Composition of the nanosized orthorhombic O′ phase and its direct transformation to fine α during ageing in metastable β-Ti alloys

The structure and chemistry of the orthorhombic O' phase after quenching of a Ti-23 at.%Nb-2 at.%O was measured using aberration-corrected transmission electron microscopy and atom probe tomography. The nanosized O' phase, formed in the vicinity of to, is enriched with oxygen and slightly depleted in Nb. Upon annealing, omega dissolves and the O' phase develops in beta up to 350 degrees C, above which temperature it transforms to colonies of fine alpha phase. Another needle-like form of a with lower Nb content is thought to nucleate from Nb-lean regions related to spinodal decomposition of beta. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.