Role of isothermal omega phase precipitation on the mechanical behavior of a Ti-Mo-Al-Nb alloy

The formation and the evolution of the athermal and isothermal omega phase precipitates in a commercial metastable β–titanium alloy, β-21S, under controlled heat treatments, and its consequent impact on tensile properties has been presented in this paper. The structural and compositional changes of the omega precipitates have been tracked using SEM, TEM, and atom probe tomography (APT). Upon quenching from above the β-transus, the microstructure consisted of a single phase β without any discrete athermal omega precipitates and this condition exhibited the highest tensile ductility. Following β -solutionizing, when the samples were aged below the omega solvus temperature, there was an increase in the size scale of the omega precipitates with increasing time period. The larger and more solute-depleted omega precipitates, resulted in increased tensile strength and reduced ductility. The details of the deformation behavior were further analyzed using electron backscattered diffraction (EBSD) and TEM. Two different modes of deformation were noted in the system, depending on the initial microstructure. While the single phase condition (β solutionized sample) exhibited deformation occurring mainly via (110)β slip, the presence of larger isothermal omega precipitates in the aged condition (β solutionized + 350 °C/100H) resulted in the activation of not only (110) β slip but also (112) β slip. The reasonably high tensile ductility noted in this aged condition has been attributed to the activation of both these slip systems.