Decomposition of the βo phase to the P63/mcm, hP18 structure in Nb–(24–36)Ti–40Al alloys

During the course of investigation of the phase equilibria in the Nb–Ti–Al system, decomposition of the ordered B2/βo phase to an ordered ω-type product was observed in three Nb–(24–36)Ti–40Al (at.%) alloys following aging at 700 °C and on quenching from high-temperatures. Using transmission electron microscopy and X-ray diffraction techniques, this phase was determined as the hP18 variant of the P63/mcm structure, with lattice parameters ao = 7.96 Å and co = 5.57 Å, and an ao≈aβ6 and co≈aβ3 relationship with the parent βo phase. Ab initio calculations were carried out to estimate the energy of formation of various phases and to also establish the role of Nb in the stabilization of the hP18 phase. The results of the evolution and stability of this structure are presented and discussed.