PNNL

Abstract

ß-titanium (ß-Ti) alloys are useful in diverse industries because their mechanical properties can be tuned by transforming the metastable ß-phase into other metastable and stable phases. Relationships between lattice parameter and ß-Ti alloy concentrations have been explored, but the lattice parameter evolution during ß-phase transformations is not well understood. In this work, the ß-Ti alloys, Ti-11Cr, Ti-11Cr-0.85Fe, Ti-11Cr-5.3Al, and Ti-11Cr-0.85Fe-5.3Al (all in at.%) underwent a 400°C aging treatment for up to 12 hours to induce the ß-to-? and ß-to-a phase transformations. Phase identification and lattice parameters were measured in-situ using high-temperature X-ray diffraction. Phase compositions were measured ex-situ using atom probe tomography. The ? and a phases formed in Ti-11Cr, and were present throughout the aging treatment. With the 0.85at.% Fe addition, the ? phase disappeared after 7.5 hours of aging. The 5.3at.% Al addition inhibited the ?-phase transformation; only the ß-to-a phase transformation occurred in both Al-containing alloys. During the phase transformations, Cr and Fe diffused into the ß phase, Ti diffused into the ? phase, and Ti and Al diffused into the a phase. The ß-phase lattice parameter decreased with increased ß-stabilizer (Cr and Fe) contents. The relationship between the ß-phase lattice parameter, the average atomic radius, the average d-shell energy, and the molybdenum equivalency is described. The c/a ratios of the a- and ?-phases are dependent on the change in the ß-phase lattice parameters. Overall, the details of the interdependence of composition and the ß-, a-, and ?-phase lattice parameters are presented for the first time.