Study On Dynamic Mechanical Behavior And Adiabatic Shear Failure Of Ultra-fine Grained Pure Titanium

Along with the fast progress in science and technology,titanium and titanium alloys with excellent characteristics are increasingly used in aerospace,marine engineering and medical and chemical industries.Ultra-fine grained(UFG)pure titanium prepared by composite deformation process has excellent comprehensive mechanical properties,which greatly broadens its application scope and field.Pure titanium is a metal that is highly sensitive to strain rate and temperature.Under dynamic impact load,the macroscopic mechanical properties and microstructure evolution law are significantly different from those under conventional quasi-static loading.In this study,UFG pure titanium prepared by equal channel angular pressing+rotary swaging composite deformation process was used as the research material,and the dynamic impact experiment of separated Hopkinson pressure bar was carried out.The various characterization methods were used such as electron backscatter diffraction(EBSD),optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM)and micro-hardness measuring instrument.The dynamic mechanical properties and the microstructure evolution in the shear zone of UFG pure titanium at high strain rate were systematically studied.The mechanical behavior of UFG pure titanium shows obvious positive strain rate sensitivity and negative temperature sensitivity.The stress-strain curve can be divided into four stages:elastic stage,yield stage,work hardening stage and local deformation stage.Under dynamic impact load,the appearance of adiabatic shear band is a prerequisite for the fracture failure of UFG pure titanium.The width of shear band becomes wider with the increase of strain rate or the decrease of temperature.Under the conditions studied in this paper,the critical strain is 28.3%and the critical strain rate is 2000 s-1.After dynamic impact,with the increase of temperature,the dislocation density at the dislocation cell wall in the nonadiabatic shear zone gradually decreases,and the adiabatic temperature rise provides enough driving force for dislocation annihilation at the large angle grain boundary,which leads to the grain boundary becoming clearer and the grain growing.The hardness in the center of adiabatic shear zone is the highest,which is obviously higher than that in the transition zone and matrix.The impact fracture morphology is a typical parabolic shear dimple,and the temperature has little effect on the dimple morphology.Based on dynamic impact test data,considering strain hardening effect,strain rate hardening and thermal softening effect,the J-C constitutive model and BP artificial neural network(BP-ANN)model of UFG pure titanium are established,the number of nodes in the input layer,hidden layer and output layer of BP-ANN is 3,11 and 1 respectively,and the two models are modified.The results show that the fitting effect of the modified J-C constitutive model(σ=[530.4+425.7ε0.3328][l+(0.03209+1.266e-5·ε0.84)lnε*][1-((T0+(0.049ε+185.4272)ε-Tr)/(Tm-Tr))0.9928])is not ideal under high strain rate loading.The average relative error of BP-ANN model modified by temperature is 4.63%,and the correlation coefficient between predicted results and experimental results is 0.97065.The model has obvious advantages at high temperature and high strain rate,and can more accurately describe the nonlinear characteristics of flow stress under the coupling effect of loading temperature and strain rate.Compared with CG pure titanium,UFG pure titanium has higher adiabatic shear sensitivity,narrower adiabatic shear band and more obvious cracking characteristics.The dislocations entangled in the grains recover dynamically,forming substructures with small orientation difference.The combined effect of adiabatic temperature rise and severe plastic deformation leads to the rotation and rearrangement of substructures in the adiabatic shear zone of CG and UFG pure titanium,which eventually merge to form dynamic recrystallization grain.The growth rate of recrystallization in the shear band of UFG pure titanium is greater than that of CG pure titanium,while the matrix is still dominated by deformed structure.Dynamic recrystallization grains are prone to local deformation of grain rotation and grain boundary slip during further plastic deformation,and adiabatic shear bands are formed along the maximum shear direction during deformation.