EFFECTS OF TERNARY ADDITIONS ON THE TWIN ENERGY AND SITE PREFERENCE IN GAMMA-TIAL

Site preference and the effects of ternary additions (Mn, V, etc.) on the twin energy in gamma-TiAl were studied by means of all-electron to tal energy self-consistent calculations based on the local density app roximation. The results show that when Mn, Ti, or V substitute on the Al-sites in gamma-TiAl, the twin energy is about 20-25% lower than tha t of stoichiometric gamma-TiAl; this may explain observations of incre ased twinning activity in 48% Al TiAl. By contrast, when ternary addit ions of V (or Al) occupy Ti-sites, the twin energy has nearly the same value as that of pure gamma-TiAl, which is consistent with the observ ation of only a few twins for Al-rich compositions. By comparing the t otal energy of Ti6(XAl5) and (Ti5X)Al6 supercells, it is found that th e 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 indica te that Mn preferentially substitutes on the Al-sites, and V (or Cr) m ay occupy both Ti- and Al-sites.