Enhanced high-temperature tensile property by gradient twin structure of duplex high-Nb-containing TiAl alloy

A pre-deformation process was employed for a TiAl alloy via high-temperature torsion, in which the stability of the constituent phases was tailored, resulting in enhanced hardening capability and ductility via a gradient microstructure. A sample with a pre-torsion of 360° exhibited a yield strength of 475 MPa and an ultimate tensile strength of 592 MPa, with a tensile ductility of 47% at 850 °C. The tensile properties were significantly enhanced compared with the as-forged sample, which exhibited values of 395 MPa, 494 MPa, and 4.6%, respectively. The physical mechanisms for the significant enhancement of the mechanical property of the TiAl alloys were studied in-depth via of transmission electron microscopy, electron-backscattered diffraction, and high-energy X-ray diffraction techniques. The high strength is mainly attributed to the twin structure formed during torsion, while high fracture elongation correlates to the recrystallization of the γ phase at twin-twin sections and the load partitioning regulated by a hierarchical microstructure. When the tensile micro-strains along the loading direction and transverse direction in the γ phase of the pre-deformed TiAl alloy, a higher mechanical performance was obtained. Moreover, the fracture mode of the pre-torsional tensile sample is a combination of pores and cleavage facets which resulted from the microstructure after torsion. The gradient twin structure approach in this study provides a strategy for developing TiAl alloys with exceptionally high-temperature tensile property, and the results of the micromechanical behavior-microstructure-property relationship may improve the understanding of the plastic deformation of TiAl alloys.