Key Takeaways & Executive Findings
- •• Introducing Ti before Nb during synthesis of Al-4Ti-1Nb-1B grain refiner yields superior grain refinement and anti-Si poisoning in Al-Si alloys, especially at high Si contents. • The Ti-first sequence reduces the ground-state energy variation (ΔE), facilitating Nb adsorption on TiB2 surfaces, as confirmed by TEM showing highest average Nb content (3.80 at.%). • First-principles calculations reveal that Nb adsorption enhances TiB2/Al interfacial adhesion (Wad) and suppresses Si segregation (κSi), thereby mitigating Si poisoning. • This study provides a novel strategy for designing advanced grain refiners for Al-Si alloys, with potential industrial applications in aerospace and automotive sectors.
Abstract
Abstract: 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.
1. Introduction
Cast Al-Si alloys are widely used in modern industries, such as aerospace, aviation, construction, and transportation, due to their excellent castability, lightweight nature, and corrosion resistance [1-4]. During the casting of Al-Si alloys, grain refinement is a critical process, as it effectively refines primary α-Al grains, thereby improving both the castability and mechanical properties of the alloys [5, 6]. Currently, the most common grain refinement strategy involves the addition of grain refiners into the Al-Si melt [7, 8], among which the Al-5Ti-1B grain refiner is the most extensively used. However, during practical applications, the presence of Si in the Al-Si melt can lead to the formation of silicides, such as TiSi2 and Ti5Si3 on the surface of TiB2 particles in Al-5Ti-1B [9, 10]. These silicides inhibit the heterogeneous nucleation of α-Al, resulting in a significant deterioration of grain refinement performance in Al-Si alloys [11, 12], especially Al-Si alloys with Si content exceeding 5wt.%, which is commonly known as the Si-poisoning effect.
In recent years, novel grain refiners have been developed to overcome the issue of Si-poisoning in Al-Si alloys. Nb has attracted attention due to its ability to form NbAl3 and NbB2 phases, which are analogous to TiAl3 and TiB2 in Al-5Ti-1B. Consequently, Al-Nb-B grain refiners are developed and have demonstrated better grain refinement performance in Al-Si alloys compared to conventional Al-5Ti-1B refiners [13-15]. This improvement is primarily attributed to the higher thermodynamic stability of NbAl3 compared to TiAl3, which makes it less reactive with Si to form Nb-based silicides such as NbSi2, Nb3Si, and Nb5Si3 [16-18]. However, the limited anti-fading performance of Al-Nb-B restricts its widespread industrial applications [19, 20]. To address this limitation, researchers have attempted to integrate the advantages of both Ti and Nb, leading to the development of Al-Ti-Nb-B grain refiners. Xu et al. [21] developed an Al-1.67Ti-3.33Nb-0.5B refiner and reported that (Nb, Ti)B2 particles acted as highly effective heterogeneous nucleation sites for α-Al in Al-Si alloys. Li et al. [22, 23] introduced Ti into the Al-Nb-B melt, forming (Nb, Ti)B2 core-shell structure with TiB2 as the core and NbB2 as the shell, which also exhibited excellent resistance to Si poisoning. Wu et al. [24] synthesized an Al-3.5Nb-1Ti-1B refiner containing (Ti, Nb)B2 particles with a distinctive sandwich-like Nb-Ti-Nb structure, achieving notable refinement performance in Al-Si alloys. Additionally, Li et al. [25-27] developed the TCB refiner based on the Al-Ti-B-C system, and Xue et al. [28] proposed an Al-4.2V-1.8B refiner. Both refiners demonstrated excellent refinement efficiency in Al-Si alloys and provide valuable insights into overcoming Si poisoning.
Among these recently developed grain refiners, Al-Ti-Nb-B has emerged as a promising and significant candidate. Our previous study [29] observed a nanoscale Nb adsorption layer on the (01 0) plan
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Hao Yi, Ying Cheng, Hua-rui Zhang, Hu Zhang (2026). Effect of Nb, Ti introduction sequence on adsorption of Nb on TiB2 surface and grain refinement performance of Al-4Ti-1Nb-1B. China Foundry. https://doi.org/10.1007/s41230-026-5182-6
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Frequently Asked Questions
What is the main finding of this study on Al-Ti-Nb-B grain refiners?
The study reveals that the introduction sequence of Ti and Nb during synthesis significantly affects the grain refinement performance of Al-4Ti-1Nb-1B. Introducing Ti before Nb yields the best refinement and anti-Si poisoning, especially in high-Si Al-Si alloys.
How does the Ti-first sequence improve grain refinement?
The Ti-first sequence lowers the ground-state energy variation (ΔE), facilitating Nb adsorption on TiB2 surfaces. This enhances the TiB2/Al interfacial adhesion and suppresses Si segregation, thereby mitigating Si poisoning.
What are the practical implications of this research?
The findings offer a novel strategy for designing advanced grain refiners for Al-Si alloys, which can improve mechanical properties and castability in industrial applications such as aerospace and automotive components.
What methods were used to validate the findings?
The study employed experimental synthesis, grain refinement tests on CP-Al and Al-Si alloys, TEM observations to measure Nb content, and first-principles calculations to analyze interfacial adhesion and Si segregation.
What is the significance of Nb adsorption on TiB2?
Nb adsorption on TiB2 enhances the interfacial adhesion energy (Wad) and suppresses Si segregation (κSi), which are critical for resisting Si poisoning and improving grain refinement efficiency in Al-Si alloys.
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