Microstructure, tensile, and creep behavior of O+BCC Ti2AlNb alloys processed using induction-float-zone melting

The microstructure, tensile, and tensile–creep behavior were studied for orthorhombic (O) plus body-centered cubic (BCC) Ti–22Al–24Nb and Ti–26Al–27Nb (at.%) alloys processed using induction-float-zone melting (IFZM). Microstructure studies were performed using scanning and transmission electron microscopy (SEM and TEM), automated electron back-scattered diffraction (EBSD), and X-ray diffraction (XRD). Upon solidification, the BCC phase evolved with [1 0 0] oriented nearly parallel to the longitudinal rod direction. During the slow cool through the O+BCC-phase field, O-variants formed in a fine lath network within the parent BCC. The as-processed Ti–26Al–27Nb rod was strongly textured with an approximately equal distribution of six resolvable O-variants. The retained BCC phase, which was sandwiched between O laths, maintained a higher volume fraction (Vf∼0.20) in Ti–22Al–24Nb compared to Ti–26Al–27Nb (Vf∼0.05). The tensile and creep behavior of the as-processed microstructures were evaluated with the tensile axis oriented parallel to the longitudinal rod direction. The fully lath O+BCC Ti–22Al–24Nb microstructure exhibited a room-temperature yield strength of 836 MPa and an elongation-to-failure of 4.5%. Surface slip traces revealed that slip was compatible between the O and BCC phases. An untransformed-BCC Ti–26Al–27Nb microstructure, oriented with [1 0 0] nearly parallel to the tensile axis, exhibited localized deformation bands resulting in an elongation of more than 5.9% and an average yield strength of 828 MPa. In terms of the creep behavior, the secondary creep rates revealed that the fully lath O+BCC Ti–26Al–27Nb microstructure significantly outperformed that for all other O-based alloys. For applied stresses greater than 300 MPa, an activation energy of 346 kJ/mol and a creep exponent of 5.1 were measured, while for lower applied stresses the creep exponent transitioned to a value of 2.3. Overall, this work shows that induction-float-zone processing produces textured fully lath O+BCC microstructures containing an attractive balance of room- and elevated-temperature properties for Al concentrations as high as 26 at.%.