Nb adsorption effect and mechanism on Al/TiB₂ interface in novel low-Nb Al-Ti-Nb-B grain refiner

Al–Ti-Nb–B grain refiner has attracted growing attention in recent years as a promising grain refiner with excellent resistance to Si poisoning. The adsorption behavior of Nb on the TiB₂ surface in Al-Ti-Nb-B grain refiners and its effect on grain refinement in Al-Si alloys are investigated in this study. The Al-4Ti-1Nb-1B, with an optimized Nb content, achieves a critical balance between the resistance to Si-poisoning by Nb and the grain growth restriction by Ti. Al-4Ti-1Nb-1B significantly refines the grain size of Al-7Si to 196.41 ± 11.41 μm, while maintaining exceptional anti-fading performance, 213.64 ± 22.67 μm after 120 min. In contrast, Al-4.5Ti-0.5Nb-1B and Al-4.9Ti-0.1Nb-1B, which contain lower Nb, show a grain size of 252.64 ± 26.11 μm and 303.31 ± 37.02 μm, larger than the prediction of free growth model due to Si-poisoning effect. TEM observations and first-principles calculations reveal a minimum Nb content of 1.92 at.% required to promote TiB₂ as nucleation sites and resist Si-poisoning. Al-4Ti-1Nb-1B, with 2.01 at.% Nb on the TiB₂ surface, slightly exceeds the threshold of 1.92 at.% and achieves an optimal balance of Ti and Nb content. This balance ensures sufficient Nb to resist Si poisoning while maintaining adequate Ti to restrict the growth of α-Al grains, thereby providing the most effective grain refinement for Al-7Si. These findings open new avenues for refining Al-Si alloys and advancing the development of grain refiners.