Abstract
The effect of oxygen content and cooling rate on phase transformations in directionally solidified (DS) intermetallic Ti-46Al-8Nb (at.%) alloy with columnar grain structure was studied. The phase transformations observed in the DS samples at various cooling rates are compared with those in the baseline as-received alloy with equiaxed grain structure. After solution annealing the baseline alloy with 500 wtppm oxygen as well as DS samples containing 1400, 1800 and 2500 wtppm were cooled to room temperature at various cooling rates ranging from 50 to about 1000 Ks^-1. The experimental results showed that the increase of oxygen content leads to a decrease in volume fraction of massively transformed γ_M(TiAl), an increase in volume fraction of retained α_R (Ti-based solid solution with hexagonal crystal structure) but has no significant effect on volume fraction of lamellar α₂(Ti₃Al)+γ(TiAl) microstructure at constant cooling rate. Increase of cooling rate leads to a decrease in volume fraction of massive γ_M and an increase in volume fraction of retained α_R at constant oxygen content. Cooling rates corresponding to oil and water quenching can suppress formation of lamellar α₂+γ microstructure. Nucleation and growth of massive γ_M is significantly enhanced within parent α grains when the nucleation at the grain boundaries is suppressed by stabilization of the α-phase by oxygen at low cooling rates. Vickers microhardness HV_m of massive γ_M as well as retained α_R increase with increasing oxygen content and cooling rate.

Key words
titanium aluminides, TiAl, heat treatment, phase transformations, microstructure, mechanical properties

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