Investigations On Impact And Tensile Properties As Well As Deformation Mechanisms Of Ti-23Nb-0.7Ta-2Zr-O Alloy

TiNbTaZrO gum metals(GMs)are a group of?-Ti alloys containing significant amounts of?-stabilizers to maintain the body centered cubic(bcc)lattice structure.It has been found that GMs had a promising future for commercial use because of their“super”mechanical properties,such as super strength,super elasticity with relatively low elastic modulus,super plasticity and so on.However,limited endeavour has been put into the research of impacting property which can be characterized by means of split Hopkinson bar(SHPB)impact and Charpy impact tests.Moreover,its dislocation-free deformation mechanism still has been under debate.In split Hopkinson bar compressive impact,adiabatic shear band(ASB)always emerges at the onset of unstable local deformation.As the precursor of fracture,formation and evolution mecanisms of ASB have received increasing attention.In Charpy impact,the pre-crack helps to evaluate the fracture toughness of materials with manufacturing defects.Therefore,investigations on impact properties of TiNbTaZrO GM not only have academic value but also have engineering meaning.In this work,bulk GM ingots were first extruded using a conventional route,then were further deformed by equal channel angular pressing(ECAP)and followed by annealing.The mechanical behaviors of the extruded GM,ECAP-processed GM and annealed GM were further studied under tensile test,quasi-static compression,SHPB compressive impact and Charpy impact with loading directions vertical and/or parallel to the extruded and ECAPed directions.X-ray diffraction(XRD),optical microscope,scanning electron microscope,electron backscatter diffraction(EBSD),transmission electron microscope(TEM)and high resolution transmission electron microscope(HR-TEM)were finally employed to characterize the microstructural evolutions of all categories of Ti-23Nb-0.7Ta-2Zr-O under deformation.Significant results are as following:1.The yield strength,ultimate tensile strength,elongation to failure and uniform elongation of the extruded GM,ECAP-processed GM and annealed GM were 903 MPa/1027 MPa/15.2%/7.8%,985 MPa/1043 MPa/8.7%/1.1%,730 MPa/749 MPa/17.5%/1.5%,respectively.TEM investigations revealed that the optimal strength-ductility combination of the extruded GM was closely related to nano-scale deformation domains which could obstruct dislocations.2.SHPB compressive impact revealed anisotropic ASBs of Ti-23Nb-0.7Ta-2Zr-O GM,i.e.,precipitous stress drops associated with ASB were observed only when loading was perpendicular to the extrusion/ECAPed directions of both extruded and ECAP-processed GM specimens.EBSD analysis indicated that the anisotropy of ASB was originated from the<110>fiber texture and ECAP texture,suggesting geometrical softening was one of the ASB formation mechanisms of Ti-23Nb-0.7Ta-2Zr-O GM.Our calculation of temperature rise during the SHPB compressive impact indicated that the global temperatures of the extruded and ECAPed Ti-23Nb-0.7Ta-2Zr-O GMs at the onset of ASB were 321 K and 331 K,respectively,both of which were much smaller than the recrystallized temperature(0.5T_m=1066 K,where T_m is the melting point of Ti-23Nb-0.7Ta-2Zr-O GM and is about 2132 K).This result hinted that the ASB was not formed through thermal softening mechanism which should have temperature increase over 0.5T_m during impacting and cause local shear deformation and recrystallization.Based on this result,we proposed that microstructural softening might be the main mechanism of the ASB formation of Ti-23Nb-0.7Ta-2Zr-O GMs.3.EBSD analysis on the ASB microstructures revealed two different deformed microstructures,i.e.,deformed ultrafine grains(UFGs,81.4%),with high dislocation density and large internal misorientations,and dynamically recrystallized UFGs,with low dislocation density and small internal misorientation,formed in the ASBs of GM samples processed by extrusion and ECAP,respectively.Additional calculation of temperature rise during dynamic compression suggested that the above microstructure differences in the ASBs were originated from their different maximum ASB temperatures(608 K for extruded GM and 1159 K for ECAP-processed GM).The temperature of the ASB of the ECAPed sample was over 0.5T_mduring impacting,resulting in recrystallized microstructure by the rotational recrystallization mechanism;while the temperature of the extruded sample was below 0.5T_m,causing deformed grains by the rotation induced grain refinement mechanism.4.Charpy impact test indicated that the absorbed energy obtained with loading direction perpendicular to the extrusion direction was higher than that with loading direction paralell to the extrusion direction.EBSD observation revealed that large amount of delaminations were nucleated at grain boundaries and mismatched textures of the impact specimen with higher abosbed energy,and propagated along grain boundaries vertically to the propagation direction of the main crack.The delaminations are able to absorb more impact energy and release the stress concentration in front of the main crack.