On the nature of ultrahigh plastic (Megaplastic) strain

The features of structural transformations occurring at ultrahigh plastic strains, which are proposed to refer to as gigascopic, have been analyzed. It is shown that, during such deformation, additional channels of elastic energy dissipation must be effectively implemented. It is concluded that structural changes are characterized by certain cyclicity. A specific route of structural rearrangements is determined by temperature, the height of the Peierls barrier for dislocations and their ability of diffusive rearranging, and the difference in the free energies of the crystalline and amorphous states.     

Molecular dynamics investigation of plastic deformation mechanism in bulk nanotwinned copper with embedded cracks

The plastic deformation of bulk nanotwinned copper with embedded cracks under tension has been explored by using molecular dynamics simulations. Simulation results show that the cracks mainly act as dislocation sources during the plastic deformation and occasionally as sinks at later stage. The dislocation pile-up, accumulation and transformation at twin boundaries (TBs) control the plastic hardening and softening deformations. The TB dislocation pile-up zone is estimated to be 5.6–8 nm, which agrees well with previous experimental and simulation results. Furthermore, it is found that the flow stress vs. dislocation density at the hardening stage follows the Taylor-type relationship.     

Local structural transformations in the fcc lattice in various contact interaction. Molecular dynamics study

The work is a molecular dynamics study of the peculiarities of local structural transformations in a copper crystallite at the atomic level in contact interaction of various types: shear loading of perfectly conjugate surfaces, local shear loading and nanoindentation. Interatomic interaction is described in the framework of the embedded atom method. It is shown that initial accommodation of the loaded crystallite proceeds through local structural transformations giving rise to higher-rank defects such as dislocations, stacking faults, interfaces, etc. In further plastic deformation, the structural defects propagate from the contact zone to the crystallite bulk. The egress of structural defects to a free surface causes deformation of the model crystallite. The deformation pattern can evolve, depending on the loading conditions, with a change in crystallographic orientation of the crystallite near the contact zone, generation of misoriented nano-sized regions, and eventually formation of a stable nanostructural state. The obtained results allow conceptually new understanding of the nature of defect generation in a crystalline structure during the nucleation and development of plastic deformation in loaded materials.     

石墨烯/铝基复合材料在纳米压痕过程中位错与石墨烯相互作用机制的模拟研究

石墨烯因其优异的力学性能已成为增强金属基复合材料的理想增强体.然而,目前对石墨烯/金属基复合材料在纳米压痕过程中嵌入石墨烯与位错之间的相互作用仍不清晰.本文采用分子动力学模拟方法,对90°,45°和0°位向的石墨烯/铝基复合材料进行了纳米压痕模拟,研究了压痕加载和卸载过程中石墨烯/铝基复合材料的位错形核及演化,以获取不同位向的石墨烯与位错的相互作用机制,并分析其对塑性区的影响.研究发现,石墨烯可以有效阻碍位错运动,并且石墨烯会沿着位错滑移方向发生弹性变形.在纳米压痕过程中,位错与不同位向石墨烯之间的相互作用差异导致塑性区的变化趋势不同.研究结果表明,在石墨烯/铝基复合材料中,位向不同的石墨烯对位错阻碍强度和方式不同,且石墨烯位向为45°的复合材料的硬度高于其他模型.此外,石墨烯/铝基复合材料的位错线总长度的演化规律与石墨烯位向紧密相关.本文研究可为设计和制备高性能石墨烯/金属基复合材料提供一定的理论指导.     

Grain-boundary dislocations and enhanced diffusion in nanocrystalline bulk materials and films

A theoretical model is suggested which describes the transformations of grain-boundary dislocation walls and their influence on diffusion processes in nanocrystalline materials fabricated under highly non-equilibrium conditions. It is shown that the decay of boundary dislocation walls of finite extent, occurring via the climb of boundary dislocations and the corresponding emission of vacancies, is capable of highly enhancing the grain-boundary diffusion in nanocrystalline materials. The enhanced diffusion, in turn, strongly affects the deformation behaviour of nanocrystalline materials. In the case of nanocrystalline films deposited on to substrates, the effects of misfit stresses on the transformations of boundary dislocation walls and the diffusion are analysed. It is demonstrated that the mean diffusion coefficient in a nanocrystalline film may increase by approximately several orders of magnitude owing to misfit stresses.     

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.     

Study of the microstructure of a HCP zirconium alloy using synchrotron radiation X-ray diffraction

Synchrotron radiation X-ray diffraction has been applied for the precision study of the micro-structure evolution of a Zr-1% Nb alloy deformed by uniaxial tension. A nonmonotonic pattern of the integral width of X-ray lines as a function of in situ deformation and acting stress has been revealed, which is in agreement with intensity fluctuations of reflections and elastic microdistortions related to structural imperfection. The obtained results correlate with transmission electron microscopy data on the cyclicity of dislocation transformations during necking.     

Physical nature of stages in plastic deformation

Conclusion In summary, the transition from one stage of plastic deformation to another is due to the phase transformations in the subsystem of deformation defects. These transformations are controlled by an internal parameter of the system, viz., the scalar dislocation density, whose value is determined by both the external force and processes retarding the shear and annihilation. The main structural level controlling this process is the level of the dislocation subsystem.We express our thanks to associate professors L. A. Telyakov, Yu. P. Sharkeev, and V. A. Starenchenko, Candidates G. V. Daneliya, D. V. Lychagin, and I. A. Lapsker, and scientific associates S. P. Zhukovskii, L. I. Trishkina, A. V. Paul', and T. S. Kunitsyna, with whose collaboration part of the results reported in this review were obtained.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 89–106, February, 1990.     

Transition of deformation mechanisms in nanotwinned single crystalline SiC

ABSTRACT

The ability to experimentally synthesise ceramic materials to incorporate nanotwinned microstructures can drastically affect the underlying deformation mechanisms and mechanics through the complex interaction between stress state, crystallographic orientation, and twin orientation. In this study, molecular dynamics simulations are used to examine the transition in deformation mechanisms and mechanical responses of nanotwinned zinc-blende SiC ceramics subjected to different stress states (uniaxial compressive, uniaxial tensile, and shear deformation) by employing various twin spacings and loading/crystallographic orientations in nanotwinned structures, as compared to their single crystal counterparts. The simulation results show that different combinations of stress states and crystal/twin orientation, and twin spacing trigger different deformation mechanisms: (i) shear localised deformation and shear-induced fracture, preceded by point defect formation and dislocation slip, in the vicinity of the twin lamellae, shear band formation, and dislocation (emission) avalanche; (ii) cleavage and fracture without dislocation plasticity, weakening the nanotwinned ceramics compared to their twin-free counterpart; (iii) severe localised deformation, generating a unique zigzag microstructure between twins without any structural phase transformations or amorphisation, and (iv) atomic disordering localised in the vicinity of coherent twin boundaries, triggering dislocation nucleation and low shearability compared to twin-free systems.     

Plastic flow macrolocalization in aluminum and the Hall-Petch relation

The parameters of plastic deformation macrolocalization are compared to the parameters of the Hall-Petch relation for the flow stress in polycrystalline aluminum samples with a grain size of 0.008–5.000 mm. Two types of the dependence of the localized plastic deformation autowave length on the grain size and two versions of hardening according to the Hall-Petch relation are found in the grain size range under study. The boundary between these versions is shown to correspond to d ≈ 0.1 mm for both cases. A relation between localized plastic flow patterns and the Hall-Petch relation is revealed.     

温度和应变速率耦合作用下纳米晶Ni压缩行为研究

本文利用低温力学测试系统研究了电化学沉积纳米晶Ni在不同温度和宽应变速率条件下的压缩行为. 借助应变速率敏感指数、激活体积、扫描电子显微镜及高分辨透射电子显微镜方法, 对纳米晶Ni的压缩塑性变形机理进行了表征. 研究表明, 在较低温度条件下, 纳米晶Ni的塑性变形主要是由晶界位错协调变形主导, 晶界本征位错引出后无阻碍的在晶粒内无位错区运动, 直至在相对晶界发生类似切割林位错行为. 并且, 在协调塑性变形时引出位错的残留位错能够增加应变相容性和减小应力集中; 在室温条件下, 纳米晶Ni的塑性变形机理主要是晶界-位错协调变形与晶粒滑移/旋转共同主导. 利用晶界位错协调变形机理和残留位错运动与温度及缺陷的相关性揭示了纳米晶Ni在不同温度、不同应变速率条件下力学压缩性能差异的内在原因.     

Dislocation dynamics in nanocrystalline nickel

It is believed that the dynamics of dislocation processes during the deformation of nanocrystalline materials can only be visualized by computational simulations. Here we demonstrate that observations of dislocation processes during the deformation of nanocrystalline Ni with grain sizes as small as 10 nm can be achieved by using a combination of in situ tensile straining and high-resolution transmission electron microscopy. Trapped unit lattice dislocations are observed in strained grains as small as 5 nm, but subsequent relaxation leads to dislocation recombination.     

Analysis of the parameters of a submicron dislocation structure in metals subjected to severe plastic deformation

Equations of dislocation kinetics are used to quantitatively compare the mechanisms of formation and evolution (with deformation) of cellular dislocation structures at moderate strains and of submicron block dislocation structures at high plastic strains. In both cases, the formation of nonuniform dislocation structures is a result of dislocation self-organization, more specifically, the self-organization of statistically random dislocations during the formation of cellular structures and the self-organization of geometrically necessary dislocations (which appear due to the nonuniform character of plastic deformation on the micron scale) during the formation of block structures.     

双模晶体相场研究形变诱导的多级微结构演化

本文采用双模晶体相场模型,计算了双模二维相图;模拟了形变诱导六角相向正方相转变过程的多级微结构演化,详细分析了位相差、形变方向对位错、晶界、晶体结构、新相形貌的影响规律.模拟结果表明:形变方向影响正方相晶核的形核位置和生长方向,拉伸时正方相优先在变形带上形核,垂直于形变方向长大,而压缩时正方相直接在位错和晶界的能量较高处形核,平行于形变方向长大;位相差对形变诱发晶界甄没过程有显著影响,体现在能量峰上为,小位相差晶界位错的攀滑移和甄没形成一个能量峰,大位相差晶界位错攀滑移和甄没因分阶段完成而不出现明显的能量峰;形变诱导相变过程中各种因素相互作用复杂,是相变与动态再结晶的复合转变.     

Formation of nanostructured states in ternary TiNiFe-based shape memory alloys during megaplastic deformation and subsequent heat treatment

The ternary metastable TiNiFe alloys that exhibit a low-temperature shape memory effect and are subjected to plastic deformation by rolling or high-pressure torsion followed by heat treatment are studied by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and electrical resistivity measurements. It is found that moderate plastic deformation of a Ti₅₀Ni₄₉Fe₁ alloy at room temperature initiates the thermoelastic B2 ? B19’ martensitic transformation and the formation of a developed banded dislocation and twin substructure in the B19’ martensite. This deformation of a Ti₅₀Ni₄₇Fe₃ alloy forms a similar dislocation substructure but in B2 austenite. Megaplastic deformation by high-pressure torsion causes amorphization in the Ti₅₀Ni₄₉Fe₁ alloy and nanofragmentation in the Ti₅₀Ni₄₇Fe₃ alloy. The evolution of the nanostructure and the martensitic transformations in TiNiFe-based ternary alloys is studied during plastic deformation and subsequent annealing at various temperatures.     

CEMS study of strain induced phase transformation in manganese Hadfield steel

A Conversion Electron Mössbauer Spectroscopy, (CEMS), study of phase transformations in a Hadfield steel induced by high rate strains is reported. Hadfield steel samples were impact deformed and the ensuing changes in the magnetic properties at the deformed zone and its surroundings have been studied by CEMS. The CEMS results are compared with wear tests and optical microscopy and show a formation of martensite by impact deformation only at the surface. Martensite is not produced by compression or tensile stresses but appears after wear tests in proportions that depend on the load and velocity conditions of test. The understanding of martensite phase formation and its evolution during deformation processes is also addressed.     

Dynamic characteristics of solids in microvolumes: Modern principles, techniques, and results of investigation

Several radically new independent in situ techniques for characterizing physicomechanical properties of materials in microvolumes are described. In particular, mechanisms behind the formation of an indent and the surrounding deformation zone were studied at a microlevel. With ionic crystals, it was demonstrated that indentation, followed by the formation of the deformation zone, passes the following stages: purely elastic deformation stage, the stage of monoatomic displacement of a material from under the indenter; and a number of final stages where dislocation plasticity is essential. Kinetic, dissipative, and activation parameters of indentation were determined, and basic mass transfer micromechanisms for each of the stages were elucidated.     

Acoustic emission during thermoelastic martensitic transformations in titanium nickelide under isothermal loading

Acoustic emission during thermoelastic martensitic transformations in titanium nickelide is investigated. Phase transitions are initiated by loading the specimen to 200 MPa under isothermal conditions. It is found out that deformation buildup and acoustic emission in the loading-unloading cycles are observed in the first cycle only, during further cycles the acoustic emission is comparable to the background, while the deformation buildup and recovery are not associated with martensitic transformations. It is shown that recovery of the deformation built up during loading occurs due to heating to 600 °C, with the major part of accumulated deformation undergoing recovery already at 250 °C and recovery of its minor part observed at 400 °C. This behavior of acoustic emission and accumulation and recovery of deformation provide evidence of martensitic phase stabilization during cycling of martensitic transformations under conditions of thermo-mechanical cycling. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 89–94, February, 2008.     

α-Fe裂纹的分子动力学研究

通过分子动力学方法，模拟了α-Fe裂纹的单轴拉伸实验中的形变过程.研究了不同晶体取向裂纹的形变特点和断裂机理，观察到各种形变现象，如位错形核和发射，位错运动，堆垛层错或孪晶的形成，纳米空洞的形成与连接等.计算结果表明，裂纹扩展是塑性过程和弹性过程相结合的过程，其中塑性过程表现为由裂尖发射的位错导致的原子切变行为，而弹性过程的发生则是由无位错区中的原子断键所导致.同时还研究了α-Fe裂纹的形变特点和断裂机理与温度场和应力场的依赖关系.     

Plastic distortion as a fundamental mechanism in nonlinear mesomechanics of plastic deformation and fracture

Any deformed solid represents two self-consistent functional subsystems: a 3D crystal subsystem and a 2D planar subsystem (surface layers and all internal interfaces). In the planar subsystem, which lacks thermodynamic equilibrium and translation invariance, a primary plastic flow develops as nonlinear waves of structural transformations. At the nanoscale, such planar nonlinear transformations create lattice curvature in the 3D subsystem, resulting in bifurcational interstitial states there. The bifurcational states give rise to a fundamentally new mechanism of plastic deformation and fracture—plastic distortion—which is allowed for neither in continuum mechanics nor in fracture mechanics. The paper substantiates that plastic distortion plays a leading role in dislocation generation and glide, plasticity and superplasticity, plastic strain localization and fracture.