Showing papers on "Microstructure published in 1992"

Generalized shape optimization without homogenization

Abstract: Two types of solutions may be considered in generalized shape optimization. Absolute minimum weight solutions, which are rather unpractical, consist of solid, empty and porous regions. In more practical solutions of shape optimization, porous regions are suppressed and only solid and empty regions remain. This note discusses this second class of problems and shows that a solid, isotropic microstructure with an adjustable penalty for intermediate densities is efficient in generating optimal topologies.

Hafnia and hafnia-toughened ceramics

Abstract: Hafnia (HfO2) and hafnium-based materials are traditionally regarded as technologically important materials in the nuclear industry, a consequence of their exceptionally high neutron absorption coefficient. Following the discovery of transformation toughening in the mid 1970s, a considerable research effort has been devoted to zirconia (ZrO2)-toughened ceramics (ZTCs). They are considered to be potentially useful materials for structural applications at low and intermediate temperatures (T 1000 °C) is related to the low temperature of the tetragonal to monoclinic transformation in ZrO2. On the basis that HfO2 exhibits a similar crystal structure and in particular that its tetragonal to monoclinic transformation temperature (∼1700 °C) is approximately 700 °C higher than that for ZrO2, it has been suggested that high-temperature transformation toughening could be possible in HfO2-toughened ceramics (HTCs). Although the concepts behind this suggestion are universally appreciated, only a limited success has been made of the fabrication and the microstructural and mechanical property evaluation of these materials. The fracture toughness values obtained so far in HfO2 toughened ceramics are, in fact, considerably lower than those obtained in their ZrO2 counterparts. A great deal of further research work is therefore required in order to understand fully and to exploit toughened ceramics in the HfO2-based and HfO2-containing systems. This review covers the science and technology of HfO2 and HfO2-toughened ceramics in terms of processing, phase transformation, microstructure, and mechanical properties.

Materials Fundamentals of Molecular Beam Epitaxy

Abstract: Preface. List of Figures. List of Tables. Bulk Phase Equilibra. Free Energies and Open Systems. Elemental Phases. Alloy Phases. Thin Film Structure and Microstructure. Ordering and Clustering. Coherency and Semi-coherency. Surface Morphology and Composition. Surface Composition. Index.

The formation of aluminum oxide scales on high-temperature alloys

Abstract: This paper is a brief review of the extensive literature relating to the formation of protective α—Al2O3 scales on alloys at high temperature. Emphasis is placed on the proposed mechanisms of scale growth based on observations of scale morphologies and microstructures, inert-marker experiments and the distribution of oxygen isotope tracers within thermally-grown oxides. Attention is also given to the determination of ionic-transport mechanisms by electrochemical methods and to the effects of reactive elements such as yttrium in modifying ionic-diffusion processes.

Microstructural evolution and mechanical properties of a forged gamma titanium aluminide alloy

Abstract: A two-phase gamma titanium aluminide alloy, Ti-47Al-1Cr-1V-2.5Nb (in at.%), was studied under forged and various subsequent heat treatment conditions, to investigate the microstructural evolution and the effect of microstructure on room temperature (RT) tensile properties and fracture toughness behavior. Four classes of microstructure and three types of lamellar formation were identified, and their formation mechanisms were analyzed using various analytical techniques including metallography, electron optics, differential thermal analysis (DTA), and crystallography. It was found that both tensile and toughness behavior were profoundly affected by the microstructural variations.

Submagmatic microfractures in granites

Abstract: Microstructures in Hercynian granite plutons of the Pyrenees indicate fracturing of plagioclase crystals in the presence of melt. This is represented mainly by quartz and/or feldspar that fills fractures. Electron-microprobe analyses demonstrate that, in compositional agreement with the rims of the zoned plagioclase, the feldspar infillings are more sodic with decreasing melt fraction. Fracturing resulted from concentration of stress at contacts between grains while the granite was submagmatic, i.e., when the melt was below the critical fraction for magmatic flow. Material compositionally far from eutectic is petrologically the best candictate to develop submagmatic fractures, because during cooling the volume fraction of melt remains low for a large temperature range. A strong tendency for microfractures to be oblique to flow foliation may constitute a new criterion for shear sense in magmas.

The microstructure evolution of a Fe73.5Si13.5B9Nb3Cu1 nanocrystalline soft magnetic material

Abstract: The microstructure evolution in the course of crystallization of a splat-quenched Fe73,5Si13.5B9Nb3Cu1 amorphous alloy was investigated by atom probe field ion microscopy (APFIM) and high resolution transmission electron microscopy (HRTEM). All the alloying elements were found to be distributed homogeneously as an amorphous solid solution in the as-quenched state. At an initial stage of annealing, a concentration fluctuation of Cu was found to occur. Cu formed clusters of a few nanometer diameter and their composition was found to be approximately 30 at.% Cu at the beginning. In the later stage, a b.c.c. FeSi solid solution and the B and Nb enriched amorphous phase with the smaller Si content were found to coexist. In addition to these two phases, Cu enriched particles containing approximately 60 at.% Cu were found to be present in the intergranular regions, although we were not successful yet to determine whether this was a crystalline or amorphous phase. Based on these observations, we discuss the crystallization process of this alloy at 550°C which leads to the emergence of excellent soft magnetic properties.

Characterization of eutectic Sn-Bi solder joints

Abstract: This report presents experimental results on 58Bi-42Sn solder joints, optical and SEM microstructures of their matrix and of their interface with copper, solidification behavior studied by differential scanning calorimetry, wettability to copper, creep, and low cycle fatigue. These results are discussed in comparison with 60Sn-40Pb solder, and with three low temperature solders, 52In-48Sn, 43Sn-43Pb-14Bi, and 40In-40Sn-20Pb. The 58Bi-42Sn solder paste with RMA flux wets Cu matrix with a wetting angle of 35° and had a 15° C undercooling during solidification. The constitutive equation of the steady state shear strain rate, and the Coffin-Manson relation constants for the low cycle shear fatigue life at 65° C have been determined. The test results show that this solder has the best creep resistance but the poorest fatigue strength compared with the other four solders.

Relationships between dopants, microstructure and the microwave dielectric properties of ZrO2-TiO2-SnO2 ceramics

Abstract: Ceramics with compositions in the solid solution region of the ZrO2-TiO2-SnO2 equilibrium diagram are finding wide application as dielectrics in filters for communications and radar systems operating at microwave frequencies. Commercially available compositions often incorporate sintering aids and dopants to reduce processing temperatures and modify the dielectric properties. However, the mechanism through which these additives influence dielectric loss is not obvious. The role of zinc oxide as a sintering aid and lanthanum and niobium as dopants, their effect upon microstructural development and their correlation with dielectric loss at microwave frequencies were investigated. For specimens of density greater than 90% theoretical, the influences of defect chemistry upon dielectric loss appear to dominate those of the microstructure. Properties close to those which might be considered intrinsic were attained through sintering for periods of up to 128h. Doping with lanthanum is detrimental to the dielectric loss, particularly after long sintering times.

Semisolid solidification of high temperature superconducting oxides

Abstract: Experiments are reported on two techniques for melt‐texture processing Ba2YCu3O6.5 by directional solidification from a semisolid melt containing particles of BaY2CuO5 and a copper‐rich liquid. One of these employs an electric resistance furnace with ambient or oxygen enriched atmosphere; the other is a laser‐heated furnace operating at 1.3 atm oxygen. Solidification interface morphologies and other structural features were examined in quenched specimens. Depending on growth rate and temperature gradient, three different types of growth morphologies of the growing 123 phase were observed: ‘‘faceted plane front,’’ ‘‘cellular dendritic’’ or ‘‘equiaxed blocky.’’ The interface temperature decreased markedly with increasing growth rate for the faceted plane front specimens. In the remaining specimens, solidification took place over a range of temperatures. The temperature of the ‘‘root’’ of the solidification front dropped, but temperature of the solidification front ‘‘tip’’ did not. A solidification model is ...

Automatic analysis of electron backscatter diffraction patterns

Abstract: The ability to measure lattice orientation in individual crystallites enables a more complete characterization of microstructure by combining lattice orientation with morphological features. Lattice orientation can be obtained by analyzing electron backscatter diffraction patterns (EBSPs). However, current computer-aided EBSP analysis techniques make it impractical to obtain the number of measurements needed for statistically reliable characterizations of microstructure. An effective fully automated technique for determining crystallographic orientation from EBSPs is described. Bands are identified by linear regions of correlation in the image intensity gradient direction. The most probable orientation is then found using the angles between the detected bands. The reliability of the technique was tested using a set of 1000 patterns obtained from annealed oxygen-free electrical grade (OFEC) copper. The orientation of each test pattern found using automatic indexing was checked against the corresponding orientation as determined by manual indexing. Ninety-nine percent of auto-indexed orientations were found to lie within 5 deg of the misorientation angle of the manual-indexed orientations. By simulating noise in the test patterns, it was found that image quality has a strong effect on the reliability of the technique. An image quality parameter is described which allows the reliability of the technique to be predicted for a pattern of given quality.

Particle morphology and chemical microstructure of colloidal silica spheres made from alkoxysilanes

Abstract: Monodisperse colloidal silica spheres with radii in the range 10–500 nm were prepared by hydrolysis and condensation of tetraethoxysilane (TES) in a mixture of water, ammonia and a lower alcohol at several temperatures. It was attempted to establish a relation between the morphology and the chemical microstructure of the particle. Particle morphologies were examined with transmission electron microscopy and static and dynamic light scattering. The microstructure of the spheres was studied with quantitative direct excitation 29Si nuclear magnetic resonance (NMR) spectroscopy and a combination of qualitative cross-polarization 13C NMR and elemental analysis. A comparison was made between these particles and particles prepared from TES in an ammonia/water in cyclohexane microemulsion and also with Ludox® silica particles. The siloxane microstructure was found to show almost no variation as a function of concentration of reagents, catalyst, co-solvent and temperature. Around 65% of the silicon nuclei was bonded through siloxane bonds with four other silicons, approximately 30% was bonded with three other silicons and a few percent with only two. It was shown that under most experimental conditions several percent of the ethoxy groups never leave the TES molecule and end up inside the silica. The Ludox particles were found to consist of a more condensed silicon structure as compared with particles synthesized in alcohol, ammonia, water mixtures, whereas the spheres prepared in the microemulsion were less condensed and contained more alkoxy groups. The differences in particle morphologies — ranging from irregularly shaped rough particles to perfect, smooth spheres — are not caused by differences in siloxane and ethoxy microstructure. It is proposed instead, that a smooth and spherical particle shape is the result of the growth by monomers or small oligomers, and that a rough, irregular shape is the result of growth by larger silicon structures.

Design and properties of glass-ceramics

Abstract: Glass-ceramics are microcrystalline solids produced by the controlled devitrification of glass. Glasses are melted, fabricated to shape, and then converted by heat treatment to a predominantly crystalline ceramic. The basis of controlled crystallization lies in efficient internal nucleation ( 1), which allows development of fine, randomly oriented grains without voids, microcracks, or other porosity. A unique manufacturing advantage of glass-ceramics over conventional ceramics is the ability to use high-speed plastic forming processes developed in the glass industry (e.g. pressing, blowing, rolling, etc.) to create complex shapes essentially free of internal inhomogeneities. Because glass-ceramic compositions are designed to crystallize, however, they can­ not be held for long periods at temperatures below the liquidus during the forming process. Therefore, the viscosity at the liquidus temperature is critical both in the choice of a forming process and in the choice of a glass composition. The properties of glass-ceramics depend upon both composition and microstructure. The bulk chemical composition controls the ability to form a glass and its degree of workability. In order to achieve internal nucleation, suitable nucleating agents are melted into the glass. Bulk com­ position also directly determines the potential crystalline phase assem­ blage, and this in turn governs the general physical and chemical charac­ teristics, e.g. hardness, density, acid resistance, etc. Secondly, but equally important, is the importance of microstructure. Microstructure is the key

Nanometer-scale lithography using the atomic force microscope

Abstract: We demonstrate a new use of the atomic force microscope (AFM) for nanometer‐scale lithography on ultrathin films of poly(methylmethacrylate) (PMMA). The PMMA films were chemically modified as both positive and negative resists due to energy transfer from a highly localized electron source provided by metallized AFM tips. We were able to fabricate a line pattern with 68 nm line periodicity with about 35 nm line widths.

Polycrystalline TiN films deposited by reactive bias magnetron sputtering: Effects of ion bombardment on resputtering rates, film composition, and microstructure

Abstract: Transmission electron microscopy, x‐ray diffraction, and Rutherford backscattering have been used to investigate the effects of ion irradiation during growth on the deposition rate, composition, and microstructure of single‐phase polycrystalline NaCl‐structure TiNx films deposited by reactive magnetron sputtering with a negative substrate bias voltage Vs. The layers were deposited on thermally oxidized Si(001) substrates in mixed Ar+4% N2 discharges at a total pressure of 4.2 mTorr. Varying Vs between 0 and 1800 V resulted in incident ion‐to‐Ti atom flux ratios of 0.3 to 0.6 at the film growth surface and increases in the substrate temperature Ts (initially Ts=300 °C) of 40 to 200 °C. The Ti resputtering yield increased from ≤0.02 (Vs≤100 V) to 0.30 (Vs=1800 V) Ti atoms per incident ion (primarily Ar+), while the N/Ti ratio in as‐deposited films increased from 1.03 for Vs=0 V to 1.12 for 100 V≤Vs≤400 V and then decreased to ≂0.95 as Vs was raised to 1800 V. Trapped Ar concentrations ranged from ≤0.5 at.% ...

Chitosan-lipid films: microstructure and surface energy

Abstract: Composite films of chitosan-laurate were shown to have low water permeability, while films containing other fatty acids or esters were not effective in this respect. Electron microscopy revealed the microstructure of these films to be very different, in terms of density, pore formation, channeling, surface contour, and packing pattern. The surface energy of the films was estimated, and the dispersion force component (γ s d ) and the polar component (γ s d ) were calculated

Gas Pressure and Temperature Dependences of Thermal Conductivity of Porous Ceramic Materials: Part 2, Refractories and Ceramics with Porosity Exceeding 30%

Abstract: A review of experimental results and theoretical models for thermal conductivities of ceramic materials with porosity less than 30% is given. It is shown that the abnormal non-monotonic pressure and temperature dependences of thermal conductivity arise from the effects of microcracks and porous grain boundaries, characterizing many industrial refractories, and from the competitive influences of classical and novel mechanisms of heat transfer in composite multiphase materials. The latter mechanisms include segregation and surface diffusion of impurities and defects in crystal structure, and the mechanism arising from chemical conversion and gas emission, occurring within pores of ceramic materials. A fractal model of porous materials' structure is proposed and used for analysis, explanation, and classification of thermophysical properties of ceramic materials measured in various thermodynamic conditions.

Influence of microstructure on crack-tip micromechanics and fracture behaviors of a two-phase TiAl alloy

Abstract: The tensile deformation, crack-tip micromechanics, and fracture behaviors of a two-phase (γ + α2) gamma titanium aluminide alloy, Ti-47Al-2.6Nb-2(Cr+V), heat-treated for the microstructure of either fine duplex (gamma + lamellar) or predominantly lamellar microstructure were studied in the 25 °C to 800 °C range.In situ tensile and fracture toughness tests were performed in vacuum using a high-temperature loading stage in a scanning electron microscope (SEM), while conventional tensile tests were performed in air. The results revealed strong influences of microstructure on the crack-tip deformation, quasi-static crack growth, and the fracture initiation behaviors in the alloy. Intergranular fracture and cleavage were the dominant fracture mechanisms in the duplex microstructure material, whose fracture remained brittle at temperatures up to 600 °C. In contrast, the nearly fully lamellar microstructure resulted in a relatively high crack growth resistance in the 25 °C to 800 °C range, with interface delamination, translamellar fracture, and decohesion of colony boundaries being the main fracture processes. The higher fracture resistance exhibited by the lamellar microstructure can be attributed, at least partly, to toughening by shear ligaments formed as the result of mismatched crack planes in the process zone.

Formation and magnetic properties of nanocrystalline mechanically alloyed Fe‐Co

Abstract: Fe100−xCox powders were prepared by mechanical alloying of the elements in a planetary ball mill They were investigated with respect to phase formation and magnetic properties using x‐ray diffraction, differential scanning calorimetry, and measurements of the saturation magnetization and the coercivity The measurement of the saturation magnetization proved the true formation of the bcc (x≤80) and fcc (x=90) solid solutions by mechanical alloying A nonequilibrium microstructure originates from a grain‐size reduction to minimum 20–30 nm and the introduction of internal strain up to 1% (root‐mean‐square strain) An improvement in the soft magnetic properties by the nanocrystalline state, as hoped for, does not occur, because the high amount of internal strain together with the high saturation magnetostriction of the Fe‐Co alloys causes relatively high coercivities of 5–40 A/cm Grain growth and strain relaxation induced by controlled heat treatment of the as‐milled powders allowed the separation of the in

Unusually high thermal conductivity in diamond films

Abstract: Chemical‐vapor‐deposited (CVD) polycrystalline diamond films have recently been reported with a thermal conductivity that is only 25% less than that of high quality single‐crystal natural diamond. By studying a series of such films of various thicknesses grown under virtually identical conditions, we have discovered a significant (factor of four) through the thickness gradient in thermal conductivity. The observed gradient is attributed mainly to phonon scattering by the roughly cone‐shaped columnar microstructure. For 350 μm films, the material near the top (growth) surface has a conductivity of at least 21 W/cm °C, i.e., comparable to the best single crystals. This remarkable dependence of thermal conductivity on microstructure has important implications for thermal management of microelectronic devices using CVD diamond.

Synthesis of ceramic membranes

Abstract: Non-supported γ-alumina films are prepared from a boehmite colloidal suspension. After calcination at 600°C, the microstructure is characterized by a mean pore diameter of 3 nm, a porosity of 50% and a tortuosity of 5.5. The structure is formed by card packed, plate-shaped particles, giving rise to slit-shaped pores. Supported γ-alumina films, made by a slipcasting process using the same boehmite precursor, have a similar structure as the non-supported films. The slipcasting process is very sensitive to support characteristics and frequently yields defect films. Polyvinylalcohol (PVA) (molecular weight 72000 g mol−1) is added to the boehmite precursor (0.25 gg−1 film) to improve this process. This addition results in a less critical and better controllable drying and calcining procedure. The addition of PVA is necessary to slipcast defect free γ-alumina films on supports with pore diameters of 0.4 μm or smaller and on multi-layer supports. It has no significant effect on the resultant microstructure of the γ-alumina film, provided all PVA is removed by appropriate thermal treatment. The slipcasting rate is slower, resulting in thinner films of 3–5 μm at identical slipcasting times.

Microstructure, composition and property relationships of plasma-sprayed thermal barrier coatings☆

Abstract: The benefits deriving from the use of thermal barrier coatings are briefly reviewed. Properties important for a thermal barrier coating are discussed and reasons for the selection of ZrO2-based alloys highlighted. Candidate zirconia alloys are critically evaluated in terms of their thermal and mechanical properties. Since coatings currently applied consist of a tetragonal phase, the long-term stability of the phase structure and the avoidance of the monoclinic phase is addressed, as is the effect of long-term ageing on the thermal properties. The relationship between microstructure and properties of plasma coatings is reviewed. Future trends, such as extending areas of application in jet engines, use of alternative alloys, and methods of application and optimization of plasma sprayed coatings by modelling the plasma spray process, are also briefly discussed.