Study On Deformation Behavior And Microstructures And Mechanical Properties Of Commercially Pure Ti Processed By ECAP At Room Temperature

Equal channel angular pressing (ECAP) can produce bulk ultra-fine grained (UFG)materials with high strength by severe simple shear. In order to develop its potentialand extend its application field, commercially pure titanium with an average grain sizeless than200nm was fabricated by ECAP for up to4passes at room temperature usinga die with the internal angle Φ of90°and outer curvature angle Ψ of20°. Advancedcharacterization and measurement techniques such as transmission electronmicroscopy (TEM), scanning electron microscopy (SEM), electron backscatterdiffraction (EBSD), X-ray diffraction (XRD), nano indentation and isothermalsimulation and so on, are used to analyze microstructural evolution, to revealmechanisms of grain refinement and plasitic deformation of CP Ti during ECAP atroom temperature and deformation behaviors of UFG CP Ti.The microstructural evolution of CP Ti during ECAP was investigated by opticalmicroscope, TEM and EBSD. The results show that dislocation slip is the maindeformation mechanism of CP Ti during the ECAP process. Deformation twins areactivated and accommodate the plastic strain by local stress concentration. Lots ofshear bands and dislocation tangles are generated during the initial stage of ECAP,which develop to dislocation cells by dynamic recovery such as the rearrangement andannihilation of dislocations. With further deformation, all the grains will be refinedinto ultrafine grains with high angle boundaries (HABs) by continuous dynamicrecrystallization (CDRX). The tensile properties and microhardness of as-received and ECAPed CP Ti wereinvestigated by electronic tensile machine and Vickers microhardness tester. Theresults show the ultimate tensile strength and microhardness increase significantly afterthe first pass and then increase by very small amounts in subsequent passes and aresaturated after4passes of ECAP. The homogeneity of microhardness distribution andmicrostructures gradually improves with increasing numbers of passes in ECAP.The texture evolution of CP Ti after ECAP was measured by X-ray diffraction.The texture of the as-received CP Ti is a typical of hot-rolled plate with the basal planeoriented toward the plate normal direction. After the first pass, the basal texture isoriented and P texture and B texture are formed. After the second and third passes, Btexture is the only one visible and P texture disappears. After the last pass, there is atypical ECAP shear texture mainly containing prism texture.The results of tensile and compressive tests show that CP Ti processed by ECAPhas low strain hardening ability and tension-compression asymmetry. To examine theeffect of loading orientation on the mechanical response, compression tests werecarried out. Anisotropy of uniaxial compressive mechanical behavior was observed inUFG CP Ti. The specimens under Y direction loading display higher flow stress valuesthan those under X direction loading. This phenomenon is related with the prismtexture developed during ECAP.In order to study the creep behavior at room temperature of as-received andECAPed CP Ti, nano indentation experiments were performed using MTS Nanoindentation’XP system equipped with a Berkovich diamond indenter. The hardness,Young’s modulus and creep behavior during the nano indentation were analyzed. Theresults show that the hardness, Young’s modulus increase with increasing of load strainrate. There is the indentation size effect (ISE) with the incorporated hardening effect.Creep resistance of CP Ti is enhanced after the first pass of ECAP and then decreasesafter4passes of ECAP.Isothermal compression tests were conducted over a wide temperature range of250~550°C with strain rate range of0.00001~1s-1for investigating deformationbehavior of UFG CP Ti. The true stress-true strain curves of UFG CP Ti display thecharacteristic of steady flow. The flow stress decreases with decreasing strain rate and increasing deformation temperature. Compared to the as-received CP Ti, there issoftening in the steady state deformation of UFG CP Ti. And UFG CP Ti has highervalues of strain rate sensitivity of flow stress and lower values of activation volume offlow stress than those of as-received CP Ti. High-angle grain boundaries can soften thematerials in steady state of deformation by enhancing the annihilation of dislocations.The constitutive relationship of UFG CP Ti was established based on theArrhenius model. The stress sensitivity exponent of UFG CP Ti is11.225and thethermal activation energy is169.008kJ·mol-1, which is close to the lattice diffusionenergy of α-Ti. It means that dislocation movement is the main plastic deformationmechanism. The processing maps of UFG CP Ti were established, and instabilitydomains were developed by the Prasad and Murty instability criteria.