Macrolocalization of plastic strain in creep of fine-grain aluminum

The evolution of the plastic strain macrolocalization pattern in low-temperature creep of commercial purity aluminum is studied. The localization pattern depends on a stage in the creep curve. At the stage of steady-state creep, localization zones propagate in the form of a wave traveling with a velocity proportional to the rate of buildup of the total strain. It is found that the volumes where the creep and strain localization wave propagation are activated equal each other. Based on estimates of the activation volumes, it is shown that the velocity of plastic strain localization waves is governed by thermally activated dislocation movement.     

On the homogeneous nucleation and propagation of dislocations under shock compression

The dynamic response of crystalline materials subjected to extreme shock compression is not well understood. The interaction between the propagating shock wave and the material’s defect occurs at the sub-nanosecond timescale which makes in situ experimental measurements very challenging. Therefore, computer simulation coupled with theoretical modelling and available experimental data is useful to determine the underlying physics behind shock-induced plasticity. In this work, multiscale dislocation dynamics plasticity (MDDP) calculations are carried out to simulate the mechanical response of copper reported at ultra-high strain rates shock loading. We compare the value of threshold stress for homogeneous nucleation obtained from elastodynamic solution and standard nucleation theory with MDDP predictions for copper single crystals oriented in the [0 0 1]. MDDP homogeneous nucleation simulations are then carried out to investigate several aspects of shock-induced deformation such as; stress profile characteristics, plastic relaxation, dislocation microstructure evolution and temperature rise behind the wave front. The computation results show that the stresses exhibit an elastic overshoot followed by rapid relaxation such that the 1D state of strain is transformed into a 3D state of strain due to plastic flow. We demonstrate that MDDP computations of the dislocation density, peak pressure, dynamics yielding and flow stress are in good agreement with recent experimental findings and compare well with the predictions of several dislocation-based continuum models. MDDP-based models for dislocation density evolution, saturation dislocation density, temperature rise due to plastic work and strain rate hardening are proposed. Additionally, we demonstrated using MDDP computations along with recent experimental reports the breakdown of the fourth power law of Swegle and Grady in the homogeneous nucleation regime.     

Mass transport in metals under intensive impulse reactions

An unusually high mobility of atoms under intensive impulse reactions is explained by the behavior of point defects at the shock wave front. It is shown that either a shock wave front or moving dislocations can capture the interstitials, or they can be thermally activated in the direction of the shock wave propagation.     

晶界对纳米多晶铝中冲击波阵面结构影响的分子动力学研究

用分子动力学方法研究了纳米多晶铝在冲击加载下的冲击波阵面结构及塑性变形机理.模拟研究结果表明:在弹性先驱波之后,是晶界间滑移和变形主导了前期的塑性变形机理;然后是不全位错在界面上成核和向晶粒内传播,然后在晶粒内形成堆垛层错、孪晶和全位错的过程主导了后期的塑性变形机理.冲击波阵面扫过之后留下的结构特征是堆垛层错和孪晶留在晶粒内,大部分全位错则湮灭于对面晶界.这个由两阶段塑性变形过程导致的时序性塑性波阵面结构是过去未见报道过的. 关键词： 晶界 塑性变形 冲击波阵面 分子动力学     

Localization of plastic strain under shock-wave loading of titanium alloy with a crack

The effect of shock-wave loading with an amplitude of 4 GPa and a duration of 10 μs on the localization of plastic flow and the dynamic fracture of VT-1 cast titanium alloy containing a disk-shaped crack is considered. It is found that the crack size decreases in the direction of shock wave propagation, the dislocation density grows and adiabatic shear bands appear near the crack, and secondary cracks originate in the areas of localized flow near the crack edges. The strain, strain rate, dislocation density, rate of dislocation generation, and dislocation velocity near and away from the healing crack are estimated.     

Instability of plastic flow in the microstructure of alkali halide crystals

Spatial localization of deformation bands in LiF and KCl single crystals caused by instability of plastic flow in the strain rate range from 5 × 10^?6 to 2 × 10^?4 s^?1 was studied experimentally. The geometrical parameters of localized shift bands (LSB) were studied as a function of strain rate and temperature. To study the LSB relief, a surface profilometry technique was used for the first time, which made it possible to determine the LSB parameters at the early stages of plastic flow (for strains in the range from 0.5 to 2%). The formation and branching of LSB steps on the surface of a deformed crystal due to the generation and motion of dislocations were found to be scaled. It was shown experimentally that the LSB formation is a thermally activated process that occurs through dislocation glide and is limited by dislocation creep.     

Thermally activated plastic deformation in the presence of double potential barriers

We consider the process by which a dislocation overcomes a double potential barrier in a thermally activated plastic deformation. We use the framework of reaction rates without taking into account the form of the barrier profile. In order to take into account the possible delay of the dislocation in the potential well we introduce the probability for overcoming the barrier with one jump and the probability for the dislocation to fall into the well. As a result we establish that the experimentally determined activation parameters of a plastic deformation not only depend significantly on the form of: the barrier, but also on the delay probability of the dislocation in the potential well, which in turn is determined by the mechanism by which the moving dislocation dissipates energy. Since the dissipation of energy by the dislocation and the delay of the dislocation in the well grow with temperature, it is possible to observe a temperature interval where the stress flow also grows with temperature.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 72–77, June, 1986.The author is grateful to V. S. Zotov for a fruitful discussion of this work and for valuable advice.     

Severe plastic deformation of amorphous alloys: I. Structure and mechanical properties

The basic regularities of variation in the structure and mechanical properties of amorphous Ni₄₄Fe₂₉Co₁₅Si₂B₁₀ alloy at severe plastic deformation (SPD) in a Bridgman cell at different temperatures are considered. It is shown that SPD is accompanied by homogeneous nanocrystallization, which is caused by the plastic flow mode. The transition from inhomogeneous mode of plastic flow to a qualitatively different one has been detected. The SPD structural model of deformational “dissolving” of crystals is proposed to explain why nanocrystals no more than 10 nm in size are observed during SPD. It is found that thermally activated nanocrystallization may occur at very low temperatures (77 K) under very high stress and with a high concentration of excess free volume.     

Change of the kinetics of shock-wave deformation and fracture of VT1-0 titanium as a result of annealing

The paper presents the results of measurements of shock-wave compression profiles of VT1-0 titanium samples after rolling and in the annealed state. In the experiments, the pressure of shock compression and distance passed by the wave before emerging to the sample surface were varied. From measurements of the elastic precursor decay and compression rate in a plastic shock wave of different amplitudes, the plastic strain and the corresponding shear stresses in the initial and subsequent stages of high-rate deformation in an elastoplastic shock wave are determined. It is found that the reduction in the dislocation density as a result of annealing reduces the hardness of the material but significantly increases its dynamic yield strengh, corresponding to the strain rate above 10⁴ s^–1. With a reduction in the strain rate, this anomalous difference in the flow stresses is leveled off.     

Structural states and specific features of the deformation behavior of metals and alloys with mixed nano-and micrograin structure

A classification of the structural states of materials with a mixed nano-and microcrystalline structure is proposed. Theoretical analysis of the structural mechanisms and peculiarities of plastic flow of singlephase and two-phase nanostructured metals and alloys with a bimodal size distribution of grains and phases is performed. The effect of grain-boundary and dislocation mechanisms of plastic flow on the specific features of the deformation behavior and plasticity of nanocrystalline materials is analyzed. A microstructural model of strain hardening of a material with two-scale nano-and micrograin structure is proposed and the condition for the loss of plastic flow stability of such a material is investigated. The dependence of the yield strength and uniform strain of nanocrystalline materials with a two-scale structure on the grain size and the ratio of the volume fractions of the nano-and microstructural components is calculated.     

On the mechanism of deformation in submicrocrystalline titanium

The mechanical behavior and structural evolution of submicrocrystalline titanium in the course of deformation are investigated at room temperature. The possible mechanisms of deformation are analyzed. It is proved that the dislocation motion inside grains is responsible for the plastic flow.     

沿〈111〉晶向冲击加载下铜中纳米孔洞增长的塑性机制研究

 用分子动力学方法计算模拟了沿〈111〉晶向冲击加载过程中，单晶铜中纳米孔洞（直径约1.3 nm）的演化及其周围区域发生塑性变形的过程。模拟结果表明，在沿〈111〉晶向冲击加载后，在面心立方（fcc）结构中的4族{111}晶面中有3族发生了滑移。伴随孔洞的增长，在所激活的3族{111}晶面上，观察到位错在孔洞表面附近区域成核，然后向外滑移，其中在剪切应力最大的〈112〉方向上，其位错速度超过横波声速，其它〈112〉方向的位错速度低于横波声速。模拟得到的位错阻尼系数范围与实验值基本符合。由于孔洞周围产生的滑移在空间比较对称，孔洞增长形貌接近球形。在恒定的冲击强度下，孔洞半径增长速率近似保持恒定，其速率随着冲击强度的增加而增大。     

冲击波在纳米金属铜中传播的分子动力学模拟

使用分子动力学方法模拟了冲击波在纳米金属铜中的传播,模拟样品由Voronoi方法得到.结果显示纳米金属铜在冲击加载下呈现多次屈服的现象,并发现冲击波具有多波结构.由于设计样品时选择了晶粒取向,晶界滑移和位错在冲击波波形上被区分开.冲击波波阵面由弹性变形区、晶界滑移主导的塑性变形区和位错主导的塑性变形区组成.样品中弹性波前沿扰动较小,而位错主导的塑性波前沿扰动较大,造成后者的主要原因是波阵面上沿冲击方向不同取向晶粒的不同屈服行为.     

Shock-induced structural phase transition, plasticity, and brittle cracks in aluminum nitride ceramic

Atomistic mechanisms of fracture accompanying structural phase transformation (SPT) in AlN ceramic under hypervelocity impact are investigated using a 209 x 10(6) atom molecular-dynamics simulation. The shock wave generated by the impact splits into an elastic wave and a slower SPT wave that transforms the wurtzite structure into the rocksalt phase. The interaction between the reflected elastic wave and the SPT wave front generates nanovoids and dislocations into the wurtzite phase. Nanovoids coalesce into mode I cracks while dislocations give rise to kink bands and mode II cracking.     

单晶Cu冲击加载及卸载下塑性行为的微观模拟

使用分子动力学方法对室温下单晶铜沿［001］和［111］方向冲击加载及卸载下的塑性行为进行了模拟,得到了Hugoniot关系以及冲击熔化压力,与实验基本符合. 加载过程中,较高的初始温度有利于位错的形核与发展. 通过对冲击波在自由表面卸载过程的模拟和分析发现：卸载过程呈现“准弹性卸载行为”;沿［001］方向卸载后大量不全位错环与堆积层错消失,而沿［111］方向卸载后只有少量层错消失,部分层错甚至会发展扩大. 关键词： 分子动力学 冲击波 塑性     

On the power-law pressure dependence of the plastic strain rate of crystals under intense shock wave loading

The plastic deformation of metallic crystals under intense shock wave loading has been theoretically investigated. It has been experimentally found that the plastic strain rate $\dot \varepsilon $ and the pressure in the wave P are related by the empirical expression $\dot \varepsilon $ ～ P ⁴ (the Swegle-Grady law). The performed dislocation-kinetic analysis of the mechanism of the origin of this relationship has revealed that its power-law character is determined by the power-law pressure dependence of the density of geometrically necessary dislocations generated at the shock wave front ρ ～ P ³. In combination with the rate of viscous motion of dislocations, which varies linearly with pressure (u ～ P), this leads to the experimentally observed relationship $\dot \varepsilon $ ～ P ⁴ for a wide variety of materials with different types of crystal lattices in accordance with the Orowan relationship for the plastic strain rate $\dot \varepsilon $ = bρu (where b is the Burgers vector). In the framework of the unified dislocation-kinetic approach, it has been theoretically demonstrated that the dependence of the pressure (flow stress) on the plastic strain rate over a wide range from 10^?4 to 10¹⁰ s^?1 reflects three successively developing processes: the thermally activated motion of dislocations, the viscous drag of dislocations, and the generation of geometrically necessary dislocations at the shock wave front.     

多孔脆性材料对高能量密度脉冲的吸收和抵抗能力

作用在脆性结构材料表面的高能量密度脉冲会以冲击波的形式传播进入材料内部, 导致压缩破坏和功能失效. 通过设计并引入微孔洞, 显著增强了脆性材料冲击下的塑性变形能力, 从而使脆性结构材料可以有效地吸收耗散冲击波能量, 并抑制冲击诱导裂纹的扩展贯通. 建立格点-弹簧模型并用于模拟研究致密和多孔脆性材料在高能量密度脉冲加载下的冲击塑性机理、能量吸收耗散过程和裂纹扩展过程. 冲击波压缩下孔洞塌缩, 导致体积收缩变形和滑移以及转动变形, 使得多孔脆性材料表现出显著的冲击塑性. 对致密样品、气孔率5%和10%的多孔样品吸能能力的计算表明, 多孔脆性材料吸收耗散高能量密度脉冲的能力远优于致密脆性材料. 在短脉冲加载下, 相较于遭受整体破坏的致密脆性材料, 多孔脆性材料以增加局部区域的损伤程度为代价, 阻止了严重的冲击破坏扩展贯通整个样品, 避免了材料的整体功能失效.     

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.     

冲击波作用下材料的极化率

 文中对材料在冲击波作用下发生极化或去极化过程，应用位错理论进行解释。导出了在冲击波作用下，计算材料极化率的公式。得出结论认为极化率不仅和位错密度有关，还和冲击波的频谱有关。     

Theory of plastic vortex creep

We develop a theory for plastic vortex creep in a topologically disordered (dislocated) vortex solid phase in type-II superconductors in terms of driven thermally activated dislocation dynamics. Plastic barriers for dislocations show a power-law divergence at small driving currents j, U(pl)( j) approximately j(-&mgr;), with &mgr; = 1 for a single dislocation and &mgr; = 2/5 for creep of dislocation bundles. This implies a suppression of the creep rate at the transition from the ordered vortex phase ( &mgr; = 2/11) to the dislocated glass and can manifest itself as an observed increase of the apparent critical current (second peak). Our approach applies to general dynamics of disordered elastic media on a random substrate.