Atomic insights into heterogeneous nucleation and growth kinetics of Al on TiB2 particles in undercooled Al-5Ti-1B melt

Titanium diboride (TiB2) is an effective grain refiner of Al alloys in the industry that facilitates casting processes by forming uniformly refined microstructures. Although our understanding of the underlying refinement mechanisms has advanced, the atomic kinetics of heterogeneous nucleation of Al on TiB2 remains unknown. Here, we report atomic-scale observations of the heterogeneous nucleation and growth kinetics of Al on self-formed TiB2 particles by in situ heating of undercooled Al-5Ti-1B films. We demonstrate that an ordered Al monolayer forms on the Ti-terminated {0001}(TiB2) surface; then, the surrounding Al atoms are initiated to form an island-shaped Al nucleus with face-centered cubic {111} stacking without the assistance of a Ti-rich buffer layer. The interfacial lattice mismatch between {111}(Al) and {00 01}(TiB2) causes remarkable out-of-plane strain that decreases gradually with Al nucleus layers increasing to 6 atomic layers. The elastic strain energy originating from this interfacial strain increases the free energy of the Al/TiB2 heterostructure, hence impeding the rapid growth of the Al nucleus. We found that TiB2 particles stabilize the Al nuclei rather than activating their free growth into grains when the experimental undercooling Delta T is lower than the onset undercooling Delta T-fg in Greer's free growth model. Our findings provide an atomic-scale physical image of the heterogeneous nucleation and growth mechanisms of Al with inoculator participation and elucidate the strain-dependent growth kinetics of Al nuclei. (C) 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Automated summary

Titanium diboride (TiB2) is an effective grain refiner for Al alloys, but the atomic kinetics of heterogeneous nucleation of Al on TiB2 are still unknown. In this study, atomic-scale observations were made to investigate the heterogeneous nucleation and growth kinetics of Al on self-formed TiB2 particles. It was found that an ordered Al monolayer forms on the Ti-terminated {0001}(TiB2) surface, and then an island-shaped Al nucleus with {111} stacking is initiated without a Ti-rich buffer layer. The interfacial lattice mismatch causes significant out-of-plane strain, which gradually decreases as the Al nucleus grows in layers. TiB2 particles stabilize the Al nuclei rather than promoting their free growth into grains when the experimental undercooling is below a certain threshold. These findings provide insights into the atomic-scale mechanisms of heterogeneous nucleation and growth of Al with the participation of TiB2 and the strain-dependent growth kinetics of Al nuclei.