Density of thermal vacancies in gamma-Ti-Al-M, M = Si, Cr, Nb, Mo, Ta or W

Modifications to alloy chemistry are often used to tailor the intrinsic flo w behavior of structural materials. Models of creep in intermetallic alloys must account for the influence of chemistry on the available intrinsic cre ep mechanisms. As in simple metals the presence of vacancies strongly influ ences bulk diffusion processes in these materials. Limiting the density of constitutional and thermal vacancies by alloying may produce materials with enhanced creep properties. The energy of intrinsic and substitutional poin t defects in L1(0) TiA1 is calculated within a first principles, local dens ity functional theory framework. Relaxed structures and energies for vacanc ies, antisites and solid solutions are calculated using a planewave-pseudop otential method. Calculated defect energies are used within a canonical ens emble formalism to estimate the point defect densities as a function of tem perature and composition. The density of vacancies is found to be sensitive to the underlying stoichiometry of TiA1. The dependence of the vacancy con centration for solid solutions of Si, Cr, Nb, Mo, Ta and W is also predicte d. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All r ights reserved.