Processing effects on the grain-boundary character distribution of the orthorhombic phase in Ti-Al-Nb alloys

The grain-boundary character distribution of the orthorhombic (O) phase in Ti(2)AINb intermetallic alloys was investigated. The alloys were thermomechanically processed either above or below the bcc transus temperature. Using electron backscattered diffraction, the twin-related O-phase variant interfacial planes were identified and quantified. For the subtransus-processed samples, the equiaxed-O/ equiaxed-O grain boundaries tended to primarily prefer 65-deg misorientations and secondarily prefer 90-deg boundaries. Of the 65-deg misoriented boundaries, which were preferentially rotated about [001], similar to 40 pct contained (110) twin-related interfacial planes. The observations were rationalized by the alpha(2)-to-O phase transformation. It is suggested that for subtransus processing within the alpha(2)-containing phase regimes, the resulting heat-treated O + bcc microstructures evolve such that the O/O boundaries tend to exhibit distinct twin-related variants with misorientations between 55 and 65 deg. For a supertransus-processed alloy, it was found that approximately equal distributions of the six resolvable O variants were formed from the dominant parent bcc orientation. The resulting O/O boundaries tended to cluster at near-90-deg misorientations, which can be explained by the bcc/O orientation relationship. It is suggested that whenever the O phase primarily transforms from the bcc structure, the resulting O + bcc microstructures evolve such that the O/O boundaries tend to exhibit misorientations near 90 deg.