Effect of alloying elements and dendrite arm spacing on the microstructure and hardness of an Al-Si-Cu-Mg-Fe-Mn (380) aluminium die-casting alloy

The microstructure of aluminium alloys based on 380 die-casting alloy was studied in detail, as a function of the alloying elements iron, magnesium, copper and manganese, and the solidification rate. Three methods of solidification were employed to simulate cooling rates obtained from investment, permanent, and die-casting processes, corresponding to 0.4, 12 and 260 °C s^–1, respectively, with emphasis on the highest cooling rate. Hardness measurements were carried out on samples obtained from the latter, in the as-cast and T5 tempered conditions (4 h at 25, 155, 180, 200 and 220 °C). The results have been discussed and the correlation between the hardness and microstructure as a function of alloying elements is presented. The effect of solution heat treatment on the variations in the microstructure and hardness has also been discussed.     

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.     

Some observations on the deformation characteristics of bulk polycrystalline zirconium hydrides

The deformation behaviour of bulk polycrystalline zirconium hydrides in the composition range ZrH_1.27 to ZrH_1.66 has been investigated by compressive loading at temperatures between room temperature and 500° C. Single-phase -zirconium hydride is brittle below 100° C. Analyses of slip traces on specimens deformed at temperatures between 100 and 250° C have shown that the glide planes are {111} types. The deformation characteristics of and ( + ) alloys at temperatures between 100 and 500° C are consistent with the hydrogen vacancies in the -phase providing significant lattice friction to the movement of dislocations in the zirconium lattice of the hydride structure. The room temperature fracture stress of ( + ) alloys increases with the volume fraction of the -phase and this can be related to the resistance offered by platelets to the propagation of cleavage cracks in the matrix. In a ( + + ) alloy the resistance to crack propagation at room temperature is further increased by the soft -zirconium phase.     

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.     

Nickel aluminide (Ni3Al) fabricated by reactive infiltration

Nickel aluminide Ni₃Al in the single phase form, with grain size 10 m, porosity 5%, tensile strength 425 MPa, modulus 92 GPa and ductility 9.5% at room temperature, was fabricated by reactive infiltration at 800 °C of liquid aluminium into a porous preform containing 78 vol % nickel and made by sintering 3–7 m size nickel particles. Without sintering, the preform contained 58 vol % nickel and reactive infiltration resulted in an aluminium-matrix NiAl₃ particle ( 50 m size) composite and extensive growth of Ni-Al needles from the preform to the excess liquid aluminium around the preform.     

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     

Extrusion and mechanical properties of mixed powder and spray co-deposited Al 2014/SiC metal matrix composites

Al 2014/SiCp metal matrix composites have been produced by a powder metallurgy route that included mixing, canning, degassing and hot extrusion (300°C, 161). Two types of SiC (12 and 4 m), two SiC contents ( 10 vol. % and 15 vol. %) and three different degassing temperatures (350, 400, 500 °C) were used. The degassing temperature was found to have no significant effect on the final properties, which were affected by remnant porosity. Porosity was mostly attributed to SiC clustering, which was related to the relative reinforcement/ aluminium powder size distributions and volume fractions. The best mechanical properties were obtained for the composite with 10 vol. % SiCp and these were very similar to the properties of a spray co-deposited metal matrix composite billet of approximately the same composition, also extruded at 300 °C, 161.     

Switching and Hysteresis on the I-V Curves of Submicron BSCCO (2212) Bridges

No Heading In thin BSCCO (2212) whisker-based superconducting bridges with width and length 1 m we observe random telegraph noise at discrete values of current. We show that the noise is associated with spontaneous processes of addition and removal of discrete vortex trains and reflects the regular structure of steps on the I–V curves. The average lifetime of the trains falls down with increasing temperature T in a complex way: intervals of steep drop ( an order of magnitude per Kelvin) are separated with a plateau of (T) with 10^–2 s. The 70 GHz irradiation with 100 W power results in the growth of the average switching frequency by 5 orders of magnitude; thus the HTSC bridges could be efficient detectors of microwave radiation. We discuss the peculiar features of (T).PACS Numbers: 74.25.Fy, 74.40.+k, 74.60.Ge, 74.72.Hs     

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.     

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.     

Effects of aluminium on the electrical and mechanical properties of PTCR BaTiO3 ceramics as a function of the sintering temperature

The effect of AI additions on the electrical behaviour of positive temperature coefficient of resistance (PTCR) BaTiO₃ ceramic sintered in air at temperatures ranging between 1220 and 1400° C have been investigated. Two batches of material, both showing a PTCR effect, were prepared identically except that additions of AI₂O₃ (0.55 mol %) were made to one of them. It has been confirmed that the presence of aluminium results in an increase in the temperature at which the maximum resistivity, _max, occurs as well as reducing the sintering temperature, in the presence of silicon, to 1240° C. Additionally, direct comparisons between the two materials have demonstrated that such additions result in an increase of 100% in the minimum resistivity, _min, at sintering temperatures beyond 1280° C. A similar increase in _max for sintering temperatures below 1360° C and a five-fold reduction in the ratio of _max/_min in samples sintered above 1320° C have also been attributed to the presence of aluminium. It was further found that aluminium increases the average grain size by 30% and promotes the formation of a liquid phase.     

Fracture of aluminium-coated carbon fibres

A degradation in the ultimate tensile strength (UTS) of aluminium-coated carbon fibres was associated with the formation of a reaction layer of aluminium carbide during annealing treatments 475° C for high tensile fibres (HT) and 550° C for high modulus fibres (HM). It was established that for a given annealing treatment, the UTS depended on the square root of the original coating thickness and proposed that fracture was controlled by cracks in the aluminium carbide, with a specific surface energy () and intrinsic crack length (c ₀) of 2.33 J m^–2 and 30 nm for HT fibres, and of 0.64 to 0.77 J m^–2 and 20 nm for HM fibres.     

Mechanical properties of dispersions of thoria in beryllia

The behaviour of the two-phase ceramic system BeO-ThO₂ as a function of thoria (ThO₂) particle size and concentration was studied by measuring changes in strength and Young's modulus. Modulus of rupture measurements were made by the four-point bending of cylindrical specimens, and Young's modulus was calculated from the deflection of rectangular beams. Examination of matching fracture faces showed that cracks passed through thoria particles, indicating they were bonded to the matrix. At 1.6 vol % thoria, the strength of a dispersion depended on the thoria particle size; 5 m particles caused little change from 33000 lb/in.² (23.1 kg/mm²) for pure beryllia (BeO), while with 110 m particles the strength was 20100 lb/in.² (14.1 kg/mm²). The strength of 1.6 vol % 200 m dispersions increased with test temperature in the range 20 to 400° C, indicating the presence of internal tensile stresses, caused by the differing coefficients of thermal expansion of beryllia and thoria. Increasing the concentration of thoria caused Young's modulus and strength to decrease; the observed values for Young's modulus agreed closely with those predicted for BeO-ThO₂ dispersions using Hashin and Shtrikman's method. It is concluded that the strength of BeO-ThO₂ dispersions is controlled by the strength of the weak thoria particles unless the particle size is restricted to less than 5 m.     

Thermal expansion of interlayer spacing and thermal vibrational displacement of carbon atoms in petroleum coke

On petroleum coke heat-treated at various temperatures in the range from 1600 to 3000°C, thermal expansion of the interlayer spacing and the mean-square displacement along the c-axis due to thermal vibration of carbon atoms were measured from room temperature to 1050°C by means of X-ray diffractometry. The static mean-square displacement due to imperfect crystallization was also estimated for each sample.The c-axis spacing increases linearly with temperature and the slopes had almost the same values (1.85×10^–4 to 1.89×10^–4Å/°C) for all samples. The mean-square thermal vibrational displacements of carbon atoms increased almost linearly from 0.012Ų at room temperature to 0.047Ų at 1000°C for all samples, except for the sample heat-treated at 1640°C.A mean free space between the layer planes, that is the interlayer spacing minus twice the total root-mean-square displacement of carbon atoms along the c-axis direction, was obtained in consideration of the mean-square thermal vibrational displacement and the mean-square static displacement. The mean free space increases abruptly with decrease of c-spacing from 6.86 to 6.85Å (heat-treatment temperatures of 1640 and 2100°C respectively), then decreases slowly and eventually again begins to increase with further decrease of c-axis spacing to 6.73Å.     

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.     

The structure of amorphous aluminium phosphate by radial distribution functions derived from X-ray diffraction

An investigation of the molecular structure of amorphous aluminium phosphate, prepared from a recently discovered complex (AlPO₄ · HCl · 4C₂H₅OH), has been carried out using radial distribution function (RDF) techniques. Studies were made on two samples, one prepared by decomposing the solid complex and the other from powder produced by evaporating to dryness a solution of the complex in methanol and then decomposing and grinding the solid obtained. RDF curves were obtained from both samples, and were found to closely resemble that previously reported for vitreous silica. The short range order in both samples is similar to that in vitreous silica, with aluminium and phosphorous atoms occupying positions in aluminium phosphate similar to the positions of silicon atoms in silica. This structure is consistent with the crystal chemistry of aluminium phosphate.It is possible to assign all the peaks in the RDF plots out to values of r5 Å to known inter-atomic distances. The peak at 1.6 Å represents the Al-O and Al-P bond lengths, and is broader than the corresponding Si-O peak in vitreous silica, as the Al-O and P-O bonds are respectively slightly longer (Al-O) and shorter (P-O) than the equivalent Si-O bonds. Subsequent peaks result from the O-O distances ( 2.6 Å); the Al-Al, Al-P and P-P distances ( 3.1Å); the Al-second O and P-second O distances ( 4.2 Å), and O-second O, Al-second Al, P-second P and Al-second P distances ( 5.0 Å). The sample prepared from the methanolic solution shows some small additional peaks in the region 4 to 5 Å, indicating more order in the second neighbour distances.Analysis of the P-O and Al-O peak area indicates that both P and Al atoms are co-ordinated to four network oxygen atoms at a distance 1.6Å, and that absorbed water also co-ordinates to one or both atoms with a P-O and/or Al-O distance of 1.6 Å.     

Effects of alloying in the dilute limit on the quantum states of electrons in magnesium

The electron quantum interference phenomenon was used to determine the effects of substitutional alloying upon the quantum-state lifetime and band gapsE _g at the Fermi energy in single crystals of pure Mg. Vapor-grown alloys containing either Zn or Cd in concentrationsC0.3–15 ppm were studied. The magnetic field dependence of the interference oscillation amplitudes for these samples indicates reductions in and significant increases inE _g relative to pure Mg (impurity concentration 10^–8). Within experimental accuracy the observed quantum-state lifetime satisfies the relation ^–1=C, with 2.7×10¹⁰ sec^–1 per ppm of Cd and 2×10¹¹ sec^–1 per ppm of Zn. The band gap corresponding to Bragg reflection from the (0001) plane of the hcp structure (which in pure Mg arises solely from spin-orbit coupling) was found to increase by more than a factor of two upon the addition of only 10 ppm Cd to Mg. These results are discussed within the framework of the pseudopotential theory of alloying. It is shown that there are discrepancies of more than three orders of magnitude between the experimental and theoretical values forE _g and of about one order of magnitude for in these dilute-limit alloys.Work supported by the National Science Foundation.Submitted to the Department of Physics, the University of Chicago in partial fulfillment of the requirements for the degree of Doctor of Philosophy.Fannie and John Hertz Foundation Fellow.     

Crystalline transformation of Ti-B-O,Al-Ti-B-O,Si-Ti-B-O and Al-Si-Ti-B-O by thermal treatments

The thermal transformations of Ti-B-O, Al-Ti-B-O, Si-Ti-B-O and Al-Si-Ti-B-O have been investigated using the methods of thermal analysis and X-ray powder diffraction. The materials are a crystalline series of TiO₂ with partial replacement of titanium by the elements, aluminium, boron and silicon. The anatase form of the materials was transformed to the rutile form at 520680 °C for Ti-B-O, 880950 °C for Al-Ti-B-O, 10801280 °C for Si-Ti-B-O and 11801330 °C for Al-Si-Ti-B-O. The rate constant for the anatase-rutile transformation of Ti-B-O was 6.908×10^–3 min^–1 under isothermal conditions at 680 °C. Analysis of the kinetic data obtained by differential thermal analysis (DTA) gave the activation energy for transformation of anatase into rutile as 663.7 Kcal mol^–1 for Al-Ti-B-O. The lattice parameters for the compounds studied at various temperatures were calculated by least-squares fitting of the X-ray powder diffraction data.     

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.     

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.