Showing papers in "Journal of Materials Science in 1998"

Electric modulus and interfacial polarization in composite polymeric systems

Abstract: The applicability of the electric modulus formalism is investigated on a Debye-type relaxation process, the interfacial polarization or Maxwell–Wagner–Sillars effect. Electric modulus, which has been proposed for the description of systems with ionic conductivity and related relaxation processes, presents advantages in comparison to the classical approach of the real and imaginary part of dielectric permittivity. In composite polymeric materials, relaxation phenomena in the low-frequency region are attributed to the heterogeneity of the systems. For the investigation of these processes through electric modulus formalism, hybrid composite systems consisting of epoxy resin–metal powder–aramid fibres were prepared with various filler contents and their dielectric spectra were recorded in the frequency range 10 Hz–10 MHz in the temperature interval 30–150°C. The Debye, Cole–Cole, Davidson–Cole and Havriliak–Negami equations of dielectric relaxation are expressed in the electric modulus form. Correlation between experimental data and the various expressions produced, shows that interfacial polarization in the systems examined is, mostly, better described by the Davidson–Cole approach and only in the system with the higher heterogeneity must the Havriliak–Negami approach be used. © 1998 Chapman and Hall

Shape-memory materials and hybrid composites for smart systems: Part I Shape-memory materials

Abstract: A review is presented of the current research and development of shape-memory materials, including shape-memory alloys, shape-memory ceramics and shape-memory polymers. The shape-memory materials exhibit some novel performances, such as sensoring (thermal, stress or field), large-stroke actuation, high damping, adaptive responses, shape memory and superelasticity capability, which can be utilized in various engineering approaches to smart systems. Based on an extensive literature survey, the various shape-memory materials are outlined, with special attention to the recently developed or emerged materials. The basic phenomena in the materials, that is, the stimulus-induced phase transformations which result in the unique performance and govern the remarkable changes in properties of the materials, are systematically lineated. The remaining technical barriers, and the challenges to improve the present materials system and develop a new shape memory materials are discussed.

Reinforcement coatings and interfaces in aluminium metal matrix composites

Abstract: The interface between the matrix and reinforcement plays a crucial role in determining the properties of metal matrix composites (MMC). Surface treatments and coating of the reinforcement are some of the important techniques by which the interfacial properties can be improved. This review reports the state of art knowledge available on the surface treatments and coating work carried out on reinforcements such as carbon/graphite, silicon carbide (SiC) and alumina (Al2O3) and their effects on the interface, structure and properties of aluminium alloy matrix composites. The metallic coatings improved the wettability of reinforcement but at the same time changed the matrix alloy composition by alloying with the matrix. Ceramic coatings reduce the interfacial reaction by acting as a diffusion barrier between the reinforcement and the matrix. Multilayer coatings have multifunctions, such as wetting agent, diffusion barrier and releaser of thermal residual stress. The roles of reinforcement coating as a means of “in situ hybridising” and “in situ alloying” are described.

Processing and properties of carbon nanotubes-nano-SiC ceramic

Abstract: Carbon nanotubes–nano-SiC ceramic has been fabricated by the hot-press method. The preparation steps involved the use of dispersing nano-SiC powders and carbon nanotubes in butylalcohol using an ultrasonic shaker. The reasonable relative density of about 95% has been achieved by hot-pressing at 2273 K (at 25 MPa in Ar for 1 h). The three-point bending strength and fracture toughness of the composite has about 10% increment over monolithic SiC ceramic which was fabricated under the same process. The reasons for the increment are the strengthening and toughening role of carbon nanotubes occuring in the matrix.

Deformation characteristics of metal foams

Abstract: The deformation behaviour of a series of aluminium and zinc foams was investigated by uniaxial testing. Because the deformation behaviour of metal foams is expected to be anisotropic owing to the existence of a closed outer skin and with respect to the foaming direction, a series of measurements was carried out where the orientation of the outer skin and the foaming direction were varied. Stress–strain diagrams and corresponding compression strengths were determined for aluminium- and zinc-based foams. The influence of an age-hardening heat treatment was investigated. Finally, the axial deformation behaviour of aluminium tubes filled with aluminium foam was tested under uniaxial loading conditions. The results of the measurements are discussed in the context of possible applications of metal foams as energy absorbers. © 1998 Chapman & Hall

ZnS precipitation: morphology control

Abstract: ZnS is the most important base material for cathode-ray tube luminescent materials. In order to meet the requirements of various types of luminescent screen, routes to ZnS powder with adjustable particle morphology were investigated. The most important morphological parameters of ZnS phosphors are the particle size and the particle size distribution. Industrially, ZnS synthesis is performed by precipitation in an aqueous medium. Precipitation methods included in this study are homogeneous precipitation, precipitation in liquid crystal phase and the introduction of seeds. The powders received were processed to luminescent materials in a standard manner. The results are discussed in relation to raw ZnS powder properties as well as to the morphology and performance of the resulting annealed phosphor powders. From the methods investigated, the use of seed sols based on nanosized CuS, ZnS and S particles yielded the best results, namely, well-defined phosphor particles of micron-scale diameter.

Shape memory materials and hybrid composites for smart systems. Part II shape-memory hybrid composites

Abstract: By hybridizing or incorporating shape-memory materials with other functional materials or structural materials, smart composites can be fabricated which may utilize the unique functions or properties of the individual bulk materials to achieve multiple responses and optimal properties, or, to tune their properties to adapt to environmental changes. A variety of shape-memory hybrid composites have been designed and manufactured, with shape-memory elements being either the matrix or the reinforcement. The hybrid composites provide tremendous potential for creating new paradigms for material–structural interactions and demonstrate varying success in many engineering applications. This review, from the standpoint of materials science, will give a state-of-the-art survey on the various shape-memory hybrid smart composites developed during the last decade. Emphasis is placed on the design, fabrication, characterization and performance of fibre-reinforced, particle-reinforced and multi-layered thin-film shape-memory composites.

A TEM study of precipitation and related microstructures in friction-stir-welded 6061 aluminium

Abstract: Residual microstructures, including dynamic recrystallization and grain growth structures and a wide range of precipitation phenomena associated with a friction-stir-weld in a thin 6061-T6 aluminium plate have been systematically investigated utilizing light metallography and transmission electron microscopy. In this rather remarkable process, a hard steel head pin rotating at 400 r.p.m. was advanced into a solid 6061-aluminium plate at a traverse velocity of approximately 2 mm s−1 to produce a solid-phase weld in its trailing side. Maximum work-piece temperatures did not exceed 425°C and there was no melt evidence. Dynamic recrystallization associated with the solid-state plastic flow therefore seems to provide the process mechanism. Weld zone hardnesses averaged roughly 55 Vickers hardness number (VHN) in contrast to the base plate or work-piece hardness of 110 VHN. Precipitation microstructures ranged from continuous to discontinuous coherent zones (∼2 nm thick) coincident with {1 0 0} planes, semicoherent and non-coherent needles and plates characteristic of Widmanstatten structures coincident with {1 1 0} planes, and a range of homogeneous precipitate particles often intermixed with these microstructures in the effective heat-affected zone (HAZ′) connecting the friction-stir-weld zone with the unaltered work piece microstructures. © 1998 Chapman & Hall

Workability, mechanical properties, and chemical stability of a recycled tyre rubber-filled cementitious composite

Abstract: The workability and mechanical properties of mortar containing shredded automobile and truck tyres were evaluated. Two different shapes of rubber particles were used as constituents of mortar: (1) granules about 2 mm in diameter, and (2) shreds having two sizes which were, nominally, 5.5 mm×1.2 mm and 10.8 mm×1.8 mm (length×diameter). As expected, the geometry of the rubber particles influenced the fracture behaviour of rubber-containing mortar. The addition of rubber led to a decrease in flexural strength and plastic shrinkage cracking of mortar. The crack width and crack length due to plastic shrinkage were reduced for mortar containing the 10.8×1.8 mm rubber shreds compared with a mortar without shreds. The rheological properties of the mortar containing rubber shreds were comparable to those of a mortar without rubber and yielded lower plastic viscosity than a mortar containing 25.4 mm×15 μm (length×diameter) polypropylene fibres. The alkaline stability of rubber in mortar was also evaluated by immersing rubber shreds in NaOH and Ca(OH)2 solutions for 4 mon and the results showed that there is less than 20% change in stress and strain value. The findings of the research suggest that automobile and truck tyres can be recycled by shredding and incorporating them into mortar and probably concrete for certain infrastructural applications. © 1998 Chapman & Hall

Metal nanoclusters in glasses as non-linear photonic materials

Abstract: Although electronics technologies have made great advances in device speed, optical devices can function in the time domain inaccessible to electronics. In the time domain less than 1 ps, optical devices have no competition. Photonic or optical devices are designed to switch and process light signals without converting them to electronic form. The major advantages that these devices offer are speed and preservation of bandwidth. The switching is accomplished through changes in refractive index of the material that are proportional to the light intensity. The third-order optical susceptibility, χ(3), known as the optical Kerr susceptibility which is related to the non-linear portion of the total refractive index, is the non-linearity which provides this particular feature. Future opportunities in photonic switching and information processing will depend critically on the development of improved photonic materials with enhanced Kerr susceptibilities, as these materials are still in a relatively early stage of development. Optically isotropic materials, e.g. glasses that have inversion symmetry, inherently possess some third-order optical non-linearities. Although this is quite small for silica-glasses at λ=1.06 μm, the absorption coefficient is extremely low, thereby allowing all-optical switching between two waveguides, embedded in a silica fibre, simply by controlling the optical pulse intensity. Different glass systems are now under investigation to increase their non-linearity by introducing a variety of modifiers into the glass-network. The incorporation of semiconductor microcrystallites enhances the third-order optical response. Metal colloids or nanoclusters, embedded in glasses, have also been found to introduce desired third-order optical non-linearities in the composite at wavelengths very close to that of the characteristic surface-plasmon resonance of the metal clusters. Ion implantation is nowadays an attractive method for inducing colloid formation at a high local concentration unattainable by the melt-glass fabrication process and for confining the non-linearities to specific patterned regions in a variety of host matrices. Recent works on metal-ion implanted colloid generation in bulk silica glasses have shown that these nanocluster–glass composites under favourable circumstances have significant enhancement of χ(3) with picosecond temporal responses. The remarkable achievements in developing such novel photonic materials seem to open the way for advances in all-optical switching devices, e.g. in inducing metal-colloids into coupled waveguides acting as a directional coupler. The present paper addresses the phenomena of optical non-linearities in metal nanocluster–glass composites that are synthesized by ion implantation, and the potential uses of these novel composite materials in photonics. © 1998 Chapman & Hall

High-temperature strength and thermal stability of a unidirectionally solidified Al2O3/YAG eutectic composite

Abstract: A unidirectional solidification method was investigated to manufacture Al2O3/YAG eutectic composites with high-temperature resistance that would make them usable at very high temperatures. We were successful in manufacturing a single-crystal Al2O3/single-crystal YAG eutectic composite with a dimension of 40 mm in diameter and 70 mm in length containing no colonies or pores. This composite also displayed excellent high-temperature strength characteristics. The flexural strength was in the range 350∼400 MPa from room temperature up to 2073 K (just below its melting point of about 2100 K) with no apparent temperature dependence. During tensile tests above 1923 K, the eutectic composite showed evidence of plastic deformation occurring by dislocation motion, and a yield phenomenon similar to many metals was observed. In addition, the microstructure of the composite was extremely stable: after 1000 h of heat treatment at 1973 K in an air atmosphere there was no growth. The above superior high-temperature characteristics are caused by such factors as the eutectic composite having a single-crystal Al2O3/single-crystal YAG structure, the formation of a compatible interface with no amorphous phase and thermal stability, and the combined effect of a YAG phase with superior high-temperature characteristics. © 1998 Chapman & Hall

Load-adaptive crystalline–amorphous nanocomposites

Abstract: Advances in laser-assisted deposition have enabled the production of hard composites consisting of nanocrystalline and amorphous materials Deposition conditions were selected to produce super-tough coatings, where controlled formation of dislocations, nanocracks and microcracks was permitted as stresses exceeded the elastic limit This produced a self-adjustment in the composite deformation from hard elastic to quasiplastic, depending on the applied stress, which provided coating compliance and eliminated catastrophic failure typical of hard and brittle materials The load-adaptive concept was used to design super-tough coatings consisting of nanocrystalline (10–50 nm) TiC grains embedded in an amorphous carbon matrix (about 30 vol%) They were deposited at near room temperature on steel surfaces and studied using X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, Raman spectroscopy, nanoindentation and scratch tests Design concepts were verified using composition–structure–property investigations in the TiC–amorphous carbon (a-C) system A fourfold increase in the toughness of hard (32 GPa) TiC–a-C composites was achieved in comparison with nanocrystalline single-phase TiC

Molten salt synthesis of lead-based relaxors

Abstract: The molten salt synthesis (MSS) of lead-based relaxors which have a perovskite structure, A(BIBII)O3 where BI is Mg2+, Fe3+, Zn2+, Ni2+ or Co2+, and BII is Nb5+, has been reviewed with regard to the formation of the perovskites, phase stability and morphology characteristics. Two relaxor materials, Pb(Mg1/3Nb2/3)O3 and Pb(Fe1/2Nb1/2)O3 were found to be successfully synthesized at a low temperature in a very short time by the MSS method. Using the example of Pb(Mg1/3Nb2/3)O3, the phase stability has been discussed on the basis of thermal and chemical analyses. The influences of the processing parameters, such as temperature, time, type and amount of salt, and non-stoichiometry, on the formation and the powder characteristics of the perovskite phase were investigated with possible explanations for the observed differences which were induced by changing the parameters. Finally, densification behaviour and dielectric properties resulting from the MSS powder were examined and compared to those of powders obtained by using the conventional mixed oxides (CMO) method. © 1998 Kluwer Academic Publishers

Single fibre fragmentation test for assessing adhesion in fibre reinforced composites

Abstract: The single fibre fragmentation test for measuring the properties of the fibre–matrix interface in fibre-reinforced composites is reviewed. Special emphasis has been paid to the recent stress transfer models in single fibre composites and its application to the development of a suitable data reduction technique for the fragmentation test. The complexities of the correlation of the micromechanical results to the properties of the macrocomposites have been highlighted.

Sol–gel processing of hydroxyapatite

Abstract: Hydroxyapatite, a calcium phosphate-based compound with numerous applications in the biological field was synthesized using the sol–gel processing route. The formation of hydroxyapatite and other compounds during the heat-treatment cycle were identified and characterized using thermal analyses and X-ray diffraction together with infrared and Raman spectroscopy. The influence of the addition of various organic alcohols (R–OH, R=CH3–, C2H5– and C3H8–) on the reaction results was studied. © 1998 Kluwer Academic Publishers

Kinetics of the anatase–rutile transformation in TiO2 in the presence of Fe2O3

Abstract: The anatase–rutile phase transition in TiO2 in the presence of Fe2O3 was investigated in air and argon atmospheres by means of X-ray diffraction and scanning electron microscopy. Isothermal curves of rutile transformed from anatase as a function of time were obtained between 825 and 950 °C. The data were well fitted by various rate laws. In the presence of Fe3+, the anatase–rutile transition temperature is lower and the transformation rate in air is higher than the corresponding one in pure TiO2. The transformation in the presence of Fe3+ in an argon atmosphere is more rapid than in air. The enhancement effect of Fe3+ on the anatase–rutile transformation in both atmospheres is understood on the basis of the formation of oxygen vacancies.

Refinement of Cast Microstructure of Hypereutectic Al-Si Alloys Through the Addition of Rare Earth Metals

Abstract: Microstructural observation and thermal analysis of Al-21 wt % Si alloys with different rare earth metals were performed to examine the effect of rare earth metal on the refinement of primary silicon phase. Simultaneous refinement of both primary and eutectic silicon morphology is achieved with the addition of rare earth and its effect increases with the amount of rare earth addition and cooling rate. Depression of 12–17 °C in primary reaction temperature and 2–7 °C in eutectic temperature is measured with the addition of rare earth. Rare earth bearing compounds were not believed to act as a nucleation agent of primary silicon phase. Some rare earth bearing compounds determined to AlCe were around primary silicon in the matrix. The twin density of eutectic silicon remains same regardless of the addition of rare earth. The refinement of silicon in rare earth treated hypereutectic Al-Si alloys is supposed to be due to the suppression of the nucleation temperature of silicon phase.

Degradation behaviour of a composite material for thermal protection systems Part I–Experimental characterization

Abstract: Thermogravimetric studies and combined thermal analysis techniques have been used to characterize an ablative composite for thermal protection systems. The aim of the work was to utilize these techniques to obtain the main parameters used in the computer simulation of the space re-entry. In particular, a phenomenological model of the degradation kinetics of a silicon-based ablative composite has been developed using thermogravimetric analysis coupled with mass spectroscopic analysis. Simultaneous thermal analysis has also been used to calculate the ablation heat. The results are used as input for a computer model, developed in Part II, to enable calculation of the temperature profiles inside a thermal protection shield during the re-entry into the earth's atmosphere. Such a program can also be used in materials selection. © 1998 Kluwer Academic Publishers

Growth of ultra-fine cobalt ferrite particles by a sol–gel method and their magnetic properties

Abstract: Ultra-fine CoFe2O4 particles are fabricated by a sol–gel method and magnetic and structural properties of powders are investigated Cobalt ferrite powders fired at and above 450 °C have only a single-phase spinel structure and behave ferrimagnetically Powders annealed at 350 °C have a typical spinel structure and are of the paramagnetic and ferrimagnetic nature, simultaneously With X-ray diffraction and Mossbauer spectroscopy measurements, the formation of nano-crystallized particles is confirmed when cobalt ferrite is annealed at 200 °C In addition, the transition from the paramagnetic to the ferrimagnetic state is observed in samples fired at 200 °C as the measuring temperature decreases from the room to liquid nitrogen temperature The magnetic behaviour of CoFe2O4 powders fired at and above 350 °C shows that an increase of the annealing temperature yields a decrease in the coercivity and, in contrast, an increase in the saturation magnetization The maximum coercivity and the saturation magnetization of cobalt ferrite powders prepared by the sol–gel method are 2020 Oe and 765 emu g−1, respectively

Laser-shock processing of aluminium-coated 55C1 steel in water-confinement regime, characterization and application to high-cycle fatigue behaviour

Abstract: 55C1 steel was irradiated with a high-power neodymium-glass laser with application to induce plastic shock waves within targets, through the expansion of a laser-induced surface plasma. Laser-shock processing experiments were conduced in the plasma-confined regime with water to increase the laser-induced peak stresses. Physical, mechanical and processings aspects were reviewed, such as the characterization of stress waves in coated steels with a VISAR velocimeter system, and the mechanical changes induced in 55Cl in terms of compressive residual stresses or work-hardening levels. With the use of convenient protective coatings, some 7-8 GPa peak stress levels could be achieved which authorized the generation of high compressive residual stress levels (nearly 80% of the compressive yield strength), but preserved the surface integrity from detrimental roughening. Surface modifications performed under different shock conditions were shown to display some 30% increase on the bending fatigue limits of 55C1 at R=0.1.

Upconversion luminescence of Er3+ in transparent SiO2—PbF2—ErF3 glass ceramics

Abstract: Oxyfluoride glasses with the composition 50SiO2 · 50PbF2 · xErF3 (x=4 and 5) by molar ratio were developed. Transparent glass ceramics were obtained by heat-treating the 50SiO2 · 50PbF2 · xErF3 glasses at the first crystallization temperatures. X-ray diffraction analysis of the transparent glass ceramics revealed that fluorite type β-PbF2:Er3+ solid solution regions of about 13.0 nm in diameter are precipitated in the glass matrix. The formation of this β-PbF2:Er3+ solid solution was also supported by Eu3+ fluorescence spectra which were measured on specimens in which Eu substituted for Er. Under 800 nm laser excitation, the Er3+ upconversion luminescence of 50SiO2 · 50PbF2 · xErF3 glasses was barely detectable, but the 50SiO2 · 50PbF2 · xErF3 glass ceramics gave Er3+ upconversion luminescence at a very high efficiency. The reason for the highly efficient Er3+ upconversion luminescence in the 50SiO2 · 50PbF2 · xErF3 glass ceramics can be explained in terms of the very small multiphonon relaxation rates that are anticipated from consideration of the Eu3+ emission spectra.

Sinterability of commercial 8 mol% yttria-stabilized zirconia powders and the effect of sintered density on the ionic conductivity

Abstract: The sintering behaviour of a number of commercially produced 8 mol% yttria-stabilized zirconia powders has been studied. The effect of different sintering regimes on the density and microstructure of the sintered ceramic was determined using density measurements, scanning electron microscopy (SEM) and dilatometry. The chemical homogeneity, particle size and the morphology of the as-received powder were related to the sintering behaviour of the different commercial powders. Powders prepared via a route which involved a spray-drying step sintered more readily than those prepared without a spray-drying step. Plasma-derived powders did not sinter to as high an apparent density as co-precipitated powders. The effect of sample density on the ionic conductivity of sintered YSZ ceramics was studied using a.c. impedance spectroscopy. This technique allowed separation of the bulk and grain-boundary components, enabling clear intepretation of the effects of sample porosity of the conduction pathways. Ceramics prepared from the three different powders achieved a bulk ionic conductivity of ∼16 S cm-1 at 1000 °C for sintered densities of 95% or greater. The results obtained are compared to values reported for a variety of other commercial powders. © 1998 Kluwer Academic Publishers

Squeeze casting of magnesium alloys and their composites

Abstract: Squeeze casting, also known as liquid metal forging, extrusion casting and pressure crystallization, is a process in which molten metal soldifies in a die under an applied high pressure. The concept of squeeze casting was invented in Russia over 100 years ago. Later the process was exploited in North America, Japan and Europe to produce various automotive components. With the rapid expansion of magnesium applications in the automotive industry, the development of squeeze casting technology for magnesium alloys and their composites has been motivated by incentives to produce high-quality components. The present paper reviews recent progress in squeeze casting, and the effects of process variables on the cast structure and properties of magnesium alloys and magnesium-based composites. Approaches to optimization of the squeeze-casting process are discussed. The significant advantages of squeeze-cast magnesium alloys and magnesium-based composites are highlighted. The on-going research work at ITM is presented.

Polypyrrole Based Microwave Absorbers

Abstract: Reflection of microwave radiations from single layer and two-layer materials is calculated. Microwave absorbing materials are formulated by mixing a commercially available paint or rubber with the conducting polypyrrole (PPy) powder. The reflection loss strongly depends on thickness and complex permittivity of the material. For a single layer material, optimum values of the real part, ɛ′, and imaginary part, ɛ′′, of the complex permittivity are found by calculations which lead to a minimum reflectivity at a given sample thickness. The ability to readily tailor the conductivity of the PPy powder enables the design of microwave absorbers according to theoretical desired values of ɛ′ and ɛ′′. A paint panel containing 2 wt% of PPy powder with a thickness of 2.5 mm exhibits a reflectivity < − 10 dB (i.e. at least 90% absorption of the incident radiation) over 12 to 18 GHz. Blending and milling during the manufacturing process can destroy the original fibrous shape of PPy aggregates leading to low radiation absorption. In an attempt to achieve a broadband absorber, a two-layer system consisting of a first layer containing PPy powder and a second layer containing carbonyl iron has been fabricated.

Surface structure of the TiO2 thin film photocatalyst

Abstract: Control of the surface structure of titanium dioxide thin film photocatalysts was successfully carried out by a polymer-doped dip-coating process. The thin films prepared were either transparent or opaque, depending on the molecular weight of the polymer doped. All the thin film photocatalysts had anatase form with similar crystallinity. The surface of the transparent thin films looked plain consisting of uniformly aggregated nanometer-size TiO2 particles, while the opaque thin films were formed of cubic crystalline TiO2 at the micrometer level. Both the transparent and opaque films showed catalytic activity for the elimination of NOx in air. The specific surface area and photocatalytic activity of the transparent thin film was almost the same as that of the opaque one. The activity of the thin films was almost equal to the commercial photocatalyst P25. Decrease in the film thickness led to a decrease of the elimination of NO in air by the thin films. The thin films were porous and the surface area was dependent on the film thickness. Adsorbed NO was photooxidized to NO2 by the thin films, while the NO2 formed was re-photooxidized to HNO3 before the desorption of NO2 from the film surface.

Incipient melting of Al5Mg8Si6Cu2 and Al2Cu intermetallics in unmodified and strontium-modified Al–Si–Cu–Mg (319) alloys during solution heat treatment

Abstract: The present work was performed on seven alloys containing in common Al–6.5 wt%Si–3.5 wt%Cu, with magnesium in the range 0.04–0.45 wt%, and strontium in the range 0–300 p.p.m. The alloys were cast in the form of tensile test bars, solution heat treated in the temperature range 480–540°C for times up to 24 h. Two types of solution heat treatment were applied: (i) single-stage, where the test bars were solution treated at a certain temperature for 12 h prior to quenching in hot water (60°C); (ii) two-stage, where the test bars were solution treated for 12 h/510°C+12 h/T°C (T=510, 520, 530, 540°C), followed by quenching in hot water. In the low-magnesium alloys (i.e. with Mg∼0.04 wt%), melting of the Al2Cu phase commenced at 540°C. Increasing the magnesium content to ∼0.5 wt% reduced the incipient melting temperature of the Al5Mg8Si6Cu2 phase to 505°C. The mechanism of incipient melting and its effect on the tensile properties have been discussed in detail. © 1998 Chapman & Hall

A TEM investigation of M23C6 carbide precipitation behaviour on varying grain boundary misorientations in 304 stainless steels

Abstract: Transmission electron microscopy (TEM) along with electrochemical potentiokinetic reactivation (EPR) testing was performed on different grades of 304 stainless steel (0.01, 0.025, 0.05, and 0.07%C) in order to assess the sensitization and precipitation behaviour on different grain boundary misorientations. The materials were heat treated at 670°C for 50 h to subject the materials to the sensitization regime. The EPR data and TEM observations revealed that when the amount of carbon was increased the degree of sensitization increased along with the density of precipitates. Large angle misorientations (Θ>15°) were prevalent in all the carbon content materials and the {1 1 0} grain surface orientation was found to be the major texturing orientation. The steels with lower carbon contents nucleated a few small precipitates on high angle grain boundaries, while larger amounts of carbides were observed on lower angle grain boundaries for the higher carbon contents. It was deemed that higher carbon contents required lower energies to nucleate and grow precipitates. A carbon content threshold was found (above 0.05% C) in which precipitates fully saturate the grain boundary. Precipitation followed the energies of different types of boundaries. The highest energy boundary (general random grain boundary) nucleated precipitates first, then precipitation followed on non-coherent twin boundaries, and was not observed on coherent twin boundaries. A “critical nucleation energy”, γgb(crit.), was therefore found to exist at which precipitation will occur on a boundary. This value was found to be in the range of 16 mJ m-2<γgb(crit.)<265 mJ m-2 which corresponds to the energies of special boundaries (coherent and non-coherent portions of twins respectively) at the ageing temperature of 670 °C. © 1998 Chapman & Hall

Single-crystal growth and characterization of Cu2O and CuO

Abstract: We have prepared a large number of crystals of cuprous oxide (Cu2O) by various procedures. Photoluminescence spectra of these crystals were studied to examine the concentration of defects, especially copper vacancy VCu to seek favourable conditions for growing Cu2O crystal. High-quality single crystals of Cu2O were prepared by the floating-zone melting method in air. Several synthetic crystals (specimens FA, FZ and GZ) and also a natural crystal were studied by X-ray analysis, inductively coupled plasma spectroscopy analysis, optical absorption, photoluminescence, photoconductivity and cyclotron resonance absorption, photoluminescence, photoconductivity and cyclotron resonance absorption to characterize their optical and electrical qualities. The best values of mobility and scattering time of photocarriers at T = 4.2 K are estimated to be μh≈1.8 × 105 cm2 V−1 s−1 and τh≈60 ps for positive holes, and μ•≈1.3 × 105 cm2 V−1 s−1 and τ•≈70 ps for electrons in Cu2O. Further, we report preliminary experimental results on transport property of crystals also of cupric oxide (CuO) purified by the floating-zone melting method.

Characterization of ultrafine γ-Fe(C), α-Fe(C) and Fe3C particles synthesized by arc-discharge in methane

Abstract: Ultrafine gamma-Fe(C), alpha-Fe(C) and Fe3C particles were prepared by arc-discharge synthesis in a methane atmosphere. The phases, morphology, structure and surface layer of the particles were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) techniques and X-ray photoelectron spectroscopy (XPS). It was found that the mean particle size ranged from 9.8 to 12.8 nm. The surface of particles mostly consisted of a carbon layer and a little oxide. Phase transformation from gamma-Fe(C) to alpha-Fe(C) was studied by annealing in vacuum and by differential thermal analysis and thermogravimetry (DTA-TGA) measurement. The abundance of gamma-Fe( C) was determined by a magnetization measurement to be approximately 30%. Phase transformation occurred between 300 and 500 degrees C in a flowing argon atmosphere. The Fe3C particles oxidized to alpha-Fe2O3 and carbon dioxide at 610 degrees C or so. (C) 1998 Chapman & Hall.

Cracking phenomena of brittle films in nanostructure composites analysed by a modified shear lag model with residual strain

Abstract: This paper proposes a modification to the shear lag model for multiple cracking of thin films in order to take into account the residual strain, and uses it to estimate the critical fracture strength of SiOx films with thicknesses from 75 to 660 nm deposited on 12 μm-thick polyethylene terephthalate (PET) substrates. It was found that: (1) The difference of residual strains in the film and substrate increased as the thickness of the film decreased. (2) In both initial and multiple formation of cracks, SiOx films failed at almost constant values of a critical stress ranging from 200 to 300 MPa when the thickness was larger than 200 nm, whereas below that it failed at higher values. © 1998 Chapman & Hall