An estimate of the kinetics of the b 0 to orthorhombic phase transformation in the Nb±Ti±Al system Joseph C. Mishurda and Vijay K. Vasudevan * Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USA Received 4 December 2000; accepted 6 March 2001 Keywords: Phase transformations; Kinetics; Nb±Ti±Al alloys; Intermetallics; Thermal analysis; Orthorhombic phase Introduction The current generation of aerospace turbine materials are near their high temperature limits 90% T m ); new high temperature materials are required to push engine perfor- mance higher. Intermetallic alloys have been extensively studied as potential candidates for these high temperature structural applications. The intermetallic compounds of interest in the Ti±Al±Nb system include the Nb 2 Al-r phase, which though possessing excellent creep resistance and strength [1], is brittle, requiring ductile phase toughening from phases such as b to enhance structural performance. Knowledge of phase trans- formation paths is essential to design new alloys. A plethel section showing the relationship between b and r phases, presented in Fig. 1, displays the tie lines between the b and r phases with decreasing temperature. An abrupt change in slope of the tie lines is observed between 970°C and 990°C; there was also a corresponding endothermic peak on the DTA thermogram of the Nb±36Ti±30Al alloy starting at 981°C and peaking at 1000°C at 5°C/min) [2]. This peak corresponds to the transformation of b 0 ordered B2) to orthorhombic O) phase as a three phase ®eld of b , r O passes through the alloy very abruptly [2]. Based on the movement of the tie lines the transformation changes in characteristics from eutectoid b 0 to r O) to peritectoid b 0 r to O). This alloy is very near the four phase plane r d b Od-Nb 3 Al) and accounts for the existence of the necessary r b O to combine with r d b to produce the two d b O and d r O three phase ®elds. Scripta Materialia 45 2001) 677±684 www.elsevier.com/locate/scriptamat * Corresponding author. E-mail address: vvasudev@uceng.uc.edu V.K. Vasudevan). 1359-6462/01/$ - see front matter Ó 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-646201)01079-X