Ultrasonic refining mechanism of ternary phase in Al–Sc based alloys prepared through molten salt electrolysis

Aluminum alloys are widely used in cutting-edge technologies and emerging strategic industries, namely aerospace, high-speed rail transportation, electric vehicles, advanced functional materials, new energy storage, and conversion devices. The processability as well as the mechanical properties of aluminum alloys can be improved via the addition of trace scandium. The ultrasonically assisted molten salt electrolysis is a promising, short technical route for large-scale preparation of low-cost, Al–Sc-based alloys characterized by uniform and fine strengthening phases. At present, it is still unclear if that is the case for the ultrasonic refining mechanism of the Sc-bearing ternary phase. This study aims at clarifying the ultrasonic refining mechanism on the strengthening phase containing scandium. Two Al–Sc based alloys were prepared using ultrasonically assisted molten salt electrolysis while the effect of ultrasound on the morphology and size of the Sc-bearing ternary phase was studied using optical microscope, scanning electron microscope, and X-ray diffraction meter. The results show that the synergetic ultrasound facilitates the transformation of the ternary AlSiSc phase from the coarse rhombic tubes (~205 μm) to the short rod (40 μm). The cluster size of ternary AlCuSc phase is also greatly reduced from ~100 μm to ~30 μm. The ultrasonic refining mechanism is mainly related to the increase of the nucleation rate of the primary Al₃Sc particles which are greatly refined and dispersed in the alloy melt before the solidification stage. The refinement of the Sc-bearing ternary phase is considered to be caused by the fine and disperse Al₃Sc particles serving as nuclei. Furthermore, ultrasound can also aid the uniform distribution of solute field and prevent the precipitation of coarse Al₃Sc phase. The effect of ultrasonic refinement of the ternary rhenium-containing phase is mainly present at the solidification stage after electrolysis.