Effect of Nb, Ti introduction sequence on adsorption of Nb on TiB2 surface and grain refinement performance of Al-4Ti-1Nb-1B

In recent years, Al-Ti-Nb-B grain refiners have attracted increasing attention due to their grain refinement performance and anti-Si poisoning ability. This study investigates the influence of the introduction sequence of Ti and Nb during the synthesis of Al-4Ti-1Nb-1B refiners on their refinement performance on CP-Al and a series of Al-Si alloys (Al-3.5Si, Al-7Si, and Al-10.5Si). It is found that Al-4Ti-1Nb-1B prepared by introducing Ti prior to Nb exhibits the best grain refinement and anti-Si poisoning compared to samples where Nb is introduced before Ti or where both are added simultaneously. This Ti-first approach demonstrates superior grain refinement performance across CP-Al, Al-3.5S1, Ai-7Si and Al-10.5Si alloys, especially at higher Si contents. It refines the grain size of Al-7Si to 150.1±27.5 µm from over 1,500 µm for the unrefined alloy. This superior performance is attributed to the variation in ground-state energy ΔE for the Ti prior to Nb sequence is lower than that of other sequences, thereby facilitating Nb adsorption on the TiB2 surface. TEM observations corroborate these findings, showing that TiB2 prepared by this sequence has the highest average Nb content of 3.80at.%. First-principles calculations reveal that this unique Nb adsorption enhances the TiB2/Al interfacial adhesion energy Wad and suppresses the segregation tendency of Si atoms at the interface, κSi(cSi). The higher the Nb adsorption at the TiB2/Al interface, the stronger the resistance to Si poisoning. These findings underscore the pivotal role of Nb-modified TiB2 in improving grain refinement and offer a novel strategy for advancing grain refiner technologies in Al-Si alloys.