THE INFLUENCE OF ALLOYING ON THE α_2/(α_2 + γ)/γ PHASE BOUNDARIES IN TiAl BASED SYSTEMS

摘要

By employing both experimental and theoretical approaches, a comparative study of the α_2/(α_2 + γ)/γ phase boundaries in some Ti-Al-X (X = Nb, Ta, V, Cr, Mn, Fe or Ga) systems has been car- ried out. The phase constitution of Ti-(33-51 at.)Al-(1-3 at.)X alloys with a composition step of 1 at. Al was determined experimentally by using the X-ray diffraction method on bulk samples equili- brated at 1173 K. The α_2/(α_2 +γ) and (α_2 + γ)/γ 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 α_2 phase, whereas Cr, Mn, Fe and Ga stabilize the γ phase.