The influence of alloying on the alpha(2)/(alpha(2)+gamma)/gamma phase boundaries in TiAl based systems

By employing both experimental and theoretical approaches, a comparative st udy of the alpha(2)/(alpha(2) + gamma)/gamma phase boundaries in some Ti-Al -X (X = Nb, Ta, V, Cr, Mn, Fe or Ga) systems has been carried our. The phas e constitution of Ti-(33-51 at.%)Al-(w1-3 at.%)X alloys with a composition step of 1 at.% Al was determined experimentally by using the X-ray diffract ion method on bulk samples equilibrated at 1173 K. The alpha(2)/(alpha(2) gamma) and (alpha(2) + gamma)/gamma phase boundaries were calculated using a model that describes the phase boundaries in terms of sublattice site oc cupancies of alloying species in the two ordered phases. The differences be tween the predicted and experimentally determined phase boundaries were fou nd to be less than about 1 at.% in most cases. The predicted volume fractio ns of constituent phases were also compared with experimental measurements by a combination of metallography and transmission electron microscopy. Agr eement between the two is good for 14 ternary and two quaternary alloys con taining 46 at.% Al. The present work suggests that, in the order of increas ing strength of stabilization at 1173 K, V, Nb and Ta stabilize the alpha(2 ) phase, whereas Cr, Mn, Fe and Ga stabilize the gamma phase. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. AN rights reserved.