Study On Dynamic Mechanical Response And Substructure Transformation Of Adiabatic Shear Zone Of TB6 And Ta-2.5W Alloy

Adiabatic shear band(ASB)is a common and important local deformation mechanism in rapid deformation processes such as mechanical processing,plastic processing,and impact deformation.In the forging process,the adiabatic shear phenomenon is a symptom of the damage of the sample.In the machining and cutting process,this phenomenon can be used by people to improve the cutting efficiency.The adiabatic shear zone is usually destructive in the high-speed impact of projectile,so it is necessary to resist the severe shear localization in the ballistic application to improve the penetration performance of projectile body.Therefore,studying the substructure transformation and formation mechanism of the adiabatic shear zone is of great significance for the processing and application of materials.The high strain rate loading process is more complicated than static loading.The formation process of the adiabatic shear zone is short,and the spatial scale is small.Therefore,it is necessary for the formation and expansion of adiabatic shear instability and the internal substructure transformation of the adiabatic shear zone.There are still disputes and differences on the conclusions of many key issues.In this dissertation,a dynamic loading experiment was conducted on TB6 and Ta-2.5W cylindrical hat-shaped specimens using a separate hopkinson pressure bar.Combined with the analysis of the original waveform,stress-strain curve,metallography and transmission electron microscopy,an attempt was made to explain the dynamic mechanical response of the two alloys and the substructural transformation mechanism in the adiabatic shear zone.Type of TB6 titanium alloy hat-shaped sample dynamic loading the original waveform,the true stress-strain curve analysis showed that four different strain rate loading conditions,TB6 titanium alloy are stress collapse phenomenon,presents the typical characteristics of adiabatic shearing localization,and present the phenomenon of "double platform" at the beginning of deformation,microstructure analysis for the stress induced martensitic(SIM)transformation.Deformation medium stress has fallen sharply and showed a trend of fluctuations in the late deformation,softening with strain hardening and strain rate hardening for competing,combined with the ASB internal transmission microscopy analysis,dynamic recrystallization(DRX)occurs inside the ASB,softening mechanism is mainly for the DRX,and the middle dominant deformation.The focused ion beam(FIB)experiment method precision of adiabatic shear area of the internal transmission microscopy analysis,combined with regional morphology in the typical regions in images and HRTEM,SAED analysis showed that the adiabatic shear zone within the formation of DRX by nanometer grain,through the analysis of the fourier transform of HRTEM images show that in the equiaxed grain interior fine needle and plate strip was observed respectively α?martensite and {111} α? Ⅰ type twin,and uses the infinitesimal deformation theory of lattice constant shear twin mechanism is verified.Combined with the calculation of the temperature in the adiabatic shear zone of TB6,it is shown that the temperature rise in the adiabatic shear zone of TB6 is relatively limited under the four sets of different strain rate loading conditions,and the highest is only 509.4 K(0.28Tm).Try to use rotational dynamic recrystallization(RDR)mechanism explains the observed phenomenon.The α?martensite observed in the adiabatic shear zone was formed due to the failure of lattice transformation and solid solution element diffusion due to the excessive cooling rate.The dynamic loading of twins continues after the formation of martensite and is produced to adapt to the strain.The analysis of the original waveform and true stress-true strain curve obtained by dynamic loading of Ta-2.5W hat-shaped specimens shows that the Ta-2.5W alloy does not undergo stress collapse under three different strain rate loading conditions and appears in the middle and late stages of deformation.The downward trend of stress fluctuation,combined with the analysis of the metallographic structure of the Ta-2.5W adiabatic shear zone,can be obtained.The Ta-2.5W shear zone is a fibrous crystal grain elongated in the shear direction,and the stress wave fluctuation in the middle and late stages of deformation decreases to dynamic recovery(DRV)dominates due to competition with strain hardening and strain rate hardening.