Anisotropy Of Deformation And Damage For Dual Titanium Alloy Ti-6Al-4V Under High Strain Rate Loading

Titanium alloy has great potential for its development because of its small density,high specific strength,good specific stiffness and great resistance to canker.With the technology development of Aeronautics,automobile industry and military,titanium alloys need to withstand high strain rate impact loading in order to better serve as a bearing component.Under a high strain rate loading(?105/s),the damage is caused by spallation,not by common shear failure.It is very necessary to know the deformation and damage response of titanium alloy at higher strain rate.As a typical hexagonal close-packed(HCP)material,titanium alloy shows anisotropy under conventional loading.However,there are lesser lectures that show the mechanism of deformation and damage anisotropy of duplex phase titanium under higher strain rate loading.The research of anisotropy is not only need of the application of the duplex phase titanium alloy,but also provides a reference for designing new construction and making properties of duplex phase titanium alloys better.High strain rate planar impact loading experiments have been performed on Ti-6Al-4V rolling sheet via a one-stage light-gas gun(14mm bore diameter).The free-surface velocity profile of samples during normal direction(ND)and transverse direction(TD)are obtained with a Duplex Pin System(DPS)to obtain different shock response.The soft-recovered specimens are examined with scanning electron microscope(SEM),electron back-scatter diffraction(EBSD),X-ray diffraction(XRD)to investigate deformation and damage of a titanium alloy Ti-6Al-4V by researching the influence of microstructure on the anisotropy of deformation and spall behavior.The main conclusions are briefly described as following:(1)In plate impact experiments,the anisotropy in hugoniot elastic limit and spallation strength for Ti-6A1-4V sheet almost is minute in two attacking directions.The spall strength along TD direction is slightly higher than ND direction.But there is strong diversity in tensile strain rate and jumping-back acceleration.The rebound acceleration is lower when the plastic variable is higher along TD direction,indicating that the deformation is not related to the damage.The rebound acceleration has a responsibility for the rate of plastic failure,and the plastic deformation may suppress damage process along TD direction.(2)In terms of deformation,the plastic deformation of the sample shocked along TD direction is stronger than that along ND direction.The deformation of ? phase is more serious than that of a phase in all impact experiment,and the deformation difference between the two phases shows more obvious with the increase of the impact velocity.As a result,the plastic deformation mismatch is aggravated at the phase boundary.Twins are not common in Ti-6A1-4V.But still be found in some stress-concentrated area,indicating that the twins can be activated in the duplex titanium alloy in some special conditions.Only the {10-12} tension twin appears.At the same impact velocity,the bigger interfacial resistance encounters during the slip-motion process along ND direction,leading to the dislocations are easy to aggregate at the interface.Consequently,the distribution of twins and LABs are more and more uniform along TD direction to coordinate more serious deformation.(3)In the aspect of damage,the non-random distribution of nucleation sites occurs at higher-angle grain boundaries.The distribution of grain orientation is parallel to the crack surface under ND loading with the high dislocation density near there,which is favorable for the rapid nucleation and expansion of micro-crack and the formation of a local shear band,resulting in a cleavage failure.However,the higher plastic flow at the interface along TD loading suppresses the nucleation and expansion of the holes,which results in an extensional failure.With the increase of the impact velocity,the horizontal distribution of micro-cracks along ND loading gradually evolves into horizontal spallation phenomenon under the function of shearing.But micro-cracks tends to parallel to shock direction along TD loading resulting in a similar zigzag zone occurs.(4)Interplay between loading direction and interface boundary can influence the deformation near the interface.Under TD loading,the direction of loading tends to parallel to interfaces in which produce non-volume shear strain and yield stress is higher,leading to promoting larger plastic deformation.Moreover,more twins are observed near the area of spallation to release stress and the resistance to dislocation spread is lower due to the interplay,suppressing the formation of voids.Besides,the grain arrangement is helpful to damage distribution.Spall surface is parallel to the grain distribution in ND direction Loading in which micro-crack is easy to propagation.