Defect-Mediated Transition from Rate Insensitivity to Rate Sensitivity of Hexagonal Titanium at Cryogenic Temperature

Strain rate sensitivity serves as a critical mechanical parameter for quantifying material behavior, particularly under dynamic loading conditions. Generally, materials that exhibit high strain rate sensitivity demonstrate substantial increases in strength, whereas those with low sensitivity maintain relatively stable properties regardless of the strain rate. In the present study, we report, for the first time, a notable strain rate insensitivity during the initial cryogenic deformation of hexagonal titanium, which subsequently transitions to strain rate sensitivity with increasing strains. This insensitivity-to-sensitivity transition of hexagonal titanium at cryogenic temperatures was elucidated through detailed characterization utilizing a combination of transmission electron microscopy and electron backscatter diffraction. Our experimental findings attribute this unusual behavior to the dynamics of defects, characterized by significant activation of 〈 c + a〉 dislocations, deformation-induced transitions of {11−22} → {10−12} → {11−22} twins, and their strong interactions. The present study therefore not only enriches the current understanding on the rate- and temperature-dependent deformation mechanisms of hexagonal metals but also provides valuable insights for advancing cryogenic high-speed forming technologies.