Grain Refinement Mechanism of Al-5Ti-1B Master Alloy by <i>Ab Initio</i> Calculations

Abstract

To exactly understand the grain refining mechanism of α-Al by the Al-5Ti-1B master alloy, the structural properties of α-Al/solid-TiB2 (S/S) and liquid-Al/solid-TiB2 (L/S) interfaces were studied using the first-principles method. Different ordered structures were formed on the interfaces with different terminations of TiB2 (0001) surface, which determines the nucleant potency of TiB2. The heterogeneous nucleation of α-Al on the B-terminated surface is frustrated by an AlB2-like structure formed at the interface. In contrast, a five-layer quasi-solid region with stacking sequence of fcc-Al (111) planes forms on the Ti-terminated TiB2 (0001) surface, which is the basis of successful heterogeneous nucleation of α-Al. Moreover, when redundant Ti solute being added into the liquid Al region of Ti-terminated liquid-Al/TiB2 interface, the quasi-solid Al region further extends until entire solidification. The reason for using the Al-5Ti-1B master alloy rather than TiB2 powders as the commercial refiner in Al industry lies in two aspects: the excessive Ti atoms in the master alloy could guarantee sufficient Ti chemical potential to form Ti-terminated surface of TiB2, and the redundant Ti solute in inoculated melts could facilitate the growth of quasi-solid Al region at the solid/liquid interface.