Twinning Behaviors And Mechanical Properties Study Of Polycrystalline Titanium

Ti and its alloys possess high specific strength,novel chemical corrosion resistance and excellent biocompatibility.Coarse-grained Ti has a relatively low strength,which restrict the further application of Ti in industry field.By introducing alloying elements,the mechanical properties of materials increase under the effect of solution strengthening and precipitation strengthening.However,additional alloying elements would reduce the corrosion resistance property and biocompatibility of Ti.Recently,material researchers found that severe plastic deformation(SPD)treatments could improve the mechanical properties of materials significantly,without changing the chemical composition of the material.During SPD process,slip and twinning are stimulated in materials,resulting in the formation of sub-structures and grain refinement,and leads to the increment of mechanical properties.Nevertheless,the majority of previous investigations focus on the influence of dislocation activities on the mechanical properties of materials,while the mutual relation between deformation twinning and mechanical properties is underestimated.Because of inadequate slip systems,twinning is important to accommodate deformation strain in HCP structured Ti,especially the deformation strain along the c-axis.The generation of deformation twins would rotates the matrix lattice and results in the variation of local strain path,thus influences further slip and twinning behaviors of materials.The c/a axis ratio of Ti is1.587,which is lower than that of the ideal value(1.633).Generally,{112(?)1},{101(?)2}extension twins(ETs)and {112(?)2},{112(?)4} compression twins(CTs)could be stimulated under different deformation conditions.Slip and twinning are activated in Ti simultaneously during SPD processes,to accommodate deformation strain and has influence on the microstructure evolution and mechanical properties.Hence,understanding the deformation mechanisms of Ti,especially the twinning behaviors,is of great significance for improving the mechanical properties of materials.The research focus on the strength/toughness and deformation mechanisms of commercially pure Ti after various SPD treatments,e.g.,rotationally accelerated shot peening(RASP)and cold rolling.Optical microscope(OM),X-ray diffraction(XRD),electron backscattering diffraction(EBSD),transmission electron microscope(TEM)are used to investigate the microstructure evolution of Ti during SPD processes,especially the interaction mechanisms between slip and twinning.Both of qualitative and quantitative analysis are carry out to investigate the twinning behaviors,and to clarify the twinning systems and twin variants selection criterions under different deformation conditions.Due to the differences between macro-stress imposes on the samples and local-stress imposes on the grain,crystal local stress simulation model is establish to predict the variant selection of Ti.Quasi-static uniaxial tensile test and micro-hardness test are used to analysis the influence of microstructure on the mechanical properties of Ti.Further,on the premise of understanding the slip and twinning behaviors of Ti,the authors tend to fabricate materials with admirable strength and plasticity by adjusting the deformation conditions and microstructure evolutions.The main contributions of this dissertation are as following:(1)A gradient structured Ti with the deformation layer thickness of 2000 ?m is fabricated via RASP treatment.EBSD and TEM investigation results show that dislocation slip and deformation twinning contributes to the grain refinement of RASP processed Ti.With the grain size decreases,deformation mechanisms change from slip and twinning to slip dominates when the grain size smaller than 750 nm.Besides to slip and twinning,twin-twin interaction is another important factor which leads to the grain refinement.Based on dislocation theory,both glissile and sessile dislocations generate under the effect of twin-twin interaction.It results in the fragment of twin pieces,and even triggers de-twinning.(2)The grain size effect on deformation mechanisms and mechanical properties of Ti is investigated by using EBSD technique.Higher twin density and more twin systems are found in Ti with larger grain size after cryogenic rolling process.The percentage of twinned grain increases rapidly first and then become steady until saturation,while the number of twin per grain keeps increasing with the increase of grain size.The generation of {112(?)2} CTs is shown a higher sensitivity to grain size than that of {101(?)2} ETs.Annealed Ti with smaller grain size possesses better work hardening.While after rolling process,Ti with larger grain size obtains better work hardening due to higher twin density and lower density of pre-existing dislocations.(3)Ti samples with strong basal texture are fabricate under the combination effect of clock rolling and annealing process,with the texture intensity of 14.33.Further investigations on textured Ti after different rolling treatments found that {101(?)2} ETs with smaller twinning shear are easier to generate,which is the major twinning system in Ti when deformed at room temperature.While {112(?)2} CTs with larger twinning shear are effective at strain accommodation,and it is the major twinning system in Ti when deformed at cryogenic temperature.Further,Ti samples with hierarchical twin structure(HTS),i.e.,high order twinning(secondary twinning and tertiary twinning)and high twin density(up to 67.32 %),are prepared by adjusting initial texture and deformation conditions.Under the combination effect of twinning and slip,the yield stress and ultimate tensile stress of Ti increase to 950 MPa and1 GPa respectively.(4)EBSD technique was use to investigate the sequential twinning phenomenon in Ti.Results show that twinning systems selection in Ti depends on crystal orientation and local stress state,while Schmid law dominates the twin variants selection.Since deformation twinning could rotate the matrix lattice to some extent,sequential twinning such as ET?CT?ET or CT?ET?CT may generate,which depends on the intersection angle of caxis and stress direction.Local stress simulation model investigated and found that local stress on the matrix is close to the macro stress on the sample,with the deviation angle smaller that10°.(5)Semi-in-situ rolling treatment is carry out to investigate the {112(?)2} twin pairs(TPs)in Ti.Results show that lattice distortion and macro stress provide the nuclear energy of TPs with variants develop along reversed orientations,while the twinning induces local stress and macro stress provide the nuclear energy of TPs with variants develop along the same orientation.Schmid law,geometrical compatibility factor(GCF)and parent grain misorientation are used to investigated the variant selection criterion of {112(?)2} TPs,and Schmid law is found dominate the formation of TPs.Further,with the increase of parent grain misorientation,GCF decreases and higher Schmid factor is required to generation TPs.