TiAl Seed Preparation & Fully Lamellar Structure Control Through Seeding And Directional Solidification

In this paper ,fully lamellar microstructure of TiAl-base alloys are studied through different methods, and alignment of the TiAl/Ti3Al lamellar microstructure is controlled through seeding and directional solidification, The main results are obtained as followed:(1) According to theoretical analysis and computation , by using self-designed mold and cold-crucible induction skull melting in vacuum, fully lamellar columnar grain microstructure of Ti54Al43Si3 alloy(as seed material) was aligned perpendicular to the growth direction in steel mould. According to relationship between lamellar microstructure orientation and phase , columnar grain was only existed a phase with consisting of TiAl (γ) and a small volume fraction of Ti3Al (α2) through SEM analysizing.(2) Directional solidification of TiAl-base alloys using a polycrystalline seed material are studied by using self-made EMCS experiment apparatus with wide range of velocity . Successful DS ingots are obtained with fully lamellar microstructure parallel to longitudinal axis using a polycrystalline seed material at different velocities of 1000μm/min and 1500μm/min in Ti54Al46 binary alloys and Ti54Al43Si3 ternary alloys. Under the drawing speed of 1000μm/min condition, the TiAl/Ti3Al lamellar microstructure was aligned parallel to the growth direction from seeding to drawing end, but in the ingots grown at the speed of 1500μm/min,lamellar microstructure is abruptly decreased after a certain point.upon solidifying beyond the transition point ,the lamellar microstructure tends to be perpendicular to growth direction . Microstructure structure appears other phase (β) or primary solidification phase a after transition zone.