Equal-channel angular pressing of an Al-6061 metal matrix composite

An Al-6061 metal matrix composite, reinforced with 10 vol % Al₂O₃ particulates, was subjected to equal-channel angular (ECA) pressing at room temperature to a total strain of 5. It is shown that the intense plastic straining introduced by ECA pressing reduces the grain size from 35 m to 1 m and this leads to an increase in the microhardness measured at room temperature. Inspection revealed some limit cracking of the larger Al₂O₃ particulates as a consequence of the ECA pressing. Tensile testing after ECA pressing gave a maximum ductility of 235% at a temperature of 853 K when testing at strain rates from 10^–4 to 10^–3 s^–1. It is suggested that high strain rate superplasticity is not achieved in this material after ECA pressing due to the presence of relatively large Al₂O₃ particulates.     

Vortex imaging in superconducting films by scanning hall probe microscopy

We have used a low noise Scanning Hall Probe Microscope (SHPM) to study vortex structures in superconducting films. The microscope has high magnetic field (2.9×10^–8T/Hz at 77K) and spatial resolution, 0.85m. Magnetic field profiles of single vortices in High T_c YBa₂Cu₃O_7– thin films have been successfully measured and the microscopic penetration depth of the superconductor has been extracted as a function of temperature. Flux penetration into the superconductor has been imaged in real time (8s/frame).     

The frictional behaviour of LiF single crystals

A study has been made of the influence of initial surface roughness, renewable and non-renewable surface contaminants, and irradiation hardening on the coefficient of friction for one LiF single crystal (A) sliding on another (B) in {100}_A<010>_A{100}_B 010_B orientation at 295 K. The normal load was 1 N, the nominal contact pressure 0.1 MPa, the sliding velocity 0.2 to 0.6 mm sec^–1, and the amplitude of the (reciprocate) motion a few millimetres. Any influence of non-renewable contaminants persisted only for cumulative relative displacements 0.1 m, and that of micrometre-scale initial surface roughness only for a few metres. At steady state in the presence of renewable contaminants the coefficient of friction varied only from a high of 0.45 in ultra-high vacuum ( 7.5 × 10^–8 Pa) and dry nitrogen-rich air ( 10⁵ Pa, relative humidity 15%) to a low of 0.38 in moist nitrogen-rich air ( 10⁵ Pa, relative humidity 50%). Irradiation hardening had no effect on the coefficient of friction at steady state. The worn surfaces created by steady-state sliding always exhibited a grooved topography partly obscured by more-or-less adherent layers of variously consolidated equiaxed debris particles 100 nm in size. Owing to the action of image forces, these particles contained no dislocations. It is suggested that the coefficient of friction was determined at steady state by the stress needed to shear these tiny particles past one another as near-rigid bodies.     

The role of Coble creep and interface control in superplastic Sn-Pb alloys

The creep behaviour of superplastic Sn-2 wt% Pb and Sn-38.1 wt % Pb is investigated at temperatures between 298 and 403 K and for grain sizes between 2.5 and 260m. In Sn-2 wt% Pb with grain sizes larger than 50 m, diffusion-controlled Coble creep is found and it is experimentally shown that this type of creep is inhibited in smallgrained specimens. Measurements covering low stresses ( 0.1 MPa) and strain rates ( 10^–10 sec^–1) rule out any explanation which relies on a threshold stress for plastic deformation. The observations are explained by a model in which, at low stresses or small grain sizes, Coble creep is rate-limited not by diffusion of vacancies but by the rate of emission and absorption at the curved dislocations in the grain boundaries which are the ultimate sources and sinks of vacancies.     

Sol-gel prepared Ni-alumina composite materials

The sol-gel method has been utilized for the preparation of dense, homogeneous ceramic-metal composites with up to 50% Ni in Al₂O₂. Examination by SEM and TEM shows that the materials consist of micrometre-size Al₂O₃ with metallic Ni in isolated regions from 50 m down to nanometre size. The density ranges from 97% (10% Ni) to 74% (50% Ni) of the theoretical number. The hardness decreases from 18 GPa for pure alumina to 10 GPa for alumina containing 50% Ni. The fracture toughness increases significantly from K _1c=3–4 MPa m^1/2 to K _1c=8.5 MPa m^1/2. The elastic and shear moduli decrease from E=400 GPa and G=160 GPa for pure alumina to E=320 GPa and G=135 GPa when containing 50% Ni. The electrical resistivity is 10⁶m with 10 to 40% Ni but decreases drastically at 50% Ni content.     

The reflection of low energy phonons at the free surface of liquid4He. II. Experiment

Using a heater and bolometer as source and detector, the reflection of a pulsed beam of phonons at the free surface of liquid⁴He at 0.05 K has been measured for angles of incidence between 30 ° and 80 °. The energy distribution of the incident beam was obtained from the theory of phonon decay in the accompanying paper (I). The average incident phonon energy was 0.3 K, corresponding to an effective beam temperature of about 0.1 K. Both heater and bolometer were made from graphite resistor board with a layer of poly aniline fibers to increase the absorptivity for rotons. The angular distribution of the phonons from the heater is quite broad, approximately (cosⁿ +cos^m )/2 with n 0.65 and m 3.4. However, the receiver response has a broad component with n 2.2 and an extraordinarily narrow one with m 106. In agreement with the theory in I, the reflection appears to be specular within the accuracy of the experiment. The reflection coefficient is unity within the experimental error; the weighted mean value is 1.001±0.025. During the experiment, the free surface became contaminated with 0.034 of a monolayer of³He, but no effect from the³He impurity was observed.     

Transport results from untwinned YBCO: Washboard frequency of the vortex lattice,and the quasiparticle mean free path

The washboard frequency of the moving vortex lattice in untwinned YBa₂ Cu₃ O_6.93 may be observed through mode-locking to an externally applied ac current of frequency _ext. The interference between and _ext results in jumps in the dc current-voltage characteristics when and _ext are harmonically related¹. The interference effect disappears in the vortex liquid state. The Hall conductivity _xy below T_c in YBCO contains contributions² from a positive quasiparticle (qp) term (H) and a negative vortex term (1/H). The qp term is surprisingly large well below T_c and implies a large gap anisotropy and a long qp mean free path (mfp). The thermal Hall effect³ _xy is closely related to the qp _xy; _xy is produced by asymmetric scattering of qp by pinned vortices. The qp mfp at H = 0, extracted from _xy and extended to low T by _xy, increases remarkably from 90 Å at T_c to more than 0.5m at 22 K.     

High-temperature creep response of a commercial grade siliconized silicon carbide

Creep studies conducted in four-point flexure of a commercial siliconized silicon carbide (Si-SiC, designated as Norton NT230) have been carried out at temperatures of 1300, 1370, and 1410°C in air under selected stress levels. The Si-SiC material investigated contained 90% -SiC, 8% discontinuous free Si, and 2% porosity. In general, the Si-SiC material exhibited very low creep rates (2 to 10×10^–10 s^–1) at temperatures 1370°C under applied stress levels of up to 300 MPa. At 1410°C, the melting point of Si, the Si-SiC material still showed relative low creep rates (0.8 to 3 × 10^–9 s^–1) at stresses below a threshold value of 190 MPa. At stresses >190 MPa the Si-SiC material exhibited high creep rates plus a high stress exponent (n=17) as a result of slow crack growth assisted process that initiated within Si-rich regions. The Si-SiC material, tested at temperature 1370°C and below the threshold of 190 MPa at 1410°C, exhibited a stress exponent of one, suggestive of diffusional creep processes. Scanning electron microscopy observations showed very limited creep cavitation at free Si pockets, suggesting the discontinuous Si phase played no or little role in controlling the creep response of the Si-SiC material when it was tested in the creep-controlled regime.     

Investigation of superplasticity by the impression creep technique

Impression creep tests have been carried out on a Zn-22% Al eutectoid alloy in two heattreatment conditions, as a function of applied stress and at temperatures of 25 to 270° C. At small grain sizes ( 0.9m) and in the temperature range 180 to 270° C, the observed deformation response conforms to Regions I, II and III typical of superplastic behaviour. Furthermore, strain-rate index values determined for Region II fall in the range 0.44 to 0.51, consistent with superplasticity. It is concluded that the impression creep technique offers considerable potential for characterization of superplastic alloy systems on small specimen volumes.     

Ohm resistivity of electroless copper layers as a function of their thicknesses

The electric Ohm resistivity of electroless Cu depositions on dielectric substrates as a function of their thicknesses is studied. Substantial deviations (up to 10–20 times) from the standard resistivity ( = 1.7 cm^–1) below 0.5 (m thicknesses are observed. The experimental data show for the entire region of thicknesses (d 0.07–5 m) a power function between the relative resistivity changes (/) and the inverse thickness of depositions (d )–(/ (1/d )^0.8. This empirical relation is discussed as an effect of the porous structure of the metallic layers deposited on the substrate. A scanning electron micrography was applied in order to visualize the morphology of the depositions. The micrographs clearly show the evolution of the deposition profile: starting from separate islands at the very beginning of the process, and gradually covering the entire area with continuous but porous metal layers.     

Hydrothermal Synthesis and Spectroscopic Properties of Crystalline ZnO Powders and Films

Polycrystalline zinc oxide is synthesized under mild hydrothermal conditions (150–250°C). The phase composition, morphology, and pulsed cathodoluminescence (PCL) spectra of ZnO powders, polycrystalline films, and single crystals are studied as a function of synthesis conditions (the nature of the precursor, the type and concentration of the solvent, solution temperature, and the redox potential of the system). The crystallite size in the resultant materials varies from 100 nm to 10 m, depending on the synthesis conditions. The PCL spectra measured under pulsed electron-beam ( 2 ns) or laser (Nd:YAG, fourth harmonic, = 266 nm, 10 ns) excitation show two bands in the UV and visible spectral regions. The intensity ratio between these bands depends on the ZnO preparation conditions. The influence of the precursor and solvent on the PCL spectrum of ZnO is studied. The redox potential of the system is shown to have a strong effect on the luminescent properties of the resultant material.     

Structural Changes in the Surface Layers of PdHx Alloys Caused by the Hydride Phase Decomposition

By the method of optical microscopy, we perform the in-situ investigation of the plane polished surfaces of PdH _x alloys (with video recording of the processes of transformation) in the case of their rapid cooling at a rate of 11–20°C·sec^–1 from T _c = 292°C ( = 1.97MPa) with simultaneous lowering of the pressure of hydrogen within the temperature range 100–160°C. In the course of decomposition of the PdH_0.24 alloy according to a scheme ₀ + , the formation of nuclei of the - and -phases and their growth were not detected. However, we observe a simultaneous coherent transformation of the entire surface accompanied by the formation of a surface topography similar to the modulated structures formed as a result of the spinodal decomposition. We discuss the possibility of realization of spinodal decomposition under the described experimental conditions.     

Prolonged laser ablation effects of YBCO ceramic targets during thin film deposition: Influence of processing parameters

Cumulative laser irradiation during high-Tc superconducting thin film pulsed laser deposition (PLD) may have a detrimental effect on film characteristics. Initial decrease of deposition rate and gradual shift of the center of the deposited material spot towards the incoming laser beam were registered on cold glass substrates. Their absorbance was used for evaluation of the film thickness distribution over the substrate area. At the initial stage, two components of the spot could be distinguished along its short axis: central (cosⁿ , n1) and peripherial (cos ), while with cumulative irradiation the thickness followed an overall cos^m (m相似文献   

Continuous,martensitic nature of the transition ß→γ Li3PO4

The transformation Li₃PO₄ shows characteristics of both continuous and martensitic transformations. Below 340° C, no detectable transformation occurs; between 340 and 410° C, the transformation goes only partially to completion; above 410° C the transformation rapidly goes to completion. At any temperature in the range 340 to 410° C, transformation proceeds rapidly in the initial stages to attain a certain degree of transformation. With prolonged isothermal heating or grinding of the samples, further transformation does not occur. The reverse transformation could not be effected under normal, dry conditions.     

Creep of ceramics

This review analyses a wide range of experimental data on the creep of ceramic materials and reveals many similarities with the creep of metals. It is demonstrated that there are two important differences in the creep behaviour of ceramics: (1) there is an enhanced role of diffusion creep, and (2) in the power-law regime, ceramics divide into two categories with stress exponents of 5 and 3, respectively. It is concluded that the behaviour with an exponent of 5 represents fully ductile behaviour as in f cc metals, whereas the behaviour with an exponent of 3 is due to dislocation climb from Bardeen-Herring sources under conditions where there is either a lack of five independent slip systems or, if five independent slip systems are available, a lack of interpenetration of these systems.     

High temperature deformation behaviour of MoSi2 and WSi2 single crystals

High temperature deformation behaviour of MoSi₂ and WSi₂ single crystals, which both oriented near 001 and near 100, have been studied by compression tests over the temperature range of 1100 to 1500° C in a high vacuum of less than 6×10^–4 Pa. At elevated temperatures, several per cent compression deformation is possible in both MoSi₂ and WSi₂. Slips on {110 and {013 planes, the dislocation with the direction of Burgers vector 331 and the stacking fault on {110 plane are observed in both deformed MoSi₂ and WSi₂. In MoSi₂, the 0.2% offset stress of the sample oriented <001> is higher than that of the sample oriented <100>. The higher strength of the sample oriented <001> is related to the higher CRSS for the main slip plane of it. The reverse orientation dependence of the strength in WSi₂ is also correlated with the difference in CRSS on {110 and {013 planes, which shows the opposite result to MoSi₂. The higher CRSS on {110 plane in WSi₂ compared to that on {013 may be caused by the formation of a large number of stacking faults on {110 plane.     

Supercarrier Effective Mass Isotope Effect by Interband Scattering

Supercarrier effective mass isotope effect (exponent ) is investigated using a two-band model with interband pair scattering. The corresponding repulsive interaction incorporates besides the dominating electronic (Coulomb) part an electron-phonon contribution inversely proportional to the ionic mass factor. Calculations illustrating the behaviour of T _c , its isotope exponent , and with doping in La _2–x Sr _x CuO ₄ type underdoped system reflect the observed tendencies. ² Both and diminish with doping, the sign of is opposite to . A typical estimation gives || 0.2.     

A Scaling Calculation of the Scattering of 4He Atomic Beams

We describe a simulation of the scattering in beams of helium atoms. The number of atoms N in the beams is reduced by a large scaling factor while the collision cross-section is increased by . This leaves the rate of scattering for each particle unchanged. As an example, we predict the outcome of a low temperature atomic beam experiment to measure the ⁴ He- ⁴ He atomic scattering cross-section at low energies. Because of the existence of a very weakly bound dimer, the low energy cross-section is expected to be unusually large, 1.83 × 10 ⁵ Å ² . In the simulation N/ is small enough for the trajectories of all the scaled atoms to be calculated numerically. The simulation shows that the experiment is quite practicable. The proposed apparatus is just over 20 cm long, and a few centimeters wide, small enough to fit in a dilution refrigerator. The heaters and bolometers are assumed to be similar to those used in previous low temperature scattering experiments. We show that, using low intensity beams, the cross-section can be measured as a function of the relative velocity v _r between 2 and 8 m/sec, corresponding to relative energies between 1 and 16 mK. By fitting (v _r) one can determine the scattering length and effective range of the interaction. We predict that, at high intensity where multiple scattering is very important, the two beams coalesce into one.     

Pyrosol deposition of fluorine-doped tin dioxide thin films

Fluorine-doped tin dioxide (SnO₂F) films were deposited from a tin tetrachloride solution in methanol utilizing a pyrosol deposition process. It is shown from thermodynamic calculations that the atmosphere during deposition is oxygen-rich and also suggested that chlorine and hydrogen chloride, which are produced during the deposition reaction, influence crystal growth. Detailed electrical, optical and structural properties of the material with respect to varying film thickness and substrate temperature are presented and discussed. Resistivity of the films deposited at 450 °C decreased from 6×10^–4 to 2×10^–4 cm, while the mobility increased from 14 to 45 cm²V^–1s^–1, respectively, when the film thickness was varied from 100 to 1650 nm. The carrier concentration was relatively unchanged for film thicknesses higher than 200 nm. Optimized SnO₂F films (600 nm) having a resistivity of 6×10^–4 cm, a carrier mobility of 20 cm²V^–1s^–1, a carrier concentration of 8×10²⁰ cm^–3 and a transmittance in excess of 80% are quite suitable as electrodes for amorphous silicon solar cells.     

Size-strength effects in sapphire and silicon nitride whiskers at 20° C

Tensile tests at 20° C have been carried out on seventy-three sapphire whiskers and on seventeen silicon nitride whiskers. The sapphire whiskers were of 0001, 1¯120, 10¯10, and 10¯11 orientations, while the silicon nitride whiskers were 0001, 11¯20, and 10¯13. Tensile strengths were in the range 45 to 1500 kg/mm², and deformation was found to be purely elastic. The tensile strength data have been analysed and fitted to empirical equations describing the effect of size on strength for different orientations. These empirical equations have been used to deduce possible fracture nucleation mechanisms. It is concluded that, in the case of 0001 sapphire whiskers, fracture nucleation may be due to dislocation pile-ups or interactions, while in the other cases a Griffith flaw mechanism is probably applicable.