THE SITE OCCUPANCIES OF ALLOYING ELEMENTS IN TiAl AND Ti_3Al ALLOYS

摘要

The site occupancies of V, Cr, Mn, Fe, Ni, Zr, Nb, Mo, Ta, Ga and Sn (1-5 at.) in TiAl alloys with different compositions, and in Ti_3Al with the compositions of Ti-26 at./Al-(1-2 at.)X, were measured by the atom location channelling enhanced microanalysis (ALCHEMl) method. For TiAl alloys, the results show that Zr, Nb and Ta atoms invariably occupy Ti sites, while Fe, Ni, Ga and Sn atoms oc- cupy Al sites, the alloy composition having no significant influence on their site preference. By contrast, the site preference of V. Cr, and Mn changes considerably with alloy composition (the Ti/Al ratio in par- ticular), the probability of these elements substituting for Ti decreasing in the above order. For quaternary Ti-Al-V-Cr alloys, the site occupancies of V and Cr do not show much mutual influence. In general, with increasing atomic number, elements in the same period show increasing tendency to substitute for Al, as is the tendency to substitute for Ti for elements in the same group of the periodic table. For Ti_3Al alloys, Ga and Sn atoms occupy Al sites, while V, Cr, Mn, Zr, Nb, Mo and Ta atoms occupy Ti sites, the site prefer- ence of V, Cr, Mn and Mo in TiAl alloys being different from that in Ti_3Al. The experimental results are interpreted in terms of a Bragg-Williams-type model and bond-order data obtained from electronic struc- ture calculation. Qualitative agreement between the model and measurements is reached.