Large-scale flow as an indicator of superplasticity

A model is proposed for superplastic deformation of materials, based on the concept of cooperative grain-boundary slip. The conditions for superplastic deformation are obtained as conditions for coherent shear bands. Analysis of the temperature dependence of the limits of the stress interval for superplastic flow is used as a basis for the introduction of two types of threshold stress that elucidate the cause of the ambiguity in the interpretation of exisiting experimental results. Fiz. Tverd. Tela (St. Petersburg) 39, 2179–2185 (December 1997)     

Broadband dielectric spectroscopy of inhomogeneous and composite weak conductors

ABSTRACT

In this paper, we discuss broadband dielectric spectroscopy from mHz up to the infrared range mainly for materials with inhomogeneous weak conductivity, including conductor-dielectric nanocomposites. Our discussion is based on the effective medium approximation (EMA) and experiments modeled by this approach are reviewed. We discuss core–shell composites modeled by coated-spheres (Hashin–Shtrikman model) and normal composites with a possible percolation of the conductor component resulting in sharp or smeared percolation threshold of the DC conductivity and diverging static permittivity in the former case. The sharp percolation threshold is modeled by the Bruggeman EMA or by general EMA with arbitrary percolation threshold and arbitrary critical exponents of the DC conductivity and static permittivity. For the case of smeared percolation threshold in the case of complex topologies, we use the Lichtenecker model allowing for partial percolation of both the components. Finally, numerous papers reporting negative permittivity in weakly conducting materials are criticized and concluded to be due to spurious effects.     

Shear stress at the interface of coaxial composites determined by the resonance of low-amplitude vibrations

The aim of this work is to determine the viscoelastic behaviour of the interface in a coaxial composite material made of a tough shield and a ductile core. The elastic modulus and the amplitude-independent internal friction are measured using a longitudinal oscillating resonant system at 50 kHz. The contribution of the interface is modelled as a shear stress that modifies the elastic behaviour of the constituents. The value of this shear stress is determined for different interfaces (epoxy resin-brass, epoxy resin-Pyrex and paraffin-Pyrex). The model is autovalidated by the excellent agreement between the calculated and experimental values of the internal friction (damping) of the composites.     

Single hole dynamics in the one-dimensional - model

We present a new finite-temperature quantum Monte Carlo algorithm to compute imaginary-time Green functions for a single hole in the t-J model on non-frustrated lattices. Spectral functions are obtained with the Maximum Entropy method. Simulations of the one-dimensional case show that a simple charge-spin separation Ansatz is able to describe the overall features of the spectral function such as the bandwidth and the compact support of the spectral function, over the whole energy range for values of J / t from 1/3 to 4. This is contrasted with the two-dimensional case. The quasiparticle weight Z_k is computed on lattices up to L =128 sites in one dimension, and scales as . Received 15 February 2000     

Linking high- and low-temperature plasticity in bulk metallic glasses: thermal activation,extreme value statistics and kinetic freezing

Abstract

At temperatures well below their glass transition, the deformation properties of bulk metallic glasses are characterized by a sharp transition from elasticity to plasticity, a reproducible yield stress and an approximately linear decrease of this stress with increasing temperature. In the present work, it is shown that when the well-known properties of the undercooled liquid regime, in terms of the underlying potential energy landscape, are assumed to be also valid at low temperature, a thermal activation model is able to reproduce the observed onset of macroscopic yield. At these temperatures, the thermal accessibility of the complex potential energy landscape is drastically reduced, and the statistics of extreme value and the phenomenon of kinetic freezing become important, affecting the spatial heterogeneity of the irreversible structural transitions mediating the elastic-to-plastic transition. As the temperature increases and approaches the glass transition temperature, the theory is able to smoothly transit to the high-temperature deformation regime where plasticity is known to be well described by thermally activated viscoplastic models.     

Formation of ripples over a sand bed submitted to a turbulent shear flow

We investigate the process of ripple formation when a sand bed is submitted to a steady and turbulent liquid flow. The sand transport dynamics is described in terms of a simple relaxation law which accounts for the fact that the transport rate does not adapt instantaneously to its equilibrium value. The equilibrium sand flux is evaluated using a standard law based on the estimation of the flow shear stress calculated at the sand bed surface. The latter is estimated from an analytical resolution of the flow over a deformed sand bed which is based on the Jackson and Hunt calculation [J.C.R. Hunt, Quart. J. R. Met. Soc. 101, 929 (1975)]. Within this model, we investigate the stability of the sand bed and are able to derive analytical scaling laws for the wavelength and phase velocity of the most dangerous mode. In the deep flow limit, the model predicts the occurrence of a single mode of instability corresponding to the formation of ripples. Predictions of our model are compared with previous models and available experimental data.     

Thin film adhesion modification by MeV ion beams: A model based on ion track concepts

Abstract

A semi-empirical model, based on concepts used to explain the process of track formation in insulators, has been applied to the experimental observation of improved film adhesion produced by MeV ion irradiation. Good agreement is obtained with experimental data for the system of gold film on native oxides of tantalum. The model uses a single free parameter, e ₀, which represents the threshold energy density for the process responsible for the improved bonding. The significance of this parameter is discussed in terms of possible ionisation induced interface phenomena.     

Long-range intensity correlation and the approach to localization

Abstract

The influence of internal surface reflectivity is incorporated into a heuristic model of long-range intensity correlation. This allows us to identify the spatial intensity correlation function in a sample of randomly positioned polystyrene spheres as the sum of a first-order term in the correlation parameter κ, which falls linearly with detector separation, and a constant term, which is of order κ². κ is a measure of the proximity to the localization threshold. In a sample which is a mixture of metallic and dielectric spheres and in which κ becomes large, the probability of large intensity fluctuations increases. The distribution of intensities is then found to be stretched exponential even for sample lengths which are comparable to the wavelength of the radiation.     

The mechanism of light induced reversible structural changes in amorphous antimony selenide films

Abstract

Photo– and thermoinduced phase transitions amorphous (a)? crystalline (c) in Sb_XSe_1-X (0.4≤x≤0.7) films are studied. For a→ c transition a new crystalline phase is found. The photostructural transitions by pulsed laser excitation are of thermal origin. The measured kinetics and threshold intensities are consistent with the proposed photothermal process model.     

A Theory of Non-Linear Elasticity Compatible With the Murnaghan Equation of State

We present an equation of state for a cubic non-linear elastic material in a general state of finite strain. For hydrostatic pressure, the predictions closely follow Murnaghan's well-known equation of state. At 170 kbar, our model differs from Murnaghan's equation by only 1.3%, which contrasts with the currently accepted non-linear elasticity theory that differs by 10% at this pressure. The theory is based on expressing the variation of the elastic constants as a linear function of stress rather than strain. We define a different set of third-order elastic constants, which involve a derivative with respect to stress, and relate these to the conventional third-order elastic constants. We apply the model to GaAs under hydrostatic pressure and we compare the predictions of the conventional non-linear theory with those of the model we present.     

The measurement problem resolved and local realism preserved via a collapse-free photon detection model

A new realislic local model of light propagation and detection is described. The authors propose a novel stochastic model of low-intensity photon detection in which background noise is added to a part of the photon prior to absorption. In this model, in agreement with Planck, there is no quantization of the propagating field. The model has some similarities to theories advanced by E. Santos and T. Marshall in the last decade, but also has substantial deviations from these. A mechanism, conserving energy and momentum, is proposed by which a sudden collapse of the wave-packet is avoided. The experimental Bell inequality violation of Aspect. Grangier and Roger [Phys. Rev. Lett.47, 460 (1981)]is discussed. The authors have carried out a computer simulation of a radio frequency (RF) analogue of the Einstein-Podolsky-Rosen thought experiment to illustrate how the manipulation of certain factors, especially signal to noise ratio, detector threshold and characteristics of the noise, enables the same Bell inequality to be either satisfied or violated by a realistic local model. Building on arguments by Santos. [Phys. Rev. A46. 3646 (1992)],the appropriateness of this Bell lest is discussed. Neither the authors' stochastic-optical model, nor their RF analogue, involves an enhancement assumption of the type defined by Clauser and Horne [Phys. Rev. D10, 526 (1974)].     

Roughening of a Si(100) surface induced by the adsorption of oxygen near the solid-oxide nucleation threshold

An investigation of the processes on a Si(100) surface interacting with oxygen near the solid-oxide nucleation threshold using x-ray photoelectron spectroscopy and atomic-force microscopy is described. The nucleation threshold is the boundary between the temperature and oxygen pressure regions where a phase transition with the formation of a submonolayer oxide and a roughening transition caused by oxygen adsorption occur. Near the nucleation threshold, either a random rough relief or a quasiperiodic structure is formed on a surface coated with chemisorbed oxygen. The formation of the rough relief due to oxygen adsorption has been interpreted within the theory of phase transitions as a result of vacancy clustering. A model that allows one to describe the dynamics of processes on the surface near the nucleation threshold in qualitative and in some cases in quantitative terms has been suggested. Zh. éksp. Teor. Fiz. 114, 239–262 (July 1998)     

Inflection point of the fluorescence excitation spectra induced by secondary inner filter effect

Abstract

The secondary inner filter effect on the fluorescence excitation spectra of rhodamine 6G aqueous solutions with concentration was demonstrated in this paper. The peak of fluorescence excitation spectrum stays at 525?nm at low concentrations, while it breaks up and turns into valley at high concentrations. The threshold concentration was determined to be 3.16?mg/L by the second derivative spectroscopy. A mathematical model was proposed to explain the inflection point of fluorescence excitation spectrum caused by the secondary inner filter effect. On the basis of it, the threshold concentration was calculated to be 2.86?mg/L, approaching to the experimental result.     

A static model for completely polarized ceramics

A model for completely polarized ceramics kept under a longitudinal compression stress σ has been advanced. The model is built upon the condition of the complete and partial stability of the domain polar c axes with due regard to the nonuniform distribution of the axes after 90° and 180° domain switches. By using this model, the ceramics piezoelectric coefficient d ₃₃, its dependence on the longitudinal compression stress σ, and the relative numbers of 90° and 180° domain switches induced by the action of the stress σ were determined.     

Propagation of waves in nonlocal porous multi-phase nanocrystalline nanobeams under longitudinal magnetic field

ABSTRACT

This article investigates wave propagation behavior of a multi-phase nanocrystalline nanobeam subjected to a longitudinal magnetic field in the framework of nonlocal couple stress and surface elasticity theories. In this model, the essential measures to describe the real material structure of nanocrystalline nanobeams and the size effects were incorporated. This non-classical nanobeam model contains couple stress effect to capture grains micro-rotations. Moreover, the nonlocal elasticity theory is employed to study the nonlocal and long-range interactions between the particles. The present model can degenerate into the classical model if the nonlocal parameter, couple stress and surface effects are omitted. Hamilton’s principle is employed to derive the governing equations which are solved by applying an analytical method. The frequencies are compared with those of nonlocal and couple stress-based beams. It is showed that wave frequencies and phase velocities of a nanocrystalline nanobeam depend on the grain size, grain rotations, porosities, interface, magnetic field, surface effect and nonlocality.     

Fiber Bundle Model Under Heterogeneous Loading

The present work deals with the behavior of fiber bundle model under heterogeneous loading condition. The model is explored both in the mean-field limit as well as with local stress concentration. In the mean field limit, the failure abruptness decreases with increasing order k of heterogeneous loading. In this limit, a brittle to quasi-brittle transition is observed at a particular strength of disorder which changes with k. On the other hand, the model is hardly affected by such heterogeneity in the limit where local stress concentration plays a crucial role. The continuous limit of the heterogeneous loading is also studied and discussed in this paper. Some of the important results related to fiber bundle model are reviewed and their responses to our new scheme of heterogeneous loading are studied in details. Our findings are universal with respect to the nature of the threshold distribution adopted to assign strength to an individual fiber.     

Recent advances in understanding the origin of martensite aging phenomena in shape memory alloys

The martensite aging phenomena (martensite stabilization and rubber-like behavior) found in many shape memory alloys have puzzled material scientists for over 60 years without a definite answer. In this article we critically reviewed previous models to understand the aging phenomena, which include pseudotwin-type model, LRO model, SRO model, twinning dislocation model, and domain (twin) boundary pinning model. We showed that these models failed to meet generality criterion, although being able to explain the phenomena to some extent. Then we focused on a very recent general model (Ren and Otsuka, Nature, 389, 579–582, 1997) which makes use of only two common features of martensitic transformation and aging, i.e., diffusionless symmetry change during martensitic transformation and diffusion during aging. This model appears to be able to explain all of the available experimental observations on the aging phenomena. In view of recent development in this field, we have reason to believe that we are approaching the final solution to the aging problem.     

Creep in particle strengthened metals

Summary The idea that the dislocations moving in a dispersion strengthened matrix meet the back stress which effectively reduces the applied stress was accepted. Assuming that the back stress does not depend on applied stress a new interpretation of the published creep data for aluminum strengthened by various volume fractions of alumina particles [4] is presented. From this assumption it follows that the back stress can be interpreted as the threshold stress; at the applied stresses lower than the threshold stress the creep controlled by the lattice dislocations overcoming dispersed particles does not take place. The threshold stress has been estimated for the temperature interval 473 to 773 K using eq. (13) and assuming the validity of the additivity rule expressed by eq. (11). In this temperature interval the creep is controlled by the diffusion in the matrix. The threshold stress increases with the volume fraction of Al₂O₃ particles. An estimate of the Orowan stress has shown that independently of the volume fraction of Al₂O₃ it is approximately twice as high as the threshold stress.Generally, the concept of the back stress solves the known difficulties of interpretation of the steady state creep data for dispersion strengthened metallic materials at least if the assumption of the back stress independency on applied stress is acceptable.     

Effects of Uniaxial Stress on X-ray Diffraction Spectra

Uniaxial stress is a typical drawback of solid pressure transmitting medium, at low temperature, which affects experimental spectra by shifting and broadening the diffraction peaks. Corrections to this effects have been proposed in the past only for the systematic shifts. We presented a simple model based on elasticity theory which rationalizes both peak shifts and broadening observed in X-ray diffraction. Our results are compared to the popular Singh model.     

Strength statistics and the distribution of earthquake interevent times

The Weibull distribution is often used to model the earthquake interevent times distribution (ITD). We propose a link between the earthquake ITD on single faults with the Earth’s crustal shear strength distribution by means of a phenomenological stick–slip model. For single faults or fault systems with homogeneous strength statistics and power-law stress accumulation we obtain the Weibull ITD. We prove that the moduli of the interevent times and crustal shear strength are linearly related, while the time scale is an algebraic function of the scale of crustal shear strength. We also show that logarithmic stress accumulation leads to the log-Weibull ITD. We investigate deviations of the ITD tails from the Weibull model due to sampling bias, magnitude cutoff thresholds, and non-homogeneous strength parameters. Assuming the Gutenberg–Richter law and independence of the Weibull modulus on the magnitude threshold, we deduce that the interevent time scale drops exponentially with the magnitude threshold. We demonstrate that a microearthquake sequence from the island of Crete and a seismic sequence from Southern California conform reasonably well to the Weibull model.