| TiNb alloys have been widely employed in the engineering fields because they possess good properties,such as shape memory effect,corrosion resistance,low elastic modulus,non-toxicity,superconductivity and workability.Chemical composition of TiNb alloys has a predominant influence on their properties.Typical Ti-37 at.%Nb alloy has been extensively applied in the field of cryogenic environment because they possess outstanding superconductivity as well as superior ductility.As an important working technique,plastic forming plays a considerable role in the application of TiNb alloys for engineering fields.Plastic working not only manufactures TiNb alloy parts with a certain shape,but also it has an important impact on microstructures of the alloy,which furthers affect the involved properties of the alloy.Therefore,in the present work,based on the typical Ti-37 at.%Nb alloy,it is of scientific significance to investigate microstructural evolution,plastic deformation mechanism,and thermal workability of TiNb alloy at high temperature by combining high-temperature mechanical experiment,X-ray diffraction(XRD),optical microscopy(OM),electron backscattering diffraction(EBSD),transmission electron microscopy(TEM),cellular automaton simulation(CA)and finite element simulation(FEM).In the present paper,the main research results are as follows.Based on high-temperature mechanical experiment results and material characterization experiment results,plastic deformation mechanism and microstructural evolution of TiNb alloy at the temperatures of 700?1000?and the strain rates of 0.0005?0.5s-1are investigated.Dislocation slip is responsible for deformation mechanism of TiNb alloy during plastic deformation at high temperature,where dynamic hardening and dynamic softening coexist.Dynamic softening mechanism is mainly attributed to dynamic recovery during plastic deformation of TiNb alloy at 700?.With the increase of deformation temperature,dynamic softening mechanism is gradually transformed to dynamic recrystallization.When the deformation temperature is elevated to 1000?,dynamic softening mechanism is transformed from discontinuous dynamic recrystallization into continuous dynamic recrystallization.According to the uniaxial compression stress-strain curves of TiNb alloy at the temperatures of 700?1000?and the strain rates of 0.0005?0.5s-1,the constitutive equation of TiNb alloy at high temperature is established based on the Arrhhenius constitutive model.Furthermore,the constitutive equation of high-temperature plastic deformation of TiNb alloy based on strain compensation is established.The results show that the established constitutive equation can accurately predict the flow behavior of TiNb alloy subjected to plastic deformation at high temperature.TiNb alloy belongs to strain rate sensitive materials in the case of plastic deformation at high temperature,where flow stress increases with the increase of strain rate,but flow stress decreases with the increase of temperature.The established constitutive equation of TiNb alloy provides an accurate material model for finite element simulation of TiNb alloy during plastic forming.According to the true stress data of TiNb alloy at the temperatures of 700?1000?and the strain rates of 0.0005?0.5s-1corresponding to the true strains of 0.3,0.6 and 0.9,processing maps of TiNb alloy in the case of various strains are established based on dynamic material model by combining microstructures of TiNb alloy subjected to plastic deformation at high temperature.According to the involved processing maps,instability region and workable region of TiNb alloy are determined and simultaneously hot workability of TiNb alloy is revealed.The results indicate that instability mode of TiNb alloy is characterized by local plastic flow in the case of plastic deformation at low temperature and high strain rate.The stable region of TiNb alloy during hot working includes the condition on which the deformation temperature range is 765?910?and the strain rate is less than 0.0007s-1as well as the condition on which the deformation temperature range is 960?1000?and the strain rate range is 0.002?0.1s-1.Cellular automaton model of discontinuous dynamic recrystallization of TiNb alloy subjected to plastic deformation at high temperatures is established and the involved cellular automaton simulation program is developed independently.Fundamental law with respect to discontinuous dynamic recrystallization of TiNb alloy is revealed by combining cellular automaton simulation,OM experiment and EBSD experiment.The results indicate that cellular automaton simulation can apparently describe the evolution of discontinuous dynamic recrystallized behaviour during plastic deformation of TiNb alloy at high temperature.The size and the volume fraction of dynamic recrystallized grains increase with the increase of deformation temperature,but they decrease with the increase of strain rate.Furthermore,random orientation of discontinuous dynamic recrystallized grains contributes to weakening the strength of deformation texture in the deformed TiNb samples.Based on the established Arrhhenius constitutive model of TiNb alloy,rigid viscoplastic finite element method is used to simulate different-temperature Cu/TiNb cladding extrusion where Cu and TiNb possess different temperature during plastic deformation.Different-temperature Cu/TiNb cladding extrusion can significantly lower the deformation temperature of Cu cladding layer so as to reduce the difference between yield stresses of Cu cladding layer and TiNb alloy core,which contributes to accommodating the deformation of the two metals.The effects of different deformation temperatures(700,800 and 900?),different friction coefficients(0.3,0.5,0.7)and different cone angles of bottom die(60,120and 180°)on Cu/TiNb cladding extrusion formability are investigated based on rigid viscoplastic finite element method.The simulation results show that deformation temperatures have a slight effect on the relative elongation between Cu cladding layer and TiNb alloy core.Increasing the friction coefficient between Cu cladding layer and die or enhancing cone angle of bottom die contributes to reducing the relative elongation between Cu cladding layer and TiNb alloy core,which is conducive to the interface bonding between Cu cladding layer and TiNb alloy core.According to the optimal prarameters based on finite element simulation,Cu/TiNb cladding extrusion die is designed,where the cone angle of bottom die is determinced as 180°.The dismention of Cu/TiNb cladding extrusion billet is completely consistent with finite element model.Cu/TiNb cladding extrusion experiment is carried out in the case of no lubrication.The experimental results show that stable flow of metal takes place during Cu/TiNb cladding extrusion,where Cu cladding layer and TiNb alloy core present compatible deformation and they do not almost exhibit the relative elongation.The experiental results agree well with the simulated ones. |
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