The initial stages of the omega phase transformation in Ti-V alloys

Abstract

The omega phase in as-quenched and aged Ti-V alloys was examined using high-resolution dark-field microscopy and selected-area diffraction. The asquenched omega morphology was shown to be based upon 〈111〉β‖〈0001〉ω rows containing particles which are 10-22 Å in diameter with an interparticle spacing of 20-25 Å. Clusters of rows were observed in the Ti-13 at. % V alloy and as the vanadium content increased the cluster size decreased.Sharp omega reflections and {111} planes of intensity were observed in the as-quenched Ti-13 at. % V alloy. As the solute content increased the omega reflections broadened and the {111} planes of intensity increased in thickness. Comparison of the omega morphology and diffraction effects showed good agreement with the suggestion that the omega reflections resulted from scattering by the clusters, while the {111} planes of intensity resulted from scattering by the isolated rows. As the vanadium content increased the 0001 and 0002 ω reflections shifted toward each other along an 〈0001〉 direction. These observations are similar to those made on the Zr-Nb system.For short ageing treatments the morphology of the omega phase was similar to that of the as-quenched alloy, but the clusters were larger. As the ageing time increased the number of isolated 〈111〉 rows decreased and the cluster shape began taking on crystallographic features. Selected-area diffraction indicated that the aged omega phase was Ti-rich and that the 1010 and 2020 reflections were not real.