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.     

New high permittivity and low loss ceramics in the BaO–TiO2–Nb2O5 composition

New dielectric ceramics with formula BaTi3Nb4O17 and Ba6Ti14Nb2O39 have been prepared and characterized. BaTi3Nb4O17 was densified to 92% of TD after firing at 1310 °C for 4 h. However, Ba6Ti14Nb2O39 fired under optimized conditions (1260 °C for 4 h) showed only 85% TD together with secondary phase. The crystal system of both of the compositions is orthorhombic. The BaTi3Nb4O17 has r56, Qu2100 (at 4.402 GHz), f+86 p.p.m. K-1 and Ba6Ti14Nb2O39 as r50, Qu650 (4.359 GHz) and f+165 p.p.m. K-1. © 1998 Kluwer Academic Publishers     

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 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.     

Particle size control of 1,3,5-triamino-2,4,6-trinitrobenzene by recrystallization from DMSO

Batches of up to 46g of the insensitive explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) have been recrystallized from DMSO in an effort to prepare larger particle-size material for recycling previously-used TATB and also for use in special formulations. The first part of the study investigated the conditions required to shift the particle-size distribution maximum from 50–70 m to several hundred micrometres in diameter. Distributions peaking at 200–246 m were successfully produced by varying the cooldown rate and degree of agitation during cooling. The second part of the study emphasized regeneration of the standard 50–70 m distribution from submicron size (ultrafine TATB) particles. The distributions peaking at 76–88 m, 27–31 m, and 15–17 m, successfully bracketed the target particle sizes, were grown by changing the degree of solution saturation. The choice of saturation temperature for the TATB/DMSO solution was based on earlier small-scale recrystallization and solubility work.     

Microstructure and hardness of as-cast in situ TiB short fibre reinforced Ti-6Al matrix composites

XD^TM technique has been successfully used to prepare TiB short fibre reinforced Ti-6Al matrix composites. Macrostructure and microstructure have been observed by optical microscopy and SEM in order to study the influence of cooling rate on the morphology, size and distribution of TiB. Due to the cooling rate, there exist three kinds of macrostructure: fine grain zone, columnar grain zone and coarse equiaxed grain zone, corresponding to the cooling rate of 100–500 K/s, 20–50 K/s and less than 10 K/s respectively. In the fine grain zone, TiB distributes randomly in matrix with main rod morphology with 3 m in width and 50 m in length. In the columnar and coarse grain zone, a colony structure was observed in which TiB distributes with a special orientation direction with matrix. A lamellar TiB with up to 50 m width and 200 m length was also formed. It was indicated that the decreasing of the cooling rate changes the morphology of TiB from rod to lamellar shape, and markedly increase the length and aspect ration of TiB, from 50 m to 200 m and from about 15 to 200, respectively. TEM results show that the rod TiB has a hexagonal cross section. Vickers hardness testing shows a little reinforcement geometry dependence, but the average hardness of 484 MPa is much higher than that of unreinforced matrix alloy.     

Sr2YRu1?uCuuO6: Evidence for SrO-Layer Superconductivity

Sr₂YRuO₆ doped on its Ru site by Cu superconducts at the below 45 K, although its Ru and Cu are magnetically ordered at 23 K and 86 K, respectively. The SrO layers superconduct. Ba₂GdRuO₆, when doped with Cu, does not superconduct, because L = 0 Gd is not crystal-field split, and so induces Cooper pair-breaking.     

Widening the Scope of Working of Solid‐State Nd‐Glass Lasers

With the example of the serial model of a pulsed Ndglass laser, it is shown that the use of an unstable resonator with a semireflecting homogeneous exit mirror makes it possible to significantly decrease the laserbeam divergence and increase the uniformity of the radiationintensity distribution in the near zone. We were the first to obtain a laserbeam quality of (40–50) mm·mrad for technological glass lasers and to attain a depth of fusing of of 6.3 mm for steel (aspect ratio 10) for an energy of 23 J. The obtained uniformity of the radiationintensity distribution was estimated at the level of ±10%. The beamquality level attained for the Ndglass laser beam allows us to recommend it for both realization of deepfusion regimes and laser hardening without the use of external integrating optical elements.     

Adsorption of polyvinylpyrrolidone (PVP) and its effect on the consolidation of suspensions of nanocrystalline CeO2 particles

CeO₂ particles with an average size of 9 nm were synthesized under hydrothermal conditions. The adsorption of polyvinylpyrrolidone (PVP) on to the particle surfaces was measured in aqueous suspensions in the pH range of 3.7 to 11.5. The amount of adsorbed PVP decreased significantly with increasing pH value. For suspensions prepared at a pH value of 3.7, complete adsorption occurred for 2.5 wt% of PVP added to the suspension. Further additions of PVP produced a gradual increase in the adsorption until a limiting value was reached when the total amount of PVP added to the suspension was 10 wt%. At this PVP concentration, 6 wt% of the PVP was adsorbed and 4 wt% remained free in solution. The effect of the adsorbed PVP on the microstructural homogeneity of films deposited by spin coating of suspensions was investigated. With no addition of PVP, crack-like voids were prevalent in the dried and sintered films. Crack-free films were obtained from suspensions containing 10 wt% of PVP. Higher PVP additions (25 wt%) produced an increase in the viscosity of the suspension but no observable change in the microstructural homogeneity of the films. The use of adsorbed polymers for steric stabilization coupled with data from the adsorption isotherms is shown to provide a rational approach to the deposition of homogeneous films from suspensions of nanocrystalline particles.     

On crystal alignment in polycrystalline YBCO materials based on TEM observation

Previously, we have observed that parallel lattice fringes show up regularly in MTG samples, indicative of good alignment of unit cells in strips of size 100 nm × a few times 100 nm. We have prepared films ( 80 nm) for TEM observation using YBCO material fabricated by the conventional solid-state reaction method in order to study the crystal texture in a length scale from a few 100 nm down to 3 Å. Parallel lattice fringes are observed to stretch across a cross-section 0.3×0.5 m². Cross lattice fringes intersecting at 90° and 45° are observed as rare events. This result suggests that the degree of alignment in the crystal texture is higher than expected inside a grain of size a few m.     

The constitution of the Ni-Al-Ru system

The constitution of the Ni-Al-Ru system has been investigated in the range 0 to 50 at% Al. Isothermal sections at 1523 and 1273 K have been determined using microstructural observations, electron probe microanalysis and X-ray diffraction. The phases present were: nickel-based solid solution (); (based on Ni₃Al); solid solutions based on NiAl and RuAl, respectively (designated ₁ and ₂), and ruthenium-based solid solution (Ru). The maximum solubility of Ru in was 5 at%. ₁, and ₂ show extensive range of solubilities, namely up to 20at% Ru in ₁ and up to 25 to 35 at% Ni in ₂. Three-phase equilibrium between , ₂ and (Ru) existed at 1523 and 1273 K. Also at 1523 K, three-phase equilibria existed between , and ₁ and ,₁ and ₂, while at 1273 K, the equilibria were between , ₁,₂ and , , ₂ indicating the occurrence of a reaction +₁, +₂ at a temperature between 1523 and 1273 K. Liquidus features have been deduced from data on as-solidified structures. Lattice parameter data and hardnesses are also reported.     

Effect of Air Oxidation on the Thermophysical Properties of a Zirconium Alloy

The results of an experimental study of the effect of air oxidation on the temperature dependence of the heat capacity and normal spectral emissivity (for the wavelength of 0.65 m) of the zirconium alloy E635 (1.3 mass% Sn, 0.3 mass% Fe, 1 mass% Nb) are presented. The subsecond resistive pulse heating technique has been used. The samples were heated in ambient air by single and multiple cyclic pulses.     

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.     

Stability of nickel coated sapphire whiskers

The effect of high temperature annealing (1100 to 1300° C) on the stability of nickel coated purified sapphire whiskers has been investigated. It was found that the initially coherent coating spheroidized to form a series of partially adherent nickel particles, which increased in size with continued time at temperature. The time for complete adhesion of the particles, which was established from the product of the number and volume of particles per unit area of surface, decreased from 72 h at 1100° C to 3 h at 1300° C, giving an activation energy of 70 kcal/mol. In addition at 1300° C, after 8 h, there was evidence for whisker breakdown from a sapphire-nickel reaction.     

On the theory of electron tunneling in NIS systems with paramagnetic impurities

Current-voltage characteristic of NIS junctions has been studied when there are paramagnetic impurities in a narrow layer of the superconductor close to the insulator. It is shown that paramagnetic impurities in such systems considerably change the current-voltage characteristic in the narrow range of the applied potentialeV andV0.     

Corrosion resistance and microstructure of stainless steel modified by pulsed high energy density plasma

Pulsed high energy density plasma has been used to deposit aluminum nitride films on the surface of the 1Cr18Ni9Ti stainless steel (0.1% C, 18% Cr, 9% Ni, 0.8% Ti). The formed films are composed of nanocrystalline-structured aluminum nitride phase, whose crystal sizes are about 10 nm. Transition areas are formed across the film to the substrate and strengthen the adhesion between them. The nanocrystalline-structured aluminum nitride films contribute to the improvement of the corrosion resistance of the modified stainless steel, whose corrosion rate has been reduced by about ten times compared with that of the unmodified stainless steel.     

The laser surface-alloying of iron with carbon

Observations are reported on the structure of iron, laser surface-alloyed with carbon. Repeated laser surface-melting of iron pre-coated with DAG graphite has produced layers containing up to 6 wt % C, showing fine-scale white iron structures. Eutectic regions (interlamellar spacing 0.5m) have been shown by transmission electron microscopy to consist of Fe₃C + ferrite, the latter having formed by decomposition of austenite during solid state cooling. Regions of fine pearlite (spacing 55 nm) have also been observed. Carbon diffusion into the substrate during alloying produces a zone containing austenite and martensite.     

CO/pH2: A Molecular Thermometer

We utilize reversible temperature dependent changes in the IR absorption spectrum of CO molecules isolated in solid parahydrogen (pH₂) to probe bulk temperature changes during rapid vapor deposition. The intensity of a well resolved feature near 2135 cm^–1 increases monotonically with temperature over the 2 to 5 K range. The thermally populated initial state of this transition lies 12 K above the CO/pH₂ ground state. During the deposition of 100 ppm CO/pH₂ samples, we detect temperature gradients 10 K/cm in 0.1 cm-thick samples subjected to heat loads 10 mW/cm². The resulting estimated thermal conductivity (TC) is 3(±2) mW/cm-K, averaged over the 2 to 5 K region. This value is 1000 times lower than the TC of single crystal solid pH₂, and 10 times lower than previously measured for pH₂ solids doped with 100 ppm concentrations of heavy impurities [Manzhelii, Gorodilov, and Krivchikov, Low Temp. Phys. 22, 131 (1996)]. We attribute this abnormally low TC to the known mixed fcc/hcp structure of the rapid vapor deposited solids.     

Microstructure-property studies in friction-stir welded, Thixomolded magnesium alloy AM60

Thixomolded magnesium alloy AM60 plates joined by friction-stir welding were observed to be as strong or stronger than the unwelded base material. The thixotropic microstructures of the as-molded plates consisted of either 3% or 18% primary solid fraction of -Mg globules (45 m average size) in a eutectic mixture consisting of -Mg grains (10 m) surrounded by Mg₁₇Al₁₂ intermetallic grains in the -Mg grain boundaries (having an average size of 1–2 m). This complex, composite microstructure became a homogeneous (Mg + 6% Al)), recrystallized, equiaxed grain (10–15 m) microstructure in the weld zone.     

Limits on Superconductivity in Nanoparticles and Nanowires

Recent experimental results are consistent with the prediction that superconductivity is suppressed in aluminum nanoparticles which are so small (10 nm diameter) that the electronic energy level spacing exceeds the superconducting energy gap in bulk material. Very recent experiments on nanowires (diameter 5–10 nm, length 150 nm) of MoGe indicate that a dissipative phase transition between superconducting and normal behavior occurs when the normal resistance of the wire equals the superconducting quantum resistance h/4e² 6.5 k.