High temperature phase equilibria near Ti-50 at% Al composition in Ti-Al system studied by directional solidification

Abstract

High temperature phase diagram near the stoichiometric composition of TiAl has been established by the directional solidification and quenching technique. The quenched dendrite morphologies showed that the first solidified phase was the beta phase in Ti-44, 46, 48 at% Al alloys and the alpha phase in Ti-50, 52 at% Al alloys. From the EDS analysis of the quenched dendrite tips and measurement of the temperature gradient directly recorded during directional solidification in Ti-(44-52 at%)Al alloys, the solid-liquid phase equilibria could be determined. The phase transformation temperatures were also confirmed by DTA. The phase equilibria established in this study agreed with the phase diagram that Okamoto proposed, while the beta+gamma two phase region, which Murray suggested, was not found near the TiAl composition. The lamellar orientation in TiAl alloys has been reported to be controlled in the growth direction in the presence of the primary beta phase from a liquid. A composition in which the liquid phase was fully transformed to the beta phase was selected, Ti-44 at% Al alloy, and directional solidification was performed at the growth rate of 45 mm/h. It was found that the lamellar orientation was aligned at nearly 0 degrees and 45 degrees to the growth direction. It is thought that the success in controlling the lamellar orientation is due to beta solidification of the Ti-44 at% Al alloy at the beginning of liquid/solid transformation. (C) 1999 Elsevier Science Ltd. All rights reserved.