ATOMIC MODELING OF NB, V, CR, AND MN SUBSTITUTIONS IN GAMMA-TIAL .1. C/A-RATIO AND SITE PREFERENCE

The Linear Muffin Tin Orbital Method was applied to calculate from first principles the electronic structure and total energies of L1(0) ordered TiXAl2 and Ti2XAl (X = Nb, V, Cr, Mn) compounds. Volumes and lattice parameters were determined by minimizing total energies. If X substitutes for Ti, the ratio c/a of the lattice parameters is always slightly larger than for the reference compound gamma-TiAl. In the case of Al substitutions, c/a decreases substantially for all X. In general, the Ti-rich Ti2XAl compounds have larger unit cell volumes than the Al-rich counterparts. For any particular X, the interatomic distances between X and Ti appear to be rather constant, irrespective of the chosen substitution site. These two findings are sufficient to explain the substantial reduction of c/a for the Ti-rich (or Al-substituted) cases obtained by the calculations. From the formation energies, DELTAE, the site preference was derived by E(site) = DELTAE(Al-rich) - DELTAE(Ti-rich) combining site preferences and c/a changes, we found that Nb and V prefer the Ti sites whereas Cr and Mn tend to substitute for Al. We conclude that X = Nb or V slightly increases c/a whereas X = Cr or Mn decreases this ratio. Although our calculations were made for 25.at% substitutions, many of our results are in agreement with experimental data for alloys with a small percentage of x.