静水压作用下Nb3Sn多晶体超导临界温度退化的耦合模型

Nb3Sn超导材料主要用于强磁场超导磁体的制造。力学变形诱导的其超导临界性能退化给强磁场超导磁体装置的电磁性能指标和安全运行造成了极其不利的影响。针对静水压作用下，Nb3Sn单晶体和多晶体表现出的不同退化行为，本文基于Maki De Gennes（MDG）关系式，建立了描述Nb3Sn单晶体变形-超导临界温度耦合响应的本构关系，并借助于多晶体有限元方法，对静水压作用下Nb3Sn多晶体超导临界温度退化响应进行了预测，预测结果与实验结果定性吻合。模型实现了从Nb3Sn单晶体到Nb3Sn多晶体变形-超导临界温度退化响应曲线的一致性预测。研究结果有助于提高对Nb3Sn高场超导材料变形-超导电性能耦合行为的认识，为发展描述运行工况下Nb3Sn超导材料力-电磁-热多物理场多尺度耦合行为的建模与数值计算方法提供了一定的基础；同时，相关结果对于特殊工况下高场超导磁体性能的评估和高应变耐受性超导材料的制备也具有一定的指导作用。     

静水压作用下Nb3Sn多晶体超导临界温度退化的耦合模型

Nb3Sn超导材料主要用于强磁场超导磁体的制造。力学变形诱导的其超导临界性能退化给强磁场超导磁体装置的电磁性能指标和安全运行造成了极其不利的影响。针对静水压作用下,Nb3Sn单晶体和多晶体表现出的不同退化行为,本文基于Maki De Gennes（MDG）关系式,建立了描述Nb3Sn单晶体变形-超导临界温度耦合响应的本构关系,并借助于多晶体有限元方法,对静水压作用下Nb3Sn多晶体超导临界温度退化响应进行了预测,预测结果与实验结果定性吻合。模型实现了从Nb3Sn单晶体到Nb3Sn多晶体变形-超导临界温度退化响应曲线的一致性预测。研究结果有助于提高对Nb3Sn高场超导材料变形-超导电性能耦合行为的认识,为发展描述运行工况下Nb3Sn超导材料力-电磁-热多物理场多尺度耦合行为的建模与数值计算方法提供了一定的基础;同时,相关结果对于特殊工况下高场超导磁体性能的评估和高应变耐受性超导材料的制备也具有一定的指导作用。     

Nb3Sn高场复合超导体临界性能力学变形效应的多尺度模拟

A15型Nb₃Sn超导体是制造高场( > 10 T)超导磁体线圈的主要材料, 被广泛应用于磁约束可控核聚变、高能物理等强磁场超导磁体装备制造领域. 力学变形诱导的Nb₃Sn超导临界性能退化给高场超导磁体装备的电磁性能指标和安全稳定运行造成了极其不利的影响. 鉴于Nb₃Sn超导体具有复杂的多尺度结构特征, 不同尺度下变形与超导电性能耦合行为是相互关联的, 本文建立了考虑微/细/宏观关联的非线性力电磁耦合本构模型, 提出了从原子尺度A15晶体结构到超导体微结构到宏观非均质Nb₃Sn复合超导体的多尺度模拟模型. 基于多晶体有限元方法, 对静水压加载条件下Nb₃Sn多晶体超导临界温度衰退和单轴拉压加载条件下Nb₃Sn复合多晶体临界性能衰退行为进行了模拟预测, 预测结果与实验观测结果定性吻合. 该模型揭示了Nb₃Sn复合超导体变形-超导电性能多尺度耦合机理, 实现对高场超导体力、电、磁、热耦合行为的预测, 有助于提高对A15型金属间化合物高场超导复合材料力、电、磁、热多尺度耦合行为的认识和描述能力, 为强磁场超导磁体的设计与制造提供有力的理论支撑.      

超导线圈在强磁场作用下的弹塑性变形分析

根据超导线圈受电磁力、热膨胀、弯曲应力作用的受力机制,首先理论研究了超导线圈在发生弹塑性变形时内部的应力、应变、位移,然后使用两段直线之间由过渡曲线相连接的简化模型作为超导线的应力-应变曲线的近似特性。利用有限元方法,数值模拟了电磁力作用下典型的Nb3Sn超导线材在不同边界条件下的应力、应变、位移分布情况。计算结果定量显示了超导线的弹塑性变形特性同电流密度的关系,初步预测了超导线圈发生塑性变形的区域和扩展以及支撑结构对超导磁体力学行为的影响。     

镁单晶变形行为的原子模拟

应用分子动力学方法模拟镁单晶在单向拉伸作用下的力学性能和微观结构演化过程.不同应变和不同温度下的模拟结果中都观测到{1011},{1012}型李晶.表明这两种孪晶是镁单晶拉伸变形的主要机制.其中{1012}型挛晶的产生伴随新晶粒生成,并且随应变增加孪晶的数量增加.此外,随着温度的增加,原子热激活效应显著.镁单晶的最大弹性形变减小,最大应力值亦变小.     

分子动力学模拟含不同初始缺陷单晶镍的断裂行为与应力演化

断裂是一个跨尺度复杂的物理过程,对宏观尺度的断裂行为已有深入的研究和发展,然而对微观尺度的断裂行为及断裂过程中应力场的变化缺乏深入的理解。本文通过分子动力学模拟,研究了具有不同初始缺陷(尖锐裂纹、钝裂纹和孔洞)的单晶镍的断裂行为和应力分布特征。结果表明,不同的初始缺陷导致了不同的断裂机制、断裂强度和抗断裂性能。含初始孔洞的单晶镍样品有最高的断裂强度和最强的抗断裂性能,这与孔洞扩展过程中堆积层错的形成密切相关。其次是含初始钝裂纹的样品,在裂纹扩展过程中出现由[100]超位错发射引起的裂尖钝化;含尖锐裂纹的样品表现为脆性断裂,裂尖原子没有出现微结构的变化,其强度和抗断裂性能最低。此外,不同的初始缺陷也会导致断裂过程中应力分布的变化,对含有尖锐裂纹的脆性断裂试样,高应力(拉伸应力、平均应力和米塞斯应力)总是出现在扩展裂纹的裂尖。而对于含有钝裂纹或孔洞的韧性断裂试样,高应力不仅分布在裂尖,也分布在位错发射和堆积层错形成的区域,在裂纹/孔洞扩展之前,应力随着加载时间的增加而迅速增加,而一旦裂纹或孔洞开始扩展,应力增加非常缓慢或几乎不增加,但拉伸应力值始终大于平均应力和米塞斯应力值。这表明,在I型...     

NiTi形状记忆合金动态压缩力学行为的实验研究

采用材料试验机和SHPB实验技术,对在不同初始温度(298～873K)和应变率(5×10-4、～2.3×103s-1)下的NiTi形状记忆合金的压缩力学行为进行了实验研究。结果表明:马氏体状态下的NiTi合金的力学行为对应变率的变化敏感,位错屈服段的硬化模量、相屈服段的硬化模量及马氏体重取向前的弹性模量对应变率的变化不敏感,而位错塑性变形前的弹性模量随应变率的提高迅速增大;奥氏体状态下的NiTi合金随着实验温度升高,无论是应力诱发马氏体相变应力还是奥氏体相屈服应力都逐渐下降,材料表现出温度软化效应。从超弹性温度范围内的卸载曲线中观察到了应力诱发马氏体到奥氏体的逆转变。     

热处理对TC4钛合金动态力学性能和微观组织的影响

为揭示热处理对TC4钛合金动态力学性能及微观组织的影响，选取2种典型热处理方式和5种加载应变率开展了TC4钛合金试样的动态力学性能实验，获取了动态应力-应变数据，并进行了试样的XRD和金相分析。结果表明：高应变率下TC4钛合金应变率强化效应显著。时效处理后，TC4钛合金流动应力、屈服强度及抗压强度得到提升，而固溶时效处理后上述性能降低。时效处理和未热处理试样应力-应变曲线均具有弹性、屈服和塑性阶段，而固溶时效处理后无明显弹性和屈服阶段。固溶时效处理后流动应力随应变率增加而增加，时效处理和未热处理试样流动应力无明显变化。时效处理后试样等轴初生α相显著增大且β相含量较低，固溶时效处理后α相晶界增大且含有针状α的β转变基体，TC4钛合金力学性能与β相和亚稳β相的马氏体转变有关。     

基于界面层模型的双周期纳米夹杂复合材料反平面问题研究

利用复变函数方法并结合双准周期Riemann边值问题理论,获得了含双周期分布非均匀相（夹杂/界面层）的复合材料在远场均匀反平面应力下弹性场的全场解答．该解答可用于对纳米夹杂复合材料的应力进行分析,结合平均场理论也用于预测纳米夹杂复合材料的有效性能．计算结果表明：当夹杂尺度在纳米量级时,应力和有效反平面剪切模量具有明显的尺度依赖性,并且随着夹杂尺寸的增加,趋近于不考虑界面效应时的结果;界面层厚度和性能对应力和有效反平面剪切模量明显变化时所对应的夹杂尺度范围和趋近于无界面效应结果的快慢有显著影响;当界面厚度足够薄时,界面层模型可用于模拟零厚度界面情况．     

考虑表面效应的形状记忆合金纳米梁弯曲特性研究

形状记忆合金(Shape Memory Alloys, SMAs)因其具有形状记忆效应和超弹性,在航空航天、生物医疗、微机电系统领域中得到了广泛的应用．当微结构尺度达到微纳米,表面效应对微结构力学性能的影响是十分显著的．本文基于梁弯曲变形理论以及Gurtin-Murdoch表面弹性理论,考虑拉压不对称、温度对于SMA纳米梁的影响,建立了考虑表面效应的SMA纳米梁相变力学模型．分析了弯曲载荷、温度、表面残余应力以及表面弹性模量对SMA纳米梁力学性能的影响规律．研究表明在SMA纳米梁相变阶段,忽略和考虑表面效应所得的截面应力及应变相对误差较为明显;在相同弯矩下,随温度的增加SMA纳米梁的截面应力随之增加,并且表面效应对其影响有减小趋势;表面残余应力对SMA纳米梁的影响显著．该文研究结果为SMA纳米梁在微机电领域的设计以及应用提供了一定基础与依据．     

考虑纤维断裂特征的Nb3Sn股线统计拉伸强度和失效概率

Nb₃Sn 超导复合股线在强磁场工程中有着重要的应用,其拉伸强度为保证和评估其长期服役安全的一个关键参数。本文从 “剪滞”理论出发,利用基于Weibull/Possion统计理论的复合材料纤维断裂的Curtin-Zhou模型,较好地描述复合股线中Nb₃Sn超导纤维碎片化过程,建立了分析超导复合股线统计拉伸强度和失效概率的模型。计算结果表明,青铜法Nb₃Sn超导复合股线的拉伸强度随着初始损伤参数的增大而迅速减小;在4.2K服役温度下,当Weibull模量为8时,随着初始损伤参数的增加,股线的拉伸强度约从900MPa衰减至480MPa,与已有实验结果吻合良好;初始损伤参数约为1时,正规化方差最大。初始损伤和Weibull模量对Nb₃Sn复合股线统计拉伸强度和失效概率函数的分布有着显著影响。     

Thermal analysis of composite superconductors subjected to time-dependent disturbances

A one-dimensional heat conduction equation with time- and temperature-dependent heat sources was employed to study the steady-state and transient response of a composite superconductor subjected to a thermal disturbance. An integral formulation was used to solve the steady-state problem of current redistribution and heat generation. The results of the integral formulation are compared with those of an analytical solution. The two solutions agree with each other except when the analytical solution fails as the temperature in the superconductor begins to exceed the critical temperature. Transient solutions were obtained by the finite-difference technique and the results are compared with a known analytical solution. Results of numerical calculations of the transient response of a composite superconductor subjected to an initial pulsed disturbance are presented. It is demonstrated that the superconductor can switch between the superconducting and the current-sharing state. The transient response and the stability of the composite conductor depend on the magnitude and duration of the disturbance, the dimensionless temperature θ^*, and the dimensionless parameter φ. Received on 18 November 1996     

基于材料相变的声子晶体带隙可调结构设计与性能分析

提出一种基于材料相变的穿孔型带隙可调声子晶体结构.其结构形式为含缝隙的形状记忆合金和环氧树脂的组合体,通过温度变化诱发相变引起的形状记忆合金材料性质的变化,实现声子晶体的带隙变化;通过合理布置缝隙与形状记忆合金相变材料的位置,实现声子晶体带隙性质的可调设计.基于有限元方法,建立了可调声子晶体的分析模型,分析了形状记忆合金的填充分数以及相变等对带隙性能的影响规律.分析结果表明,通过合理设计微结构形式,材料相变可实现带隙位置和宽度的调节,同时可实现特定频段内带隙的有无.     

Quench characteristics and mechanical responses during quench propagation in rare earth barium copper oxide pancake coils

Quench and mechanical behaviors are critical issues in high temperature superconducting(HTS)coils.In this paper,the quench characteristics in the rare earth barium copper oxide(REBCO)pancake coil at 4.2K are analyzed,and a two-dimensional(2D)axisymmetric electro-magneto-thermal model is presented.The effects of the constituent materials,background field,and coil size are analyzed.An elastoplastic mechanical model is used to study the corresponding mechanical responses during the quench propagation.The variations of the temperature and strain in superconducting layers are compared.The results indicate that the radial strain evolutions can reflect the transverse quench propagation and the tensile hoop and radial stresses in superconducting layers increase with the quench propagation.The possible damages are discussed with the consideration of the effects of the background field and coil size.It is concluded that the high background field significantly increases the maximum tensile hoop and radial stresses in quenching coils and local damage may be caused.     

基于纳米压入法Sn-Ag-Cu无铅焊料高温力学性能的研究

无铅化和微型化已经成为电子封装的发展趋势,温度对无铅焊点的可靠性产生了不可忽视的影响。本文对Sn96.5Ag3Cu0.5无铅焊料进行回流处理,采用纳米压入法研究其在实际工况下的高温力学性能。结果表明,温度对焊料试样的力学性能影响显著。随着温度的升高,弹性模量和硬度逐渐降低,焊料发生软化;较高温度下的蠕变应力指数较小,焊料的蠕变抗力降低,其相应的蠕变激活能为76kJ/mol。由此可知,随温度的升高,焊料的蠕变机制由位错攀移逐渐转变为晶界滑移。     

The Phase Transition between Chiral Nematic and Smectic A* Liquid Crystals

 In this paper we study the Landau-de Gennes free energy used to describe the transition between chiral nematic and smectic A liquid crystal phases. We consider the phenomenology of the transition and discuss the behavior of the material constants. Within the present mathematical framework, the physically observed growth behavior of the twist and bend Frank constants, K ² and K ³ respectively, plays a major role in determining the transition regime. We show existence of minimizers in a large class of admissible fields. Then, under the hypothesis that K ² and K ³ are large, we establish estimates for the transition regime separating the two phases. The work emphasizes the interplay between two competing effects: the layer formation of the smectic A phase and the twist tendency of the chiral nematic phase. Our discussion also illustrates the analogies as well as the discrepancies in modeling and behavior between smectic A^* liquid crystals and superconducting materials described by the Ginzburg-Landau theory. (Accepted May 7, 2002) Published online November 5, 2002     

Vacancy effect on the elastic constants of layer-structured nanomaterials

Layer-structured nanomaterials where alternating layers of nanocrystallites meet along high angle grain boundaries constitute a special category of nanomaterials. In the present study we investigated the effect of the presence of a vacancy on the elastic constants of such materials by the use of atomistic simulation methods. The calculations were performed on a model system where atoms interact via a Lennard–Jones potential and the elastic constants were obtained in the frame of homogeneous deformations, for nanocrystallite layer widths ranging from 2.24 up to 37.12 nm. The results show that the favoured position of the vacancy is located in the GB core. The state of relaxation of the structure is an important factor that affects the obtained results. In both the unrelaxed and relaxed structures results converge to a given value after the 5th (3 1 0) layer. This value seems to depend on the size of the nanocrystallite L and approaches the bulk value above a given size L. It is also concluded that in the case of a relaxed system there is a smoother variation of the system energy and elastic constant as a function of the distance of the vacancy from the GB plane when the size L increases. The way that external stresses are applied on the system affects the values of the obtained elastic properties, with the elastic constants related to the characteristic directions of the grain boundary being the most affected ones. These findings are of particular interest for fabrication methods of nanostructured materials, experimental methods for the measurement of their elastic properties as well as multiscale modelling schemes.