Phase transformations in Ti-35Nb-7Zr-5Ta-(0.06-0.68)O alloys

The phase transformations occurring in Ti-35Nb-7Zr-5Ta-(0.06-0.68)O β solution treated and aged between 427 and 593 °C for 8 h have been investigated. Aging at 427 °C resulted, respectively, in ω, ω + α and α phase formation for 0.06, 0.46 and 0.68 wt.% O. A modification in ω phase morphology, from near circular to ellipsoidal, was also observed with increasing O from 0.06 to 0.46 wt.%. Aging at higher temperatures resulted in resolution of the ω phase in 0.06 wt.% O. Lenticular α precipitation was observed at higher O content, the volume fraction of α increasing with increasing O at a constant aging temperature and with increasing aging temperature at a constant O content. The latter also resulted in coarsening of the α precipitates and an increase in their aspect ratio. Finally aging of these alloys resulted in the formation of precipitate free zones (PFZs) along prior β grain boundaries, the width of these zones increasing with an increasing aging temperature. These observations are consistent with the ability of O to suppress ω phase formation through interruption of the 〈111〉 lattice displacement required for this phase's formation, while promoting α phase formation at higher O content, presumably through local ordering within the β phase.