Morphology and stability of orthorhombic and hexagonal phases in a lamellar γ-Ti-42Al-8.5Nb alloy-A transmission electron microscopy study

Intermetallic titanium aluminides are often alloyed with niobium to improve oxidation resistance and mechanical properties. Depending on the overall alloy composition and on the solubility of the present phases for niobium, new phases may be introduced in the alloys' microstructure upon niobium addition. Here we describe the microstructure of the alloy Ti-42Al-8.5Nb produced by powder metallurgy after annealing at 550 degrees C and 650 degrees C. We identify the orthorhombic O-phase within alpha(2) laths of lamellar (alpha(2) + gamma) colonies by single crystal electron diffraction and high-resolution imaging. Domains of O-phase variants and alpha(2) phase form columnar crystallites in alpha(2) (0001) planes having low indexed alpha(2) {11-20} and {1-100} interface planes. The nm-sized domains are rotated with respect to each other resulting in elastic strains across the domain boundaries. No elemental segregation of Nb was detected in alpha(2)/O-phase lamellae. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.