Primary Phase Selection Related to Liquid Local Structure Within Ti–Al–V Alloy Solidified During Free Fall

The primary phase and liquid local structure during the solidification of Ti–45 pct Al–x pct V (x = 0, 4, 8 and 12, atomic percent) alloys were systemically investigated by combining the drop tube technique and molecular dynamics simulations. In drop tube experiments, the microstructure of Ti–45 pct Al–4 pct V is composed of well-developed α₂ dendrites and its path is liquid → α (hexagonal close packed structure) → α₂. Whereas in Ti–45 pct Al–8 pct V and Ti–45 pct Al–12 pct V alloys, it is composed of lamellar α₂ and its path is liquid → β (body centered cubic structure) → α → α₂. In order to explore the reasons for the primary phase selection, classical molecular dynamics was performed to obtain the liquid local structure. The results reveal that the attractive effect between Al and V atoms is much stronger than Ti-Al and Ti-V atomic pairs in liquid Ti-45% Al-4% V alloy. The total content of 1441+1661 (BCC-type pairs) HA indices is broadly lower than that of 1421+1422 (HCP-type pairs) in liquid Ti-Al-V alloys. The addition of V can enlarge the content of 1441 + 1661 and reduce the content of 1421 + 1422, and it also causes the decline of the fraction of HCP atoms in Ackland–Jones analysis. Moreover, V addition makes the content of 〈0, 0, 12, 0〉 Voronoi polyhedron increase significantly. On the other hand, the total content of 1421 + 1422 goes down with the decrease of cooling rate when the temperature is below the solidification temperature, while the fraction of HCP atoms decreases. Liquid Ti–45 pct Al–4 pct V alloy solidified into crystals with HCP structure at a cooling rate of 10¹⁰ K/s. These results have contributed to the effect of liquid local structure on the primary phase selection during the solidification of Ti–Al based alloys.