Abstract: 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 {0001}_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 $\text{ }\!\!\Delta\!\!\text{ }T$ is lower than the onset undercooling $\text{ }\!\!\Delta\!\!\text{ }{{T}_{\text{fg}}}$ 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.

Key words: In situ tem, Heterogeneous nucleation, Lattice strain, Aluminum, TiB₂ particles