Dynamic Mechanical Properties and Adiabatic Shear Behavior of Ultrafine-Grained Pure Titanium AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY23080007

Chinese Library Classification Number:TG146.23

Citation Information:

The ultrafine-grained (UFG) pure titanium prepared by composite deformation process (equalchannelangularpressingandrotary swaging) has many excellent properties such as high strength, excellent fatigue strength and good corrosion resistance, which can be widely used in aerospace, marine exploration, and petrochemical fields. Pure titanium is a metal that is highly sensitive to strain rate and temperature. Under dynamic impact loading such as high-speed impacts, dynamic fracture, and explosions, pure titanium presents macroscopic mechanical properties and microstructure evolution laws that are significantly different from those of conventional quasi-static loading. The difference between parts with ultrafine microstructure subjected to dynamic loading and quasi-static loading is mainly reflected in the inhomogeneity of the material deformation, non-isothermal, inertia effects and strong shock wave effects. Meanwhile, the adiabatic shear phenomenon is usually one of the concerns of researchers when studying the dynamic mechanical behavior of UFG materials, because the formation of an adiabatic shear band (ASB) is often a precursor to the fracture failure of a material in a dynamic impact process. In order to investigate the mechanical properties and adiabatic shear behavior of UFG pure titanium under dynamic loading, dynamic impact experiments were carried out at strain rates of 2000~3000 s^-1 and temperatures of 300~450 ℃ by the split Hopkinson press bar (SHPB) technique, and the microstructure after dynamic deformation was characterized using scanning electron microscopy (SEM) and electron backscattering diffraction (EBSD) techniques. The results showed that UFG pure titanium exhibitednegative temperature sensitivity and positive strain rate sensitivity, and the true stress-true strain curve showedan obvious yielding effect. The true stress-true strain curve couldbe divided into three stages：the yielding, the work-hardening, and the localized deformation stage. Furthermore, all the curves showed a significant double-stress peak characteristic, and the softening phenomena appearance caused by the dislocation annihilation, rearrangement. Heat aggregation and the subsequent formation of the ASB werethe main reasons for the two stress reductions. With the increased of temperature, the yield strength under different strain rates decreased. When the temperature rose from 300 ℃ to 350 ℃, the yield strength of UFG pure titanium decreasedby 9.98%；From 350 ℃ to 400 ℃, the yield strength decreasedby 6.95；From 400 ℃ to 450 ℃, the yield strength decreasedby 7.91%, which was only 77% of the yield strength at 300 ℃. The values of strain rate sensitivity index m of UFG pure titanium in the range of strain rate of 2000~3000 s^-1 were in the range of 0.12 to 0.17, which indicatedthat the flow stresses increased with the increased of strain rate under different impact loads, and UFG pure titanium hadstrong strain rate sensitivity. When the strain rate was larger than 2500 s^-1, ASBs with an angle of 45° or 135° to the impact direction appeared in the UFG pure titanium specimens. There was a clear boundary between the shear band and the matrix. The width of the shear band increasedwith the increased of the strain rate and the decreased of the temperature. The width of the shear band increasedfrom 4.3 μm to 42.8 μm with the increased of the strain rate from 2000 to 3000 s^-1 when the temperature was 350 ℃. And the width of the shear band decreasedfrom 48.7 μm to 16.4 μm with the increased of the temperature 300 to 450 ℃ when the strain rate was 2500 s^-1. There were many microviods along the direction of shear band expansion in the ASB, and the convergence between adjacent microviods led to the formation of crack and fracture failure. The impact fracture morphology of UFG pure titanium under dynamic loading exhibitedthe typical parabolic shear dimples, and there were some microwoids in the shear dimples. The temperature hadlittle effect on the morphology of the shear dimples. The microhardness distribution near the adiabatic shear zone of UFG pure titanium hadthe same trend after dynamic loading in different conditions. The microhardness in the center of the ASB was the highest, which was obviously higher than that in the transition zone and the matrix. With the increased of strain rate, the microhardness of the center zone in the ASB, the transition zone and the matrix increased slightly. The microhardness of the center zone in the ASB was greatly affected by the temperature. The higher the temperature was, the lower the microhardness of the center zone in the ASB was. The microstructures of the shear band were mainly composed of a large number of fine dynamic recrystallizationequiaxed grains, which was induced by local high temperature and large deformation. With the increased of the temperature, dynamic recrystallization grain size in the ASB increasedfrom 332 nm at 300 ℃ to 1159 nm at 450 ℃. As the local softening points, dynamic recrystallization grains contributed to the adiabatic shear instability under the action of thermal-mechanical coupling and ultimately led to the fracture failure of UFG pure titanium.