Microstructure Evolution and Mechanical Properties of Ti-44Al-5Nb-1Mo-2V-0.2B Alloys in the Cross Hot-Pack Rolling Process

TiAl alloys are highly promising for high-temperature structural applications in the aerospace and automotive industries because of their low density, excellent high-temperature strength, and resistance to creep and oxidation. Nevertheless, low-temperature brittleness and poor deformability are the main factors severely restricting the widespread application of TiAl alloys. The process of beta-solidifying gamma-TiAl alloys results in alloys that consist primarily of alpha(2), gamma, and B2 phases, and have superior hot workability. Further thermomechanical treatments are applied to achieve a fine microstructure and enhance the inherent ductility of gamma-TiAl alloys. In this work, Ti-44Al-5Nb-1Mo-2V-0.2B alloy sheet with ultrahigh plasticity at 800 degrees C was achieved by cross hot-pack rolling (CHPR) and one-step annealing processes. SEM, EBSD, TEM, and tensile methods were used to investigate the hot deformation behavior, and the effects of different rolling processes and heat treatments on the microstructural evolution and mechanical properties of the alloy. The results show that the CHPR sheet had a more highly uniform deformation microstructure along the thickness direction and sheet plane compared with that of a unidirectional hot-pack rolled (UHPR) sheet, which consisted of residual lamellar colonies and equiaxed gamma, alpha(2), and B2 grains at colony boundaries. The size of the residual lamellar colonies was significantly smaller and the content was lower in the CHPR sheet compared with the UHPR sheet. This was due to a large number of broken residual lamellae and complete recrystallization under the combined action of a bidirectional shear force and compressive stress during the CHPR process. The high-temperature flow-softening mechanisms of TiAl alloy in the CHPR process mainly included bending and kinked lamellae, beta/B2 coordinated deformation, phasetransformation decomposition of alpha(2)/gamma lamellar, and dynamic recrystallization induced by primary and secondary twinning. To achieve further grain refinement, subsequent annealing of the CHPR-processed TiAl alloy was performed at 1200-1340 degrees C. A multiphase equiaxed microstructure with fine lamellar colonies was obtained at 1200 degrees C and a nearly complete lamellar microstructure was obtained at 1340 degrees C. Moreover, the room-temperature and high-temperature tensile properties of UHPR and CHPR sheets in the horizontal and vertical directions were compared with samples annealed at 1200 degrees C. The tensile properties of the CHPR sheets were more uniform in both directions. The multiphase equiaxed microstructure obtained in the CHPR alloy annealed at 1200 degrees C had the best strength-plasticity balance with a tensile strength of 624 MPa (515 MPa) and elongation of 1.32% (107.0%) at room temperature (800 degrees C). According to the fracture behavior, the fracture mode of these alloy sheets was translamellar or cleavage fracture at room temperature. Conversely, the fracture mode changed to ductile fracture at 800 degrees C, and the failure mechanism was mainly via microhole coupling. The fractures in the annealed sheets (1200 degrees C) had small and deep dimples, indicating optimal tensile elongation. The uniform and fine lamellar structure and equiaxed microstructure can hinder crack propagation and achieve enhanced mechanical properties.