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The mechanical response of modern alloys results from a complex interplay between existing microstructure and its evolution with time under stress. To unravel these processes, in situ approaches intrinsically have a critical advantage to explore the basic mechanisms involving dislocations, grain boundaries (GBs), and their interactions in real time. In this article, we discuss recent findings using in situ nanomechanical testing techniques and refined crystallographic analysis tools. Advancements in in situ nanomechanics not only include multiaxial loading conditions, which bring us closer to real-world applications, but also high strain-rate testing, which is critical to compare experiments and simulations. In particular, unraveling the details of GB-based mechanisms and related microstructural changes will facilitate significant breakthroughs in our understanding of the behavior of materials on macroscopic length scales.
In situ transmission electron microscopy (TEM) nanomechanical
testing has benefited from a number of recent technical developments related to
both how deformation is imaged and how deformation is induced and measured
inside a TEM instrument. These developments have led to new insights into the
deformation mechanisms of a wide range of metals and alloys, as well as
measurements of the unusual mechanical properties of small-scale objects such as
whiskers and nanocrystals. Herein, we describe this recent progress through
selected highlights of recent findings on the dynamic behavior of defects such
as dislocations, twins, and grain boundaries.
Magnetophosphenes are described as flickering lights appearing in the visual field, due
to retinal exposure to time-varying magnetic fields (MF). Human magnetophosphene
perception (MP) serves as a scientific basis for international guidelines intending to
limit exposure to electromagnetic fields in the extremely low frequency range. However,
the flux density threshold at which MP occurs, as well as the dose and frequency responses
of the phenomenon, are not clearly experimentally established. The 50–60 Hz threshold is
extrapolated from data in the lower frequency range. The objective of this paper is to
provide a descriptive anecdotal report of MP from 8 individuals exposed to 50 mT MF at 20,
50 and 60 Hz. They describe variations of flickering light perceptions in the visual
field, matching the description by D’Arsonval (1896). This preliminary testing introduces a new experimental protocol, which will
test the threshold for MP and other associated neurophysiological responses in humans.
In situ TEM straining experiments have been performed on a Ti3Al single crystal, along the c-axis, in order to study the slip of 2c+a dislocations in pyramidal planes. The results show that slip takes place in π1 planes, in contrast with what has been observed after compression tests (slip in π2 planes), and that rows of loops are nucleated in the slip plane. The mechanisms which may control slip in the π1 planes are briefly discussed.
Tension compression fatigue tests and subsequent TEM observations were conducted on single crystalline silicon in a temperature and strain rate domain where lattice friction is still effective: 800-900°C and 1.5 to 6x10-4s-1. Samples oriented for single slip conditions were cyclically loaded under plastic strain amplitude control. For amplitudes ranging from 6x10-4 to 10-2, cyclic stress-strain curves exhibit two different stages of hardening and pass through a maximum before saturation is reached. TEM observations suggest that strain localization takes place near the maximum cyclic stress and beyond. Before mechanical saturation, edge dislocation dipoles sit mainly in thick rectilinear walls. Once the maximum stress is reached, these thick walls “condense” in much thinner walls that seem to carry out the imposed deformation while other regions become inactive. In this case, the dislocation structure anneals out and a loop structure is created from the dipolar walls.
Plastic deformation due to thermal stresses has been investigated for different metallic films deposited on Si or α-alumina substrates. We conducted post-mortem TEM and SEM investigations of samples that underwent thermal cycles in order to capture the microstructural changes imposed by thermal stresses. The ultimate goal is to determine the dominant plasticity mechanisms responsible for such changes. In-situ thermal cycles performed inside the TEM allowed direct and real-time observations of dislocation behaviour under stress. It is shown that dislocation density drops in Al/Si, Au/Si and in Cu/α-alumina thin film systems. Except in the case of pseudo-epitaxial Cu on sapphire, the interaction of dislocations with the interfaces (passivation, oxide, adhesion layer) is attractive and leads to the disappearance of interfacial dislocations. In this light, the generalized observation of high tensile stresses that arise in metallic films at the end of cooling is explained in terms of insufficient dislocation sources instead of classic strain hardening. Diffusional processes can substitute for a lack of dislocation, but the low relaxation strain rate that would be excpected should lead to high stresses during the cooling stages of thermal cycles.
A novel micro testing machine has been used to perform tensile tests on nanocrystalline Al/Zr microsamples with grain sizes ranging from 10 to 250 nm. The problems associated with testing such small specimens (200μm × 200μm in the gage section) were overcome by using a contact-free interferometrie strain gage (ISDG) and alignment and low friction loading were assured by use of a linear air bearing. The postulated relationship between yield stress and hardness was investigated and will be discussed. The effect of the microstructure and the grain size of the compacts on their mechanical behaviour are also analysed.
The vast majority of micro electro-mechanical systems fabricated today depend on polycrystalline silicon thin films for structural support. Studies involving the mechanical performance of these thin films have progressed to the point where the elastic properties and tensile strength of the films can routinely be measured using a specially designed microsample tensile testing machine. However, a fundamental understanding to predict the mechanical behavior of the polycrystalline silicon films requires that these experimental measurements be complemented with detailed observations of the underlying thin film microstructure. This paper describes some of the plan view and cross-section transmission electron microscope observations that have been performed on different deposition runs of double layer polycrystalline films obtained from the Microelectronics Center of North Carolina. The emphasis has been placed on determining the flatness and dimensions of the polycrystalline films, grain morphology and distribution, texture, and dislocation substructure and microtwinning in the undeformed films.
We have performed investigations of resonance effects inside a gallium nitride one-dimensional photonic crystal slab in order to enhance the second-harmonic generated from an beam incident on the surface of the slab. Convenient conditions on the incident beam propagation direction and polarization are first identified by experimental or theoretical linear optical studies. Giant enhancements in the second-harmonic conversion have been obtained by comparison with the unpatterned GaN layer. The combined role of the resonant coupling of the fundamental field and of the second-harmonic field has been observed by rotating the polarization of the fundamental beam.
This article is devoted to recent progress in the area of in situ electron microscopy (scanning and transmission) and will focus on quantitative aspects of these techniques as applied to the deformation of materials. Selected recent experiments are chosen to illustrate how these techniques have benefited from improvements ranging from sample preparation to digital image acquisition. Known for its ability to capture the underlying phenomena of plastic deformation as they occur, in situ electron microscopy has evolved to a level where fully instrumented micro- and nanomechanical tests can be performed simultaneously.
The primary outcome was staff vaccination status. Independent variables included staff demographic and employment characteristics, health status, attitudes and beliefs about the vaccine, and implications for its use.
The staff vaccination rate was 51%. Leading motivators of vaccine receipt were self-protection (77%) and patient protection (49%). The most common reasons for nonreceipt were concerns about side effects (49%), preventive quality (20%), and inconvenience (14%). Logistic regression results suggested that age of 50 years or older (OR, 1.47; P = .021), male gender (OR, 2.50; P < .001), strong belief in vaccine effectiveness (OR, 19.03; P = .008), and importance of HCW vaccination (OR, 20.50; P = .005) significantly increased the probability of vaccination. Recommending the vaccine to coworkers, patients, or patients' families was also associated with HCW vaccination (OR, 3.20; P < .001). Providers who did not believe the vaccine was protective (P < .001) or effective P < .001) were less likely to recommend it to patients.
Strategies to increase vaccination rates among HCWs should address concerns about side effects, effectiveness, and protective value of the vaccine and access to it. The impact of provider recommendations should be stressed. Vaccination and subsequent prevention of illness may limit morbidity and mortality, thus benefiting HCWs, healthcare facilities, and patients.
We examine the form of the free surface flows resulting from the collision of equal jets at an oblique angle. Glycerol-water solutions with viscosities of 15–50 cS were pumped at flow rates of 10–40 cc/s through circular outlets with diameter 2 mm. Characteristic flow speeds are 1–3 m/s. Figures 2–4 were obtained through strobe illumination at frequencies in the range 2.5–10 kHz.
At low flow rates, the resulting stream takes the form of a steady fluid chain, a succession of mutually orthogonal fluid links, each comprised of a thin oval sheet bound by relatively thick fluid rims (Fig. 1). The influence of viscosity serves to decrease the size of successive links, and the chain ultimately coalesces into a cylindrical stream.
As the flow rate is increased, waves are excited on the sheet, and the fluid rims become unstable (Figs. 2 and 3). Droplets form from the sheet rims but remain attached to the fluid sheet by tendrils of fluid that thin and eventually break. The resulting flow takes the form of fluid fishbones, with the fluid sheet being the fish head and the tendrils its bones. Increasing the flow rate serves to broaden the fishbones.
In the wake of the fluid fish, a regular array of drops obtains, the number and spacing of which is determined by the pinch–off of the fishbones (Fig. 4). At the highest flow rates examined, the flow is reminiscent of that arising in acoustically excited fan-spray nozzles.
An oscillatory instability mechanism is identified for a horizontal liquid layer with
undeformable open surface heated from the air side. Although buoyancy and surface
tension gradients are expected to play a stabilizing role in this situation, we show that,
acting together, they may lead to the instability of the motionless state of the system.
The instability is a consequence of the coupling between internal and surface waves,
whose resonant interaction and resulting mode mixing are discussed. Predictions
amenable to experimental test are given together with a thorough analytical and
numerical study of the problem.
In situ transmission electron microscopy (TEM) was performed to study dislocation motion during temperature cycles in aluminum films passivated with a SiO2 layer. The films were cycled from room temperature to 450 °C. Wedge-haped cross-sectional TEM samples were used to retain the constraint of the Si substrate. Besides interactions between dislocations and interfaces, the movement of threading dislocations within the constrained aluminum film was observed. This observation provides an experimental corroboration of the occurrence of threading dislocation motion, which is the basis for rationalizing the high-ield strength of thin films in available models of thin-film plasticity.
This paper presents the first experimental results on Marangoni–Bénard instability
in a symmetrical three-layer system. A pure thermocapillary phenomenon has been
observed by performing the experiment in a microgravity environment where buoyancy
forces can be neglected. This configuration enables the hydrodynamic stability
of two identical liquid–liquid interfaces subjected to a normal gradient of temperature
to be studied. The flow is driven by one interface only and obeys the criterion
based on the heat diffusivity ratio proposed by Scriven & Sternling (1959) and Smith
(1966). The measured critical temperature difference for the onset of convection is
compared to the value obtained from two-dimensional numerical simulations. The
results of the simulations are in reasonable agreement with the velocimetry and the
thermal experimental data for moderate supercriticality. Numerically and experimentally,
the convective pattern exhibits a transition between different convective regimes
for similar temperature gradients. Their common detailed features are discussed.
We measured the growth hormone (GH) response to clonidine (an alpha-2-adrenergic agonist) and to apomorphine (a dopaminergic agonist) in 15 major endogenous and 15 minor depressive in-patients matched for gender and age. Results showed a significantly smaller GH response in the major depressives to both Clonidine (P<0.01) and apomorphine (P<0.001). No significant difference existed between the two groups with regard to changes in blood pressure and pulse rate during either test. While major depressives showed a trend toward smaller sedative side-effects than minor depressives after Clonidine, they showed significantly smaller sedative and gastro-intestinal side-effects after apomorphine. No significant correlation was present either in the major depressive or in the minor depressive group between the GH responses following Clonidine and apomorphine challenges. These results support the hypothesis of both noradrenergic and dopaminergic neurotransmitter disturbances in major depression, with individual variability with regard to those biochemical anomalies.
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