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A bimodal microstructure containing very fine α2(Ti3Al) particles was produced by over-aging a Ti-6Al-4V alloy. The effects of α2 precipitation on quasi-static and dynamic deformation behavior were investigated in comparison with an unaged bimodal microstructure. Quasi-static and dynamic torsional tests were conducted on them using a torsional Kolsky bar. The quasi-static torsional test data indicated that the over-aged bimodal microstructure showed higher fracture shear strain than the unaged bimodal microstructure, while their maximum shear stresses were similar. Under dynamic torsional loading, both maximum shear stress and maximum shear strain of the over-aged microstructure were higher than those of the unaged microstructure. The possibility of the adiabatic shear band formation under dynamic loading was quantitatively analyzed by introducing concepts of critical shear strain, absorbed deformation energy, and void initiation. In the over-aged microstructure, the energy required for forming adiabatic shear bands was higher than that in the unaged microstructure, thereby lowering the possibility of the adiabatic shear band formation. The α2 precipitation in the over-aged microstructure was effective in both the improvement of quasi-static and dynamic torsional properties and the reduction in the adiabatic shear banding, which suggested a new approach to improve ballistic performance of Ti alloy armor plates.