Disclination mechanism for plastic deformation of nanocrystalline materials

A model is developed for the plastic deformation of nanocrystalline materials in terms of the evolution of a spatial grid of disclinations located at the triple junctions of grains. Plastic deformation takes place as the result of plastic rotation of grains, the mismatch of whose rotations causes the nucleation of partial disclinations at the junctions of intergrain boundaries. It is shown that the distinctive feature of the mechanical behavior of nanocrystals is a deviation from the Hall-Petch law up to a critical grain size D _cr⩽25 nm. Fiz. Tverd. Tela (St. Petersburg) 39, 2023–2028 (November 1997)     

Structural mechanisms of fracture of nanocrystalline materials

Structural mechanisms and features of brittle and quasi-brittle fracture of nanocrystalline materials are theoretically analyzed. The role of size effects and internal stresses caused by a nonequilibrium structure during brittle trans-and intercrystallite fracture is studied. The dependence of the nanocrystalline material durability on the working stress and grain size is calculated. The conditions for certain mechanisms of plastic deformation to be operative in nanocrystalline materials are analyzed. The influence of the grain-boundary and dislocation mechanisms of plastic deformation on the conditions of nanocrack formation is studied. The dependence of the fracture toughness of nanomaterials on structure parameters is calculated.     

Grain boundary sliding and lattice dislocation emission in nanocrystalline materials under plastic deformation

A theoretical model is proposed to describe the physical mechanisms of hardening and softening of nanocrystalline materials during superplastic deformation. According to this model, triple interface junctions are obstacles to glide motion of grain boundary dislocations, which are carriers of grain boundary glide deformation. Transformations of an ensemble of grain boundary dislocations that occur at triple interface junctions bring about the formation of partial dislocations and the local migration of triple junctions. The energy characteristics of these transformations are considered. Pileups of partial dislocations at triple junctions cause hardening and initiate intragrain lattice sliding. When the Burgers vectors of partial dislocations reach a critical value, lattice dislocations are emitted and glide into adjacent grains, thereby smoothing the hardening effect. The local migration of triple interface junctions (caused by grain boundary sliding) and the emission of lattice dislocations bring about softening of a nanocrystalline material. The flow stress is found as a function of the total plastic strain, and the result agrees well with experimental data.     

Mechanism of the formation of shear microbands under plastic deformation of nanocrystalline materials

The mechanism of deformation localization and formation of shear microbands under plastic deformation of submicrocrystalline and nanocrystalline materials is theoretically discussed in the framework of the dislocation-kinetic approach. An equation of evolution of the density and self-organization of dislocations in these materials is formulated taking into account that the grain boundaries are the main sources, sinks, and barriers to moving dislocations. By solving this equation, it is found that the width of microbands and the distance between them depend on the nanograin size and the degree of plastic deformation. It is also demonstrated that there exists a critical grain size (350 nm in the case of α-Fe) above which no microbands are formed in the nanomaterial. The theoretical results are compared with the data available in the literature.     

Structural mechanisms of plastic deformation of amorphous alloys containing crystalline nanoparticles

Patterns of plastic deformation of amorphous nanocrystalline composites, caused by the local action of an indenter on a thin electron microscopy foil, have been experimentally investigated for the first time in structural analysis. Classification of the observed types of interaction of shear bands with crystalline nanoparticles is performed. This classification is in good agreement with the theoretically predicted interaction mechanisms.     

Grain growth and collective migration of grain boundaries during plastic deformation of nanocrystalline materials

A theoretical model is proposed for the collective migration of two neighboring grain boundaries (GBs) in a nanocrystalline material under applied elastic stress. By analyzing the change in the energy of the system, it is shown that GBs can remain immobile or migrate toward each other depending on the values of the applied shear stress and misorientation angles. The process of GB migration can proceed either in a stable regime, wherein the GBs occupy equilibrium positions corresponding to a minimum of the energy of the system under relatively small applied stress, or in an unstable regime, wherein the motion of GBs under relatively high stress is accompanied by a continuous decrease in the system energy and becomes uncontrollable. The stable migration of GBs leads to a decrease of the grain bounded by them at the cost of growth of the neighbor grains and can result in complete or partial annihilation of the GBs and the collapse of this grain. Unstable migration leads either to annihilation of GBs or to passage of them through each other, which can be considered as the disappearance of the grain and nucleation and growth of a new grain.     

Structural methods for studying nanocrystalline materials

The most important methods for determining the grain size, the grain size distribution and also the actual microstructure of nanocrystalline materials are: X-ray diffraction line profile analysis, transmission electron microscopy, and small-angle neutron scattering. For each of these three methods their specific advantages and disadvantages are discussed and an experimental example is given.     

Structural signature of plastic deformation in metallic glasses

The structure feature of a model CuZr metallic glass during deformation is investigated by molecular dynamics simulations. A spatially heterogeneous irreversible rearrangement is observed in terms of nonaffine displacement. We find that regions with smaller nonaffine displacement have more Voronoi pentagons, while in those with larger nonaffine displacement other types of faces are more populated. We use the degree of local fivefold symmetry (LFFS) as the structural indicator to predict plastic deformation of local structures and find that the plastic events prefer to be initiated in regions with a lower degree of LFFS and propagate toward regions with a higher degree of LFFS.     

Evolution of mechanisms of plastic deformation in porous metals

An experimental study was made of the structure of deformed porous iron with a porosity P ranging from 4 to 40% within the range of strains up to fracture. Measurements are made of grain-boundary slip and rotations of structural elements and their individual contributions to the total strain as a function of P and . It is established that there is a change in the dominant mechanisms of plastic deformation with an increase in porosity, this change being connected with a change in the topological characteristics of the system and corresponding to the transition from intergranular slip to a group of mechanisms that ensures movement of the grains as a whole. The established laws are analyzed on the basis of the concept of structural levels of deformation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 101–105, January, 1996.     

Investigation of relaxation processes in nanocrystalline cobalt produced by severe plastic deformation

It has been shown that the dependence of the width of X-ray diffraction lines of plastically deformed cobalt on the annealing temperature is described by the exponential function. Characteristic temperature regions corresponding to the processes of recovery and recrystallization have been established. It has been shown that the values of the activation energy of recrystallization determined from the experimental data are comparable with the activation energy of the grain-boundary diffusion in metals. The activation energy for the recovery region is considerably lower than the activation energy of migration of nonequilibrium grain boundaries in nanocrystalline metals. The X-ray diffraction data have been confirmed by the investigations of the microstructure and microhardness.     

Structural changes in aluminum alloys upon severe plastic deformation

The structure and phase composition of Al-Zn, Al-Mg, and Al-Mg-Zn alloys were studied before and after severe plastic deformation of these alloys. The deformation was performed by high pressure torsion with true strain of ～6. It was established that, as a result of severe plastic deformation, the grains of Al and Zn and also of the β and τ phases revealed in the structure decrease significantly in size and reach nanometer scales. A supersaturated solid solution of Zn in Al decomposes completely in this case and achieves the equilibrium state corresponding to room temperature. The decomposition is less pronounced for the magnesium-containing alloys. Based on the obtained experimental data, a conclusion is drawn concerning the possible mechanisms of this process. Microhardness measurements revealed softening of the alloys as a result of the deformation, which is due to the decomposition of the supersaturated solid solution.     

Texture evolution and operative mechanisms during large-strain deformation of nanocrystalline nickel

The large-strain deformation of nanocrystalline nickel was investigated at room temperature and cryogenic (liquid N₂) temperature. Deformation mechanisms ranging from grain boundary sliding to slip, operate due to a wide distribution of grain sizes. These mechanisms leave their finger print in the deformation texture evolution during rolling of nanocrystalline nickel. The occurrence and severance of different mechanisms is understood by a thorough characterization of the deformed samples using X-ray diffraction, X-ray texture measurements, electron back-scattered diffraction and transmission electron microscopy. Crystal plasticity-based viscoplastic self-consistent simulations were used to further substantiate the experimental observations. Thus, a comprehensive understanding of deformation behavior of nanocrystalline nickel, which is characterized by simultaneous operation of dislocation-dominated and grain boundary-mediated mechanisms, has been developed.     

Structural kinetic aspects of the physics of evolution of plastic deformation

Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 7–22, March, 1991.     

Dynamics of plastic deformation: Waves of localized plastic deformation in solids

Conclusions It has been shown here that a localized plastic deformation in structurally inhomogeneous media can be of a wave nature and can propagate in the form of nonlinear plastic waves, not only at the microscopic level but also at the mesoscopic level. It has been established that there is an interrelationship between this new effect and grain-boundary slippage (an effect which has been under study for a long time) and also with certain types of quasiviscous fracture in plastically deformable materials.We have discussed certain specific practical problems in the mechanics of plastic deformation, and for certain types of fracture. In the future, these problems will be discussed at a more profound level and in greater detail, because of experimental studies which are presently being carried out on the dynamics of deformation for various types of loading and fracture [17, 18, 31]. We hope that the approach proposed here for a theoretical study of the localization of deformation and fracture can be taken to study such effects as splitting off [31], the influence of defect fluxes on grain-boundary slippage [22], superplasticity [23], the behavior of tectonic faults and boundaries of various types [32], electroplastic and magnetoplastic effects, and high-temperature localization of deformation [25].The general nature of the approach proposed here results from the circumstance that a localization of deformation is present explicitly or implicitly during plastic deformation, and the behavior of this deformation plays a role of fundamental importance in the propagation of plastic deformation through a material.The author wishes to thank V. E. Panin for a constant discussion of this problem and I. O. Nedavnii for carrying out the numerical calculations.V. V. Kuibyshev Tomsk State University. Institute of Strength Physics and Materials Science. Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 19–41, April, 1992.     

晶体相场法模拟纳米晶材料反霍尔-佩奇效应的微观变形机理

采用晶体相场模型模拟获得了平均晶粒尺寸从11.61–31.32 nm的纳米晶组织, 研究了单向拉伸过程纳米晶组织的强化规律的微观变形机理. 模拟结果表明: 晶粒转动、晶界迁移等晶间变形行为是纳米晶材料的主要微观变形方式, 纳米晶尺寸减小, 有利于晶粒转动, 使屈服强度降低, 显示出反霍尔-佩奇效应.当纳米晶较小时, 变形量超过屈服点达到4%, 位错运动开启, 其对变形的直接贡献有限, 主要通过改变晶界结构而影响变形行为, 位错运动破坏三叉晶界, 引发晶界弯曲, 促进晶界迁移. 随纳米晶增大, 晶粒转动困难, 出现晶界锯齿化并发射位错的现象. 关键词： 晶体相场 纳米晶 反霍尔-佩奇效应 微观变形