Synergistic Effect of La and TiB₂ Particles on Grain Refinement in Aluminum Alloy

Synergistic effect of TiB₂ (in form of Al-5Ti-1B) and La on grain refining results in Al-2Cu alloy was investigated. α-Al grains are significantly refined by Al-5Ti-1B. When trace La is added to the melt, further refinement is exhibited. Average grain size and nucleation undercooling of α-Al reduce first and then almost remain unchanged with La addition. Satisfactory grain refining result achieves when La addition level reaches 600 ppm. When more than 600 ppm La is added to the melt, La-rich particles form and the effect of solute La left in matrix on the microstructure almost no longer changes. Theoretical calculation results demonstrate that solute La segregates to Al melt/TiB₂ particles interface along with Ti and Cu prior to α-Al nucleation and the synergistic effect of La and TiB₂ particles on grain refinement mainly attributes to the enhancement in the potency of TiB₂ particles to heterogeneously nucleate α-Al by trace La addition.