Y.Y. Cui

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

The effect of Ti/Al ratio on the site occupancies of alloying elements in γ-TiAl

Abstract The site occupancies of V, Cr, Mn, Fe, Ni, Zr, Nb, Ta, Mo, Ga and Sn (1∼5 at%) in TiAl alloys with different nominal Ti/Al ratios were measured by the atom location channelling enhanced microanalysis (ALCHEMI) method. The results showed that Zr, Nb and Ta invariably occupy Ti sublattice sites, whereas Fe, Ni, Ga and Sn occupy Al sublattice sites, the alloy composition having no significant influence on their site preference. By contrast, the site preference of V, Cr, and Mn changes significantly with alloy composition (the Ti/Al ratio in particular), the probability of these elements occupying Ti sites decreasing in the above order. In general, with increasing atomic number, elements in the same period show increasing tendency to substitute for Al, so is the tendency to substitute for Ti for elements in the same group of the periodic table. A discussion is made in terms of the Bragg–Williams model by using published bond order data as input, allowing qualitative interpretation of the experimental findings.

Atom location by channeling enhanced microanalysis study of site distributions of alloying elements in titanium aluminides

The original equation of Spence and Tafto for quantitative determination of site occupancy using atom location by channeling enhanced microanalysis method has been extended to take into account both delocalization effect and the influence of point defects (antisite atomic distribution and vacancies). The outcome of this treatment suggests that, for crystals free of antisite defects, the accuracy of site occupancy is influenced by delocalization effect but is independent of both thermal and structural vacancies. For crystals free of structural vacancies, the accuracy of site occupancy is influenced by both delocalization effect and antisite defects, but is independent of thermal vacancies. The delocalization effect was shown to vary with channeling strength at a given channeling condition. For a binary ordered phase in which at least one of the host elements exhibits weak delocalization effect las is the case for many transition-metal aluminides), a tangent-line method for obtaining the value of k (a parameter necessary for the calculation of site occupancy) was proposed, allowing the determination of the delocalization correction factor for the other host element. Application of this method to estimating the delocalization effect of Al in Ti3Al and TiAl under axial and planar channeling conditions, respectively, was demonstrated.