Effects of interstitial oxygen on microstructure and mechanical properties of Ti-48Al-2Cr-2Nb with fully lamellar and duplex microstructures

The effect of added oxygen in the range of 1000 to 4000 wt ppm on the microstructures of a Ti-48Al-2Cr-2Nb alloy has been investigated and compared to the microstructures for a high-purity alloy. For specimens cooled from the a phase, interstitial oxygen stabilizes the lamellar microstructure with respect to gamma grains, with higher than equilibrium values for the alpha(2) volume fraction. For specimens cooled from the alpha + gamma phase field, the lamellar microstructure still tends to be favored by oxygen, but it is found that the phase volume fractions are not significantly different from equilibrium values. This suggests that interstitial O essentially reduces the kinetics of the a to alpha + gamma transformation. Thus, interstitial oxygen will have a strong effect on microstructures obtained by continuous cooling from alpha, but significantly less on those, such as the duplex microstructure, obtained by long treatment in the alpha + gamma phase field. In general, increased O content is well correlated with reduced ductility. Finally, the role of interstitial oxygen on this phase transformation is discussed.