Study On Mricrostructure And Superconducting Properties Of Nb₃Sn Strands For ITER Project

International Thermonuclear Experimental Reactor（ITER）is an international nuclear fusion research and engineering project,which is one of the most ambitious energy projects in the world today.420 tons Nb₃ Sn strands will be utilized for toroidal field（TF）coils in the ITER project for its high critical current density.The TF coils will produce a maximum magnetic field of 11.8 T.So the technical requirements for Nb₃ Sn superconducting strands,with fixed Cu to non-Cu ratio and heat treatment schedule,are challenging: critical current density（Jc）should be larger than 720 A/mm2（4.2 K,12 T）and hysteresis loss（Qh）should be less than 500 mJ/cm3（4.2 K,±3 T）.Manufacture process,microstructure and superconducting properties of Nb₃ Sn strands for ITER project were studied in this thesis.High-performance Nb₃ Sn strands were prepared via both internal tin process and bronze process.Influence of strand structure and processing optimization on properties has been studied.And effect of different heat treatment on the properties of Nb₃ Sn strand has been investigated.Focusing on transformation of the Cu-Sn intermediate phase,distribution of the third elements and critical temperature change during heat treatment,critical current sensitivity to strain,the performance characteristics are researched systematically.The key factors of high-performance strand design for both internal tin process and bronze process have been obtained via optimizing the structure and processing.For bronze process Nb₃ Sn strands,small Nb filaments will lead to increase of Jc and filament space larger than 1μm will benefit for reducing Qh.Qh of strand with pure Nb diffusion barrier is much bigger than that with pure Ta diffusion barrier.Annealing temperature below 500 ℃ could prevent pre-reaction and result in higher Jc and n value.For internal tin process Nb₃ Sn strands,Nb content 29.2% will bring to appropriate Jc and Qh.Sub-element with splits used in strand will reduce Qh largely.Influence of heat treatment on microstructure and superconducting properties have been researched systematically.For bronze process Nb₃ Sn strands,extension of holding time at 575 ℃ made no difference on Jc.Extension of holding time at 650 ℃ will result a little reduction of Jc.Increasing the final holding temperature from 650 ℃ to 675 ℃ reduce Jc and Qh obviously.For internal tin process Nb₃ Sn strands,the degree of filament reaction is incomplete at 630 ℃ and grains grow bigger at 670 ℃.An appropriate filament reaction degree and grain size has been obtained at 650 ℃,which result in a good performance.Extension of holding time at 650 ℃ from 50 h to 150 h will make the effective filament diameter（deff）increase from 31 μm to 50 μm.The heat treatment schedule of 575 ℃/100 h and 650 ℃/100 h is the best for both two kinds of strands.The mechanism and kinetics of growth of Nb₃ Sn layer was investigated.The time exponent n for bronze process Nb₃ Sn strand is less than 0.5,which means the rate controlling process is grain boundary diffusion.The time exponent n for internal tin process Nb₃ Sn strand is about 0.5,which means the rate controlling process are both grain boundary diffusion and volume diffusion.The reaction heat treatment has been researched systematically by ex situ method and rule of Cu-Sn intermediate phase transformation during heat treatment has been revealed.Function of the first stage of 210 ℃ is making Sn transform to ε and ε phase.The ε and ε phase become more uniform during the second stage of 340 ℃ and the third stage of 450 ℃.Homogenous α phase appeare at the fourth stage of 575 ℃ and Nb₃ Sn phase is formed at the last stage of 650 ℃.The regularity of distribution of the third element Ti during heat treatment has been studied by EDS.Ti6Sn5 transform to CuSnTi ternary compound during 450-575 ℃ in internal tin process Nb₃ Sn strand and Ti content in Nb₃ Sn filament is 0.57⁰.90 wt.% after heat treatment.Ti diffused into Nb-Ta filament when temperature is higher than 340 ℃ in bronze process Nb₃ Sn strand and Ti content in Nb₃ Sn filament is about 0.4 wt.% after heat treatment.No Ti element has been found in the Cu-Sn matrix after heat treatment in both kinds of strands.The reason for holes formation has been discussed.Kirkendall holes are formed between filaments in two kinds of strands.The holes in Sn core area are formed since the difference densities of phases in internal tin process Nb₃ Sn strand.The Ic sensitivity to the axial strain of internal tin process Nb₃ Sn strands have been researched.The pre-strain of strand WST01CW0007 is about-0.078% and irreversibility limit for axial tensile strain is +0.23%.The pre-strain of strand WST01CW0014 is about-0.066% and irreversibility limit for axial tensile strain is +0.3%.The sharing temperature Tcs of SULTAN samples made by two kinds of Nb₃ Sn superconducting strands at different magnetic field and operating current are able to meet ITER requirements.