Behavior of the nickel-titanium alloys with the shape memory effect under conditions of shock wave loading

The behavior of the Ti_51.1Ni_48.9 and Ti_49.4Ni_50.6 alloys with shape memory effects has been studied under submicrosecond shock wave loading in the temperature range from −80 to 160°C, which includes both the regions of the stable state of the specimens in the austenite and martensite phases and the regions of thermoelastic martensitic transformations. The grain size of the studied alloys varies from initial values 15–30 to 0.05–0.30 μm. The dependences of the dynamic elastic limit on the temperature and on the elemental composition are similar to the dependences of the yield stress of these alloys under low strain rate loading. The rarefaction shock wave formation as a consequence of the pseudoelastic behavior of the alloy during a reversible martensitic transformation has been revealed. A decrease in the grain size leads to an increase in the dynamic elastic limit and decreases the temperatures of martensitic transformations.     

纳米多晶铜中冲击波阵面的分子动力学研究

冲击波阵面反映材料在冲击压缩下的弹塑性变形行为以及屈服强度、应变率条件等宏观量, 还与冲击压缩后的强度变化联系. 本文使用分子动力学方法, 模拟研究了冲击压缩下纳米多晶铜中的动态塑性变形过程, 考察了冲击波阵面和弹塑性机理对晶界存在的依赖, 并与纳米多晶铝的冲击压缩进行了比较. 研究发现: 相比晶界对纳米多晶铝的贡献而言, 纳米多晶铜中晶界对冲击波阵面宽度的影响较小; 并且其塑性变形机理主要以不全位错的发射和传播为主, 很少观察到全位错和形变孪晶的出现. 模拟还发现纳米多晶铜的冲击波阵面宽度随着冲击应力的增加而减小, 并得到了冲击波阵面宽度与冲击应力之间的定量反比关系, 该定量关系与他人纳米多晶铜模拟结果相近, 而与粗晶铜的冲击压缩实验结果相差较大.     

10 MA多支路汇流装置上钽的强度实验研究

磁驱动准等熵加载技术通过电流产生的磁压力加载材料,加载路径由负载电流波形和负载结构决定。作为应变率介于静高压加载和冲击加载之间的新型实验技术,熵增小、温升低。10 MA装置是典型的多支路汇流装置,包括24个电流支路,可在较大范围内控制负载电流波形,实现mm厚、cm直径样品在不同应变率下的准等熵加载。基于10 MA装置,通过调节负载电流波形实现样品加载路径控制,在一定压力-应变率范围,开展金属钽的强度实验研究,获取了不同厚度金属钽样品的加-卸载波剖面速度历史,分析获得了钽在系列峰值压力下的强度数据,比较了多个加载平台不同加载路径下的强度数据,实验结果与美国圣地亚国家实验室的磁驱动准等熵结果接近（平均应变率都约为10⁵ s⁻¹）,明显高于冲击加载的流动强度,低于准静态加载获取的流动强度,与应变率增高强度会有所下降的理论预测一致。基于多支路汇流装置,未来将可开展更为丰富的材料动力学特性实验研究。

     

On the conditions of strain localization and microstructure fragmentation under high-rate loading

The paper reports on research in the deformation and fragmentation mechanisms of coarse- and fine-grained materials under high-rate loading. The study was performed by an experimental procedure based on collapse of thick-walled hollow cylinders and by molecular dynamics simulation. The key issue was to study the formation of plastic strain localization bands. It is found that the pattern of plastic deformation is governed by loading conditions and characteristic grain sizes. For a coarse-grained material, the governing mechanism is dislocation deformation resulting in localization bands. For a fine-grained material, the governing mechanism is grain boundary sliding with attendant fragmentation of the material. A dependence of the strain rate and degree on the critical grain size was disclosed. The computer simulation revealed mechanisms of grain boundary sliding on the scales studied.     

Effect of preliminary strain hardening on the flow stress of titanium and a titanium alloy during shock compression

The effect of preliminary strain hardening of VT1-0 titanium and a Ti-6 wt % Al-4 wt % V alloy on their mechanical properties under quasi-static and high-rate (τ;10⁵ s^?1) loading is studied. Preliminary hardening is accomplished using equal-channel angular pressing (which results in a significant decrease in the grain size and a twofold increase in the quasi-static yield strength) and shock waves. High-rate deformation is attained via shock-wave loading of samples. The experimental results show that structural defects weaken the dependence of the yield strength on the strain rate. The difference in the rate dependences can be so high that the effect of these defects on the flow stress can change sign when going from quasi-static to high-rate loading.     

Effect of a fullerene C60 addition on the strength properties of nanocrystalline copper and aluminum under shock-wave loading

The Hugoniot elastic limit and the spall strength of aluminum and copper samples pressed from a mixture of a metallic powder and 2–5 wt % C₆₀ fullerene powder are measured under a shock loading pressure up to 6 GPa and a strain rate of 10⁵ s^?1 by recording and analyzing full wave profiles using a VISAR laser interferometer. It is shown that a 5% C₆₀ fullerene addition to an initial aluminum sample leads to an increase in its Hugoniot elastic limit by an order of magnitude. Mixture copper samples with 2% fullerene also exhibit a multiple increase in the elastic limit as compared to commercial-grade copper. The elastic limits calculated from the wave profiles are 0.82–1.56 GPa for aluminum samples and 1.35–3.46 GPa for copper samples depending on the sample porosity. The spall strength of both aluminum and copper samples with fullerene additions decreases approximately threefold because of the effect of high-hardness fullerene particles, which serve as tensile stress concentrators in a material under dynamic fracture.     

孪晶对Be材料冲击加-卸载动力学影响的数值模拟研究

在高压、高应变率加载条件下,孪晶变形对材料的塑性变形具有重要的贡献,而目前孪晶对金属材料的动态屈服强度、冲击响应等的影响还没有被充分揭示.为此,本文考虑孪晶变形和晶粒碎化,针对铍(Be)材料在高应变率加载下的动态力学响应发展了含孪晶的热弹-黏塑性晶体塑性模型.经过和实验结果的对比,发现该模型可以更准确地预测Be材料在动态加载下,尤其是高压动态加载下的屈服强度.进一步,基于该塑性模型研究了Be材料在冲击加载下的准弹性卸载行为,结果表明剪切波速随着压力和剪应变的变化而发生变化是材料产生准弹性卸载现象的主要原因.此外,研究了冲击波卸载过程中Be材料孪晶的演化过程,发现Be材料卸载过程中也伴随着孪晶的产生.     

冲击加载下20钢中弹性前驱波衰减与应力松弛行为实验研究

 通过改变样品厚度，对平面冲击加载下20钢的弹性前驱波的波幅衰减和应力松弛进行了实验研究。采用激光速度干涉测速仪（VISAR）实测了样品后自由面速度历史，采样频率达到1 ns，保证了实验结果的准确性。实验结果显示：Hugoniot弹性极限随着传播距离呈指数衰减，在所研究的样品厚度范围内，Hugoniot弹性极限减小了44%；应力松弛行为和弹性前驱波的上升沿时间也依赖于传播距离；冲击加载的强度对材料动态屈服行为的影响很小。     

Comparative analysis of uniaxial strain shock tests and Taylor tests for maraging steels

High-strength constructional 38XH3MΦA steel and three kinds of maraging steel were tested to determine the yield stress under dynamic loading. The 38XH3MΦA steel was used as central test material to work out the experimental technique and compare the results obtained in different test methods. Results obtained in the plane shock tests under uniaxial strain condition show roughly the same yield stress values as those obtained in Taylor tests.     

含微孔洞脆性材料的冲击响应特性与介观演化机制

微孔洞显著地影响着脆性材料的冲击响应,理解其介观演化机制和宏观响应规律将使微孔洞有利于而无害于脆性材料的工程应用.通过建立能够准确表现材料弹性性质和断裂演化的格点-弹簧模型,本文揭示了孔洞的演化对于脆性材料的影响.冲击下孔洞导致的塌缩变形和从孔洞发射的剪切裂纹所导致的滑移变形产生了显著的应力松弛,并调制了冲击波的传播.在多孔脆性材料中,冲击波逐渐展宽为弹性波和变形波.变形波在宏观上类似于延性金属材料的塑性波,在介观上对应于塌缩变形和滑移变形过程.样品中的气孔率决定了脆性材料的弹性极限,气孔率和冲击应力共同影响着变形波的传播速度和冲击终态的应力幅值.含微孔洞脆性材料在冲击波复杂加载实验、功能材料失效的预防、建筑物防护等方面具有潜在的应用价值.所获得的冲击响应规律有助于针对特定应用优化设计脆性材料的冲击响应和动态力学性能.     

Study on the effect of temperature rise on grain refining during fabrication of nanocrystalline copper under explosive loading

Nanocrystalline (NC) copper was fabricated by severe plastic deformation of coarse-grained copper at a high strain rate under explosive loading. The feasibility of grain refinement under different explosive loading and the influence of overall temperature rise on grain refinement under impact compression were studied in this paper. The calculation model for the macroscopic temperature rise was established according to the adiabatic shock compression theory. The calculation model for coarse-grained copper was established by the Voronoi method and the microscopic temperature rise resulted from severe plastic deformation of grains was calculated by ANSYS/ls-dyna finite element software. The results show that it is feasible to fabricate NC copper by explosively dynamic deformation of coarse-grained copper and the average grain size of the NC copper can be controlled between 200～400 nm. The whole temperature rise would increase with the increasing explosive thickness. Ammonium nitrate fuel oil explosive was adopted and five different thicknesses of the explosive, which are 20 mm, 25 mm, 30 mm, 35 mm, 45 mm, respectively, with the same diameter using 20 mm to the fly plate were adopted. The maximum macro and micro temperature rise is up to 532.4 K, 143.4 K, respectively, which has no great effect on grain refinement due to the whole temperature rise that is lower than grain growth temperature according to the high pressure melting theory.     

The role of surface states in magnetic properties of nanocrystalline CuO

The temperature behavior of the magnetic susceptibility of a CuO low-dimensional antiferromagnet subjected to spherically converging isentropic shock wave loading is investigated. The grain sizes in CuO polycrystalline samples range from 5 to 110 nm. It is demonstrated that a decrease in the grain size leads to an increase in the role of the surface states in the magnetic properties. Unlike polycrystals prepared by standard methods, the susceptibility of nanocrystalline samples in the range 78 K<T<150 K decreases with an increase in temperature, which is explained by the formation of Cu²⁺ paramagnetic ions on the surface of nanocrystalline particles.     

爆炸载荷下纳米晶铜晶粒度分布及影响因素研究

 采用爆炸动态加载使粗晶铜发生高应变率塑性大变形的方法制备了纳米晶铜。利用X射线衍射法对其晶粒度进行了检测,借助于LS-DYNA3D非线性有限元程序对试样变形过程进行了数值模拟,在此基础上对应变和应变率进行了统计,分析了宏、细观应变对晶粒细化程度的影响。结果表明：采用爆炸加载法可制备出纳米晶铜,平均晶粒度范围可有效控制在100 nm以内;爆炸加载过程中应变率高达10⁴ s^-1,应变的提高有利于晶粒细化;在爆炸加载方向晶粒度成不均匀分布。     

钽在冲击载荷下的动态力学特性

 在对称碰撞实验中,用VISAR测试系统记录了自由面粒子速度剖面。结合材料常压弹性纵波声速的测量,利用冲击加载的双波结构,测量了钽在30 GPa以内的冲击波速度,给出了线性相关性非常好的D-u_p 关系,并与高压下的测量结果作了比较。同时对由自由面粒子速度剖面来计算层裂强度的模型进行了讨论,发现模型差异对钽的层裂强度的影响超过30%。     

Metastable states,relaxation mechanisms,and fracture of liquids under severe loading conditions

The relaxation properties and fracture of glycerol, silicone oil, transformer oil, and water have been studied experimentally under shock wave loading. The power-law strain rate dependences of the stress amplitude and spall strength were found for the compression and rarefaction fronts, respectively. It was shown that temperature has a strong influence on the spall strength of glycerol near the phase transformation temperature. The power laws reflect a self-similar nature of the momentum transfer and fracture mechanisms of liquids that are conventionally observed in solids and governed by the mechanisms of defect-induced structural relaxation. The mechanisms of viscoelasticity are related to the metastable states that may give rise to a collective behavior of displacement field fluctuations (microshearing) in liquids and thus provide a viscoplastic response of liquids under high strain rate loading.     

Influence of the reversible α–ε phase transition and preliminary shock compression on the spall strength of armco iron

Full wave profiles are used to determine the Hugoniot elastic limit and the spall strength of armco iron samples with an as-received structure and the samples recovered after preliminary loading by plane shock waves with an amplitude of 8, 17, and 35 GPa. The measurements are performed at a shock compression pressure below and above the polymorphic a–e transition pressure. Metallographic analysis of the structure of armco iron shows that a developed twinned structure forms inside grains in the samples subjected to preliminary compression and recovered and that the twin concentration and size increase with the shock compression pressure. The spall strength of armco iron under shock loading below the phase transition pressure increases by approximately 10% due to its preliminary deformation twinning at the maximum shock compression pressure. The spallation of samples with various structures at a shock compression pressure above the phase transition proceeds at almost the same tensile stresses. The polymorphic transition in armco iron weakly affects its strength characteristics.     

激光冲击强化“残余应力洞”的形成机制

利用ABAQUS有限元软件进行了单个圆形高斯光斑的激光冲击强化数值模拟,分析材料表面光斑中心区域形成的"残余应力洞"现象,并通过分析材料的动态力学响应特征揭示了"残余应力洞"的形成机制。结果表明:在冲击波加载时,光斑边界处会产生很强的剪切应力,形成向四周传播的表面稀疏波和向材料内部传播的剪切波。当稀疏波同时传播到光斑中心,发生相遇、汇聚,使材料产生急剧的上下位移过程,造成冲击波加载塑性变形后的二次塑性变形。二次塑性变形中形成了较大的剪切塑性应变,并降低了冲击波加载阶段产生的轴向和径向塑性应变,使残余压应力降低,从而形成"残余应力洞"。     

岩石Hopkinson层裂的流形元法模拟

 利用二阶流形元法,通过引入裂纹产生及扩展判据,对冲击载荷作用下岩石Hopkinson动态层裂过程进行了数值模拟,再现了拉伸波作用下Hopkinson层裂过程,计算得到的层裂片厚度和速度等与理论值符合较好,验证了流形元法在模拟冲击载荷作用下材料动态破坏过程方面的有效性和优越性。     

多孔脆性介质冲击波压缩破坏的细观机理和图像

本文采用一种具有良好定量性质的离散元模型研究了带孔洞的各向同性脆性介质在细观尺度上的压缩破坏特征. 通过对孤立孔洞、三种简单的孔洞排布方式和大量孔洞随机排布等几种情况的模拟, 认识到了剪切破坏和局域拉伸破坏是冲击波压缩下多孔介质的基本破坏模式; 孔洞之间的损伤贯通会促进孔洞在较低应力下发生塌缩, 但损伤区的应力松弛过程却会对一定范围内的介质起到损伤屏蔽作用; 不同区域中损伤促进和损伤屏蔽的综合效果是在多孔脆性介质中形成一种高损伤区与低损伤区间错排布的奇特损伤分布. 本文的研究结果为深入理解脆性材料冲击波压缩破坏的演化过程和机理提供了细观尺度上的初步物理图像.