Universality and scaling of unstable plastic flow

The statistics of the jumplike plastic deformation of a Cu–Be alloy under the conditions of a low-temperature unstable plastic flow is studied experimentally. At a high strain rate, the parameters of the load jumps are found to be related by power laws, which corresponds to a scale-invariant behavior. A comparison with the data obtained for another mechanism of plastic instability, namely, the Portevin–Le Chatelier effect, points to the existence of universal laws governing the dynamics of a dislocation ensemble in the conditions of plastic instability.     

The nature of large-scale correlations in plastic flow

The nature of large-scale correlations in the locations of the centers of plastic flow during the deformation of crystalline solids is discussed. It is shown that regularities in the generation and development of such centers can be described as various types of autowaves arising from processes of self-organization. Examples are presented of the excitation-wave and phase-wave types of autowave processes experimentally observed during the deformation of single-crystal and polycrystalline metals and alloys. Some numerical parameters of the autowave processes are estimated. Fiz. Tverd. Tela (St. Petersburg) 39, 1399–1403 (August 1996)     

Distinctive features of plastic strain localization under severe plastic deformation of metals

Mechanical testing is performed and the structure of zirconium and aluminum predeformed by ∼450% using multiaxial forging (MAF) and equal channel angular pressing (ECAP) is investigated. Tensile loading tests of the severely deformed specimens exhibited their tendency to necking, with the ductility of the material in the neck, however, being superior to that in the neck of initial coarse-grained specimens. The results of the experiments imply that a fundamental stage of plastic flow of solids under severe plastic deformation (SPD) is the formation of cellular-banded structure and strain localization in the fine-grain bands. This considerably retards further deformation-induced refinement of the structure by SPD, and also results in the rapid formation of a fracture neck in the materials with this structure. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 43–49, November, 2007.     

非晶力学流变的自组织临界行为

非晶材料是由液体快冷冻结而成的结构无序的亚稳态固体.在受力条件下,非晶材料表现出独特和复杂的流变行为,具有跨尺度的高度时空不均匀特征,并在一定条件下表现出自组织临界行为,和自然界以及物理系统中许多复杂体系的动力学行为相似.本文结合作者近年来在非晶合金流变行为方面的研究结果,对非晶材料流变的研究进展和物理机制的认识进行介绍,包括非晶材料流变的跨尺度特征、表征和微观结构机制,以及近年来发现的非晶力学流变的自组织临界行为、物理机制等.最后,对非晶材料流变行为研究中亟需解决的问题进行了总结和展望.     

Analysis of strain hardening of crystals under large plastic strains

A strain-hardening mechanism is considered theoretically for crystals with large shear strains (γ=1–10). According to this mechanism, the emergence of the fourth and fifth stages in the strain-hardening curve for a crystal is associated with geometrically essential dislocations (GEDs) appearing as a result of accommodation processes accompanying large plastic deformations. Geometrically essential dislocations are concentrated in the boundaries of strongly disoriented fragments of the crystal. A comparison of the experimental and theoretical results leads to the conclusion that the density evolution of GEDs with deformation at the fourth and fifth stages is described by the same kinetic equation as for the evolution of the density of statistically random dislocations at the second and third stages, but with lower values of the kinetic coefficients.     

Critical behavior of sandpile vortices under variable charge

In the following, we consider a sandpile cellular automaton model (height version), which also takes account of lattice cyclicity for variable charge of grains projected upon the board. The size of the charge or additional grains of sand used to upset the equilibrium and to induce an avalanche is found to affect the distributional forms and in particular the convergence of the estimate for the dynamic exponent. Exponent estimates show slow variation with lattice dimensionL and some evidence of evolution with charge. The scaling region is limited in all cases with noisy decay at all levels of charge. Results presented extend the work of Duarte and Goncalves (1990) on the triangular lattice.     

On the localization of plastic strain under compression of LiF crystals

The plastic flow localization patterns for alkali halide LiF crystals under compression have been investigated. The main spatiotemporal regularities of the strain localization at different stages of deformation hardening in the single crystals have been established. The relation has been traced between the orientation of localized strain zones and the crystallography of slip systems of the test specimens studied simultaneously by the double-exposure speckle photography and photoelasticity methods.     

Decomposition of solid solution in an alloy under high plastic strain

The kinetics of impurity atmosphere formation and phase precipitation near the triple joints of deformation boundaries of the cellular structure is considered. The decomposition of solid solutions of substitutional impurities when an alloy is under high plastic strain is theoretically analyzed.     

Electrical and aerodynamic behavior of arc under shock conditions

Arc-shock interaction in a supersonic nozzle has been investigated for a current range from 200 to 1500 A and for three pressure ratios. The adverse pressure gradient associated with the shock causes flow separation and a broadening of arc cross section. Compared with the shock position in the absence of the arc, the shock center is moved toward upstream and to a region close to the wall. The shock is no longer plane. The center of the shock is not very sensitive to the current, but the shock strength reduces when the current is increased, V-I characteristics under shock conditions are slightly modified     

Localized strain autowaves at the initial stage of plastic flow in single crystals

Plastic strain localization in single crystals of pure metals and alloys is studied on the yield plateau and at the easy glide stage with a zero or small strain hardening coefficient. The difference between localization patterns in the two cases is explained, and strain localization mechanisms are suggested. At these stages of plastic deformation, various types of autowaves are observed.     

Critical behavior in the relaminarization of localized turbulence in pipe flow

The statistics of the relaminarization of localized turbulence in a pipe are examined by direct numerical simulation. As in recent experimental data [J. Peixinho and T. Mullin, Phys. Rev. Lett. 96, 094501 (2006)10.1103/PhysRevLett.96.094501], the half-life for the decaying turbulence is consistent with the scaling (Rec-Re) -1, indicating a boundary crisis of the localized turbulent state familiar in low-dimensional dynamical systems. The crisis Reynolds number is estimated as Rec=1870, a value within 7% of the experimental value 1750. We argue that the frequently asked question, of which initial disturbances at a given Re trigger sustained turbulence in a pipe, is really two separate questions: the "local phase space" question (local to the laminar state) of what threshold disturbance at a given Re is needed to initially trigger turbulence, followed by the "global phase space" question of whether Re exceeds Rec at which point the turbulent state becomes an attractor.     

局部塑性变形下铁基金属玻璃的致密化和非均匀性增强

本文研究了经局部塑性变形后, Fe₇₈Si₉B₁₃金属玻璃在原子尺度上的结构演变及其对合金显微硬度的影响.借助砂纸作为传力的媒介,充分放大了作用于带材表面上的等效压力,发生塑性变形后合金表面产生了大量的剪切带.基于倒空间和实空间的同步辐射X射线衍射分析,在塑性变形后,合金结构的致密度增大,过剩自由体积被排出,并由此揭示了Fe₇₈Si₉B₁₃金属玻璃在短程及中程尺度上原子协同重排行为.结合高分辨透射电子显微镜观察的结果, Fe₇₈Si₉B₁₃金属玻璃在发生塑性变形后,结构不均匀的程度将会加剧.此外,不同于单轴加载下金属玻璃的加工软化, Fe₇₈Si₉B₁₃金属玻璃在发生局部塑性变形后,维氏硬度增大,表现出局部的加工硬化行为.从自由体积的角度看,合金表面的大量剪切带可能是由于剪切带影响区域的重叠和交叉发生相互作用,并加速原子迁移,...     

Build-up of strain defects and the energetics of plastic strain

Calculations are presented for the concentrations of strain defects in a strained crystal and from these data the values of the stored energy E are computed. The contribution of dislocations and the strain points of defects to the stored energy is evaluated as a function of the strain for monocrystals of copper. The work of the strain is determined. A comparison is made of the theoretical results with those obtained by experiment.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 12–16, April, 1984.     

动态拉伸加载下高纯铝破坏的临界行为

以高纯铝(99.999%)作为延性金属的模拟材料，在一级气体炮上开展了一维应变平面冲击波加载实验.通过对不完全层裂的回收样品进行细观微损伤统计分析，讨论了高纯铝动态破坏时发生的临界行为.文中定义拉伸应力和拉伸作用时间的乘积为拉伸(作用)冲量，统计发现：随着拉伸冲量的增加，样品中的损伤呈现明显的临界行为特征：当拉伸冲量较小时，损伤以线性方式缓慢增长，当拉伸冲量足够大，且超过一定的临界阈值以后，损伤以幂指数形式快速增长.初步实验结果表明，高纯铝的拉伸冲量临界阈值约为0.34GPa·μs，对应的损伤临界值约为0.12.     

Mesoscale plastic flow instability in a solid under high-rate deformation

Here we consider high-rate deformation in solids in the context of a nonlocal transport theory, present a dynamic stress-strain diagram with elastic and plastic portions defined from a single standpoint, determine the conditions for pulse stress accumulation, and propose a mathematical model of momentum and energy exchange between scales and an instability criterion for transient plastic flow under shock loading. Phe instability criterion for high-rate deformation is verified by the example of shock loading of high-strength 30CrNi4Mo steel.