Showing papers on "Microstructure published in 1971"

Nonohmic Properties of Zinc Oxide Ceramics

Abstract: Nonohmic properties of ZnO ceramics with five additives of Bi2O3, CoO, MnO, Cr2O3, and Sb2O3 are studied in relation to sintering temperature, additive content, and temperature dependence. The observation of electron photomicrographs and X-ray microanalysis proves a ceramic microstructure such that ZnO and these five oxides form, at the grain boundaries, segregation layers which are responsible for the nonohmic properties. The electrical resistivity and dielectric constant of segregation layers are estimated to be 1013 ohm-cm, and 170, respectively by using a simple model. The electric field strength corresponding to the steep rise in the current is also estimated to be 104 V/cm by taking account of the concentration of applied voltage at the segregation layer. In view of these data and simple model, a possible explanation for nonohmic properties is discussed.

Improved Hot-Pressed Electrooptic Ceramics in the (Pb,La)(Zr,Ti)O3 System

Abstract: Physical, electrical, and optical properties of selected compositions within the lead lanthanum zirconate-titanate (PLZT) system are presented, including chemically and thermally etched microstructures, examples of residual porosity defects, temperature dependence of remanent polarization and coercive field, lateral switching strains, and thermal expansion and optical transmission data. Optical-quality ceramics were produced by stringent raw material selection and improved ceramic processing and hot-pressing. Optimum results were obtained from starting oxides with higher purity (99.9%) and smaller particle size (<2 μm) than those used in previous investigations. These oxides were processed by methods developed to minimize segregation and promote chemical homogeneity and hot-pressed at temperatures (1050° to 1250°C) higher than those used previously to remove residual porosity.

Study of microstructure of chrysotile asbestos by high-resolution electron microscopy

Abstract: Several samples of chrysotile asbestos from different localities, including a synthetic sample, were electron-microscopically observed by the lattice imaging method along two directions parallel and perpendicular to the fibre axis. The results are as follows: (a) The lattice fringes of 4.5 A corresponding to 020 were often tilted to the edge of the fibrils with an angular distribution ranging up to about 10° with a peak value at a few degrees, depending on the sample. (b) Most of the fibrils examined were hollow cylinders and their circumferential lattice layers form spiral or multi-spiral layers. The perfectly concentric cylindrical layers were also found with a frequency depending on the sample. (c) Unusual growth patterns which cannot be explained by Jagodzinski and Kunze's model were observed. (d) The lattice images of the cortical fibrils (cone-in-cone shape) were observed in the synthetic sample. (e) Most fibrils greater than about 350 A in diameter showed traces of discontinuous growth in two or three steps, depending on the growth conditions, and this gave rise to various distributions of the fibril diameters.

The rapid heat treatment of steel

Abstract: The technique of austenitizing steel by heating rapidly through the Ac1 to Ac3 range, limiting the maximum temperature to the minimum required for complete austenitization and quenching immediately is discussed. Rapid austenitizing refines the austenite grain size if the initial microstructure is a fine aggregate, such as martensite or tempered martensite. Additional grain refinement usually results from two or more cycles and most steels hardenable by heat treatment become ultrafine grained and hence exhibit increased strength and toughness. Rapid austenitizing can also be applied, particularly to high-carbon steels, to develop a unique microstructure comprised of a uniform dispersion of very small carbide particles in an ultrafine grained martensitic matrix.

The variation in flow stress and microstructure during superplastic deformation of the Al-Cu eutectic

Abstract: Stress-strain curves have been obtained for the superplastically deformed Al-Cu eutectic tested in tension under constant true strain-rate conditions. It is shown that constant flow stress conditions do not obtain and that, after an initial transient, the flow stress is linearly related to natural tensile strain. Optical metallography has been employed to follow the variation of both inter-phase particle separation and α-Al grain size with strain and it is concluded that the observed strain hardening is due mainly to grain coarsening.

Effect of Microstructure on Pulse Electrical Strength of MgO

Abstract: Pulse electrical strengths of hot-pressed MgO containing 0.3 wt% LiF were measured. Strengths as high as 5.9×108 V/in. were obtained on samples 99.8% of theoretical density with 1-to 3-μm grain size. An increase of either porosity or grain size resulted in a monotonic decrease in strength. Single-crystal strengths of 4.0×106 V/in. were obtained by polishing samples chemically. A simple recessed-electrode configuration eliminated corona effects.

The strength and fracture toughness of 18 Ni (350) maraging steel

Abstract: The influence of microstructure on the strength and fracture toughness of 18 Ni (350) maraging steel was examined. Changes in microstructure were followed by X-ray and neutron diffraction and by optical and electron microscopy. These observations have been correlated with the fracture morphology established by scanning electron microscopy. Air cooling this alloy from the austenitizing temperature results in a dislocated martensite. During the initial stage of age hardening, molybdenum atoms tend to cluster (forming preprecipitates) and the cobalt assumes short range ordered positions. Subsequent aging results in Ni3Mo and σ-FeTi with overaging being associated with the formation of equilibrium reverted austenite and Fe2Mo. The fracture behavior is examined in terms of elementary dislocation precipitate interactions. It is suggested that the development of coplanar slip in the underaged conditions leads to its increased stress corrosion susceptibility and decreased fracture toughness. The optimum aged condition is then associated with cross-slip deformation. The fracture behavior of the overaged condition is a dynamic balance between a brittle matrix and the ductile (crack blunting) reverted austenite.

Preparation of Small‐Grained and Large‐Grained Ceramics from Nb‐Doped BaTiO3

Abstract: Barium titanate powders with average grain sizes of 0.07 and 0.65 μm were coated with an organic Nb ester, using a liquid-solid mixing process. Small-grained (1 to 3 μm) ceramic dielectrics from both powders and large-grained (>50 μm) dielectrics from the 0.65 μm powder were fired using 0 to 2 at.% Nb. Microstructures, densities, and electrical resistivities were investigated.

The interrelationship of fracture toughness and microstructure in a Ti-5.25Al-5.5V-0.9Fe-0.5Cu alloy

Abstract: Fracture toughness of anα-Β titanium alloy heat treated to a constant yield strength has been found to depend upon the morphology of α produced or remaining after the initial solution treatment. In equiaxed α structures, fracture toughness depends linearly upon the grain boundary area per unit volume,Sv, and is independent of equiaxed α particle size or spacing. In a grain boundary α structure fracture toughness depends both onSv and, within limits, linearly on the thickness of the α. Explanations are offered for the observed propagation of cracks at α-@#@ Β interfaces and for the observation that high fracture toughness can accompany low tensile ductility.

Salt fingers in the presence of grid-generated turbulence

Abstract: The transport of heat and salt across a density interface containing salt fingers is investigated when turbulence produced by vertically oscillating horizontal grids is imposed on the deeper layers above and below the interface. The fluxes of heat and salt are measured as functions of the stirring frequency. The results are discussed with reference to the parameter $\lambda = w^2/(\overline{u})^2$ , where w is the velocity of the fluid in the fingers in the undisturbed state and $\overline{u}$ is the r.m.s. horizontal velocity of the turbulence. It is found that the salt flux has a minimum (as a function of the stirring frequency) when Λ ∼ 0·3 and that when Lambda; [lsim ] 0·05 the transport across the interface is dominated by mechanical mixing. The ratio r of the contributions of heat and salt to the buoyancy flux increases with decreasing Λ and r > 1 when Λ [lsim ] 1·3. The latter result implies that if turbulent intensities are such that Λ [lsim ] 1·3 in a particular oceanic situation, mechanical mixing will affect the vertical transport of heat and salt in such a way that the salt finger mechanism will be unable to produce layered temperature and salinity microstructure in the manner described by Turner (1967).

New porous biomaterials by replication of echinoderm skeletal microstructures.

Abstract: AN important aspect of materials science is the development of new porous solids for prosthetic applications, especially for synthetic teeth and bone replacement. To be acceptable, such biomaterials must meet some rather demanding specifications, among which are high strength and compatibility with-body fluids or tissues. The structure of the porous medium is important as the pores must be both interconnected and have the proper size, shape and uniformity to permit ingrowth and attachment of body tissue. The problem lies in attaining the optimum combination of these properties to yield a high strength to weight ratio material, with a microstructure which best facilitates firm attachment of tissue. Ideally these materials should be easily machined or fashioned into particular shapes for different individual implants, and should be relatively inexpensive.

A study of stress-corrosion in Al-Zn-Mg alloys

Abstract: This study of Al-Zn-Mg alloys was undertaken to establish the specific combination of mechanical and thermal conditions which is necessary for stress-corrosion susceptibility to occur in these alloys. The work included a study by electron transmission microscopy of the microstructure of this alloy system. A new mechanism is proposed to account for the observations made. It suggests that the high corrosion potential at grain boundaries in stress-corrosion susceptibile Al-Zn-Mg alloys is due to segregation of zinc and magnesium. By taking precautions to reduce this segregation, the stress-corrosion life can be increased.

Microstructure and Electrical Properties of Sc2O3‐Doped, Rare‐Earth‐Oxide‐Doped, and Undoped BaTiO3

Abstract: Polycrystalline BaTiO3 prepared from alkoxy-derived high-purity submicron powders was studied. Highly dense bodies with uniform grain size were obtained typically by uniaxial cold-pressing at 3000 psi and isostatic pressing at 30,000 psi followed by sintering at 1300° to 1350°C in air for 0.5 to 1 h. Using the same consolidation parameters and intimate mixing of residual concentrations of highly active fine-particulate rare-earth oxides to act as grain-growth inhibitors, nearly theoretically dense bodies with a uniform microstructure and 1 to 1.5 μm grain size were obtained. Typical microstructures with well-defined 90° and 180° domain patterns characteristic of BaTiO3: were observed. Also, an example of a checkerboard pattern resulting from a 〈111〉 ingrown twin plane in the structure which is independent of the Curie temperature was found. Electrical measurements on the undoped material indicated room-temperature dielectric constant and tan δ values of 5000±500 and 4×10−3, respectively. Very high k values and dissipation factors were observed with the La2O3- and Nd2O3-doped samples.

Effect of gamma-gamma prime mismatch, volume fraction gamma prime, and gamma prime morphology on elevated temperature properties of Ni, 20 Cr, 5.5 Mo, Ti, Al alloys

Abstract: This study was designed to provide a critical test for the postulate that the mismatch between the lattices of austenite (γ) and the age-hardening gamma-prime (γ′) precipitates and the resultant coherency strains have a significant influence on the elevated temperature, particularly stress rupture, properties of a nickel-base superalloy. Two experimental alloys with a base analysis of Ni, 20 Cr, 5.5 Mo were designed with variable titanium and aluminum additions. To discern the effect of mismatch, an alloy without molybdenum was also experimented with. By manipulating the mismatch and volume fraction γ′ by heat treatment and chemistry, it was shown that a lower γ-γ′ mismatch indeed is beneficial to stress rupture life. Importance of volume fraction γ′ on this elevated temperature was also established.

Resorbable ceramic implants

Abstract: The purpose of the investigation was to show the ability of two calcium aluminate ceramic compositions to act as suitable implants for partial replacements of long bones. It was anticipated from previous studies that the ceramic would be well tolerated by the autogenous bone and surrounding soft tissue while the ceramic was slowly dissolving in the body fluids, allowing new osseous tissue to penetrate the resulting pore structure. It was expected that after a period of time the ceramic would no longer exist as a separate entity but would be completely infiltrated with mineralized osseous tissue and ultimately be completely replaced by bone. Two ceramic implants were prepared by slightly different techniques and CaO to Al2O3 ratios. The result was a polyphase ceramic and a nearly single phase ceramic. The polyphase material, after implantation in the test subject for 32 weeks, was completely impregnated with connective tissue while undergoing considerable alteration in microstructure. The single-phase material was not altered significantly in any way. This implant could not be fully evaluated for its ability to allow for osseous tissue formation on its surface, or within its open pore structure, due to slight variations in the original fixation of the implant and resulting attempts to correct the problems.

Effect of nitrogen on the electrical and structural properties of triode‐sputtered tantalum films

Abstract: The structural and electrical properties of oxygen‐doped tantalum films, sputtered in a triode system, have been investigated. X‐ray diffraction traces from films with a low oxygen content indicate the presence of bcc Ta with a strained lattice. The intensity of the (110) bcc Ta diffraction peak decreases with increasing oxygen content and is interpreted as a decrease in the amount of bcc Ta phase present. Over this same range the appearance and rise in intensity of the β‐Ta (200) peak indicates the formation of β‐Ta. As the oxygen content is further increased, the structural data indicate a reorientation of the tetragonal β‐Ta (200) to the (202) planes with respect to the film surface, an effect which has been previously observed in diode‐sputtered tantalum; further increase in oxygen content produces a stretching of the β‐Ta lattice by as much as 10% of the value reported by Read and Altman. The room‐temperature resistivity increases with increasing oxygen content until a ``plateau'' region is reached a...

Carbon fibre/nickel compatibility

Abstract: The effects of evaporated pure nickel coatings on the room-temperature fracture strength and microstructure of individual carbon fibres have been investigated. The fracture strength of a nickel-coated fibre was not affected by anneals in a 10−6 torr vacuum below 1000°C. However, after higher temperature anneals a reduction in strength was noted, the magnitude of which was time-dependent, but almost independent of the thickness of the coating. The reduction in strength was not related to fibre recrystallisation, but appeared to be controlled by the formation of a carbon-nickel interface, with an energy for adhesion of approximately 110 kcal mole−1.

The tensile properties of a graphite-fiber-reinforced Al-Si alloy

Abstract: Room temperature uniaxial tensile tests have shown that composites of an Al-13 wt pct Si alloy and Thornel 50 graphite fibers have strengths greater than a theoretical value that was calculated on the basis of the law of mixtures. At 28 vol pct fibers, the average uniaxial tensile strength was found to be 106,000 psi, and several values between 130,000 and 144,000 psi were obtained. The modes of deformation and failure in the composites have been studied by the microexamination of polished surfaces and fractures of tested specimens. The reasons for the high strengths and the unusual modes of fracture that were observed cannot be explained on the basis of the presented data. Specimens of the composite have been thermally cycled between −193° and +20°C twenty times and others between −193° and +500°C twenty times. Tensile tests and microexamination of these thermally cycled specimens show that thermal cycling does not degrade the tensile properties of the composites or change their microstructure.

Influence of Milled Powder Particle Size Distribution on the Microstructure and Electrical Properties of Sintered Mn‐Zn Ferrites

Abstract: The grain growth and densification rates of Mn-Zn ferrites during sintering are closely linked to the characteristics of the calcined and milled powders used. Long milling times enlarge powder particle size distributions and tend to promote discontinuous grain growth during sintering. For fixed sintering conditions, an optimum milling time, which corresponds to minimal eddy current and hysteresis losses, exists. The electrical properties of overmilled powders deteriorate greatly because duplex structure occurs. Theoretical analysis of the probability of discontinuous grain growth occurring during sintering in relation to powder particle size distribution agrees with the experimental results.

Phase relations in concentrated Ta-Hf and Nb-Hf alloys

Abstract: The precipitation reactions occurring in concentrated Ta-Hf and Nb-Hf alloys over the temperature range 600° to 1400°C were investigated by X-ray diffraction and transmission microscopy. In Ta-Hf alloys containing 30 to 62 at. pct Hf and Nb-Hf alloys containing 25 to 50 at. pct Hf a stable bcc solid solution exists at elevated temperature. When quenched from the single phase region and aged at lower temperatures both alloys precipitate α phase, an hcp hafnium-rich phase. Transmission microscopy showed that in the Nb-Hf system the precipitation reaction begins with the formation of coherent zones. Further aging causes coarsening into discs, still at least partially coherent. When coherency is lost the α morphology changes to rod-type. The effect of small amounts of oxygen added to these alloys during aging is an increase in the unit cell size of theα phase. Oxygen concentrations up to ~0.5 at. pct do not cause observable changes in the phase relations. These results cannot be reconciled with the miscibility gap-monotectoid type phase diagrams reported by some earlier investigators and indicate that the proper phase diagram for both systems is theβ-isomorphous type.

Correlation of microstructure and strength during stage III annealing of irradiated vanadium

Abstract: The microstructure of Vanadium irradiated to a fluence of 1.0 × 1019 n/cm2 E < 1 MeV at a temperature of 55 °C was quantitatively analyzed after irradiation and post-irradiation annealing and correlated with changes in the strength as determined from microhardness measurements. Radiation anneal hardening during Stage III annealing was not accompanied by a change in the size or number of defect aggregates. It is shown that the strength changes and migration kinetics of Stage III are consistent with the migration of oxygen and/or carbon to the defect aggregates. It is suggested that the simultaneous observation of no growth of defect aggregates and radiation anneal hardening in Stage III can be used to differentiate between impurity migration and intrinsic defect migration.

Electron Microscopic Study on the Structure of TinO2n-1 (4≤n≤10) Phases

Abstract: Transmission electron microscopy and diffraction study of the homologous series of oxides TinO2n-1 (4≤n≤10) reveals that the structures of these oxides are derived from rutile by regular crystallographic shear on planes parallel to (121), as previously suggested by Andersson and Jahnberg (Arkiv Kemi. 21 (1963) 413). Electron microscope images clearly show microstructures. Micro-twin structures of fine scale are observed very often for specimens with n≥7 but never detected when n≤6. The interface plane of the twin bands is parallel to the (200)r in most cases, although the twin structure with the interface plane (002)r or (020)r is seen occasionally. A mode of two phase mixture for the specimen in the two phase region is clarified.