Anisotropic tensile behavior of Ti-47Al-2Cr-2Nb alloy fabricated by direct laser deposition

Additive manufacturing is emerging as an increasingly promising technology for TiAl alloys, which are attracting significant attention for their special microstructure and mechanical properties. In the present study, direct laser deposition was applied to fabricate single-track Ti-47Al-2Cr-2Nb alloy walls with a back-and-forth scanning strategy. The resulting alternative-band microstructure, comprising complicated microstructure bands and coarse lamellar colony bands, was characterized, and electronic back-scatter diffraction analysis showed that the lamellae in coarse colonies present a consistent orientation, nearly parallel to the substrate. The tensile properties of the as-deposited specimens varied greatly with the angle (theta) between the loading direction and the substrate. The specimens exhibited the best comprehensive tensile properties of the maximum ultimate strength and elongation (706 MPa and 0.51 %) with the loading direction parallel to the substrate (theta= 0 degrees). The ultimate strength and elongation dropped to minimum values of 273 MPa and 0.16 %, respectively, when the loading direction was perpendicular to the substrate (theta= 90 degrees). At the middle state of loading direction, theta= 45 degrees, the ultimate strength decreased to 358 MPa, and the elongation remained as high as 0.49 %. Transmission electron microscopy observations revealed that dislocations and deformation twinning were both activated under different loading modes, and their morphologies were dramatically different. The formation mechanism of the alternative-band microstructure was proposed from the aspects of rapid solidification and the cyclic heat-treatment effect, and its possible fracture mechanism and crack propagation mode were discussed.