The grain boundary character distribution (GBCD) of the body-centered-cubic (BCC) phase in the orthorhombic (O) + BCC class of Ti-Al-Nb alloys was investigated. The alloy compositions ranged between 12-23at.%Al and 25-38at.%Nb and the processed sheet microstructures were either fully-BCC or O+BCC. The alloys were processed by pancake forging and hot-pack rolling followed by heat treatments varying within the O, BCC, and O+BCC phase regimes. Through scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) the effects of sheet orientation, alloy composition, and grain size on the GBCD were evaluated. No strong significant correlation was observed between any of the studied microstructural parameters. The low-angle boundaries (LAB) fraction was least prevalent throughout the fully-BCC microstructures and always less than 0.1, while general high-angle boundaries (GHABs) were dominant. The fraction of special boundaries was the minority and typically ranged between 0.25-0.3. The fine-grained BCC-phase present in the subtransus O+BCC microstructures exhibited more LABs than coincident site lattice boundaries (CSLBs), though the total GHAB fractions were not significantly different than the supertransus microstructures. Using EBSD, the twin-related O-phase variant interfacial planes were identified and quantified. The variant boundaries are near {110} or {130} and represented over 33% of all O-phase boundaries. This indicates the potential to modify the GBCD in the O-phase to enhance properties in Ti-Al-Nb alloys.
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