Synergistic effect of aluminum distribution and in-situ annealing on layered microstructure formation: Microstructural evolution and mechanical anisotropy mechanisms in EBM-fabricated Ti-48Al-2Cr-2Nb

This study fabricated Ti-48Al-2Cr-2Nb alloys with a layered microstructure via electron beam melting (EBM), elucidating the synergistic effects of the periodic distribution of Al and intrinsic annealing on the microstructural evolution. During EBM process, the evaporation of Al element occurred, leading to a periodic Al distribution along the building direction. Al-depleted regions exhibited a higher α₂ phase content (with a content of approximately 3.27 vol %), where Zener pinning inhibited grain growth, retaining fine γ/α₂ lamellae and equiaxed γ grains. Conversely, Al-enriched regions showed a weakened α₂ pinning effect, under the intrinsic annealing, resulting in the abnormal grain growth of γ grains. Thus, alternating coarse-grained (CG) and fine-grained (FG) regions were formed along the building direction. CG regions have lower hardness than that of FG regions. Tensile tests showed that different printing directions have anisotropic behavior: when the tensile direction was perpendicular to the building direction, the material achieved an optimal elongation of up to 1.3 % at room temperature. This enhanced ductility resulted from the coordinated deformation between coarse and fine grain regions. However, when the tensile direction was parallel to the building direction, the poor cooperative deformation ability of the coarse and fine grains triggered preferential crack initiation at the interfaces, resulting in brittle fracture characteristics at both room temperature and elevated temperatures. This research provides a theoretical foundation and experimental guidance for optimizing the EBM process and for the design and regulation of heterogeneous structures in TiAl alloys.