Showing papers on "Microstructure published in 1975"

The physics of metal oxide varistors

Abstract: This paper outlines our present understanding of the conduction mechanisms and physical processes relevant to the performance of ZnO−based ceramic varistors. Varistor behavior is determined by the gross ceramic microstructure of the device as well as by the localized conduction processes which occur between grains. We show that the qualitative features of the highly nonlinear conductivity are largely independent of the details of varistor composition or processing but rather appear to be a general effect engendered by a microstructure of conducting grains surrounded by thin insulating oxide barriers. Evidence is presented from a variety of sources that this intergranular layer is ∼100 A in thickness resulting in grain−to−grain fields of F∼106 V/cm. The conduction mechanism at breakdown is consistent with a Fowler−Nordheim tunneling process obeying a current−density−vs−field relation given by J∝exp(−γ/F), where γ is a constant. At somewhat lower fields (prebreakdown region) the conduction process follows a...

Microstructure and phase transformation in a highly non−Ohmic metal oxide varistor ceramic

Abstract: The microstructure of a complex multicomponent varistor ceramic based on ZnO with small additions of antimony, bismuth, cobalt, manganese, and tin oxides has been elucidated using a variety of structural techniques. Three crystalline phases are found to coexist in the sintered material. The bulk phase consists of a polycrystalline matrix of ZnO doped with Co(II). The ZnO grains are separated from one another by a Bi2Zn4/3Sb2/3O6 pyrochlore phase which forms a three−dimensional threadlike network, indicative of a liquid phase at high temperatures. A Zn(Zn4/3Sb2/3)O4 spinel constitutes the third phase and forms well−faceted octahedral crystals located frequently at grain boundaries between the ZnO and occasionally within the grains. This phase acts as a grain growth moderator for ZnO by anchoring the boundaries during sintering, but plays no role in the nonlinear characteristic of the varistor. A quasiequilibrium between the pyrochlore and spinel has been established such that formation of the spinel is favored at high temperatures. The over−all varistor microstructure may be described as a three−dimensional series−parallel network of ZnO−pyrochlore junctions.

Strength and Microstructure in Lithium Disilicate Glass-Ceramics

Abstract: The effect of heat treatment on the microstructure of Li2O-Al2O3·SiO2 glass-ceramics which contain crystals of either Li2SiO3, Li2Si2O5, or both was investigated quantitatively. Strength determinations for abraded rods were correlated with heat treatment on the basis of both size and distribution of crystals and the type and amount of crystal phases present. The presence of Li2Si2O5 crystals enhanced the strength, whereas the presence of Li2SiO3 crystals did not change the strength of the abraded parent glass. The interrelation between strength and microstructure is discussed.

Metallurgical factors affecting high strength aluminum alloy production

Abstract: With the advent of linear elastic fracture mechanics, the detailed effects of processing and microstructure on toughness can be evaluated. The effect of microstructure on plane stress and plane strain fracture toughness is considered in detail together with strength, fatigue behavior and corrosion resistance. It is concluded that second phase particles in all size ranges can influence toughness. Increasing the size and amount of particles or decreasing precipitate coherency all lead to decreases in toughness. Grain structure is also shown to play a prominent role in determining plane stress fracture toughness ; at a given strength level, a fibrous grain structure and the prevention of recrystallization are desirable. The ability to influence fatigue crack propagation by control of processing is more remote though relatively little systematic work has been carried out in this field. Thermomechanical processing is considered to offer another possible route to achieving a desirable balance of toughness, strength and corrosion resistance.

Effect of stress during aging on the precipitation of θ′ in Al-4 Wt pct Cu

Abstract: Experiments have been made on Al-4 wt pct Cu single crystals to determine the influence of stress, applied during aging, on precipitation. Without stress, θ platelets precipitate on all (100) planes. Transmission electron microscopy showed a marked effect of aging stress on the microstructure. A compressive stress of approximately 48 MPa (∼ 7,000 psi) parallel to [001] favored precipitation on (010) and (100) and inhibited precipitation on (001). When a tensile stress of similar magnitude was applied parallel to [001], precipitation was favored on (001) and inhibited on (010) and (100).

The mechanism of crazing in polystyrene

Abstract: Under tensile strain the inhomogeneous surface microstructure of thin polystyrene films gave rise to the formation of ∼300 A nuclei in which large strains were localized even at the glass transition. At lower temperatures these nuclei coalesced into disk-shaped microneck zones. Fibril formation within these zones and a stable craze structure was possible only with molecular weights higher than 10,000 to 37,500.

The rheological properties of molten mixtures of polyoxymethylene and copolyamide: Their fibrillation and microstructure

Abstract: The rheological properties of molten mixtures of polyoxymethylene and copolyamide have been investigated by the methods of capillary viscometry over a wide range of shear stresses. It has been found that at relatively close values of the melt viscosity polyoxymethylene (POM) and copolyamide (CPA) form mixtures which differ markedly in viscosity and elastic properties from the original polymers. Depending on the magnitude of the shear stress, the viscosity of mixtures of definite composition may be either higher or lower than the viscosity of the components. Three- to tenfold swelling of the stream emerging from the capillary is observed for such mixtures, whereas at the same temperatures and stresses no swelling of the POM and CPA extrudates is practically observed. The data obtained are accounted for by the fact that the flow of mixtures through the capillary involves the formation of hundreds of thousands of ultrathin fibres of POM in the matrix of CPA, which are oriented along the flow axis.

Observations on the growth of whisker crystals from zinc electroplate

Abstract: Spontaneous growth of zinc whiskers from electroplate was studied at 20°C and 50°C for a period of two years after electroplating and a number of physical and metallurgical properties of the electroplate were determined. The growth rate correlated well with the internal macrostress of the deposit as determined with a dilatometer during plating. Crystallographic texture and a microstructure of flat lath-like grains also tended to favor growth somewhat, while grain size, microhardness and microstrain had no apparent effect.

Microstructure and Intrusions in the California Current

Abstract: Two microstructure records taken at shallow depths off Cabo San Lucas, at the southern tip of Baja California, are compared. One is similar to records previously taken in the mid-gyre, and has an “irregularly steppy” appearance, a linear T-S relation, and a Cox number of approximately 10. It is suggested that this type of profile may represent the background condition of the ocean in which the levels of vertical turbulence are quite low and the principal dissipation occurs by small-scale shear instabilities at the “step“ structures. The other record exhibits a very irregular T-S relation, due to multiple interleavings of the water masses present in the area. Coupled with this is an average Cox number of at least 6000 and a much greater variability in the local microstructure levels along the record; half-meter averages of the dissipation rate of temperature fluctuations show a range greater than 106. In some cases these differences occur over vertical separations of a few meters. In general, the ...

Grain refinement through thermal cycling in an Fe-Ni-Ti cryogenic alloy

Abstract: A thermal cycling technique which allows the grain refinement of Fe-12 Ni-0.25 Ti alloy from 40∼60 µm (ASTM #5∼6) to 0.5 ∼ 2 µm (ASTM #1∼18) in four cycles has been developed. The process consists of alternate annealing in γ range and (α + γ) range with intermediate air cooling. The transformation behavior, the change of microstructures and cryogenic mechanical properties on each cycling step are described. Due to the ultrafine grain size, the ductile-brittle transition temperature of this ferritic alloy in Charpy impact testing was suppressed below 6 K. In fracture toughness testing at 77 K, the mode of fracture was altered from brittle quasi-cleavage to complete ductile rupture through the grain refining.

Deformation and microstructure of extended‐chain polydiacetylene crystals

Abstract: The mechanical properties (Young's modulus, ultimate tensile strength, deformation processes) of extended-chain polydiacetylene crystals are investigated. The properties observed are similar to those of metal and ceramic whiskers. The elastic modulus is strain-dependent and the ultimate tensile strength increases with decreasing crystal size. The maximum tensile strength observed was 1700 Nmm−2. The ultimate tensile strength seems to be controlled by the presence of a small number of defects near the surface at which fracture nucleates. Irreversible deformation of the crystals was observed to occur by crack propagation normal and parallel to the direction of the macromolecules. The observed mechanical behavior corresponds to exceptionally high per-chain properties. The per-chain modulus obtained for these crystals is nearly as high as that of diamond. A chain-aligned polyethylene fiber with the same per-chain mechanical properties would have an ultimate strength as high as 0.9 × 104 Nmm−2.

Structural changes caused by age-hardening in a dental gold alloy

Abstract: The correlation between microstructure and hardness of dental gold alloy, Au-35.7 at.% Cu-11.2 at.% Ag, was studied by electron microscopy. The age-hardening peak was due to the formation of plate-like AuCu I on the matrix {100} planes. The microstructure at this stage showed a striation contrast, being parallel to the 〈111〉 directions on the (110) plane, as found in the AuCu alloy. The structure of the final state was identified as AuCu I-type ordered platelets which had the c -axis distributed in the three cube directions and had the [011] twin relation, although relatively higher hardness was maintained at the later stage of ageing.

Microstructure and toughness of structural steels

Abstract: The effects of notch acuity, inclusion content, and strength level on the toughness of a variety of ductile steels have been investigated infully plastic single edge notched bend testpieces. Results for specimens containing fatigue precracks and sharp notches indicate that accurate predictions of a material's resistance to the initiation of fibrous fracture ahead of afatigue crack may be inferred from tests on notched testpieces andfrom a knowledge of the microstructure of the material; an experimental procedure has been proposed whereby this may be achieved for quality control and material evaluation purposes. The spacing of optically visible inclusions is found essentially to define both the unit of ductile crack extension and, for low-strength steels, the limiting lateral dimensions of the high-strain field ahead of the crack tip. As a consequence, the notch-tip ductility isfound to be invariant with the changes in notch acuity for sharp stress concentrators. The effect of increasing the purity...

SEM investigations of iron surface ion erosion as a function of specimen temperature and incidence angle

Abstract: Ion bombardment of iron surfaces produced an ion eroded surface microtopography strongly dependent on the incidence angle and specimen temperature. For bombardment temperatures exceeding the self-diffusion temperature, T d, a microstructural smoothing process due to the thermally activated diffusion effects, assisted by ion bombardment, can be observed by SEM. The role of migrational processes which should be taken into account for a correct interpretation of the final surface topography formed by “hot sputtering”, was also considered.

The effect of quench rate on the properties and morphology of ferrous martensite

Abstract: The effect of high quench rate on theMs temperature, percent transformed, martensite morphology and austenite hardness has been studied for several Fe-Ni-C steels. For these steels the quench rate was varied only in the austenite region. TheMs temperature was found to increase with increased quench rate for both high- and low carbon steels while the percent transformation increased or decreased depending upon the morphology of the steel. No variations in martensite hardness were found in the as-quenched condition, but a difference in tempering rate was found between fast and slow quenched specimens. Austenite hardness decreased slightly with increasing quench rate while the martensite morphology changed from lath to plate. Parallel aligned plate structures were observed which resemble a twinned lath morphology. It was demonstrated that the actual difference between this morphology and a true lath morphology is the self-accommodating nature of the lath structure. The morphology changes were compared to the measured changes in martensite properties in order to identify the mechanism of the morphology shift. It was concluded that for these alloys the morphology was controlled by the austenite shear mode.

Powder metallurgy approach for control of microstructure and properties in high strength aluminum alloys

Abstract: High-strength products made from atomized Al-Zn-Mg-Cu-Co alloy powders have good combinations of strength, ductility, resistance to stress-corrosion cracking and fracture toughness. Powder Metallurgy (PJM) methods produce fine metallurgical structures and compositions which cannot be produced by Ingot Metallurgy (IJM) methods.

Microstructural studies of a Ni-W directionally solidified eutectic composite

Abstract: The influence of heat treatment on the microstructure of a Ni-45.5 wt pct W directionally solidified eutectic composite has been studied. Four types of microstructural conditions have been examined using light and thin foil electron microscopy techniques. The as-grown composite consists of W fibers in a matrix which contains some WNi4 precipitates. Quenching the composite from above the peritectoid temperature results in W fibers in a Ni-W solid solution matrix. Subsequent aging results in precipitation of coherent WNi4. Long time aging at temperatures just under the peritectoid temperature promotes the peritectoid reaction leading to envelopment of the W fibers by a layer of WNi4. The envelopment reaction is accompanied by a change from faceted to round fiber morphology. The presence of W fibers in the Ni-W solid solution influences aging in two ways when compared to a solid solution alloy. First, the fiber/matrix interfaces act as dislocation sources on quenching; these dislocations are absorbed into the WNi4 precipitate/matrix interfaces leading to early loss of coherency. Second, the WNi4 precipitates undergo a preferential coarsening due to elastic interaction between the fibers and the WNi4 precipitates; this leads to a mosaic structure, the occurrence of which is uniquely dependent on the presence of W fibers.

Formation of bimodal crystal textures in polypropylene

Abstract: Electron microscopy, selected area electron diffraction, X-ray diffraction, and dynamic-mechanical testing have been used to study flow-crystallized and hot drawn isotactic polypropylene. As a result of these investigations, it was found that bimodal crystal textures can apparently be formed by at least two different treatments, but the corresponding morphologies are completely different. Flow-induced crystallization was observed to result in a microstructure of lamellae oriented perpendicular to the flow direction, while hot drawing of polypropylene films above a critical temperature produced a morphology of microfibrils lying parallel to the draw direction. Below this critical temperature, drawing produced a fibrillar morphology having only a typical unimodal fibre texture. As a result of information obtained here, a mechanism involving epitaxial deposition of chain segments onto growing lamellae is concluded to be responsible for formation of the bimodal crystal texture in flow-crystallized material.

An investigation of the relationships between processing conditions, microstructure and mechanical properties of an injection moulded semi-crystalline thermoplastic

Abstract: The semi-crystalline polymer TPX has been injection moulded into standard tensile bars using a range of barrel temperatures, mould temperatures and injection pressures. Acid etching, microhardness testing and selected volume X-ray pole figures were used to characterize the changes in microstructure and crystalline texture which occurred throughout the range of mouldings. A strong correlation was shown to exist between microstructure and crystalline texture and processing conditions. Three-point bend tests on the injection moulded bars indicated that a ductile-to-brittle transition in failure mode could be induced by changing the moulding conditions, and that this change could be correlated with microstructural and dimensional changes.

Microstructure of ZnO films used for acoustic surface‐wave generation

Abstract: Zinc oxide, vacuum deposited as a crystalline aggregate with preferential orientation of its fiber grains, has been used extensively as a film transducer for generating micro‐acoustic waves. The orientation and size of the crystallite grains have a pronounced effect on the film’s electromechanical properties. The microstructure of dc‐triode compound sputtered ZnO films was investigated using the analytical tools of scanning electron microscopy (SEM), reflection electron diffraction (RED), and x‐ray diffraction (XRD). The ZnO films investigated had been deposited on fused quartz and oxidized silicon having surface interdigital electrode arrays from which the acoustic surface‐wave properties of coupling efficiency, velocity, and propagation loss had been determined. Optically clear films having electromechanical and propagation properties approaching those of single‐crystal zinc‐oxide layers were found to have a smooth surface, a dense, amorphous‐like vertical structure, an average crystallite size estimate...

The Study of Microstructure

Abstract: Materials science is totally concerned with the structure of materials, with the understanding and control of structure and with the prediction and understanding of its ultimate consequences—the macroscopic properties of matter. Materials scientists, therefore, use extensively those techniques that have been developed to investigate the structure of matter at all levels from the macroscopic down to the atomic. It is commonly found that most of the significant features of the structure of a material are of a ‘microscopic’ size, that is, from a few tens of microns down to atomic dimensions. Techniques for investigating structure at this level may be classified into methods based upon the analysis of diffraction patterns, which give information averaged from a relatively large sample, and imaging methods, which give detailed information about a small region. For very many applications the imaging methods are more useful, since the local arrangement of the microstructure is often extremely important. A few examples will make this clear. Plastic flow in a crystalline solid is controlled by the generation and movement of dislocations and their interaction with other microstructural features. A solid-state precipitate may greatly improve the mechanical properties of a material if it is finely dispersed within the grains, but can cause catastrophic brittleness if it collects at a grain boundary.

Effect of Reactant Nitrogen Pressure on the Microstructure and Properties of Reactively Sputtered Silicon Nitride Films

Abstract: The quality of silicon nitride films deposited by rf reactive sputtering of silicon in a nitrogen discharge is found to be strongly dependent on the discharge nitrogen pressure. For a given substrate to target spacing there is a "critical" sputtering pressure which, if exceeded, results in deposition of films with noticeably inferior properties. The properties of silicon nitride films deposited both above and below this critical pressure are compared. The degradation of film properties accompanying an increase in the nitrogen discharge pressure is correlated with the presence of microvoids (~50A), revealed by transmission electron microscopy. It is suggested that the microvoids result from a reduction in the reemission of deposited material as the discharge pressure is increased.