Orientation-dependent performance has been demonstrated in different materials consisting of nanoparticles on substrates. The fabrication of desirably oriented nanoparticles requires knowledge of orientation selection rules. Based on the Wulff–Kaishew theory, our analysis shows that the energy-favorable orientation(s), is influenced by the surface energy of particles, in addition to the dominant factor, i.e., the energy difference between particle/substrate interfacial energy and surface energy of the substrate. To verify this, a model system of dewetted Au nanoparticles on SrTiO₃ is studied. The {111}-terminated SrTiO₃ supports only {111}-orientated Au particles, with the lowest interfacial energy. On the other hand, {100}-terminated SrTiO₃ supports multiple Au particles, with {111}-, {100}-, {110}- orientations, as a possible result of close surface energy contributions. The above orientations can be additionally manipulated by changing the heat treatment temperature. Our results provide fundamental insights into fabricating supported nanoparticles for practical applications.

This study aims to bridge the critical knowledge gap in understanding the dynamic microstructural evolution during high-solid-fraction semi-solid rheo-die casting process, including slurry preparation (0.1–0.3 K/s) and rheo-die casting (10–150 K/s). A novel phase-field model coupling continuous cooling with explicit nucleation was developed, enabling the dynamic simulation of continuous solidification microstructure evolution, considering two-stage cooling rate transition characteristics. Integrated the Swirled Enthalpy Equilibration Device (SEED) slurry preparation and graded-cooling mold experiments established variable cooling rate and solid fraction conditions for quantitative analysis of α-Al morphological evolution during rheo-die casting solidification. Through experimental and simulation investigations of the Al-7Si alloy, it is concluded that during Stage I slurry preparation, the primary α₁-Al phase coarsened due to Ostwald ripening. In Stage II rheo-die casting, primary α₁-Al undergoes continued growth under a moderate cooling rate (15 K/s). Meanwhile, secondary α₂-Al formation exhibits a cooling-rate and solid fraction dependence: a high cooling rate (150 K/s) promotes explosive nucleation with the volume fraction decreasing from 4.78% to 0.33% as the solid fraction rises, whereas a mid-cooling rate (15 K/s) substantially suppresses its formation. Mechanistically, a high cooling rate promotes solute trapping, which intensifies constitutional undercooling, thereby elevating both the nucleation and growth driving forces to facilitate the formation of secondary α₂-Al, whereas higher solid fractions restrict secondary phase formation by narrowing the solidification windows from 22 °C to 7 °C.

The synergistic effects of solute concentration and cooling rate on the evolution of secondary α₂-Al during high-solid-fraction rheo-diecasting of Al-xSi (x = 1, 4, 7 wt.%) alloys was studied. Combined gradient-cooling experiments (100 vs. 10 K/s) and phase-field simulations show that the population and morphology of secondary α₂-Al are co-governed by initial Si content and cooling rate. Higher cooling rates promote finer, more uniform secondary α₂-Al in Al-1Si and Al-4Si, while lower cooling rates cause coarsening and coalescence. In addition, the formation of α₂-Al is severely suppressed in Al-7Si. Crucially, a lower initial solute concentration significantly amplifies cooling rate-induced solute enrichment, quantitatively evidenced by the final liquid concentration difference (Al-1Si: 0.83 wt.% > Al-4Si: 0.29 wt.% > Al-7Si: 0.13 wt.%). This enrichment governs the dynamic competition between constitutional and thermal undercooling, contributing a substantially greater driving force for early-stage nucleation in Al-1Si compared to Al-7Si. As solidification progresses in all three systems, the enrichment of the residual liquid narrows the solidification interval, thereby progressively elevating the role of thermal undercooling.