Effects of ternary additions on the twin energy and site preference in y-TiAl

Site preference and the effects of ternary additions (Mn, V, etc.) on the twin energy in y-TiAl were studied by means of all-electron total energy self-consistent calculations based on the local density approximation. The results show that when Mn, Ti, or V substitute on the Al-sites in y-TiAl, the twin energy is about 20-25% lower than that of stoichiometric y-TiAl; this may explain observations of increased twinning activity in 48% A1 TiAl. By contrast, when ternary additions of V (or Al) occupy Ti-sites, the twin energy has nearly the same value as that of pure y-TiAl, which is consistent with the observation of only a few twins for Al-rich compositions. By comparing the total energy of Ti6(XAl5) and (Ti5X)Al6 supercells, it is found that the relative stability of Ti6(XAl5) over (Ti5X)Al6 (i.e., the preference for occupying Al-sites) is increased in going from Nb, V, Cr, and Ti to Mn, in agreement with observation that excess Ti occupies Al-sites, and Nb preferentially substitutes on the Ti-sites. The results indicate that Mn preferentially substitutes on the Al-sites, and V (or Cr) may occupy both Ti- and Al-sites.