Neutron diffraction study of oxygen dissolution in α2-Ti3Al

Rietveld refinements of neutron powder diffraction data on α2-Ti3Al have been performed to determine the crystal structure as a function of interstitial oxygen (O) concentration for three alloys with a Ti/Al ratio of ≈2.34 and O concentrations of 0.25, 3.99, and 7.71%. The structures of the alloys are hexagonal in space group P63/mmc where Ti and Al atoms populate unique sites with excess Al at the Ti site and O atoms occupy octahedral interstitial sites surrounded by six Ti sites. The length of the c-axis was found to increase linearly as the O occupancy of the interstitial sites increased; this lattice lengthening effect was much less pronounced along the a-axis. Correspondingly, the increases in the lengths of Ti–Al and Ti–Ti bonds with a major component of their direction parallel to the c-axis were roughly an order of magnitude greater than the increases in the lengths of Ti–Al and Ti–Ti bonds more closely aligned with the a-axis. Densities calculated from the lattice parameters and occupancy factors fall in the range (4.118±0.004) to (4.194±0.004) g/cm3, and exhibit a nearly linear increase with oxygen concentration. Measured densities of (4.113±0.001), (4.146±0.009), and (4.191±0.002) g/cm3 for these alloys agree with the results of the refinements.