TIAL-BASED ALLOYS WITH NICKEL

The alloying potential of nickel as a minor constituent has been inves tigated in TiAl-based intermetallics with respect to its constitutiona l ability to provide the necessary microstructures for higher ductilit y and low fracture toughness. Six alloys (Ti0.60Al0.40)(1-x1)Ni-x1 and (Ti0.56Al0.44)(1-x2)Ni-x2 with X(1,2) = 0.01, 0.02 and 0.05 have been studied by means of X-ray diffraction, optical microscopy, EDS and WD -EMP analyses on alloys as-cast and annealed at 900 degrees C for 240 h under vacuum. The microstructures obtained confirm the phase triangu lation at 900 degrees C near the Ti-Al boundary. The solubility limit of Ni in the alpha(2)-phase is at lower Al/Ni contents than hitherto s hown. The Ti, Al-rich vertices of the three-phase equilibrium at 900 d egrees C, alpha(2)-Ti3Al(Ni) + gamma-TiAl(Ni) + tau(2)-TiNiAl2, were l ocated at Ti51.5Ni0.7Al47.8 (in at.%) and at Ti62.0Ni0.9Al37.1 respect ively. Thus TiAl-based alloys with nickel can only develop a lamellar alpha(2) + gamma structure for small amounts of Ni (less than 1 at.% N i at 900 degrees C). To compare the effects of Mn and Ni additions on gamma-TiAl, self-consistent FLAPW bandstructure calculations were perf ormed for model compounds Ti(2)XAl and TiXAl(2) (X = Mn, Ni). Lattice parameters were determined by minimizing the total energy, From the co mparison of the formation energies of the compounds, we found that Ni prefers Ti sites whereas Mn preferably substitutes Al. Taking into acc ount the site preference both ternary additions tend to reduce the c/a ratio.