Beta-titanium (β-Ti) alloys contain elements that promote enhanced retention of the β phase, termed β-stabilizers. This class of Ti alloys can exhibit a range of mechanical properties, making them suitable for applications requiring low Young's moduli, such as biomedical implants, as well as applications that require high Young's moduli and strengths, such as aerospace engine components. The mechanical properties of β-Ti alloys can be tuned to meet the needs of specific applications by changing the microstructure. As the β-phase is metastable, phase transformations to other metastable and stable phases can be induced by thermomechanical processing. Aging β-Ti alloys in the 300°C to 500°C temperature range allows certain β-Ti alloys to undergo the β-to-ω and β-to-α phase transformations, both of which promote strengthening. The ω phase in particular is known to significantly increase Young's modulus and strength, but is generally avoided because it also severely decreases elongation-to-failure (εf). Recently, the ω phase has been shown to assist in the formation of a nanoscale α phase. Composition and processing affect both the β-to-ω and β-to-α transformations. In this dissertation, four β-Ti alloys, Ti-11Cr, Ti-11Cr-0.85Fe, Ti-11Cr-5.3Al, and Ti-11Cr-0.85Fe-5.3Al (all in at.%) were investigated to determine how Fe and Al additions affect the phase transformations, phase volume fractions, and phase lattice parameters during aging, and how the microstructural changes affect the mechanical properties. Each alloy underwent 400°C aging treatments to induce the β-to-ω and β-to-α phase transformations. A microstructural characterization of the β-homogenized condition of each alloy was performed. Each alloy was found to contain only the β phase with randomly distributed alloying elements using X-ray diffraction (XRD), scanning electron microscopy (SEM), and atom probe tomography (APT). The Fe and Al additions did not affect the hardness of the alloys, but reduced the shear and Young's moduli of the alloys. The alloys’ yield strength (σy) and ultimate tensile strength (UTS) increased with decreased grain size and εf increased as the grain size approached that necessary for the minimum critical stress for stress-induced martensite. A microstructural characterization of each alloy during and after aging was also performed to determine the effects of Fe and Al on the microstructural evolution. Ti-11Cr underwent the β-to-ω and β-to-α transformations. The Fe addition reduced the volume fraction of the ω phase and increased the lattice parameter of the β phase. The Al addition inhibited the β-to-ω phase transformation and increased the volume fraction of the α phase. When Fe and Al were both added, Al prevented the β-to-ω transformation and Fe reduced the α-phase volume fraction. Cr and Fe diffused from the ω and α precipitates into the β matrix during the β-to-ω and β-to-α transformations. Al diffused from the β matrix into the α precipitates. The β-phase lattice parameter decreased with increasing Cr and Fe contents, and an empirical relationship between the β-phase lattice parameter and the β-phase stability, involving the molybdenum equivalency, was proposed. The β-phase lattice parameter also affect the c/a ratios of the α and ω phases, which has implications for the ω-assisted α-phase transformation. Higher contents of impurity element O were measured in the α and ω phases. Each alloy exhibited an increase in σy and UTS with precipitation of the α and ω phases. The aged alloys containing Fe exhibited larger εf values than the Fe-free alloys. The alloys containing ω-phase microstructures exhibited significantly higher shear moduli than the ω-free alloys. Through this work, the composition-processing-microstructure-property relationships of the Ti-Cr-Fe-Al system were revealed. The relationships established between processing, β-phase stability, microstructure, and mechanical properties are expected to be applicable to β-Ti alloys outside of the Ti-Cr-Fe-Al system.
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