Deformation Behavior Of Ti-Al Intermetallics Under High Strain Rate

In recent years,the theory of impact resistance for advanced lightweight materials has been put forward in the field of aerospace,which requires in-depth study of the dynamic mechanical behavior under high strain rate,so as to provide consultation for the optimization,design and preparation of high-performance materials.As a substitute for Ni-based superalloy,Ti-Al intermetallic compound can be the ideal lightweight and high temperature structural materials for aerospace applications with higher specific strength,specific stiffness,creep resistance and oxidation resistance.It will make sense to investigate the deformation behavior under high strain rate for engineering guidance.Ti₃Al and Ti₂AlNb were taken as the research object in this paper.In order to study the flow stress characteristics and dynamic plastic constitutive relation under high strain rate,the quasi-static tensile curves and the dynamic compressive flow stress curves with the strain rate of 500⁷500s^-1 were obtained with the electronic universal testing machine and the Split Hopkinson Pressure Bar（SHPB）at room temperature.The microscopic deformation behavior under high strain rate was investigated,by observing the adiabatic shear deformation and fracture morphology of the two kinds of alloys.Both of the two materials show a certain strain rate strengthening effect,strain rate sensitivity and significant strain rate plasticizing effect under high strain rate.The strain rate strengthening effect Ti₃Al alloy was significantly stronger than Ti₂AlNb alloy,as well as the strain rate sensitivity.The dynamic plastic flow tendency of the two alloys are significantly enhanced with the increase of strain rate,and the elongation of Ti₂AlNb alloy is greater than that of Ti₃Al alloy at the same strain rate condition.The thermal softening effect of adiabatic temperature rise has a certain effect on the flow stress of the two materials at relatively high strain rate,and the thermal softening effect of the Ti₃Al alloy is stronger than Ti₂AlNb.Johnson-Cook（J-C）model was chosen to investigate the constitutive relation of the two materials.The strain rate coefficient of the J-C model was modified.At the same time,the thermal softening term was improved by establishing the function relation of adiabatic temperature rise with strain and strain rate to fit the constitutive relation of Ti₃Al;the constitutive relation of Ti₂AlNb was fitted by introducing the heat softening substitution related to the strain rate and strain rate.The improved J-C model can reflect the strain rate sensitivity and the softening effect of adiabatic temperature rise better.Ti₃Al has weak bearing capacity and lower failure strain rate under high strain rate.The adiabatic shear band in Ti₃Al is faster in appearance,larger in width,more in number and longer in crack growth than Ti₂AlNb.And the unique crack in the deformation zone can be found in Ti₃Al.The Ti₂AlNb has better dynamic plasticity because of the thinning and elongation of the matrix grains and more uniform deformation under high strain rate.The parabolic shear dimples are observed at the fracture surface of both materials under high strain rate.In general,the size of shear dimples increases with the increase of the strain rate.Under high strain rate,the shear dimples grow insufficiently with the sharp emergence of the crack,and part of the shear dimples are worn flat because of the friction and extrusion of the crack.With larger area and great size of shear dimples,the dynamic plasticity of Ti₂AlNb is better than that of Ti₃Al.