Complex nonlinear deformation of nanometer intergranular glassy films in beta-Si3N4

The mechanical properties of a model of Y-doped intergranular glassy film in silicon nitride ceramics are studied by large-scale ab initio modeling. By linking directly to its electronic structure, it is shown that this microstructure has a complex nonlinear deformation under stress and Y doping significantly enhances the mechanical properties. The calculation of the electrostatic potential across the film supports the space charge model in ceramic microstructures.     

Nucleation of misfit dislocations and plastic deformation in core/shell nanowires

During fabrication of metal nanowires, an oxide layer (shell) that surrounds the metal (core) may form. Such an oxide-covered nanowire can be viewed as a cylindrical core/shell nanostructure, possessing a crystal lattice mismatch between the core and shell. Experimental evidence has shown that, in response to this mismatch, mechanical stresses induce plastic deformation in the shell and misfit dislocations nucleate at the core/shell interface. As a result, the mechanical, electrical and optoelectronic properties of the nanowire are affected. It is therefore essential to be able to predict the critical conditions at which misfit dislocation nucleation at the nanowire interface takes place and the critical applied load at which the interface begins deforming plastically. Two approaches are explored in order to analyze the stress relaxation processes in these oxide-covered nanowires: (i) energy considerations are carried out within a classical elasticity framework to predict the critical radii (of the core and shell) at which dislocation nucleation takes place at the nanowire interface; (ii) a strain gradient plasticity approach is applied to estimate the flow stress at which the interface will begin deforming plastically (this stress is termed “interfacial-yield” stress). The interfacial-yield stress, predicted by gradient plasticity, depends, among other material parameters, on the radii of the core and shell. Both approaches demonstrate how the geometric parameters of nanowires can be calibrated so as to avoid undesirable plastic deformation; in particular, method (i) can give the radii values that prevent misfit dislocation formation, whereas method (ii) can provide, for particular radii values, the critical stress at which interface deformation initiates.     

Dislocation Plasticity Effects on Interfacial Fracture

Analyses are reviewed where plastic flow in the vicinity of an interfacial crack is represented in terms of the nucleation and glide of discrete dislocations. Attention is confined to cracks along a metal-ceramic interface, with the ceramic idealized as being rigid. Both monotonic and fatigue loading are considered. The main focus is on the stress and deformation fields near the crack tip predicted by discrete dislocation plasticity, in comparison with those obtained from conventional continuum plasticity theory. The role that discrete dislocation plasticity can play in interpreting interface fracture properties in the presence of plastic flow is discussed.     

Experimental study of different modes of block sliding along interface. Part 2. Field experiments and phenomenological model of the phenomenon

The paper reports the results of field experiments on studying different modes of gravitational sliding of a block on the natural fault surface. Various materials were used as interface filler to model the whole range of deformation events that can be arbitrarily divided into three groups: accelerated creep, slow slip, and dynamic slip. The experiments show that the type of modeled deformation events is defined by both structural parameters of contact between blocks and material composition of the contact filler.Foundations for a new geomechanical model of occurrence of different-type dynamic events were developed. The model is based on the idea that “contact spots” form subnormally to the crack edges during shear deformation; the “spots” are clusters of force mesostructures whose evolution governs the deformation mode. The spatial configuration of “contact spots” remains unchanged during the entire “loading-slip” cycle but changes after the dynamic event occurrence. The destroyed force mesostructures can be replaced by similar structures under intergranular interaction forces when the external influence is fully compensated. Unless “contact spots” are incompletely destroyed, the deformation process dynamics is defined by their rheology. The migration of “contact spots” during deformation of a crack filled with heterogeneous material causes changes in deformation parameters and transformation of the mode itself due to changing rheology of local contact areas between blocks.It is found by fractal analysis that in order for dynamic slip to occur, spatially structured “contact spots” characterized by low fractal dimension must be formed; slow slip events can exist only in a certain parametric domain called the “dome of slow events”. It is found that the probability of slow slip occurrence is higher on fault regions characterized by maximum fractal dimension values: fault tips, fault branching and fault intersection zones.     

Deformation behavior and enhanced plasticity of Ti-based metallic glasses with notches

The plasticity of Ti-based metallic glasses with different aspect ratios can be improved by introducing two semicircular notches on the edges of the samples, owing to the interactions of shear bands (SBs) under conventional compression tests. The interaction of SBs can be ascribed to the easy initiation of SBs around the notches due to the large stress gradient, and the consequent blocking effect of notches on the propagation of shear bands. Additionally, the current findings provide a new way to understand the physical nature for the plastic deformation behavior of some brittle metallic glasses and supply an effective approach to enhance the plasticity to some extent.     

Multiscale simulation from atomistic to continuum – coupling molecular dynamics (MD) with the material point method (MPM)

A new multiscale simulation approach is introduced that couples atomistic-scale simulations using molecular dynamics (MD) with continuum-scale simulations using the recently developed material point method (MPM). In MPM, material continuum is represented by a finite collection of material points carrying all relevant physical characteristics, such as mass, acceleration, velocity, strain and stress. The use of material points at the continuum level provides a natural connection with the atoms in the lattice at the atomistic scale. A hierarchical mesh refinement technique in MPM is presented to scale down the continuum level to the atomistic level, so that material points at the fine level in MPM are allowed to directly couple with the atoms in MD. A one-to-one correspondence of MD atoms and MPM points is used in the transition region and non-local elastic theory is used to assure compatibility between MD and MPM regions, so that seamless coupling between MD and MPM can be accomplished. A silicon single crystal under uniaxial tension is used in demonstrating the viability of the technique. A Tersoff-type, three-body potential was used in the MD simulations. The coupled MD/MPM simulations show that silicon under nanometric tension experiences, with increasing elongation in elasticity, dislocation generation and plasticity by slip, void formation and propagation, formation of amorphous structure, necking, and final rupture. Results are presented in terms of stress–strain relationships at several strain rates, as well as the rate dependence of uniaxial material properties. This new multiscale computational method has potential for use in cases where a detailed atomistic-level analysis is necessary in localized spatially separated regions whereas continuum mechanics is adequate in the rest of the material.     

A composite model of the plastic flow of amorphous covalent materials

A model based on the data available in the literature on the computer simulation of amorphous silicon has been proposed for describing the specific features of the plastic flow of amorphous covalent materials. The mechanism of plastic deformation involves homogeneous nucleation and growth of inclusions of a liquidlike phase under external shear stress. Such inclusions experience plastic shear, which is modeled by glide dislocation loops. The energy changes associated with the nucleation of these inclusions at room and increased temperatures have been calculated. The critical stress has been found, at which the barrierless nucleation of inclusions becomes possible. It has been shown that this stress decreases with an increase in temperature. According to the calculations, the heterogeneous (homogeneous) plastic flow of an amorphous material should be expected at relatively low (high) temperatures. Above the critical stress, the homogeneous flow is gradually replaced by the heterogeneous flow.     

Evaluating the effects of loading parameters on single-crystal slip in tantalum using molecular mechanics

This study is aimed at developing a physics-based crystal plasticity finite element model for body-centred cubic (BCC) metals, through the introduction of atomic-level deformation information from molecular dynamics (MD) investigations of dislocation motion at the onset of plastic flow. In this study, three critical variables governing crystal plasticity mediated by dislocation motion are considered. MD simulations are first performed across a range of finite temperatures up to 600K to quantify the temperature dependence of critical stress required for slip initiation. An important feature of slip in BCC metals is that it is not solely dependent on the Schmid law measure of resolved shear stress, commonly employed in crystal plasticity models. The configuration of a screw dislocation and its subsequent motion is studied under different load orientations to quantify these non-Schmid effects. Finally, the influence of strain rates on thermal activation is studied by inducing higher stresses during activation at higher applied strain rates. Functional dependence of the critical resolved shear stress on temperature, loading orientation and strain rate is determined from the MD simulation results. The functional forms are derived from the thermal activation mechanisms that govern the plastic behaviour and quantification of relevant deformation variables. The resulting physics-based rate-dependent crystal plasticity model is implemented in a crystal plasticity finite element code. Uniaxial simulations reveal orientation-dependent tension–compression asymmetry of yield that more accurately represents single-crystal experimental results than standard models.     

Substructure formation in high-strength dispersely strengthened alloys

Conclusions Analysis of new experimental laws of plastic flow observed in high-strength alloys with dispersional strengthening (such as the formation of substructure with high crystal-lattice curvature, high-temperature localization of deformation from the earliest stages, with reorientation of the localized-shear zones and the adjacent undeformed structural elements) leads to the conclusion that deformational point defects play an important role in the realization of collective deformational modes in the high-strength state.In conditions of high nonequilibrium concentration, deformational point defects, first, permit the inclusion of quasi-viscous diffusional mechanisms of crystal-lattice reorientation by point-defect drift in the local fields of high inhomogeneous stress and, second, by facilitating dislocational deformation mechanisms, may lead to local weakening of the shear zones, localization of the plastic flow, and stability loss, in particular, as a result of mutually consistent autocatalytic defect multiplication.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 81–92, March, 1991.     

Influence of elastic strains on the adsorption process in porous materials. Thermodynamics and experiment

If we disregard the shape of the boundary hysteresis loop, H1 for SBA-15, MCM-41 and KIT-6, H2 for p⁺-type porous silicon and porous glass, the hysteretic features inside the loop are qualitatively the same for all these systems and show that none of them are composed of independent pores whether the pores are interconnected or not. We hence believe that the physical parameter which couples the pores is not the interconnectivity but the elastic deformation of the porous matrix. The thermodynamic approach we develop includes the elastic energy of the solid. We show that the variation of the surface free energy, which is proportional to the deformation of the porous matrix, is an important component of the total free energy. With porous silicon, we experimentally show that a stress external to the porous matrix exerted by the substrate on which it is supported significantly increases the total free energy and the adsorbed amount and decreases the condensation pressure compared to that of the same porous matrix detached from its substrate which is the relaxed state of the supported layer. This stress can be partly relaxed by making thicker porous layers due to the breaking of Si-Si bonds. This results in the shift of the isotherms towards that of the membrane. We propose a new interaction mechanism occurring through the pore wall elastic deformation in which the external mechanical stress is imposed on a given pore by its neighbours.     

设计脆性材料的冲击塑性

通过微结构设计提升脆性功能材料的冲击塑性, 将有助于避免或延缓失效的发生. 提出在脆性材料中植入特定的微小孔洞以改善其冲击塑性的设计方法. 采用一种能够定量表现脆性材料力学性质的格点-弹簧模型, 研究了孔洞排布方式对脆性材料冲击响应的影响. 孔洞随机排布的多孔脆性材料具有明显高于致密脆性材料的冲击塑性, 而设计规则的孔洞排布方式将有助于进一步提升脆性材料的冲击塑性. 对150 m/s活塞冲击下气孔率5%的多孔样品的介观变形特征分析表明, 孔洞规则排布的样品中孔洞贯通和体积收缩变形占主导, 而孔洞随机排布的样品中剪切裂纹长距离扩展和滑移与转动变形占主导. 尽管在宏观的Hugoniot应力-应变曲线上, 两种孔洞排布方式的样品都表现出三段式响应特征(线弹性阶段、塌缩变形阶段和滑移与转动变形阶段), 但孔洞规则排布时孔洞塌缩变形阶段对整体冲击塑性的贡献更大. 研究揭示的规则排布孔洞增强脆性材料冲击塑性的原理, 将有助于脆性材料冲击诱导功能失效的预防.     

非晶合金中的流变单元

非晶合金是一类具有诸多优异性能的先进金属材料,同时也是研究非晶态物质的模型体系.最近,大量的实验和模拟证据显示,在非晶合金中可能存在类似晶体中缺陷的"流变单元",这些动力学单元和非晶合金的的流变、物理、力学性能密切关联.本文主要综述了流变单元提出的背景、实验证据、流变单元的特征、激活与演化过程、相互作用以及相关的理论.文中提供了大量实验证据证明流变单元模型不仅可以帮助理解非晶态物质中如形变、玻璃转变、弛豫动力学以及非晶结构和性能的关系等重要的基本物理问题,而且可以指导非晶合金性能的调控和设计,获得性能优异的非晶合金材料.     

Relaxation waves in plastic deformation

Conclusion Experimental study of distortion fields of plastically deformed solids performed on a wide range of materials including fine- and coarse-grain body- and face-centered polycrystals, as well as amorphous alloys reveals that in these materials plastic deformation develops in the form of waves having translational and rotational components. This fact is in accordance with the currently developed theory of a turbulent mechanical field, which also has translational and rotational components.The plastic deformation waves are observable at a macroscopic structural level, and their spatial period (wavelength) is determined by the dimensions of the deformed object and dimensions of the basic structural elements (for a polycrystal, the grain size). The propagation rate of these waves is significantly less than the characteristic propagation rate of an elastic excitation and the velocity of previously described plastic waves which are produced by shock deformation, which latter speed is determined by the hardening coefficient.The character of plasticity waves depends on the form of the material's deformation curve, and on the stage of the hardening curve. The distribution of plastic distortion components changes especially significantly in prefailure regions, which allows detection of the latter long before formation of a macroscopic crack. The role of rotations in forming the failure process has been established.A synergetic interpretation of plasticity wave formation has been proposed, based on synchronization of relaxation acts occurring at stress concentrators during the deformation process.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 19–35, February, 1990.     

Pressure-enforced plasticity in MAX phases: from single grain to polycrystal investigation

Ti₄AlN₃, Ti₃AlC₂ and Ti₃Al_0.8Sn_0.2C₂ MAX phases were plastically deformed at room temperature (RT) under gaseous confining pressure. Microstructures of as-grown and deformed samples are carefully analysed using scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM). It is demonstrated that high level of plastic deformation can be reached under confining gas pressure; the later suppresses the brittle failure at RT to the profit of plasticity. Multiscale characterization techniques are shown to provide a unique insight into all the scales of the plastic deformation; in particular, the effect of the mesoscale. Indeed, grain shape and orientation relative to the compression axis are shown to play a key role in the deformation process, intergranular stresses leading to a complex stress field in the polycrystalline samples. The TEM results show that dislocation activity highly depends on the grain orientation. The observation of dislocation entanglements unambiguously demonstrates that dislocations may be organized in such a configuration so that their glide in the basal plane can be hindered when deep plastic regime is reached.     

Indenter size effect on the reversible incipient plasticity of Al(001) surface:Quasicontinuum study

Indenter size effect on the reversible incipient plasticity of Al(001) surface is studied by quasicontinuum simulations.Results show that the incipient plasticity under small indenter, the radius of which is less than ten nanometers, is dominated by a simple planar fault defect that can be fully removed after withdrawal of the indenter; otherwise, irreversible incipient plastic deformation driven by a complex dislocation activity is preferred, and the debris of deformation twins, dislocations,and stacking fault ribbons still remain beneath the surface when the indenter has been completely retracted. Based on stress distributions calculated at an atomic level, the reason why the dislocation burst instead of a simple fault ribbon is observed under a large indenter is the release of the intensely accumulated shear stress. Finally, the critical load analysis implies that there exists a reversible-irreversible transition of incipient plasticity induced by indenter size. Our findings provide a further insight into the incipient surface plasticity of face-centered-cubic metals in nano-sized contact issues.     

Plastic deformation transfer through the amorphous intercrystallite phase in nanoceramics

A three-dimensional model is proposed for plastic deformation transfer through the amorphous intercrystallite phase in mechanically loaded nanoceramics. In this model, glide dislocation loops are pressed against amorphous intercrystallite boundaries by the applied local shear stress and initiate in them local longitudinal plastic shears, which causes emission of new glide dislocation loops into neighboring grains. The energy characteristics of these processes and the critical applied stress required for barrierless nucleation of grainboundary and intragrain loops are calculated. As an example, a nanoceramic based on cubic silicon carbide is considered. It is shown that plastic deformation transfer through the amorphous intercrystallite phase in such nanoceramics is energetically favorable and can occur athermically over wide ranges of values of the applied stress and the structural characteristics of the material.     

Autowave processes in stage i easy glide in the flow curve for single crystals

A material under loading is treated as a dissipative bistable medium described by a nonlinear equation of the reaction-diffusion type for order parameters. The coefficients of the equation are temperature and stress dependent. It is shown that in the easy glide stage I deformation the plastic flow occurs by the switching wave propagation mechanism. The action of the temperature on the critical shear stress and on the length of the region of easy glide under quasi-static loading is analyzed. Institute of Strength Physics and Materials Science, Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 8–13, June, 1999.     

Mechanical research and development of a monocrystalline silicon neutron beam window for CSNS

The monocrystalline silicon neutron beam window is one of the key components of a neutron spectrometer. Monocrystalline silicon is brittle and its strength is generally described by a Weibull distribution due to the material inhomogeneity. The window is designed not simply according to the mean strength but also according to the survival rate. The total stress of the window is stress-linearized into a combination of membrane stress and bending stress by finite element analysis. The window is a thin circular plate, so bending deformation is the main cause of failure and tensile deformation is secondary and negligible. Based on the Weibull distribution of bending strength of monocrystalline silicon, the optimized neutron beam window is designed to be 1.5 mm thick. Its survival rate is0.9994 and its transmittance is 0.98447, which meets both physical and mechanical requirements.     

Work-hardening of Fe,Fe-0·5 wt% Si and Fe-0·9 wt% Si single crystals

Work-hardening curves of single crystals of iron and its alloys with 0·5 and 0·9 wt.% Si and paths of the tensile axis of specimens during deformation were investigated. Single crystals oriented for single glide were deformed in tension at temperatures 113, 201, 295 and 403 K at a nominal strain rate of 5·5×10^?5 sec^?1. It appears that with increasing silicon content (a) the transition between the high-temperature regime of plastic deformation (characterized by three-stage work-hardening) and the low-temperature regime is shifted to lower temperatures, (b) stage I-hardening is enhanced and (c) the flow stress level increases. These observations are explained by strong solution hardening of iron by silicon atoms which suppresses the differences between mobilities of screw and non-screw dislocations, increases the flow stress level and consequently the density of primary dislocations. As a result of this the latent hardening in the secondary slip system increases and stage I extends to a large strain.