|The effect of oxygen content and cooling rate on phase transformations in directionally solidified intermetallic Ti-46Al-8Nb alloy|
LAPIN, J., GABALCOVA, Z.
vol. 46 (2008), no. 4, pp. 185 - 195
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 γM(TiAl), an increase in volume fraction of retained αR (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 γM and an increase in volume fraction of retained αR 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 γM 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 HVm of massive γM as well as retained αR increase with increasing oxygen content and cooling rate.
titanium aluminides, TiAl, heat treatment, phase transformations, microstructure, mechanical properties
Full text (1062 KB)