Revealing the mechanism of grain refinement and anti Si-poisoning induced by (Nb, Ti)B2 with a sandwich-like structure

Abstract

As a promising anti Si-poisoning grain refiner, Al-Nb-(Ti)-B has gained increasing attention in the past several years. However, its exact underlying mechanism remains to be explored, and further improving the grain refinement efficacy whilst maintaining the anti Si-poisoning performance is imperative to promote its industrial application. In this work, a new master alloy (Al-3.5Nb-1Ti-1B) was developed by incorporating Ti into Al-3.5Nb-1B, which presents an excellent grain refinement performance for Al-10Si. To disentangle the role of Ti in enhancing the anti Si-poisoning ability of Al-Nb-B, the Ti-adsorption reconstructed NbB2/Al interface and its effect on Si adsorption and α-Al nucleation were investigated by using the high-resolution electron microscopy supplemented with ab initio calculations. The NbB2 substrate, TirichB2 region and NbrichB2 shell make up a sandwich-like (Nb, Ti)B2 substrate, whose formation is attributed to the smaller interfacial energy of Al with (0001) NbB2 than that with (0001) TiB2. It is revealed that existence of the NbrichB2 shell decreases the Si adsorption propensity (κSi) and increases the chemical affinity of Al (Wad); meanwhile, formation of the TirichB2 region reduces the lattice mismatch (f) of the (Nb, Ti)B2/Al interface. These three factors contribute to an excellent grain refinement ability of the sandwich-like (Nb, Ti)B2 substrate for Al alloys even with a high Si content. This study sheds new light on designing innovative anti Si-poisoning grain refiners by controllable preparation of multi-structure (Nb, Ti)B2 particles through the analyses of interfacial energy and the κSi, Wad and f factors.