Identification of a new tetragonal phase in the Nb-Ti-Al system

During the course of investigation of the phase equilibria in the Nb-Ti-Al system, a new phase, designated theta, was observed in three Nb-Ti-40 at.% Al alloys following ageing at low temperatures (less than 1000 degrees C). Using X-ray diffraction and transmission electron microscopy techniques, th is phase was determined to have a bet structure, with lattice parameters a( 0) = 5.106 Angstrom and c(0) = 28.168 Angstrom, a space group of I4(1)/amd (number 141) and composition near 45 at.% Nb-25 at.% Ti-30 at.% Al. An orie ntation relationship between the new tetragonal phase and the gamma-TiAl ph ase of [101](gamma)//[120](theta) and (11 (1) over bar)(gamma)//(001)(theta ) was observed within the microstructure.