Observation of a novel stacking-fault nucleation mechanism in melt-textured YBa2Cu3O7

The dislocation substructure of melt-textured YBa₂Cu₃O₇-Y2BaCuO₅ composites deformed under uniaxial compressive stress and a superposed gaseous confining pressure has been investigated by transmission electron microscopy. Observations reveal the generation of perfect dislocations in (001) as well as out of this plane. Screw segments of the latter lying on (001) are seen to act as Frank-Read sources for ? 〈301〉 partials bounding YBa₂Cu₄O₈-type stacking faults, according to a mechanism previously anticipated from theoretical considerations by one of the present authors.     

Determination of Peierls stress from acoustic harmonic generation

The interaction of acoustic waves with dislocations leads to aperiodic oscillations in the magnitude of the acoustic nonlinearity parameter β as a function of the acoustic drive amplitude σ _ampl. The magnitude and spacing of the oscillations depend on the value of the Peierls stress. A least-square curve fit of the β(σ _ampl) equation to experimental data taken of 99.999% pure aluminum monocrystals oriented for wave propagation along the [1?0?0] crystal axis yields the value 6.2?×?10⁴?Pa for the Peierls stress. The value is consistent with the smallest values reported in the literature for aluminum where for both theoretical and experimental studies the reported values range over three orders of magnitude.     

Dynamic and quasi-static mechanical properties of fibre-reinforced metallic glass at different temperatures

We have investigated the dynamic and quasi-static mechanical properties of a tungsten fibre-reinforced Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5 bulk metallic glass composite (W/BMG) at temperatures ranging from 77 to 473?K. It was found that the yield stress and elastic modulus of the composite increase while strain-to-failure decreases with decreasing temperature. Tests under uniaxial dynamic compression showed that the W/BMG has a higher yield stress than under quasi-static compression. It was shown that yielding of the composite material is dominated by a shear banding process in the glass matrix, which is essentially a viscous flow as a consequence of local adiabatic heating. The interval between the nominal testing temperature and the temperature at which shear bands initiate is a key factor affecting the shear banding process, rendering the properties of the W/BMG very sensitive to temperature.     

Dislocation dissociation in the intermetallic compound MoSi2

Abstract

The identity of dislocations which contribute to plastic deformation of polycrystalline MoSi₂ when compressed at 1400°C has been determined using transmission electron microscopy. It has been confirmed that dislocations with Burgers vectors lying parallel to ? 100 ? and ? 111 ? are activated in response to the applied stress. In addition, the deformation microstructure is characterized by the presence of networks containing dislocations with Burgers vectors parallel to ? 100], ? 110] and ? 111 ?. It has been shown that dislocations with Burgers vectors lying parallel to ? 111 ? are dissociated. A simple explanation has been developed to account for the occurrence of dissociation of particular dislocations, and on the basis of this model the dissociation is represented by

½? 111 ?→ ½? 111 ?+SISF+¼? 111 ?

where SISF stands for a superlattice intrinsic stacking fault. The SISF energy has been estimated from the separation of the partial dislocations to be about 261 mJ m^?2. Other observations of the dissociation of dislocations in MoSi₂ have been interpreted in terms of the model developed in the present work.     

Microstructural development during transient directional solidification of a hypomonotectic Al–In alloy

Upward directional non-steady-state solidification experiments have been performed on a hypomonotectic Al–5.5?wt%In alloy. The alloy developed cellular as-solidified microstructure for tip growth rates, V _L, higher than 0.95?mm/s. The casting regions associated with V _L?<?0.95?mm/s were shown to be characterized by a microstructure formed by In droplets disseminated in the Al matrix. Tip growth rate and microstructural features, such as cell spacing and interphase spacing, have been experimentally determined. The experimental cell spacings have been compared with theoretical predictions furnished by the Hunt–Lu model. It was found that the experimental scatter lies below the minimum range of values theoretically predicted. Moreover, the experimental cell spacing evolution with V _L is characterized by a ?1.1 power law. The droplets’ interphase spacing, λ, is related to the growth rate by the Jackson–Hunt relationship (λ ² V _L?=?constant).     

One hundred years of laws in psychology

Mainstream psychology in the 20th century has been conceived as a nomothetic science, but few psychological "laws" have been proposed. A PsycLit search of journal abstracts from 1900 to 1999 yielded a total of 3,093 "law" citations, or 22 per 10,000 entries, with two psychophysical laws (Weber's law and Stevens's power law) and two learning laws (Herrnstein's matching law and Thorndike's law of effect) as the most frequently cited. The number of law citations has been decreasing throughout the century, to 10 per 10,000 entries in the last decade, with few references to laws of recent origin. This could be the result of increasing doubts about the lawfulness of psychological processes coupled with a general preference for less ambitious terms (such as effects, principles, models, or functions).     

Negative incremental bulk modulus in foams

Negative incremental stiffness is known to occur in structures such as post-buckled flexible tubes and single-cell models. A single foam cell under uniaxial loading buckles and exhibits a non-monotonic S-shaped deformation curve, which is indicative of negative incremental stiffness. Negative stiffness is not observed in bulk materials. For example, individual foam cells display negative stiffness but foams tested in uniaxial compression exhibit a plateau in the stress–strain curve because the buckled cells localize in bands. This behaviour is consistent with the continuum view in which strong ellipticity and, hence, a positive shear modulus G and positive C ₁₁ modulus are required for stability, even for a constrained object. It is hypothesized that a solid with negative bulk modulus can be stabilized by control of the surface displacement. Experimentally, foams were hydrostatically compressed by controlled injections of small volumes of water into a plastic chamber, causing volumetric deformation. A negative incremental bulk modulus was observed in a foam with 0.4-mm cell size beyond about 20% volumetric strain. A foam with large cells, 2.5–4?mm in size, was anisotropic and did not exhibit the cell buckling required for negative modulus.     

Experimental evidence of relaxation arising from the motion of geometrical kinks on screw dislocations in iron

Abstract

Internal friction measurements with a superimposed bias stress have provided evidence for geometrical kink migration on screw dislocations in iron. This intrinsic process causes a relaxation phenomenon in internal friction which has been identified with the occurrence of a subpeak (below 20 K) of the α-peak. The effect of different bias stress has allowed us to evaluate the kink migration energy, E ^m _k ? 0·001eV.     

Absorption and photoluminescence spectroscopy of Er3+-doped SrAl2O4 ceramic phosphors

A spectroscopic characterization of Er³⁺-doped SrAl₂O₄ phosphor materials synthesized by a solid-state reaction method with Er concentrations varying from 0.1 to 1?mol% has been performed by studying photoluminescence (PL) in the temperature range 10 to 360?K and absorption spectra. PL signals containing five emission bands at 1492, 1529, 1541, 1558, and 1600?nm, respectively, have been observed at room temperature for Er³⁺ transitions in the near infrared region. The samples exhibit a main luminescence peak at 1.54?µm, which is assigned to recombination via an intra-4f Er³⁺ transition. Sharp bands centered at around 378, 488, 521, 651, 980, 1492, and 1538?nm in the absorption spectra can be associated with transitions from ⁴I_15/2 level to ²H_9/2, ⁴F_7/2, ²H_11/2, ⁴F_9/2, ⁴I_11/2, ²H_11/2, and ⁴I_13/2 levels, respectively. The sharp emission peaks and excellent luminescence properties show that SrAl₂O₄ is a suitable host for rare-earth-doped phosphors, which may be suitable for optical applications.     

Origin of the hardness enhancement in superhard nc-TiN/a-Si3N4 and ultrahard nc-TiN/a-Si3N4/TiSi2 nanocomposites

The ideal shear strengths of a variety of TiN/SiN _x /TiN interfaces have been calculated with the ab initio density functional theory. Using these data, the high hardness H?≥?105?GPa, which has been reported for the quasi-ternary nc-TiN/a-Si₃N₄/TiSi₂ nanocomposites, can be explained on the basis of the Sachs average of polycrystal plasticity of randomly oriented equiaxed grains and pressure-enhanced flow stress.     

Single stacking faults in high-stress deformed semi-insulating GaAs

Abstract

Single intrinsic stacking faults in semi-insulating undoped GaAs have been studied by transmission electron microscopy. The crystals were deformed at room temperature by uniaxial compression and under hydrostatic pressure. It is shown that the stacking faults are produced by the dissociation of 60°(β) dislocations under very high stress (τ?0·75 GPa). The partial dislocations bounding the stacking faults are systematically 30°(β) in character. These observations are consistent with the classification of mobilities of partial dislocations that had been previously established when studying deformation microtwins in the same material: 30°(β)<30°(α) <90°(α or β).     

Effect of pressure on the mechanical and dynamical stability of NaAlSi

The effects of pressure on the mechanical and dynamical stability of NaAlSi have been fully investigated by first-principles calculations within the generalised gradient approximation (GGA). The obtained results show that the optimised lattice constants, atomic positions and elastic constants at ambient pressure are in good agreement with the available experimental and theoretical data. The lattice constants decrease monotonically with increasing pressure. The elastic constant C₁₁, C₁₂, C₁₃, C₃₃ and C₆₆ increase while C₄₄ decreases with increasing pressure. It is found that NaAlSi is mechanically unstable when the pressure is above 25.97?GPa. The phonon-dispersion curves along several high symmetry lines are obtained. The phonon frequencies at the Z point decrease with increasing pressure and completely soften to zero at 27.52?GPa, further confirming the structural instability of NaAlSi.     

The form of the psychophysical function near threshold

Psychophysical functions typically depart from a simple power law in the vicinity of the absolute threshold. Five versions of the psychophysical power law have been proposed to describe the function near threshold. An account is given of some of the difficulties encountered in attempts to decide among the various versions by means of empirical tests.     

Pressure-induced structural transition in UGa2

High-pressure X-ray diffraction has been performed on UGa₂ up to 20?GPa using a diamond anvil cell. UGa₂ exhibits the AlB₂-type structure with space group P6/mmm at room temperature and atmospheric pressure. At about 16?GPa a reversible structural transformation to a tetragonal phase was observed. The bulk modulus of the AlB₂-type phase has been determined to be ～100?GPa, which is comparable to rare earth digallides like TmGa₂ and HoGa₂.     

On the plasticity event in metallic glass

Based on a systematic molecular dynamics analysis, this study reveals that plastic deformation of metallic glass is not through a uniform configuration change but via many localized plasticity events. These events are manifested by the atomic clusters of high kinetic energy and high strain rate, emerging even in the elastic deformation regime. The life of such a plasticity event is on the order of 10^?12?s, during which the distribution of kinetic energy follows a power law. The study shows that yielding in metallic glass occurs at the sudden surge point of the number of plasticity events. In the steady plastic deformation regime, the continuous nucleation and annihilation of the plastic events lead to a steady flow stress and stabilized total potential energy.     

Ultrafine grain development in copper during multidirectional forging at 195 K

Strain-induced evolution of ultrafine grains in pure copper was studied in multidirectional forging (MDF) at 195?K. The stress–strain behaviour was characterized by rapid strain hardening during early processing and the rate of strain hardening gradually decreased with straining, leading to an apparent steady-state flow at large cumulative strains of more than 5. The structural changes were associated with the development of high-density microshear bands crossed by MDF. The new fine grains 0.16?µm in size, which was smaller than the subgrain size evolved during early deformation, were evolved primarily at microshear band intersections, and then the new fine grains filled out the whole sample as the number of microshear band intersections increased at large strains. This is essentially similar to continuous dynamic recrystallization. The size of new grains can be expressed by a power law function of flow stress with a grain size exponent of about –0.3. The kinetics of the strain-induced grain evolution is analyzed and the mechanisms are discussed.     

Individual half-judgment brightness functions

Nine Ss made half-judgments of each of nine brightness standards. Individual half-judgment brightness functions were constructed, one for each of the two threshold forms of the power law. The ω-Law (translation on the psychological axis) provided a better fit to the half-judgment data than did the ?-Law (translation on the intensity axis). A test of scale consistency for the fractionation method was confounded with the power law hypothesis, but the data are interpreted as providing fair support for the method independent of the form of the power law. The effect of Standard on exponent estimates was significant for both forms of the law, and the effect of Standard on threshold estimates was significant for the ?-Law but not for the ω-Law. Both forms of the law contain a so-called threshold parameter, but the interpretation of this parameter as a threshold was rejected for the ?-Law and accepted for the ω-Law.     

Facile route to synthesising mesoporous SBA-15 using phosphoric acid

A facile route has been developed to synthesise mesoporous SBA-15 using mild H₃PO₄ (replacing the most commonly used strong acid HC1) as the medium and the effects of H₃PO₄ concentration and synthesis temperature on the mesostructure and morphology of the resultant materials investigated. The results demonstrate that a more ordered structure can be obtained at low H₃PO₄ concentration (1–2?M), while a higher H₃PO₄ concentration (3–4?M) results in a less ordered structure. Furthermore, the morphology of SBA-15 particles transforms from rod-like to sphere-like with increasing H₃PO₄ concentration.     

Les remèdes du droit aux souffrances de la personne malade

Can law eliminate suffering? For a long time, the suffering has been only compensated. Indeed, the compensation of the physical and moral sufferings was allowed. But can law remove suffering? In fact, the French national law provides humans and technical means so as to eliminate pains. Now the law states that people's suffering justifies medical care and treatments.     

Temperature dependence of the low-cycle fatigue behaviour of a Cu-SiO2 bicrystal with a [011], 18° twist boundary

The temperature dependence of the low-cycle fatigue behaviour of a Cu bicrystal containing dispersed SiO₂ particles and having a [011], 18° twist boundary has been studied. Failure occurred at shorter times with increasing temperature and stress amplitude. Crack nucleation took place at the particles' surfaces on the grain boundary where slip lines intersected. The crack tended to propagate along primary slip lines and this tendency became stronger as the temperature was increased.