| NbTi alloy has good mid-low magnetic field supercpnducting properties,excellent machining properties,low processing costs and low application costs.NbTi/Cu multi-core composite wire is the main form of application of the alloy,and widely used in the maglev trains,nuclear magnetic resonance imaging instruments,international thermonuclear experimental reactor and other engineering projects.The critical current density is one of the most important supercpnducting properties indicators in the application of NbTi/Cu multi-core composite wire.The microstructure of NbTi alloy can be tailored by optimizing the processes of pre-deformation and multi-stage aging system to increase the Jc of superconducting wire,thereby broadening the application field and enhance commercial competitivenessThis paper aims to NbTi alloy.The ?/? phase transition temperature is determined and the?/? phase transition process is studied and analyzed,which lays a theoretical foundation for determining a reasonable aging temperature interval and subsequent experimental results analysis.The effects of the degree of pre-deformation on the microstructure and hardness of NbTi alloy after aging was studied.On this basis,the effects of multi-stage aging treatment on the microstructure and hardness of NbTi alloy is finally studied,and a multi-stage aging system suitable for preparing high performance superconducting wires is obtained.The main research results are as follows:(1)The ?/? phase transition temperature of NbTi alloy calculated by JMatPro software is 616?,while that measured by DTA is 620?.The ?/? phase transition temperature of NbTi alloy is determined comprehensively to be 618?.The main phase transition process of NbTi alloy at low temperature aging is metastable ???+?',?'??+?iso?+?iso+???+?,and that at medium and high temperature is metastable ???+?'??+?.(2)After a large degree of pre-deformation including three-way free forging and 70%unidirectional cold rolling,and three-stage aging at 420?,The micro structure of NbTi alloy has ? phase,? phase and ?' phase.The main phase transion process is metastable ???+?'??+?.However,after a small degree of pre-deformation and three-stage aging at 420?,the micro structure of NbTi alloy has mainly ? phase.Increase the degree of pre-deformation,which is conducive to the precipitation of ? phase.The ? phase are mainly distributed along with the long dislocation cell walls and grain boundaries,and its size is mostly a few to hundreds of nanometers.The lattice mismatch of ? pahse and ? phase is 61.3%,which is a non-coherent interface.The ? phase contain Nb element and Ti element respectively are 9.81wt%and 90.19wt%,and 59.26wt%and 40.74wt%in the ? phase.(3)After pre-deformation and three-stage aging at 420?,The ? phase of NbTi alloy are dominated by recrystallized grains.While the ? phase mainly are deformed grains after the low degree of pre-deformation and sub-grains after the high degree of pre-deformation.When the pre-deformation degree is low,the small-angle grain boundaries and large-angle grain boundaries respectively account for 75.46%and 24.54%in the ? phase,and 33.82%and 66.18%in the ? phase.When the pre-deformation degree is high,the small-angle grain boundaries and large-angle grain boundaries respectively account for 64.00%and 36.00%in the ? phase,and 9.18%and 90.82%in the ? phase.The ? phase change from deformation texture to annealing texture,which show the transformation from {112}<110>texture dominated by Brass{110}<112>texture to Brass-R{111}<110>texture dominated by {110}<110>texture.The hardness of NbTi alloy increases first and then decreases with the increase of the degree of pre-deformation.(4)The volume percentage of ? pahse in NbTi alloy increases first and then decreases with the increase of aging temperature,and increases with the increase of aging stages.The kinetic model of isothermal phase transition of NbTi alloy at 420? is f=1-exp(-0.0135t0.550).The hardness of NbTi alloy continuously decreases with the increase of aging temperature,and decreases first and then increases with the increase of aging stages.Obtaining a reasonable NbTi alloy multi-stage aging system is 420? five-stage aging... |
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