Texture Regulation And Mechanical Property Of TA1 Based On Phase-transformation Effect Induced By Pulsed Electric Current

With the development of modern industry,pursuing higher strength and plasticity in titanium and its alloys becomes the main aim for the researchers in order to meet the more stringent requirements.Improving fabrication process and controlling microstructures(type/grain size/morphology/distribution of phases)are considered as the most effective approaches to improve the mechanical properties of titanium.Recently,a pulsed electric current(PEC)treatment was proposed as an effective method to improve the mechanical properties of metallic materials.Therefore,based on the related PEC effects and crystallographic theories,a PEC treatment generated by spark plasma sintering(SPS)system was applied to treat CP-Ti TA1,to study its influence on the texture evolution and mechanical behavior,and to compare with the results by radiation heating(without PEC treatment)under the same conditions of temperature,pressure and cyclic indexes,and the effect of pressure on the microstructure and mechanical property of the CP-Ti.Moreover,the hcp?bcc transition mechanism of CP-Ti was analyzed based on the Burgers orientation relationship and interface sliding model.The results obtained here provide a new insight into the fabrication of high-performance CP-Ti by tailoring microstructure and controlling texture.Firstly,different mechanical properties of CP-Ti with different texture orientation and microstructure were obtained by PEC treatment with different PEC intensities(heating rates),cycle indexes,initial structures,and holding times.Morphology observation shows that the equiaxed structure in the initial hot-rolled specimen transforms into basketweave structure under PEC intensity of 1400A(50K/min),and lamellar structure under PEC intensity of 1600A(100K/min)and 1900A(200K/min),respectively.The as-treated CP-Ti shows excellent plastic strain of 43.2% with high strength,surpassing any CP-Ti fabricated by other methods reported so far.Texture analysis indicates that the as-treated TA1 specimens have the gradually weakened texture with the increased PEC intensity,which is attributed to an enhancement in long-range atomic diffusion induced by the thermal and athermal effects.However,there is a significant change in texture with different cyclic indexes,which increases and then decreases as the cyclic index increases.The specimen shows the maximal yield strength(297.5MPa)induced by the maximal texture after three-cycle phase transformations,while the corresponding activation energy of nucleation is smaller than that of the initial hot-rolled specimen.The results change with the initial structures,and the texture increases with increasing holding time.The results obtained herein confirm the positive effect of PEC treatment on texture and mechanical properties of CP-Ti.Subsequently,combined with the above results,different experimental results of CP-Ti were obtained by radiation heating(without PEC treatment)under the same conditions of heating rate,pressure and cyclic indexes.It is found that the morphology and texture remain relatively unchanged except for the enhanced strength with increasing heating rates.Morphology observation shows that the equiaxed structure in the first two-cycle phase transformations transforms into lamellar structure with more cycle indexes,and the grain size increases with increasing cyclic indexes.Moreover,the texture and plasticity remain relatively unchanged but the higher yield strength and ultimate strength.Thirdly,the combination of pressure and PEC treatment on CP-Ti were studied.Results show that the as-treated CP-Ti specimens have the typical lamellar structure,and the grain size,which is much smaller than that obtained with no-pressure condition,increases with increasing PEC intensity.Texture analysis indicates that the as-treated TA1 specimens have a significant change along the direction parallel to PEC.The stronger diffraction intensity of plane(10-10)after 1400A(50K/min)transforms into the stronger(0002)after 1600A(100K/min),and then both back to the initial level after 1900A(200K/min).The CP-Ti treated by pressure and PEC displays the highest ultimate strength of 706 MPa with a high plastic strain of 39.1%.The research results obtained here further comfirm the validity of tailoring the fabrication technology,which is the designed pathway toward high-strength and large-ductile materials.Finally,based on the Burgers orientation relationship and interface sliding model,the transition mechanism hcp?bcc and bcc?hcp in CP-Ti under radiation heating and PEC treatment were analyzed.Results indicate that the transitions of hcp?bcc and bcc?hcp are processed regularly and reciprocally in CP-Ti under the condition of radiation heating,which has no significant effect on the texture orientation.However,the hcp?bcc transition shows a significant difference under the PEC treatment.Correspondingly,the hcp has rotated each other after experienced specific hcp?bcc transition and conventional bcc?hcp transition,and then changes the texture orientation of CP-Ti.In summary,the PEC treatment can improve and tailor texture orientation of metallic materials and thus contribute to an enhancement in long-range atomic diffusion,and then greatly improve the mechanical properties.Therefore,the method of PEC treatment is a very promising pathway for the fabrication of high-performance metallic materials.