinforcement of the A356 alloy with multi-walled carbon nanotubes (MWCNTs) markedly refines the solidification microstructure by increasing the nucleation density of the primary aluminium phase. Atomic-scale simulations are performed in the present work to capture the mechanism behind the increased nucleation of α-Al grains during the solidification of the A356 alloy due to the presence of a CNT within the melt, employing density functional theory and molecular dynamics simulations. First-principles DFT calculations are performed within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) functional with projector augmented wave (PAW) pseudopotentials to reliably capture the highly localized chemical bonding, charge transfer, and electronic hybridization at the metallic Al-CNT contact plane. Interfacial energy parameters, electronic density of states (DOS), and phonon density of states (PDOS) are extracted using density functional theory for the Al-CNT composite structure. Furthermore, the changes in the diffusion coefficient of Al atoms in the liquid state due to the presence of the CNT are calculated using the molecular dynamics simulation technique. The simulation results demonstrate strong chemical bonding at the Al-CNT interface, the presence of the CNT as a stiff substance within the Al matrix in the melt state and a reduction in the diffusion coefficient of Al atoms due to CNT inclusion, which are in excellent agreement with the experimental observations of the increased heterogeneous nucleation of α-Al grains during the solidification of the Al-CNT composite.