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

Abstract

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 °C and 650 °C. We identify the orthorhombic O-phase within α2 laths of lamellar (α2 + γ) colonies by single crystal electron diffraction and high-resolution imaging. Domains of O-phase variants and α2 phase form columnar crystallites in α2 (0001) planes having low indexed α2 {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 α2/O-phase lamellae.