Prevalence Of Shear Banding In Compression Of Zr41Ti14Cu12.5Ni10Be22.5 Pillars As Small As 150 Nm In Diameter

Recently, the size dependence of mechanical behaviors, particularly the yield strength and plastic deformation mode, of bulk metallic glasses (BMG) has created a great deal of interest. Contradicting conclusions have been drawn by different research groups, based on various experiments on different BMG systems. Based on in situ compression transmission electron microscopy (TEM) experiments on Zr41Ti14Cu12.5Ni10Be22.5 (Vit 1) nanopillars, this paper provides strong evidence that shear banding still prevails at specimen length scales as small as 150 nm in diameter. This is supported by in situ and ex situ images of shear bands, and by the carefully recorded displacement bursts under load control its well as load drops under displacement control. Finite element modeling of the stress state within the pillar shows that the unavoidable geometry constraints accompanying such experiments impart a strong effect on the experimental results, including non-uniform stress distributions and high level hydrostatic pressures. The seemingly improved compressive ductility is believed to be due to such geometry constraints. Observations underscore the notion that the mechanical behavior of metallic glasses, including strength and plastic deformation mode, is size independent at least in Vit 1. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Strong Strain Hardening in Nanocrystalline Nickel

Low strain hardening has hitherto been considered an intrinsic behavior for most nanocrystalline (NC) metals, due to their perceived inability to accumulate dislocations. In this Letter, we show strong strain hardening in NC nickel with a grain size of 20 nm under large plastic strains. Contrary to common belief, we have observed significant dislocation accumulation in the grain interior. This is enabled primarily by Lomer-Cottrell locks, which pin the lock-forming dislocations and obstruct islocation. motion. These observations may help with developing strong and ductile NC metals and alloys.

A Wide Band Strong Acoustic Absorption in a Locally Network Anechoic Coating

Composite materials with interpenetrating network structures usually exhibit unexpected merit due to the cooperative interaction. Locally resonant phononic crystals (LRPC) exhibit excellent sound attenuation performance based on a periodical arrangement of sound wave scatters. Inspired by the interpenetrating network structure and the LRPC concept, we develop a locally network anechoic coating (LNAC) that can achieve a wide band of underwater strong acoustic absorption. The experimental results show that the LNAC possesses an excellent underwater acoustic absorbing capacity in a wide frequency range. Moreover, in order to investigate the impact of the interpenetrating network structure, we fabricate a faultage structure sample and the network is disconnected by hard polyurethane (PU). The experimental comparison between the LNAC and the faultage structure sample shows that the interpenetrating network structure of the LNAC plays an important role in achieving a wide band strong acoustic absorption.

Effect of coal-over-coal reburn on furnace temperature and heat flux distributions in 1 MW tangentially fired furnace

The furnace temperature and heat flux distributions of 1 MW tangentially fired furnace were studied during coal-over-coal reburn, and the influences of the position of reburn nozzle and reburn fuel fraction on furnace temperature and heat flux distributions were investigated. Compared with the baseline, the flue gas temperature is 70–90 C lower in primary combustion and 130–150 C higher at furnace exit, and the variations of the flue gas temperature distributions along furnace height are slower. The temperature distribution along the width of furnace wall decreases with the increase of the relative furnace height. In the primary combustion zone and the reburn zone, the temperature and heat flux distributions of furnace wall are much non-uniform and asymmetric along the width of furnace wall, those of furnace wall in the burnout zone are relatively uniform, and the temperature non-uniformity coefficients of the primary combustion zone, the reburn zone and the burnout zone are 0.290, 0.100 and 0.031, respectively.

The calculation of photoelectron angular distributions with jet-like structure from scattering theory

Photoelectron angular distributions produced in above-threshold ionization (ATI) are analysed using a nonperturbative scattering theory. The numerical results are in good qualitative agreement with recent measurements. Our study shows that the origin of the jet-like structure arises from the inherent properties of the ATI process and not from the angular momentum of either the initial or the excited states of the atom.

A scaling law of photoelectron angular distributions in one-cycle laser pulses

The photoelectron angular distributions (PADs) from above-threshold ionization of atoms irradiated by one-cycle laser pulses satisfy a scaling law. The scaling law denotes that the main features of the PADs are determined by four dimensionless parameters: (1) the ponderomotive number u(p) = U-p/hw, the ponderomotive energy U-p in units of laser photon energy; (2) the binding number E-b = E-b/h(w), the atomic binding energy E-b in units of laser photon energy; (3) the number of absorbed photons q; (4) the carrier-envelope phase phi(0), the phase of the carrier wave with respect to the envelope. We verify the scaling law by theoretical analysis and numerical calculation, compared to that in long-pulse case. A possible experimental test to verify the scaling law is suggested.

The intensity distributions of collected signals in coherent anti-Stokes Raman scattering microscopy

Coherent anti-Stokes Raman scattering (CARS) microscopy with the combining of confocal and CARS techniques is a remarkable alternative for imaging chemical or biological specimens that neither fluoresce nor tolerate labeling. The CARS is a nonlinear optical process, the imaging properties of CARS microscopy will be very different from the conventional confocal microscopy. In this paper, we calculated the propagation of CARS signals by using the wave equation in medium and the slowly varying envelope approximation (SVEA), and find that the intensity angular distributions vary considerably with the different experimental configurations and the different specimen shapes. So the conventional description of microscopy (e.g.. the point spread function) will fail to descript the imaging properties of CARS microscopy. (c) 2004 Elsevier B.V. All rights reserved.

Ionization suppression of diatomic molecules in strong laser fields

The ionization rate of molecules in intense laser fields may be much lower than that of atoms with similar binding energy. This phenomenon is termed the ionization suppression of molecules and is caused by the molecular inner structure. In this paper, we perform a comprehensive study of the ionization suppression of homonuclear diatomic molecules in intense laser fields of linear and circular polarizations. We find that for linear polarization the total ionization rate and the ionization suppression depend greatly on the molecular alignment, and that for circular polarization the ionization suppression of molecules in the antibonding (bonding) shells disappears (appears) for laser intensities around 10(15) W/cm(2). We also find that the molecular photoelectron energy spectra are greatly changed by the interference effect, even though the total ionization rate of molecules remains almost the same as that of their companion atoms.

Quantum interference in laser-induced nonsequential double ionization in diatomic molecules: Role of alignment and orbital symmetry

We address the influence of the orbital symmetry and the molecular alignment with respect to the laser-field polarization on laser-induced nonsequential double ionization of diatomic molecules, in the length and velocity gauges. We work within the strong-field approximation and assume that the second electron is dislodged by electron-impact ionization, and also consider the classical limit of this model. We show that the electron-momentum distributions exhibit interference maxima and minima due to electron emission at spatially separated centers. The interference patterns survive integration over the transverse momenta for a small range of alignment angles, and are sharpest for parallel-aligned molecules. Due to the contributions of the transverse-momentum components, these patterns become less defined as the alignment angle increases, until they disappear for perpendicular alignment. This behavior influences the shapes and the peaks of the electron-momentum distributions.

Photoelectron angular distributions of molecules in bichromatic laser fields of circular polarization

Photoelectron angular distributions (PADs) from above-threshold ionization of O-2 and N-2 molecules irradiated by a bichromatic laser field of circular polarization are Studied. The bichromatic laser field is specially modulated such that it can be used to mimic a sequence of one-cycle laser pulses. The PADs are greatly affected by the molecular alignment, the symmetry of the initial electronic distribution, and the carrier-envelope phase of the laser pulses. Generally, the PADs do not show any symmetry, and become symmetric about an axis only when the symmetric axis of laser field coincides with the symmetric axis of molecules. This study shows that the few-cycle laser pulses call be used to steer the photoelectrons and perform the selective ionization of molecules. (C) 2008 Elsevier B.V. All rights reserved.

Spectral distributions of harmonic generation from electron oscillation driven by intense femtosecond laser pulses

The characteristics of harmonic radiation due to electron oscillation driven by an intense femtosecond laser pulse are analyzed considering a single electron model. An interesting modulated structure of the spectrum is observed and analyzed for different polarization. Higher order harmonic radiations are possible for a sufficiently intense driving laser pulse. We have shown that for a realistic pulsed photon beam, the spectrum of the radiation is red shifted as well as broadened because of changes in the longitudinal velocity of the electrons during the laser pulse. These effects are more pronounced at higher laser intensities giving rise to higher order harmonics that eventually leads to a continuous spectrum. Numerical simulations have further shown that by increasing the laser pulse width broadening of the high harmonic radiations can be limited. (C) 2005 Elsevier B.V. All rights reserved.

Vacuum heating in the interaction of ultrashort, relativistically strong laser pulses with solid targets

An analytical fluid model for vacuum heating during the oblique incidence by an ultrashort ultraintense p-polarized laser on a solid-density plasma is proposed. The steepening of an originally smooth electron density profile as the electrons are pushed inward by the laser is included self-consistently. It is shown that the electrons being pulled out and then returned to the plasma at the interface layer by the wave field can lead to a phenomenon like wave breaking since the front part of the returning electrons always move slower than the trailing part. This can lead to heating of the plasma at the expense of the wave energy. An estimate for the efficiency of laser energy absorption by the vacuum heating is given. It is also found that for the incident laser intensity parameter, a(L)> 0.5, the absorption rate peaks at an incident angle 45 degrees-52 degrees and it reaches a maximum of 30% at a(L)approximate to 1.5.

Anomaly in photoelectron angular distributions

The recently observed anomaly in photoelectron angular distributions (PADs), the disappearance of the main lobes of PADs which should be usually in the direction of laser polarization, is reinterpreted as a minimum of generalized Bessel functions in the laser-polarization direction with the theory of nonperturbative quantum electrodynamics. The reinterpretation has no artificial fitting parameters and explains more features of the experimentally observed PADs, in contrast to the existing interpretation in which the anomaly is interpreted as a quantum interference of angular momentum partial waves. Some hierarchy anomalies are predicted for further experimental observations.

Generation of strong quasistatic magnetic fields in interactions of ultraintense and short laser pulses with overdense plasma targets

An analytical fluid model is proposed for the generation of strong quasistatic magnetic fields during normal incidence of a short ultraintense Gaussian laser pulse with a finite spot size on an overdense plasma. The steepening of the electron density profile in the originally homogeneous overdense plasma and the formation of electron cavitation as the electrons are pushed inward by the laser are included self-consistently. It is shown that the appearance of the cavitation plays an important role in the generation of quasistatic magnetic fields: the strong plasma inhomogeneities caused by the formation of the electron cavitation lead to the generation of a strong axial quasistatic magnetic field B-z. In the overdense regime, the generated quasistatic magnetic field increases with increasing laser intensity, while it decreases with increasing plasma density. It is also found that, in a moderately overdense plasma, highly intense laser pulses can generate magnetic fields similar to 100 MG and greater due to the transverse linear mode conversion process.