High temperature deformation and control of homogeneous microstructure during hot pack rolling of Ti-44Al-5Nb-(Mo, V, B) alloys: The impact on mechanical properties

Hot workability and deformation mechanism of beta-solidifying gamma-TiAl alloy with a nominal composition of Ti-44Al-5Nb-1Mo-2V-0.28 (in at%) were systematically studied under uniaxial hot compression. The strain rate sensitivity exponent (m) and processing maps at strains of 0.2 and 0.6 were developed. Based on an analysis of the processing maps and the corresponding microstructure, 1200 degrees C/0.1 s(-1) was defined as the optimum deformation condition. At low temperatures and high strain rates, pileup of dislocations at lamellar colonies was found to be responsible for deformation, which provided the driving force for sub-grain boundary formation. As deformation progressed, twinning-induced dynamic recrystallization (DRX) and near complete breakdown of lamellae through the rearrangement of sub-boundaries were the main deformation mechanism. Moreover, nanolamellar structure and stacking faults (SF) were generated. The hot isostatically pressed (as-HIPed) alloy had relatively good workability, and crack-free sheets were obtained from the as-HIPed alloy with different reductions. A multiple phase, homogeneous as-rolled microstructure without coarse residual lamellar colonies (RL) or shear bands was obtained when the total rolling reduction was 78.8%. The ultimate tensile stress (UTS) and ductility at 800 degrees C ranged from 523 to 553 MPa and from 21% to 32%, respectively, for different deformation reductions.