Showing papers on "Microstructure published in 1985"

Microstructure of Fish Otoliths

Abstract: Otolith microstructure examination has found an increasing number of applications in recent years. However, few workers have critically assessed the assumptions upon which the age and growth infere...

Strength of metals and alloys

Abstract: This book presents the papers given at a conference on the mechanical properties of metals and alloys. Topics considered at the conference included hardening, anisotropy and texture, phase transformations, creep resistance, plasticity, deformations, microstructure, fracture properties, fatigue, wear resistance, temperature effects, stress analysis, and recrystallization.

Controlled Transformation and Sintering of a Boehmite Sol-Gel by α-Alumina Seeding

Abstract: The transformation, microstructural development, and densification of an α-alumina-seeded boehmite sol-gel was studied. α-Alumina particles are shown to act as nuclei for the trans- formation of θ- to α-alumina and to result in an increase in the transformation kinetics and lowering of the transformation temperature by as much as 170°C. By increasing the seed concentration (i.e., nucleation frequency), a submicrometer aggregate-free microstructure develops, rather than the vermicular microstructure that usually characterizes the α-alumina transformation. As a result, the transformed α-alumina sinters to full density with a submicrometer grain size at 1200°C. It is believed that seeding may represent a unique method for microstructure control in the many ceramic systems that transform by nucleation and growth.

Rhenium additions to a Ni-base superalloy: Effects on microstructure

Abstract: Refractory metal alloying to superalloy single crystals has resulted in alloys possessing higher temperature capabilities than have previously been known. In this study, additions of rhenium to a modified MAR-M200* alloy were made at the expense of tungsten. A study was carried out on the γ′ precipitate growth characteristics, morphology, and lattice misfit as a function of rhenium content and aging time and temperature. Rhenium additions substantially lowered the γ′ coarsening kinetics and resulted in negative γ-γ′ misfit alloys. A qualitative model is developed which explains these results. Particle growth was shown to bevia the widely accepted diffusion controlled coarsening mechanism. Precipitates were also shown to grow by particle coalescence into irregular blocky shapes at high aging temperatures while remaining cuboidal at lower temperatures, and the necessity of using large precipitate volume fraction modifications to theoretical coarsening models is illustrated.

The microstructure of rapidly solidified Al 6 Mn

Abstract: The microstructure of rapidly solidified Al−Mn alloys containing 18 to 25.3 wt pct Mn was studied by transmission electron microscopy. One of the phases found in the microstructure exhibits icosahedral symmetry manifested in electron diffraction patterns having five-fold symmetry. A new structural concept is proposed to account for the observed electron diffraction patterns. The structure is assumed to be composed of many connected polyhedra. Although not forming a regular lattice, such structures are able to produce sharp diffraction peaks. The terminal stability and transformation of the icosahedral phase was also studied and reported.

Microstructure and formation mechanism of porous silicon

Abstract: A systematic study is presented of the effects of silicon dopant type, resistivity, current density, and hydrofluoric acid concentration on the formation and properties of porous silicon. Cross‐section transmission electron microscopy revealed the presence of two distinct microstructures. The structure formed is determined by the doping level with the transition occurring near degeneracy. A model of the anodisation process is presented which is based on the semiconducting properties of the material and which explains the formation of the two different types of porous structure observed.

Microstructure-strength properties in ceramics. I: Effect of crack size on toughness

Abstract: A systematic study of the inert-strength characteristics of ceramics as a function of crack size relative to grain size has been made using controlled indentation flaws. The focus of the test program is on aluminas, with barium titanates and glass-ceramics providing support data in confirmation of general trends. On progressively diminishing the indentation load, the strengths first show a steady increase, but subsequently tend to a plateau, as the contact size begins to approach the characteristic grain size. A simple extension of conventional indentation fracture mechanics theory (incorporating residual contact stresses) is developed to describe this scale transition. The basis of the analysis is the postulated existence of a “microstructural driving force,” grain-localized at the center of the pennylike radial crack, in direct analogy to the indentation driving force. This description provides closed-form solutions to the fracture mechanics equations, such that the data are interpretable in terms of an apparent R-curve function. Only two quantities are required to specify the function completely, one relating to the macroscopic toughness determined from large-scale crack specimens and the other to a microstructure-associated stress intensity factor. These quantities are advocated as useful reliability parameters. It is found that the second quantity can vary widely from material to material, even within a given class, to the extent that materials which show superior strength characteristics at large indentation loads may be dramatically weaker at low loads. The indications are that, at least for aluminas, the key to such weakening effects is to be found in the grain-boundary structures. The study emphasizes the need for extreme caution in extrapolating macroscopic-crack data unconditionally into the microscopic-flaw region, and for more fundamental investigations into the underlying physical processes actually responsible for the microstructural driving forces.

The Formation of Filamentous Carbon on iron and Nickel Catalysts

Abstract: The microstructure of primary carbon filaments formed on supported iron and nickel catalysts has been investigated using transmission electron microscopy, dark-held imaging, and (selectedarea) electron diffraction. It has been established that the filaments consist of cone-shaped graphite layers, stacked with their c-axes in a direction normal to the metal-carbon interface. A growth mechanism is proposed involving the excretion of cone-shaped graphite layers. To explain the constancy of the filament diameter, slippage of these layers over one another is invoked. Edge dislocations are brought about by differences in the rate of carbon excretion.

Dependence of thin-film microstructure on deposition rate by means of a computer simulation

Abstract: A computer simulation is employed to demonstrate, in a two‐dimensional growth model, that a vapor‐deposited thin film of low adatom mobility undergoes a sudden change from a porous columnar microstructure to a densely packed film if the substrate temperature is increased to a certain value. The temperature where this structural transition occurs is shown to be related to the lower boundary temperature of the empirical structure‐zone model. The dependence of the transition temperature and range on the vapor deposition rate is discussed.

On elastic relaxation and long wavelength microstructures in spinodally decomposed InxGa1-x.AsyP1-y epitaxial layers

Abstract: The origin of the long-wavelength (100–300 nm) quasi-periodic microstructure observed in transmission electron microscopy studies of spinodally decomposed In Ga1−x As y P1−y alloys has been investigated. The contrast is well explained by diffraction effects arising from lattice plane bending near the surfaces of the thinned specimens, such as would be induced by elastic relaxation of shear stresses accompanying a quasi-periodic lattice modulation. Excellent qualitative agreement between calculated and experimental contrasts lends weight to the claim that these contrasts, observed over part of the composition range of liquid phase epitaxy grown InxGa1−xAs y P1−y alloys, are associated with spinodal decomposition. The accompanying strain modulation amplitude is of the order of 10−3. It is suggested that speckle contrast on a smaller scale (5–15 nm) in this material may also be related to composition variations. More generally, the nature of near-surface elastic relaxation in compositionally modulat...

Microstructures of SrTiO3 Internal Boundary Layer Capacitors During and After Processing and Resultant Electrical Properties

Abstract: The microstructure of strontium titanate internal boundary layer capacitors at various stages in their processing was studied by transmission electron microscopy of rapidly quenched and normally cooled samples. Compositions containing excess TiO2, Al2O3, and SiO2 have a completely wetting liquid phase at the sintering temperature; during cooling TinO2n−1, Magneli phases precipitate at multiple grain junctions. Diffused metal oxides and flux (Bi2O3, PbO, CuO, and B2O3) rapidly penetrate as a liquid phase along boundaries in postsintering heat treatment. This liquid phase disappears during slow cooling.

Effect of ion bombardment during deposition on the x‐ray microstructure of thin silver films

Abstract: The effect of argon ion bombardment, during deposition on the microstructure of several tens of nanometers thick Ag films, has been studied. The structure of the Ag films was analyzed by x‐ray powder diffraction method. Results show that Ar ion bombardment not only influenced the film growth process but had a significant effect on the structure of the resulting films. In comparison to an evaporated thin Ag film, our films showed much less [111] preferred orientation and a lattice expansion normal to the film surface instead of contraction, with compressive rather than tensile surface strain and plane stress. We also observed much smaller grain sizes, and higher twin fault probabilities, microstrains and dislocation densities. These structural parameters varied systematically with the normalized energy En, that is, the energy deposited by incident energetic Ar+ at the film surface per arriving Ag atom; at first rapidly, then leveling off when En≥42% eV/Ag atom. Preferential orientation is believed to be dependent on film thickness as well as on En. Unlike other parameters, twin fault probability increased to a maximum at En=20 eV/Ag atom and then decreased as En increased further due to self‐annealing during deposition.

Silicon carbide filaments - Microstructure

Abstract: The microstructure of chemically vapour deposited silicon carbide filaments has been examined using transmission electron microscopy. The filament bulk consisted of heavily faulted columnar subgrains ofβ-SiC which were preferentially oriented such that {1 1 1} planes were parallel to the surface of the carbon fibre substrate. The protective coating on the filament surface was characterized by several microstructurally distinct layers, all of which consisted primarily of carbon. The first layers of the coating contained small crystallites of SiC in addition to turbostratic carbon, while the outer layers showed no evidence of SiC. Implications of the filament microstructure with respect to mechanical properties are discussed.

The influence of cobalt, tantalum, and tungsten on the microstructure of single crystal nickel-base superalloys

Abstract: The influence of composition on the microstructure of single crystal nickel-base superalloys was investigated. Co was replaced by Ni, and Ta was replaced by either Ni or W, according to a matrix of compositions based on MAR-M247. Substitution of Ni for Co caused an increase inγ′ solvus temperature, an increase inγ-γ′ lattice mismatch, and the precipitation of W-rich phases in the alloys with high refractory metal levels. Substitution of Ni for Ta caused large decreases inγ′ solvus temperature,γ′ volume fraction, andγ-γ′ lattice mismatch, whereas substitution of W for Ta resulted in smaller decreases in these features. For the alloys withγ′ particles that remained coherent, substitution of Ni for Co caused an increase inγ′ coarsening rate. The two alloys with the largest magnitude of lattice mismatch possessedγ′ particles which lost coherency during unstressed aging and exhibited anomalously low coarsening rates. Creep exposure at 1000 °C resulted in the formation ofγ′ lamellae oriented perpendicular to the applied stress axis in all alloys.

High resolution electron microscope study of epitaxial CdTe‐GaAs interfaces

Abstract: CdTe films have been grown on (100) GaAs substrates with two different epitaxial relations: (111)CdTe∥(100)GaAs and (100)CdTe∥(100)GaAs. High resolution electron microscope observation of these two types of interfaces was carried out in order to investigate the role of the substrate surface microstructure in determining which type of epitaxy occurs. The interface of the former type shows a direct contact between the CdTe and GaAs crystals, while the interface of the latter type has a very thin layer (∼10 A in thickness), which is most likely an oxide, between the two crystals. These observations suggest that the GaAs substrate preheating cycle prior to CdTe film growth is crucial in determining which type of epitaxy occurs in this system.

Layered structures, epitaxy, and interfaces

Abstract: This book contains papers presented at a symposium on layered structures - epitaxy and interfaces. Topics include the following: processing; mechanical properties and reliability; interface and films; polymers; microstructural and chemical compounds of ceramic metals interfacial fracture energies.

The microstructure of a ZnO varistor material

Abstract: The microstructure of a ZnO varistor material has been investigated by a combination of X-ray diffractometry and analytical electron microscopy (SEM, TEM, STEM, EDX). The material was found to consist of: ZnO grains (doped with manganese, cobalt and nickel); smaller spinel grains which hinder the growth of ZnO grains during sintering; intergranular Bi-rich phases (namely α-Bi2O3, pyrochlore and an amorphous phase); and a small proportion of ZnO-ZnO interfaces which did not have any intergranular film but to which bismuth had segregated. The intergranular microstructure is largely a result of processes which occur during liquid phase sintering and subsequent cooling to room temperature.

Microstructure and sintering kinetics of highly reactive ZrO2-Y2O3 ceramics

Abstract: Ultra-fine stabilized zirconia powders, which only contain extremely small aggregates were prepared. The control of agglomerates and aggregates in these powders is of utmost importance in order to obtain highly sinter-reactive ceramics. The very small aggregates appear to be the smallest microstructural units which determine the ultimate packing situation after compaction. Resulting green microstructures and sintering behaviour were studied extensively. The sintering process is seen to proceed via several stages of micro-structural development. During the most important stage, where the ceramic material approaches full density, the observed occurrence of abnormal grain growth strongly influences the ultimate grain size. The extent of abnormal growth is highly dependent on aggregate sizes present in the starting powder.

Microstructure of silicon implanted with high dose oxygen ions

Abstract: Buried implanted oxide layers have been formed by high dose implantation of oxygen ions (3×1018 ions cm−2) into 〈100〉 silicon wafers, at a constant temperature of 500 °C. The implanted layers were studied by cross‐sectional transmission electron microscopy and secondary ion mass spectroscopy. The defects at both the top Si/SiO2 and the SiO2/bulk Si interfaces are shown to be SiO2 precipitates. The precipitates are unstable and can be eliminated by heat treatment, and a homogeneous top silicon layer with a low density of dislocations can be obtained.

The microstructure of the steel fibre-cement interface

Abstract: The microstructure of the steel fibre-cement paste interface was studied by scanning electron microscopy. The interfacial zone surrounding the fibre was found to be substantially different from the “bulk” paste further away from the fibre surface. The interfacial zone consisted of (a) a thin (1 or 2Μm thick) duplex film in actual contact with the reinforcement, (b) outside of this, a zone of perhaps 10 to 30Μm thickness, which, in reasonably well hydrated systems, is largely occupied by relatively massive calcium hydroxide crystals, with occasional interruptions of more porous regions, and (c) outside of this, a highly porous layer parallel to the interface. The interaction of cracks initiated in the matrix with this interfacial zone was observed.