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 beta solution treated and aged between 427 and 593 degrees C for 8 h have been investigated. Aging at 427 degrees C resulted, respectively, in omega, omega+alpha and alpha phase formation for 0.06, 0.46 and 0.68 wt.% O. A modification in omega 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 to phase in 0.06 wt.% O. Lenticular alpha precipitation was observed at higher O content, the volume fraction of alpha 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 alpha precipitates and an increase in their aspect ratio. Finally aging of these alloys resulted in the formation of precipitate free zones (PFZs) along prior O grain boundaries, the width of these zones increasing with an increasing aging temperature. These observations are consistent with the ability of O to suppress omega phase formation through interruption of the (111) lattice displacement required for this phase's formation, while promoting alpha phase formation at higher O content, presumably through local ordering within the beta phase. (c) 2005 Elsevier B.V. All rights reserved.