The influence of alloying on the α2/(α2+γ)/γ phase boundaries in TiAl based systems

By employing both experimental and theoretical approaches, a comparative study 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 phase 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 diffraction 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 occupancies of alloying species in the two ordered phases. The differences between the predicted and experimentally determined phase boundaries were found to be less than about 1 at.% in most cases. The predicted volume fractions of constituent phases were also compared with experimental measurements by a combination of metallography and transmission electron microscopy. Agreement between the two is good for 14 ternary and two quaternary alloys containing 46 at.% Al. The present work suggests that, in the order of increasing 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.