Lamellar orientation control of TiAl alloys under high temperature gradient with a Ti–43Al–3Si seed

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 β grains nucleate among the original α columnar crystals, and the lamellar orientation changes to an angle of 45° 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 β 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 α phase growing in a stable way and to prevent nucleation of the β phase.