Research On Deformation Behavior And Constitutive Relation Of NbTi And NbTi/Cu

NbTi alloy is one of the typical low temperature superconducting materials because of its excellent superconductivity,processing and low cost.NbTi/Cu multi-filamentary wires are widely applied in many fields.At present,NbTi/Cu multi-filamentary wires are prepared by multi-path bundle drawing technology.However,the strain mismatch of NbTi and Cu during the processing of NbTi/Cu filamentary wires tends to lead defects such as distortion of wire cross-section,wire breaking,non-uniform of wire diameter due to the great differences in the physical property between NbTi and Cu.The defects ultimately deteriorate the comprehensive properties of the NbTi/Cu multi-filamentary wires.Hence,it is urgent to develop the precise forming technology for high performance NbTiCu multi-filamentary wires.In this paper,the compression experiments of NbTi alloy with different strain rates and temperatures and NbTi/Cu composite with constant strain rates at the room temperature are separately conducted in Gleeble-3500 test machine.The deformation behavior of NbTi alloy at high temperature and NbTi/Cu composite at room temperature are researched to establish the constitutive equations based on the Arrhenius exponential equation,which lays the foundation for the finite element simulation.The effects of deformation temperature and strain rate on the micro-deformation behaviors of NbTi alloy are studied by means of XRD,SEM and EBSD.The hot processing maps of NbTi alloy with different strains are constructed to optimizethe hot extrusion process of NbTi/Cu composite.The main results are listed as follows:(1)When the deformation temperature is less than 700℃,the true stress-strain curve of NbTi alloy is similar to the dynamic recrystallization curve,which is similar to the dynamic recovery curve when the deformation temperature is more than 700℃.The constitutive equations of NbTi alloy at high temperature are listed as follows:(2)The deformed micro structure of NbTi alloy consists of a single(β-Ti,Nb)solid solution.The preferred orientation of grains and the texture strength decrease with the increase of deformation temperature at the same strain rate.However,the average grain size increases with the increase of deformation temperature at the same strain rate.Furthermore,the number of low angle grain boundaries increases while that of high angle grain boundaries decreases.When the deformation temperature is low,the softening mechanism is mainly dynamic recrystallization,while that changes to dynamic recovery when the deformation temprature is high.The types of the texture in NbTi alloy remain basically unchanged with the change of deformation temperature.When the deformation temperature is 650℃,the {011}<211>textureis dominant in NbTi alloy in which the {123}<634>textureis dominantat other deformation temperatures.(3)The preferred orientation of grains and the texture strength increase with the increase of the strain rate at the same deformation temperature.However,the average grain size decreases with the increase of the strain rate at the same deformation temperature.Furthermore,the number of low angle grain boundaries increases while that of high angle grain boundaries decreases.The softening mechanism of NbTi alloy changes from dynamic recrystallization to dynamic recovery with the increase of the strain rate.The types of the texture in NbTi alloy remain basically unchanged with the change of the strain rate.When the strain rate is less than 0.01s-1,the {123}<634>textureis dominant in NbTi alloy in which the {011}<211>textureis dominant when the strain rate is more than 0.1s-1.(4)When the strain rate is less than 10s-1,the true stress-strain curve of NbTi/Cu composite is similar to the dynamic recovery curve,which has some softening characteristics when the strain rate is more than 10s-1.The constitutive equation of NbTi/Cu composite at room temperature is listed as follows:lnσ=6.27568-0.13459lnε-0.03524(/lnε)2+0.010843lnε(5)The hot processing maps of NbTi alloy with different strains are constructed to optimizethe hot extrusion process of NbTi/Cu composite with the sirain rate 0.001s-1～0.0302s-1 at 650℃.