ATOMIC MODELING OF NB, V, CR AND MN SUBSTITUTIONS IN GAMMA-TIAL .2. ELECTRONIC-STRUCTURE AND SITE PREFERENCE

The effect of Nb, V, Cr and Mn on the lattice parameters of gamma-TiAl and the site preference of these transition metals were studied based on first-principles supercell calculations of the electronic structur e and total energy of ordered Ti-n-1 XAl(n) and Tl(n)XAl(n-1) compound s with X = Nb, V, Cr and Mn. For the calculation of optimized volumes by total energy minimization 4-, 8- and 32-atom cells were taken into account. Trends of c/a changes in connection with the analysis of the electronic structure are derived from 4-atom supercell results in cont inuation of a previously published Part 1 of our investigations. We fi nd that Mn has the strongest preference for Al sites because of strong stabilizing electronic structure effects. The substantial decreased v alue of the c/a ratio found for the case of Mn substituting for Al is related to a bcc-like local arrangement of Ti around the Mn atom. This geometrical situation is accompanied by the formation of a deep pseud ogap in the density of states. The Fermi energy falls precisely into t his pseudogap which distinctly separates Ti-Mn bonding and antibonding states. The site preference trend for the other substitutional compou nds containing X = Nb, V, Cr are also discussed in combination with th eir corresponding density of states.