极低温下Nb_(3)Sn超导体单晶裂纹动态扩展模拟

Simulation of Dynamic Crack Propagation in Superconducting Nb_(3)Sn at Extreme Low Temperature

研究Nb_(3)Sn超导体的损伤断裂行为对于揭示超导临界性能弱化背后的力学机制具有重要的意义。采用分子动力学模拟方法,研究了极低温下不含裂纹和含中心裂纹的Nb_(3)Sn单晶在力学拉伸变形作用下的断裂机制和裂纹扩展行为,同时分析了应变率效应对Nb_(3)Sn单晶断裂机制与裂纹扩展行为的影响。结果表明:不含裂纹的Nb_(3)Sn单晶在结构受力后出现滑移,滑移带上位错塞积导致应力集中,应力集中使原子键断裂从而萌生裂纹致使Nb_(3)Sn单晶断裂;而含中心裂纹的Nb_(3)Sn单晶则由于裂纹尖端应力集中使得原子键断裂形成微裂纹,裂纹扩展致使Nb_(3)Sn单晶断裂。Nb_(3)Sn单晶在不同的应变率下表现出不同的断裂机制,在低应变率下表现为脆性断裂,而在高应变率下表现为韧性断裂。

The study on damage and fracture in superconducting Nb_(3)Sn is an indispensible part of understanding the origin of strain sensitivity in Nb_(3)Sn.In this paper,by using molecular dynamic simulations,the fracture and crack propagation behavior of single crystal Nb_(3)Sn with ideal lattice and with central crack at extreme low temperature are studied,respectively.The strain rate effects on the crack initiation and growth in both Nb_(3)Sn sample cases are also carefully analyzed.The results show that for stressed Nb_(3)Sn single crystal with ideal lattice,it slips with the dislocations plugging emerging on the slip band,which contributes to stress concentration and atomic bond breaking,resulting in the failure of Nb_(3)Sn.While for Nb_(3)Sn single crystal with central crack,the atomic bonds break due to the stress concentration concurring at the crack tip,microcracks form and propagate to induce the Nb_(3)Sn fracture.The analysis on the damage fracture and failure mechanism of single crystal Nb_(3)Sn at different strain rates reveals that it shows brittle fracture at low strain rate and ductile fracture at high strain rate.