The transition from rate-independent to rate-controlled ductility of hexagonal titanium upon cryogenic deformation

Strain rate is a critical factor influencing the mechanical response of hexagonal close-packed titanium under cryogenic conditions. In this study, uniaxial tensile tests were performed on commercially pure titanium at 77 K over a broad strain rate range from 0.001 to 1 s⁻¹. A critical strain rate of approximately 0.5 s⁻¹ was identified, above which ductility exhibits a pronounced reduction, whereas below this threshold, ductility remains relatively stable. Through comprehensive analyses of strain evolution, deformed microstructure, and fracture morphology, this behavior is attributed to severe localized adiabatic heating resulting from inhomogeneous deformation, rather than conventional twin or shear mechanisms. At high-strain rates, the lack of sufficient heat dissipation facilitates thermal softening, accelerating the transition from microvoid coalescence to premature necking and fracture. These findings not only deepen the understanding of deformation mechanisms in titanium under extreme conditions but also provide guidance for the design of high-rate forming processes in cryogenic environments.