Microstructural evolution during directional solidification of TiAl alloys grown from Ti-43Al-3Si (at%) seeds was studied. By using an appropriately oriented seed, the TiAl/Ti3Al lamellar microstructure was aligned parallel to the growth direction at growth rates of 36 and 54 mm/h. When the growth rate reached 90 mm/h, the lamellae could not remain parallel to the growth direction after a certain solidification length. It was found that new beta grains nucleate among the original alpha columnar crystals, and the lamellar orientation changes to an angle of 45 degrees to the growth direction beyond the transition point. Based on the equilibrium binary TiAl phase diagram, the solidification behavior of hypo-peritectic and hyper-peritectic alloys was analyzed. The results suggest that hyper-peritectic alloys should be used as master ingots to obtain a successful seeding of TiAl alloys. Using the nucleation and constitutional undercooling criteria, the nucleation undercooling for the beta phase in the Ti-47Al alloy during the seeding procedure was calculated to be 5.74 K at the given temperature gradient of 400 K/cm. Therefore, a high temperature gradient and a low growth rate were required to keep the alpha phase growing in a stable way and to prevent nucleation of the beta phase. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.