The site occupancies of alloying elements in TiAl and Ti3Al alloys

The site occupancies of V, Cr, Mn, Fe, Ni, Zr, Nb, Mo, Ta, Ga and Sn (1-5 a t.%) in TiAl alloys with different compositions, and in Ti3Al with the comp ositions of Ti-26 at.%Al-(1-2 at.%)X, were measured by the atom location ch annelling enhanced microanalysis (ALCHEMI) method. For TiAl alloys, the res ults show that Zr, Nb and Ta atoms invariably occupy Ti sites, while Fe, Ni , Ga and Sn atoms occupy Al sites, the alloy composition having no signific ant 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 particular), the probability of these elements substituting for Ti decr easing in the above order. For quaternary Ti-Al-V-Cr alloys, the site occup ancies of V and Cr do not show much mutual influence. In general, with incr easing 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 Ti3Al alloys, Ga and Sn atoms occupy Al sites, while V, Cr, Mn, Zr, Nb, Mo and Ta atoms occupy Ti sites, the site preference of V, Cr, Mn and Mo in TiAl alloys being different from that in Ti3Al. The experimental results are interpreted in terms of a Brag g-Williams-type model and bond-order data obtained from electronic structur e calculation. Qualitative agreement between the model and measurements is reached. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.