The Grain Boundary Character Distribution of the Orthorhombic Phase in Ti2AlNb Intermetallic Alloys

The grain boundary character distribution (GBCD) of the orthorhombic (O) phase in near Ti2AlNb intermetallic alloys was investigated. The alloys were prepared by either induction heating above the body centered cubic (BCC) transus or by subtransus processing through pancake forging and hot-packed rolling. Using electron backscatter diffraction (EBSD), the twin-related O-phase variant interfacial planes were identified and quantified. For the subtransus processed samples, which were subsequently heat-treated within the single-phase O or two-phase O+BCC regimes, the equiaxed-O/equiaxed-O grain boundaries tended to primarily prefer 65° misorientations (~30% of all O boundaries) and secondarily prefer 90° boundaries and 55° boundaries (~5-10% of all O boundaries). Of the 65° misoriented boundaries, which were preferentially rotated about [001], a large percentage contained (110) twin-related interfacial planes. A smaller percentage (5-10%) of O boundaries were observed at ~55° misorientations about [001] and exhibited (130) twinrelated interfacial planes. These tendencies can be rationalized by the α2-to-O phase transformation. For the supertransus processed alloy, the BCC-to-O transformation was characterized using EBSD and XRD and it was found that approximately equal distributions of the 6 resolvable O variants were formed from the dominant parent BCC orientation. The resulting O/O boundaries tended to cluster at near-90° misorientations, which can be explained by the BCC/O orientation relationship (OR). Introduction Grain boundary engineering (GBE) is a means to potentially improve the mechanical properties of alloys through thermomechanical processing by altering the GBCD. GBE has yet to be thoroughly investigated for Ophase Ti2AlNb intermetallic family of alloys. This work describes the differences in the O-phase GBCD of supertransus and subtransus processed microstructures. The reason for the observed differences is expected to be a result of the phase transformation behavior and orientation relationships (ORs) which exist between O/α2 ([0001]α2//[001]O; (10-10) α2//(110)O) and O/BCC ([111]BCC//[1-10]O; (110)BCC//(001)O) structures. Experimental Procedures Details of the subtransus processed fully-O Ti-25Al-24Nb and O+BCC Ti-23Al-27Nb(at.%) microstructures can be found in reference [1], while the supertransus processed Ti26Al-27Nb alloy is described in reference [2]. Both the fully-O and O+BCC subtransus microstructures exhibited equiaxed-O/equiaxed-O boundaries which were characterized by EBSD. The supertransus processed material exhibited lath-O/lath-O boundaries and this material was characterized using both XRD and EBSD. Details of the EBSD specimen preparation and mapping techniques are described in reference [3]. Results and Discussion Subtransus-Processed Microstructures For each of the subtransus processed microstructures, the fraction of the low angle boundaries, defined by 5-15° misorientations, for the equiaxed-O phase was less than 0.02 indicating the microstructure was nearly fullyrecrystallized. Figures 1a and b illustrate misorientation charts for the fully-O and O+BCC subtransus microstructures, respectively. Most of the O/O boundaries clustered between 50-70° misorientations. In both microstructures, the O/O boundaries exhibited the highest