Alignment of the TiAl/Ti3Al lamellar microstructure in TiAl alloys by growth from a seed material

Abstract By using an appropriately oriented seed from the TiAlSi system, the TiAl/Ti 3 Al lamellar microstructure was aligned parallel to the growth direction in a number of directionally solidified TiAl-based alloys. The seed composition was kept constant at Ti-43Al-3Si (at. %) and the composition of the master ingots was varied for alloys in the TiAlSi, TiAlNbSi, and TiAl systems. The lamellar microstructure could be aligned for alloys containing up to approximately 47 at.% (Al + Si) in each of these systems. For the composition of seed material, Ti-43Al-3Si, where alpha is the primary solidification phase, the original orientation of the lamellar microstructure was maintined after heating to and cooling from the single-phase alpha region making seeding of the alpha phase possible. When this silicon containing seed was used, the nucleation of the primary beta phase in master alloys of Ti−(47 − x )Al− x Si( x = 0 to 1 at.%) could be suppressed resulting in the nucleation and growth of only the alpha phase with an orientation determined by the seed crystal. After steady state conditions were reached, single PST crystals with an aligned lamellar microstructure could be grown at growth rates as high as 40 mm/h.