Dislocation nucleation at amorphous intergrain boundaries in deformed nanoceramics

A theoretical model is proposed for lattice dislocation nucleation in deformed nanocrystalline ceramics with amorphous intergrain boundaries. According to the model, a lattice dislocation dipole nucleates at an amorphous intergrain boundary through a local plastic shear along the boundary cross section. The energy parameters of this nucleation process are calculated. It is demonstrated that the dislocation nucleation at amorphous intergrain boundaries is energetically favorable and can occur as an athermic process (without energy barrier) in the nanocrystalline phase of cubic silicon carbide 3C-SiC and in the TiN/a-Si₃N₄ nanocomposite over wide ranges of structural parameters and mechanical loads.     

Differential dislocations in martensite-type interphase boundaries

The tensor describing the density of differential dislocations, formed on interphase surfaces with a martensite transformation and on boundaries of mechanical twins, is calculated. Differential dislocations arise due to the transformation of Burgers vectors and dislocation lines in the process of their succession from the parent crystal. The dependence of the density of differential dislocations on the starting dislocation structure, transformation geometry, and the transformation matrix of the lattice with the transformation is demonstrated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 76–79, June, 1981.     

Capillary-driven interdiffusion along interphase boundaries in solids

We consider chemical interdiffusion along interphase boundary (IB) between two immiscible solid phases. We derive explicit expressions for capillary-related excess chemical potentials of the atoms diffusing along the IB. The obtained expressions contain both a local term which depends on local curvature of the IB, and a non-local one which depends on overall system geometry and on energy of all surfaces and interfaces in the system. The obtained expressions are employed for describing the sintering of two immiscible two-dimensional solid particles controlled by surface and IB diffusion processes. We demonstrate that the sintering of particles is accompanied by relative rigid-body rotation of the particles even for fully isotropic surfaces and interphase boundaries.     

Dislocation nucleation from symmetric tilt grain boundaries in body-centered cubic vanadium

We perform molecular dynamics (MD) simulations with two interatomic potentials to study dislocation nucleation from six symmetric tilt grain boundaries (GB) using bicrystal models in body-centered cubic vanadium. The influences of the misorientation angle are explored in the context of activated slip systems, critical resolved shear stress (CRSS), and GB energy. It is found that for four GBs, the activated slip systems are not those with the highest Schmid factor, i.e., the Schmid law breaks down. For all misorientation angles, the bicrystal is associated with a lower CRSS than their single crystalline counterparts. Moreover, the GB energy decreases in compressive loading at the yield point with respect to the undeformed configuration, in contrast to tensile loading.     

Directional emission from whispering-gallery modes in deformed fused-silica microspheres

Highly directional emissions from whispering-gallery modes are demonstrated in deformed nonaxisymmetric fused-silica microspheres. Ray trajectory analysis indicates that ray dynamics in a three-dimensional reso-nator differs qualitatively from that of a two-dimensional system, with Arnol'd diffusion playing an essential role in the three-dimensional system.     

Dislocation arrangement in deformed very dilute Cu-Cd alloy single crystals

The influence of the precipitation annealing on the CRSS and dislocation arrangement of the Cu-0·03 at. % Cd single crystals is observed experimentally. Dislocation distributions are determined by TEM of sections (111) and (101) in stage I and in the transition region. The CRSS increases due to precipitation annealing. There are not observed any second phase particles, but nevertheless there exist some differences in the dislocation arrangement of the solution annealed single crystals and the precipitation annealed ones.     

Dislocation emission by pores in nanocrystalline metals

A theoretical model is proposed to describe the emission of lattice and grain-boundary dislocations from pores in nanocrystalline metals during mechanical loading. In this model, dislocation emission occurs via an ideal nanoshear. A dislocation nucleates at a finite distance from a pore, and the modulus of its Burgers vector increases continuously from zero to the modulus of the Burgers vector of a lattice or grain-boundary dislocation. The applied stress and the critical pore-dislocation distance at which dislocation emission via an ideal nanoshear in nanocrystalline Ni, Al, and Cu becomes an energetically favorable and barrier-free process are determined.     

Dislocation unpinning model of acoustic emission from alkali halide crystals

The present paper reports the dislocation unpinning model of acoustic emission (AE) from alkali halide crystals. Equations are derived for the strain dependence of the transient AE pulse rate, peak value of the AE pulse rate and the total number of AE pulse emitted. It is found that the AE pulse rate should be maximum for a particular strain of the crystals. The peak value of the AE pulse rate should depend on the volume and strain rate of the crystals, and also on the pinning time of dislocations. Since the pinning time of dislocations decreases with increasing strain rate, the AE pulse rate should be weakly dependent on the strain rate of the crystals. The total number of AE should increase linearly with deformation and then it should attain a saturation value for the large deformation. By measuring the strain dependence of the AE pulse rate at a fixed strain rate, the time constantτ _s for surface annihilation of dislocations and the pinning timeτ _p of the dislocations can be determined. A good agreement is found between the theoretical and experimental results related to the AE from alkali halide crystals.     

Dislocation emission in A1 during recrystallization

Summary The dislocation structure evolution during isothermal recrystallization of Al has been followed by means of internal friction, modulus defect and X-ray diffraction analyses accompanying the microscopical observations. Many peaks ofQ ⁻¹ and of modulus defect and X-ray diffraction line width have been found, both in nucleation and grain growth, all referable to dislocations. The first of these peaks, at the beginning of recrystallization, presents recovery characteristics, different from those shown by the subsequent, secondary peaks. Secondary peaks are interpreted in terms of dislocation emission from grain boundaries, whereas the first peak appears connected with subboundaries, walls present in the initial stages of recrystallization. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Particle emission from a hot,deformed, and rotating nucleus

The emission of nucleons from a hot, deformed and rotating nucleus is treated within the Thomas-Fermi approximation. We study in particular the dependence of the transmission coefficient on the deformation and the rotational frequency of the emitting nucleus. A tractable form of the transmission coefficient is given.     

Dislocation emission from an elliptically blunted crack tip with surface effects

The explicit expressions for critical stress intensity factors are derived for edge dislocation emission from an elliptically blunt crack with surface effects under mode I and mode II loadings. The influence of surface effects on dislocation emission criterion is analyzed. The result indicates the impact of the surface stress becomes remarkable for nanoscale blunted cracks and some particular materials, which cannot only affect the value of the critical stress intensity factors for dislocation emission, but also alter the emission angle.     

The effective diffusivity in polycrystalline material in the presence of interphase boundaries

The problem of calculating the long-time-limit effective diffusivity in stable two-phase polycrystalline material is addressed for the first time. We make use of a phenomenological model where the high-diffusivity interphase boundaries are treated as connected ‘coatings’ of the individual grains. The derivation of expressions for the effective diffusivity with segregation is along the lines of the analysis by Maxwell in . Monte Carlo computer simulation using lattice-based random walks on a very fine-grained mesh is employed to test the validity of the expressions. It is shown that, for the specific cases analysed, the derived expressions for the effective diffusivity are in very good agreement with results from the simulations. Since the pattern of behaviour is not entirely clear at present, it is difficult to guide the choice for the best expression in a given case. The equivalent of the Hart equation for this problem is also derived. This equation is shown to be invariably in poor agreement with simulation results.     

Magnetic Barkhausen emission in lightly deformed AISI 1070 steel

The Magnetic Barkhausen Noise (MBN) technique can evaluate both micro- and macro-residual stresses, and provides indication about the relevance of contribution of these different stress components. MBN measurements were performed in AISI 1070 steel sheet samples, where different strains were applied. The Barkhausen emission is also analyzed when two different sheets, deformed and non-deformed, are evaluated together. This study is useful to understand the effect of a deformed region near the surface on MBN. The low permeability of the deformed region affects MBN, and if the deformed region is below the surface the magnetic Barkhausen signal increases.     

Computer simulation of the matrix-inclusion interphase in bulk metallic glass based nanocomposites

Atomistic models for matrix-inclusion systems are generated. Analyses of the systems show that interphase layers of finite thickness appear interlinking the surface of the nanocrystalline inclusion and the embedding amorphous matrix. In a first approximation, the interphase is characterized as an amorphous structure with a density slightly reduced compared to that of the matrix. This result holds for both monatomic hard sphere systems and a Cu(47.5)Zr(47.5)Al(5) alloy simulated by molecular dynamics (MD). The elastic shear and bulk modulus of the interphase are calculated by simulated deformation of the MD systems. Both moduli diminish with decreasing density but the shear modulus is more sensitive against density reduction by one order of magnitude. This result explains recent observations of shear band initiation at the amorphous-crystalline interface during plastic deformation.     

Dislocation structure and transport properties of low-angle tilt boundaries in high-temperature superconductors

A theoretical model that describes split grain-boundary dislocations in low-angle tilt boundaries of high-temperature superconductors is suggested. It is shown that the dissociation of dislocations in low-angle tilt boundaries is usually accompanied by a decrease in their elastic energy and causes an increase in the critical current density across the boundaries in high-temperature superconductors.     

Dislocation emission from a crack under mixed mode loading studied by molecular statics

The dislocation emission surface in (k _I,?k _II,?k _III) space is calculated by means of atomistic simulations for the {111}?110? crack in Al. For each relevant combination of loading mode, the precise nature of the dislocations and of the emission process are determined. When appropriate, the analytic formulas proposed by Rice are used by calculating the unstable stacking energy including the effect of the mixed mode loading. Quantitative agreement with the full atomistic calculation is found in the case where dislocations glide in the crack plane. This clearly identifies when and how ab initio data can be introduced in the calculation.     

The effect of interphase adhesion on nanofiller structure in polymer/organoclay nanocomposites

The role of interphase adhesion in the organoclay plate surface-polymer matrix system is shown to be crucial for the degree of dispersion of silicate plates. Quantitative methods for assessing the major structural characteristics of organoclay are discussed. The proposed interpretation of an organoclay packet (tactoid) agrees well with the effective particle model that was elaborated earlier.     

Structure and properties of interphase boundaries of gallium arsenide-metal (dielectric)

To study the nature and properties of potential barriers in gallium arsenide devices, we have investigated structural phase transitions in GaAs contacts with multilayer films containing refractory transition-metal borides (TiB₂, LaB₆). We verified the important role in degrading Schottky barrier device performance played by local mechanical stresses introduced at the interface by lateral nonuniformities in interphase interactions. We examine the electrical properties of MIS gallium arsenide devices, taking into account the high density of electronic surface states (ESS). We show it is possible to control the density of ESS by selecting the dielectric, and we discuss its deposition and annealing with a pulsed laser. We discuss the nature of potential barriers in gallium arsenide devices, drawing upon our data and previously published data and modern theoretical models.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, No. 10, pp. 52–62, October, 1993.