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8/30/04

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Theory of Interface Properties for Carbide Precipitates in TiAl R. BENEDEK, D.N. SEIDMAN, and C. WOODWARD Among the additives to TiAl alloys that have been investigated in recent years with the objective of improving high-temperature mechanical properties, particular attention has been given to carbon, which forms the carbide precipitates Ti3AlC (cubic perovskite) and Ti2AlC (hexagonal). Using the first-principles density-functional-theory code VASP, calculations of host-precipitate interface energies were performed for these two carbides. Calculations were first applied to coherent interfaces to determine the favored termination layers and parallel translation states. For the favored interface configurations, a correction is applied for the effect of misfit, to obtain an estimated interface energy for semicoherent interfaces. The correction is based on an approximate formulation recently presented by the authors. The perovskite is found to have a lower interface energy than the hexagonal phase, consistent with the experimental finding that the former nucleates homogeneously and the latter inhomogeneously.

I. INTRODUCTION

NUMEROUS dopants have been added to two-phase TiAl alloys in order to improve their high-temperature mechanical properties and corrosion resistance. Among those additives, the first-row elements B, C, and N, as well as Si, are known to form precipitates during aging at elevated temperatures,[1] as the
2-phase component of the two-phase mixture dissolves. Carbon has received considerable attention for its improvement of high-temperature strength and creep resistance as well as its contribution to the refinement of the lamellar structure.[2] This manuscript presents theoretical calculations of properties of the interface with -TiAl of the two carbide phases that have been observed experimentally to occur in C-doped alloys.[3] One of these is the perovskite-structure Ti3AlC, referred to as P-type, and the other is the hexagonal (H-type) phase Ti2AlC. Another ternary hexagonal phase, Ti3AlC2, is mentioned in the literature.[4] Basal-plane lattice constants for the two hexagonal phases are similar, and the structures differ only in the stacking sequence. Perhaps Ti3AlC2 would be observed as a precipitate in -TiAl only for higher C concentrations than are normally employed (tenths of a percent) and at higher annealing temperatures. II. METHOD Although continuum methods are widely employed to treat the elastic energy of inclusions in a matrix[5] and epitaxial interfaces,[6] such methods do not address the detailed chemistry of the constituent materials. The cohesive properties of

R. BENEDEK, Consultant, and D.N. SEIDMAN, Professor, are with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 30208. Contact e-mail: [email protected] C. WOODWARD, Senior Scientist, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, OH 45433-7817. This article is based on a presentation made in the