Microstructural development during directional solidification of α-seeded TiAl alloys

Microstructural selection during the directional solidification of binary TiAl alloys grown from Ti–43Al–3Si (at.%) seeds was examined. By using a seed crystal, the high-temperature hexagonal close packed α-phase can be correctly oriented so that an aligned lamellar TiAl/Ti3Al microstructure results from the subsequent solid-state transformations upon cooling. From the equilibrium phase diagram, primary body-centered cubic β-phase solidification is expected in the compositional range for which the binary TiAl alloys were successfully seeded. Thus successful crystal growth of TiAl was found to be dependent upon the undercooling necessary for β nucleation. From these data, a microstructural selection map for the seeded growth of the α-phase was constructed and used to predict the compositional range for seeding TiAl alloys grown in CaO crucibles. The compositional range of the seeding criterion was then extended to multicomponent alloys by using an equivalent-Al concept to predict the shift in the liquidus surface for primary α and β solidification.