Interface Structure and Energy Calculations for Carbide Precipitates in -TiAl R. Benedek , D. N. Seidman , C. Woodward [*] Materials Science and Engineering Department, Northwestern University, Evanston, Il 60208 Air Force Research Laboratory Materials and Manufacturing Directorate, Wright Patterson AFB, OH, 45433-7817 November 22, 2002 Abstract Ternary carbide precipitates improve the high-temperature creep strength of 2-phase TiAl al- loys. The perovskite (P-type) Ti AlC nucleates at relatively low temperatures (750 deg. C), whereas hexagonal (H-type) Ti AlC precipitates occur at somewhat higher temperatures. Calculations are performed, based on first- principles-local-density-functional theory, of the interface structure and energy of these two carbides with a -TiAl matrix. Calculations are first done on coherent interfaces, and approximate corrections are then made for the effect of misfit. The perovskite is known to form needle-shaped precipitates oriented along the c-axis of the host. Our calculations yield a relatively low energy for the (100) perovskite-host interface, which is a favorable orientation owing to its low misfit, and because the terminating carbide layer for the coherent interface is pseudomorphic with the host. Predictions are given for the critical thickness for coherence and the critical nucleation size for a P-type precipitate. Calculations for interfaces of H-type platelets with the host show a much larger interface energy than that for the P-type precipitate. INTRODUCTION The possible utilization of TiAl as a superalloy replacement material is a tantalizing prospect, but techni- cal obstacles remain, amongst which the improvement of high temperature strength and creep resistance are particularly significant. Dispersion strengthening has been explored as a way to improve high tem- perature mechanical properties of TiAl. The first row elements, particularly C, N and O, have been investigated as additives, to introduce hard precipitates into the alloy matrix. Carbon has received the most attention [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], and is the subject of the present article. Two carbides have been observed in carbon-doped alloys. The perovskite (P-type) carbide Ti AlC, forms after aging at about 750 degrees C, and the hexagonal (H-type) phase, Ti AlC, forms at aging temperatures in the rage 800-900 degrees C. Another hexagonal carbide, with composition Ti AlC , is also known [11], but has not been observed as a precipitate in TiAl. To gain a better understanding of the strengthening behavior of carbide precipitates, it would be helpful to characterize their properties in a -phase TiAl host. In this article, we present calculations, based on first- principles methods [12], of the energy and chemistry of interfaces of the P- type and H-type carbides with -TiAl. This may be viewed as a first step toward the calculation of the interaction Mat. Res. Soc. Symp. Proc. Vol. 753 © 2003 Materials Research Society BB3.5.1