Simulation Of Fracture Of Nb/Nb₃Sn Composite Superconductor At The Extreme Low Temperature

Superconducting Nb₃Sn has demonstrated great potential for applications in the fields of magnetic confinement fusion and superconducting magnets in high energy physics.The strain-dependent critical performance degradation is a critical issue in the material application in Nb₃Sn superconducting magnet design and manufacture.The superconducting critical performance of Nb₃Sn superconductor is extremely sensitive to stress and strain.When it is subjected to external load,the superconducting performance changes accordingly.When the applied strain exceeds its irreversible strain limit,cracks will occur,resulting in the sharp superconducting critical performance degradation,and then quench will be initiated.The study of the damage and fracture behavior of Nb₃Sn superconductor plays an important role in revealing the mechanical mechanism behind the superconducting critical performance degradation and the safe design of superconducting magnet equipment.In this paper,the molecular dynamics simulation method is used to establish the Nb₃Sn single crystal model based on the experimental observation results of Nb₃Sn crystal structure.By calculating the elastic constants and lattice constants of Nb₃Sn single crystal,the reliability of the empirical potential function is verified by comparing with the first-principle simulations.On this basis,the fracture mechanism,the crack propagation behavior and the strain rate effect of Nb₃Sn single crystal without microcrack and with central microcrack under tensile load at extremely low temperature are studied.The analysis of the crystal structure evolution and the atomic stress distribution shows that brittle fracture occurs in Nb₃Sn single crystal.The fracture of the Nb₃Sn single crystal without microcracks is caused by stress concentration arising from the dislocation accumulation on the slip band,while the fracture of Nb₃Sn single crystal with central microcracks is caused by stress concentration originating from the dislocation accumulation at the crack tip.The strain rate has little effect on the elastic deformation behavior of Nb₃Sn single crystal.With the increase of the strain rate,the mechanism of the fracture of Nb₃Sn single crystal changes from the brittle fracture at low strain rate and the ductile fracture at high strain rate.For Nb₃Sn single crystal with central microcrack,the crack propagation leads to fracture at low strain rate,and the dislocation emitted by the crack tip at high strain rate is the main cause of fracture.Based on the Nb₃Sn single crystal model,the calculation model of Nb/Nb₃Sn composite superconductor without microcrack and with central microcrack is established.The fracture process of the Nb matrix and the fracture mechanism and the crack propagation behavior of Nb₃Sn single crystal without microcrack and with central microcrack are discussed from the microscopic point of view.The results show that the Nb matrix of the Nb/Nb₃Sn composite without microcrack and with central microcrack shows phase transformation and stacking fault in the fracture process,and the stacking fault delivery leads to the fracture of the Nb matrix.Under the action of the Nb matrix,both Nb₃Sn single crystal without microcrack and Nb₃Sn single crystal with central microcrack undergo ductile fracture,however,the crack propagation behavior is different.The fracture of Nb₃Sn single crystal without microcrack is caused by the stress concentration after the intersection of multiple slip bands.In Nb₃Sn single crystal with central microcrack,the crack tip is passivated due to the emission of dislocation,the crack propagates slowly and breaks.The crack propagation results obtained by the molecular dynamics simulations provide a certain reference for revealing the physical mechanism behind the mechanical and electromechanical coupling of Nb₃Sn superconductors with complex structures.It also contribute to the preparation and fabrication of high strain tolerant superconducting materials.The findings reported in this paper lay a certain foundation for the development of multi-scale crack propagation model in practical superconductors,and provide a theoretical guide for the interpretation of fracture characteristics of engineering superconductors.