Effect of grain size dispersion on the strength and plasticity of nanocrystalline metals

The effect of the dispersion of the grain size distribution on the yield stress, ultimate stress, and uniform strain of nanocrystalline metals is analyzed theoretically. It is shown that, as the grain size dispersion increases, the degree of grain boundary hardening (Hall-Petch effect) of nanocrystalline materials decreases, the onset of the grain boundary softening (inverse Hall-Petch effect) shifts to smaller nanograin sizes, and the uniform strain at which necking occurs increases.     

Mechanism of decrease in the strength of submicron-sized specimens of FCC metals with a nanocrystalline structure

The effect of decrease in the strength of submicron-sized specimens of face-centered cubic (fcc) metals with a nanocrystalline structure and a cross-sectional size D < 5d, as compared to the strength of the specimens with D ? 5d (where d is the grain size), has been considered theoretically on the basis of the dislocation–kinetic equations and relationships. Previously, it has been found that this decrease is caused by the escape of a part of the dislocations through the surface of the specimen under the action of single-pole dislocation sources in grains adjacent to the surface. In this study, it has been shown that the absorption of lattice dislocations by grain boundaries and its accompanying grain boundary sliding lead to a further decrease in the flow stress of specimens, which is equally related to both thin (D < 5d) and thick (D ? 5d) specimens.     

A continuum model for the jumping sandbox

We propose a continuum model for the motion of sand oscillating vertically. We derive a nonlinear parabolic equation with a nonlinearity corresponding to a bistable potential multiplied by a switching function. This is obtained by modifying the equation for a free surface and incorporating the thermodynamics of the interface.     

一个描述合作网络顶点度分布的模型

讨论一类社会合作网络以及一些与其拓扑结构相似的技术网络的度分布.建议一个最简化模型，通过解析的方法说明这些网络演化的共同动力学机理，而且说明顶点的度分布和项目度分布之间具有密切的一致关系，而项目所含的顶点数分布对度分布的影响较小；对模型的更一般情况进行数值模拟，说明上述结论具有一定的普遍性.这个模型显示这类广义的合作网络一般具有处于幂函数和指数函数这两种极端情况之间的度分布.简要介绍对一些实际合作网络做统计研究的结果，说明本模型的合理性. 关键词： 合作网络 度分布 项目度分布 项目含顶点数     

Size effects of nano-spaced basal stacking faults on the strength and deformation mechanisms of nanocrystalline pure hcp metals

The size effects of nano-spaced basal stacking faults (SFs) on the tensile strength and deformation mechanisms of nanocrystalline pure cobalt and magnesium have been investigated by a series of large-scale 2D columnar and 3D molecular dynamics simulations. Unlike the strengthening effect of basal SFs on Mg alloys, the nano-spaced basal SFs are observed to have no strengthening effect on the nanocrystalline pure cobalt and magnesium from MD simulations. These observations could be attributed to the following two reasons: (i) Lots of new basal SFs are formed before (for cobalt) or simultaneously with (for magnesium) the other deformation mechanisms (i.e. the formation of twins and the < c + a > edge dislocations) during the tensile deformation; (ii) In hcp alloys, the segregation of alloy elements and impurities at typical interfaces, such as SFs, can stablilise them for enhancing the interactions with dislocation and thus elevating the strength. Without such segregation in pure hcp metals, the < c + a > edge dislocations can cut through the basal SFs although the interactions between the < c + a > dislocations and the pre-existing SFs/newly formed SFs are observed. The nano-spaced basal SFs are also found to have no restriction effect on the formation of deformation twins.     

描述光伏效应的新解析模型

对陆启生等人提出的描述光伏效应的解析模型涉及的边界条件进行了讨论,提出了一个适用性更宽的解析模型。通过对新模型、陆的模型以及另一个解析模型的比较,对前两个模型能够描述光伏型光电探测器在强光辐照时的信号饱和效应的原因进行了解释。     

A continuum model of a multilayer nanosheet

A continuum model for describing the bending and free vibrations of a crystalline graphite sheet consisting of graphene layers is proposed. Graphene is modeled by a two-dimensional layer having a finite rigidity under extension and bending. The interval between graphene layers through which their Van-der-Waals interaction occurs is modeled by a fictitious layer with relatively low rigidity. In the solution, formulas describing the bending of a multilayer sheet with alternating rigid and soft layers are used.     

A fibrous dynamic continuum model of the tympanic membrane

The geometry and anisotropic ultrastructure of the tympanic membrane are used in combination with curvilinear shell equations to formulate a general continuum model describing its dynamic behavior. Primary terms appearing in the model are associated with shell membrane restoring forces, bending-type structural damping, and transverse inertia. Since the model is based extensively on the physical characteristics of the membrane, it is relatively easy to account for differences between species as well as pathological conditions. The fibrous structure and cone-shaped geometry, readily apparent in mammalian eardrums, introduce several small parameters into the model that are exploited in order to construct a closed-form asymptotic solution. The solution includes the coupling to the three-dimensional motion of the ossicular chain and it includes the frequency-dependent pressure distribution in the auditory canal. When applied to the cat eardrum, this asymptotic solution is shown to reproduce a large manifold of experimentally observed frequency and excitation-dependent vibrational shapes. In addition to the shapes, transient amplitude and phase data for the cat are reproduced.     

Mechanism of deformation-twin formation in nanocrystalline metals

A theoretical model is proposed to describe the nucleation of deformation twins at grain boundaries in nanocrystalline materials under the action of an applied stress and the stress field of a dipole of junction or grain-boundary wedge disclinations. The model is used to consider pure nanocrystalline aluminum and copper with an average grain size of about 30 nm. The conditions of barrier-free twinning-dislocation nucleation are studied. These conditions are shown to be realistic for the metals under study. As the twin-plate thickness increases, one observes two stages of local hardening and an intermediate stage of local flow of a nanocrystalline metal on the scale of one nanograin. In all stages, the critical stress increases with decreasing disclination-dipole strength. The equilibrium thickness and shape of the twin plate are analyzed and found to agree well with the well-known results of experimental observations.     

Atomistic mechanisms of fatigue in nanocrystalline metals

We investigate the mechanisms of fatigue behavior in nanocrystalline metals at the atomic scale using empirical force laws and molecular level simulations. A combination of molecular statics and molecular dynamics was used to deal with the time scale limitations of molecular dynamics. We show that the main atomistic mechanism of fatigue crack propagation in these materials is the formation of nanovoids ahead of the main crack. The results obtained for crack advance as a function of stress intensity amplitude are consistent with experimental studies and a Paris law exponent of about 2.     

A new method of increasing the fatigue strength of metals

High-intensity ultrasonics at 23 kHz was applied to mild-steel specimens. No change in hardness could be detected at a temperature of 20°C, but when the temperature was allowed to increase by energy-absorption the hardness increased in a subsurface layer. This change in hardness was accompanied by changes in density and in the arrangement of dislocations, and resulted in an increase of 20–30% in the low-frequency fatigue limit. An explanation of this phenomenon is attempted.     

Temperature dependence in the Schumann-Runge photoabsorption continuum of oxygen

The photoabsorption cross section in the Schumann-Runge continuum of oxygen has been measured with high precision over the wavelength region 140–174 nm at temperatures in the range 295–575 K. Models for the upper state potential and the electronic transition moment were used in the calculation of the cross section and its temperature dependence. By comparing this theoretical cross section with measured values, curves for the upper state potential and the transition moment in the continuum region have been obtained independently for the first time.     

Strength of magnesium oxide under high pressure: evidence for the grain-size dependence

X-ray diffraction patterns from magnesium oxide compressed in a diamond anvil cell up to 55 GPa have been recorded and the differential stress (a measure of compressive strength) and grain-size (crystallite size) determined as a function of pressure from the line-width analysis. The strength agrees well with the uniaxial stress component (another measure of compressive strength) derived earlier from the line-shift data. The strength increases while the crystallite size decreases steeply as the pressure is raised from ambient to ∼10 GPa. The increase in strength is much smaller at higher pressures. The strength-pressure data are explained by combining the grain-size dependence of strength and the shear-modulus scaling law. The dependence of strength on grain-size has not been considered in the past in the discussion of high-pressure strength data.     

Crystals and quasicrystals: A continuum model

We construct the first model of particles in the plane with completely symmetric, short range, two body interactions which has quasiperiodic, but no periodic, ground states.Supported in part by NSF Grant No. DMS-8501911     

High strength and superplasticity of nanocrystalline materials

The part played by grain boundaries in nanocrystalline materials produced by equal-channel angular pressing is considered. The tensile strength and tensile and compressive yield stresses of various materials and alloys were studied over a broad low-temperature range. It was found that, at close-to-liquid helium temperatures, the strength is the highest possible for the given material and that strain localization is better pronounced than in conventional materials. The results obtained are interpreted in terms of the influence of boundaries, which act as the dominant hardening factor increasing the resistance to dislocation motion. By contrast, experiments conducted at elevated temperatures showed the boundaries to become mobile, thus imparting superplasticity to materials in some cases. The actual maximum tensile and torsional shears are compared. It is demonstrated that, despite the closeness between the tensile-and torsional-deformation activation energies, the stress-strain curves and the shears differ strongly, which implies that these characteristics are affected by the actual type of deformation involved.