Localized Plastic Instability of Titanium Alloy under Cryogenic Impact

The study of local instability of metals under ultra-low temperature and high strain rate is of crucial importance for the safety assessment of aerospace structures. To achieve the ultra-low temperature environment, a refrigeration system based on liquid helium circulation was developed. The specimen to be tested was cooled by a contact-conduction method to achieve the temperature in the liquid helium temperature range. Combined with the Hopkinson pressure bar experiment, impact compression experiments were conducted on titanium alloy specimens to study the dynamic properties of titanium alloys as low as the liquid helium temperature range (30K). The titanium alloy specimens adopted a circular ring structure, with the hollow structure having sufficient deformation space, which is conducive to the generation of shear bands. The experimental results show that: low temperature enhances the dynamic strength of titanium alloys; the material as a whole exhibits brittle deformation characteristics, and low temperature makes shear bands more likely to occur in titanium alloys; the width of shear bands gradually narrows with the decrease in temperature; Conjugate shear bands are more likely to occur under ultra-low temperature and high strain rate. Based on the generalized variational principle and constitutive relation incorporating microstructural evolution, a governing equation for localized dynamic instability was established. From this governing equation, an angular criterion for shear band formation was derived, which effectively characterizes the formation of shear bands. And the three development stages of localized shear bands were studied (i.e., shear deformation, shear band, and shear fracture). The results show that: low temperature changes the plastic stiffness of the material, promoting the generation of shear bands at lower strains; shear bands form before the peak point of the stress-strain curve. This work provides experimental and theoretical references for ultra-low temperature experimental techniques and the plastic instability of titanium alloys under ultra-low temperature and high strain rate.