The alloying effects of tantalum on the microstructure of an α₂+γ titanium aluminide

Atom probe field ion microscopy has been used to investigate the alloying effects of tantalum in a hot extruded two-phase (α₂+γ) Ti-47 at.% Al-2% Cr-1% Nb-0.80% Ta-0.20% W-0.15% B alloy in both stress-relieved (2 h at 900°C) and aged (2160 h at 800°C) conditions. Slightly over half of the tantalum additions were found to be in solution in the main matrix phases. The concentration of tantalum in the aged alloy was determined to be 0.43±0.14% Ta and 0.47±0.04% Ta in the α₂ and γ phases, respectively, indicating that tantalum does not partition strongly to either phase. The presence of tantalum does not reduce the strong oxygen partitioning to the α₂ phase, or modify the low boron concentrations measured in the α₂ and γ phases. However, the addition of tantalum does appear to change the most prevalent boride phase from MB₂ to MB and possibly to increase the rate at which near-equilibrium levels of Al and Ti are achieved in the α₂ and γ phases. In addition, tantalum appears be responsible for the precipitation of a Ti(Cr, Al)₂ phase which contains significant amounts of Ti, Al, Cr, Ta and W. © 1999 Elsevier Science S.A.