Study On Mechanical Behavior And Microstructure Evolution Of Annealed Pure Titanium Under Quasi-Static/Dynamic Forced Shearing

As a kind of adiabatic shear sensitive material,it is necessary to study the adiabatic shear behavior of pure titanium at high strain rate in addition to a clear understanding of the quasi-static properties and micro mechanism at low strain rate.The microstructure evolution mechanism of adiabatic shear band has attracted much attention in the field of materials science.However,there is not enough experimental evidence on the formation process of ultrafine grains within adiabatic shear band,and the specific recrystallization mechanism is still controversial.Besides,hexagonal close-packed metals usually exhibit strong anisotropy,and the initial orientation of materials may have a certain influence on the adiabatic shear behavior.Therefore,in this paper,the annealed pure titanium plate is employed as the initial material,and the hat-shaped specimens are prepared along the rolling direction(RD)and transverse direction(TD)of the plate plane.The quasi-static and dynamic forced shear tests are carried out on the hat-shaped specimens by using the universal testing machine and the split Hopkinson pressure bar(SHPB)respectively.The samples with different deformation are obtained by controlling the reduction displacement,and the mechanical behavior and its anisotropy of quasistatic and dynamic forced shear deformation are analyzed.The microstructure and texture evolution of quasi-static and dynamic forced shear deformation of pure titanium with different initial orientation were investigated by means of optical microscopy(OM),electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM).The main conclusions are as follows:(1)The initial microstructure of annealed pure titanium sheet is composed of inhomogeneous equiaxed grains.Most grains of RD sample have the major orientation of<1010>or<1120>parallel to the loading direction,and the grains of TD sample have the weak orientation of<0001>parallel to the loading direction.The difference of initial orientation leads to the consistent anisotropy of quasi-static and dynamic mechanical behavior:the yield stress in RD sample is lower than that in TD sample,but the peak stress is slightly higher than that in TD sample.The comparison of quasi-static and dynamic mechanical curves shows that the yield stress and peak stress of dynamic deformation are significantly higher than that of quasi-static deformation,showing obvious strain rate strengthening effect,and the anisotropy of dynamic mechanical behavior is more obvious.(2)The quasi-static and dynamic forced shear deformation mainly occurs in the shear region between the notches at both ends of the pure titanium hat-shaped specimen.The inhomogeneous deformation occurs in the shear region:the deformation of the bottom end is more severe than that of the top end.The grains in the shear central region of the quasi-static sample develop into elongated narrow band structure and fragmented fine grains under the synergistic action of multiple generations of twinning and subsequently elongation and subdivision along the shear direction(SD).The shear central region of dynamic sample is highly localized,the early stage of shear localization involves the formation of twin/matrix lamellae,and the lamellae subsequently undergo longitudinal splitting and transverse breakdown into fine elongated subgrains,the continuing thermally/force assisted subgrains breakdown and lattice rotations,ultimately leads to the formation of elongated or equiaxed ultrafine grains(formation of adiabatic shear band),and then the ultrafine grains coarsen during subsequent deformation.(3)The initial orientation affects the microstructure evolution of the shear region under quasi-static deformation by affecting the twinning behavior:at the early stage of deformation,RD sample mainly forms{1012} extension twins,while TD sample mainly forms {1122}contraction twins and {1012} extension twins;{1012} extension twin variants merge with each other and {1122} contraction twin variants intersect with each other.Thus,the grain refinement caused by twin intersection in the TD sample is earlier than that in the RD sample,and the grain refinement degree in the shear central region of TD sample is finally more significant.The initial orientation also has a certain influence on the formation and evolution of adiabatic shear band under dynamic deformation:the TD sample has stronger adiabatic shear sensitivity than the RD sample,and shear localization occurs earlier to form adiabatic shear band.Therefore,the coarsening of ultrafine grains within shear band is more obvious during subsequent deformation,resulting in a larger average grain size in the adiabatic shear band.(4)The difference of microstructure evolution between quasi-static and dynamic deformation leads to the obvious difference of texture evolution:the grain orientation in the shear central region of quasi-static deformation is affected by multiple generations of twinning and shear strain,and the c-axis of most grains tends to deflect towards the shear plane normal(SPN),forming the strong texture;The orientation deflection of TD sample is more significant than that of RD sample,and the<0001>direction of most grains is parallel to the SPN.Adiabatic shear band appears in the shear central region of dynamic deformation,and the grain orientation within shear band shows regional differences.Then,the grains rotate along some certain direction during subsequent deformation,and finally form the texture characteristic which is independent of the initial orientation:the<1120>direction of the grains is parallel to the shear direction,and the {1010} plane is parallel to the shear plane.The whole article contains 75 figures,2 tables,and 120 references are cited.