Showing papers on "Microstructure published in 1990"

On the contribution of triple junctions to the structure and properties of nanocrystalline materials

Abstract: Nanocrystalline materials, having a crystal size less than ~10 rim, have been shown to possess unusual properties [1-3]. These properties are primarily the result of a substantial fraction of the atoms (20 50%) lying in intercrystalline regions [3]. On the basis of x-ray scattering [4], EXAFS [5], hydrogen solubility [6], small angle neutron scattering [7], and self diffusivity measurements [8] conducted on bulk nanocrystalline materials, it has been suggested [3] that the grain boundaries in these materials are more disordered than those in conventional polycrystals. Recent studies[9,10] involving direct observation of nanocrystalline interfaces by HREM, provide contradictory results. Wunderlich and co-workers[9] have shown that interfaces in nanocrystalline Pd show an 'extended' structure not typically observed in conventional systems. However, Thomas et al [10] observed that the interfacial structure of nanocrystalline Pd is consistent with that typically observed in coarse-grained materials.

Use of ion beam assisted deposition to modify the microstructure and properties of thin films

Abstract: Ion beam assisted deposition, the bombardment of a thin film with a beam of energetic particles during deposition, provides a powerful technique for modifying the microstructure and properties of thin films and coatings. Various experimental approaches used for ion beam assisted deposition are described and the physical basis for the effects is examined. Observations on modification of nucleation and growth behaviour, microstructure development, compound synthesis, and applications to the modification of properties such as intrinsic stress, adhesion, surface mechanical properties, corrosion and oxidation resistance, optical properties, and electrical properties, are reviewed.

Mechanical properties of nanophase TiO 2 as determined by nanoindentation

Abstract: Nanoindenter techniques have been used to determine the hardness. Young’s modulus, and strain rate sensitivity of nanophase TiO2, which is currently available only in very small quantities and which cannot be tested by most conventional techniques. Hardness and Young’s modulus both increase linearly with sintering temperature over the range 25–900°C but come to within only 50–70% of the single crystal values. Strain rate sensitivity, on the other hand, is measurably greater for this material than for single crystal rutile, and the value of strain rate sensitivity increases as the grain size and the sintering temperature are decreased. In its as-compacted form, the strain rate sensitivity of nanophase TiO2 is approximately a quarter that of lead at room temperature, indicating a potential for significant ductility in these ceramic materials. Finally, a significant scatter in hardness values has been detected within individual nanophase samples. This is interpreted as arising from microstructural inhomogeneity in these materials.

Microstructure-bearing composite plastic articles and method of making

Abstract: A microstructure-bearing composite plastic article can be superior, both in microstructure and in physical properties, when it is a composite of a tough, flexible substrate, at a surface of which is microstructure formed of a cured oligomeric resin having hard segments and soft segments, which cured resin is substantially confined to the microstructure portion of the composite. Such a composite plastic article can be made by depositing an uncured oligomeric resin composition (16) onto a master negative molding surface in an amount barely sufficient to fill the cavities (14) of the master (10), filling the cavities by moving a bead of the compositions between a substrate (22) and the master, and curing the deposited composition by ultraviolet radiation while keeping the temperature during curing to not more than 50°C above the typical use temperature of the finished composit plastic article.

The influence of microstructure on the properties of ferroelectric ceramics

Abstract: The domain twinning in ferroelectric ceramics is dependent on grain size. In fine grained ceramic a simple lamellar structure allows two- dimensional stress relief, in coarse grained ceramic a banded lamellar structure takes away homogeneous stress in three dimensions. The different domain configurations and internal stresses lead to different dielectric properties and to different hysteresis curves. Inhomogeneous grains of BaTiO3 with some CdBi2Nb2O9 have a core with a normal domain pattern and a shell without domains at room temperature. Core and shell have different transition temperatures. The macroscopic dielectric constant therefore has very high values in a very broad temperature range. Ceramics which are properly prepared in order to have oriented grains exhibit properties which come near to the properties of single crystals.

Grain Growth in Sintered ZnO and ZnO-Bi2O3 Ceramics

Abstract: Grain growth in a high-purity ZnO and for the same ZnO with Bi2O3 additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: Gn—Gn0=K0t exp(—Q/RT). For the pure ZnO, the grain growth exponent or n value was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn2+ lattice diffusion mechanism. Additions of Bi2O3 to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi2O3 content. The preexponential term K0 was also independent of Bi2O3 content. It is concluded that the grain growth of ZnO in liquid-phase-sintered ZnO-Bi2O3 ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi2O3-rich liquid phase.

Powder metallurgy of titanium alloys

Abstract: The powder metallurgy of titanium alloys is reviewed for both the blended elemental (BE) and prealloyed (PA) approaches. The BE technique allows low cost processing with mechanical behaviour at ing...

The production of nanocrystalline powders by magnetron sputtering

Abstract: Magnetron sputtering has been used for the production of nanoscale particles of pure metals, binary alloys, intermetallics, and ceramics. Al, Mo, Cu91Mn9, Al52Ti48, and ZrO2 particles with diameters of 7–50 nm were synthesized. The structures and grain sizes of the particles were examined by means of x‐ray diffraction and both transmission and scanning electron microscopy. The particles were collected and compacted in situ in the sputtering chamber. The structure of these nanocrystalline materials is also described. This novel application of magnetron sputtering at high gas pressures is of interest for both the production of nanocrystalline materials and isolated clusters.

Organic aerogels: microstructural dependence of mechanical properties in compression

Abstract: Aerogels are a unique class of ultrafine cell size (<1000 A), low-density foams. These materials have continuous porosity and a microstructure composed of interconnected colloidal-like particles or chains with characteristic diameters of 100 A. Traditional aerogels are inorganic, made via the hydrolysis and condensation of metal alkoxides (e.g. tetraisopropoxy titanate). Recently, the authors reported the development of organic aerogels from the sol-gel polymerization of resorcinol with formaldehyde. Because these new aerogels are composed of a highly crosslinked aromatic polymer, they can be pyrolyzed in an inert atmosphere to form vitreous carbon aerogels. This work describes how the microstructure of these organic aerogels can be manipulated and controlled. The microstructural dependence of the compressive mechanical properties of both resorcinol-formaldehyde and carbon aerogels is examined in detail.

Cold deformation microstructures

Abstract: The evolution of the cold deformation microstructure is described for medium to high stacking fault energy, single phase fcc metals. Macroscopic strain accommodation for polycrystalline metals is considered, and it is suggested that grains subdivide during deformation on a smaller and smaller scale, and that each volume element is characterised by an individual combination of slip systems. A number of microstructural observations (especially of aluminium, nickel, and copper) are described, and dislocation arrangements are discussed on the basis of the general principle that they are low energy dislocation structures. It is shown that the microstructural evolution is quite similar in polycrystalline metals and in single crystals deforming by multislip, and ways in which metallurgical parameters such as stacking fault energy and grain size can affect the microstructure are examined. The general principle of grain subdivision during cold deformation is discussed with reference to the microstructural ...

Synthesis and properties of YBa2Cu3O7 thin films grown in situ by 90° off-axis single magnetron sputtering

Abstract: High quality superconducting films of YBa 2 Cu 3 O 7− x were deposited in situ using single target 90° off-axis sputtering. We have investigated their superconducting DC and RF properties, their normal state properties, and their microstructures. These films are distinctly different from bulk crystals and post-deposition annealed films. Sharp superconducting transition temperatures can be reproducibly obtained by control of deposition parameters. The T c can be varied from 75 to 89 K. The optimization of properties other than T c and the control of film texture occur under conditions different from those for which the highest T c is obtained. Normal state conductivities are as high as or higher than those of single crystals. Critical current densities reach 6 × 10 7 A/cm 2 at 4.2 K. All the above properties are relatively insensitive to compositional variations. The T c 's have a much weaker dependence on the c -axis lattice parameters than do those of bulk samples. The measured low-temperature penetration depth is 1400 A and surface resistance at 4.2 K and 10 GHz is as low as 16 μΩ. Microstructural studies show sharp interfaces between films and their substrates and a variety of defect structures. Many of the properties of in situ films can be explained by clean grain boundaries and the characteristics of the surface growth occuring during in situ deposition.

Crack initiation and growth toughness of an aluminum metal-matrix composite

Abstract: The effects of systematic changes in matrix microstructure on crack initiation and growth toughnesses were determined on an Al-Zn-Mg-Cu alloy containing 0, 15, 20 percent by volume of SiC particulates. Materials were heat treated to underaged (UA) and overaged (OA) conditions of equivalent matrix microhardness and flow stress. Although both the fracture initiation and growth toughnesses, as measured by JIc and tearing modulus, were similar for the unreinforced materials in the UA and OA conditions, significant effects of microstructure on both JIc and tearing modulus were observed in the composites. SEM and TEM observations of fracture paths in the two conditions are utilized to rationalize these observations in light of existing theories of ductile fracture propagation.

Analysis of solidification microstructures in Fe-Ni-Cr single-crystal welds

Abstract: A geometric analysis technique for the evaluation of the microstructures in autogenous single-crystal electron beam welds has been previously developed. In the present work, these analytical methods are further extended, and a general procedure for predicting the solidification microstructure of single-crystal welds with any arbitrary orientation is established. Examples of this general analysis are given for several welding orientations. It is shown that a nonsymmetric cell structure is expected in transverse micrographs for most welding geometries. The development of steady-state conditions in the weld pool is also examined in terms of the weld pool size, its shape (as revealed by the dendritic growth pattern), and the size of the dendritic cells. It is found that steady state is established within a few millimeters of the beginning of the weld. Furthermore, steady state is achieved faster in welds made at higher welding speeds. A general analysis of the three-dimensional (3-D) weld pool shape based on the dendritic structure as revealed in the two-dimensional (2-D) transverse micrographs is also developed. It is shown that in combination with information on the preferred growth direction as a function of the solidification front orientation, the entire dendritic growth pattern in single-crystal welds can be predicted. A comparison with the actual weld micrographs shows a reasonable agreement between the theory and experiment. Finally, the theoretical analysis of the dendrite tip radius is extended from binary systems to include the case of ternary systems. The theoretical dendrite trunk spacing in a ternary Fe-Ni-Cr alloy is calculated from the dendrite tip radius and is compared with the experimental values for several weld conditions. Good agreement between experiment and theory is found.

Ceramic Thin Films: Fabrication and Applications

Abstract: Ceramics are a distinct class ofmaterials whose properties range from extreme hardness to unique electrical behavior. New methods of creating thin films of complex oxides and electronic ceramics allow the integration of these properties with semiconductor technology and raise the possibility of a new range of electronic devices.

Growth of YBa2Cu3O7 thin films on MgO: The effect of substrate preparation

Abstract: We discuss the results of a study of the effect of substrate preparation on the microstructure and superconductive properties of YBa2Cu3O7 thin films formed by laser ablation on (001) MgO substrates. Thermal annealing of the substrates is found to be highly effective in producing at fairly low growth temperatures (670 °C), epitaxial, c‐axis normal films with good superconductive properties. Alternative surface treatments result in the formation of large angle tilt boundaries and inferior superconductive properties.

The Role of Excess Magnesium Oxide or Lead Oxide in Determining the Microstructure and Properties of Lead Magnesium Niobate

Abstract: Near-phase pure perovskite lead magnesium niobate (PMN) with MgO or PbO additives was produced by reacting PbO with MgNb2O6 at 800°C and sintering at 1200°C. Dense ceramics were characterized by scanning electron microscopy, X-ray diffraction, and dielectric measurements. The microstructural studies showed that excess MgO exists as micrometer spherical particles either in the grain boundary as a discrete particle or in the perovskite grain as an inclusion. The pyrochlore phase exists in large isolated grains in the microstructure. The 10 mol% MgO excess composition had a peak dielectric constant of 19 500 at 100 Hz, which suggests very “clean” or uninhibiting grain boundaries. The excess addition of PbO did not improve the yield of perovskite PMN phase and decreased the dielectric constant. PMN grain boundaries are the dominant path of fracture. This paper, to a certain degree, explores the chemistry and characteristics of these grain boundaries.

The structure of sodium silicate glass

Abstract: Atomistic scale computer simulations can provide a more detailed understanding of the structure of glasses. Our results on sodium silicate glasses show that sodium ions are associated with nonbridging oxygens and that the sodium ions and nonbridging oxygens tend to aggregate to form silica rich regions and alkali rich regions. Interatomic distances of our simulated glasses are in good agreement with experimental results. O–Si–O bond angles are centered around 109° with narrow distributions. Si–O–Si bond angles are broadly distributed from 120° to 180° with an average of about 153°.

The effect of cell size on the mechanical behavior of cellular materials

Abstract: The Young's modulus, strength and fracture toughness, of a brittle reticulated vitreous carbon foam, was measured as a function of cell size at a constant density and compared to a theoretical model Image analysis was used to characterize the macrostructure of the samples and provided a basis for evaluating the mechanical behavior It was determined that both the compressive and bend strength scale inversely with cell size The change in compressive strength is due to a change in the strut strength with cell size The bend strength behavior may be due to a reduction in the critical flaw size, as well as the increasing strut strength at smaller cell sizes The fracture toughness and elastic modulus were found to be independent of cell size Comparison of these results with previous work on open cell alumina clearly indicates a very different behavior and is attributed to a change in the microstructure of the solid phase with cell size in the alumina materials

New cubic phases formed by alloying Al3Ti with Mn and Cr

Abstract: It has been found that Mn or Cr can substitute for Al in Al 3 Ti and change the crystal structure from tetragonal DO 22 to cubic L1 2 . The new phases have been produced with negligible porosity and second phases, and have compositions near Al 67 Mn 8 Ti 25 and Al 67 Cr 8 Ti 25 . As with other cubic forms of Al 3 Ti, the ternary addition can range from bout 5 to 15 at. pct. while still maintaining the cubic phase. These new cubic phases have significantly reduced hardnesses and greatly improved crack resistance compared to binary Al 3 Ti, as well as other cubic forms based on alloying with Fe, Ni, or Cu. In addition to appreciable ductility in compression, the new phases show some tensile ductility

Early formation of chemical vapor deposition diamond films

Abstract: Nanometer‐size diamond particles formed on a silicon substrate by the hot‐filament chemical vapor deposition method were examined by a high‐resolution electron microscope. The particles developed well‐faceted cuboctahedral habits. Examination of their morphologies and microstructures provides a wealth of information on their crystal growth mechanism. The effect of the pretreatment of the substrate by diamond powder, which has been known to enhance thin‐film growth, was found to be due to seeding by ‘‘diamond dust’’ on the substrate surface.

Amorphous material in high strain experimental fault gouges

Abstract: The microstructures of gouges produced in room temperature, rotary shear sliding experiments were examined by transmission electron microscopy. Gouges were produced by sliding on ground surfaces of granite, quartzite, or marble except for one experiment in which a 1-mm-thick simulated gouge layer was used. Water was added to the sliding surfaces of all but one sample. Crystal plastic processes play no role in the granite and quartzite gouges and a minor role in the marbles. All of the gouges consist of mostly submicron crystalline fragments; in addition, the granite gouges contain 5–60% amorphous material, and the quartzite gouge contains −50% amorphous material. In the granite samples the composition of the amorphous material commonly lies between K-rich and Na,Ca-rich feldspars, although portions may be silica-rich. The microstructural relations suggest that the amorphous material forms by comminution of fragments rather than by melting. The amount of amorphous material increases, and the size of the largest crystalline fragments decreases, with an increase in average shear strain, although the microstructure is nearly uniform throughout each granite gouge layer. These observations suggest that after slip becomes localized on “Y” shear surfaces and/or R1 Riedel shears the entire gouge layer must continue to undergo deformation. It is suggested that cyclic deformation in the gouge must occur to accommodate the passage of geometric irregularities on the active slip surfaces.

Thermally induced structural modification of Mo‐Si multilayers

Abstract: The effect of elevated temperature on the structural stability and performance of Mo‐Si multilayer mirrors is investigated. Mo‐Si multilayers deposited by magnetron sputtering are annealed at temperatures ranging from 200 to 800 °C. A detailed and consistent picture of the thermally induced changes in the microstructure is obtained using an array of complementary measurement techniques including small‐ and large‐angle x‐ray scattering and high‐resolution electron microscopy. The first significant structural changes are observed at 400 °C, characterized by an increase in the width of the amorphous interlayer regions, as well as the nucleation of microcrystallites of silicide in these regions. At higher temperature the Mo layers transform completely into polycrystalline mixtures of Mo5 Si3 and MoSi2 in both the hexagonal and tetragonal phase. The layers of silicide remain intact but exhibit a structural instability, resulting in severely warped layers surrounded by pockets of amorphous Si and voids. By 800 °C the layered structure is completely destroyed and the composition is predominately tetragonal MoSi2 . The performance of the multilayers as normal‐incidence x‐ray mirrors is measured and correlated with the observed structural modifications. Finally, our results are compared and contrasted with other annealing studies of the Mo‐Si system.

Mechanical properties and microstructures of Al-Mg-Sc alloys

Abstract: The mechanical properties of Al-(Mg)-0.5Sc alloys have been investigated. Room-temperature tensile and toughness properties were found to reflect a superposition of the properties of Al-Mg and Al-0.5Sc alloys and are quite competitive with high-performance Al alloys. A combination of substructure refinement by Mg and stabilization by Al3Sc precipitates produces exceptional superplasticity as exemplified by superplastic forming (SPF) elongations in excess of 1000 pct at a strain rate of 0.01 s-1. Overall, these alloys demonstrate an extremely attractive combination of strength, toughness, density, and SPF fabricability.

Synthesis of an alumina coating from chelated aluminium alkoxides

Abstract: Chelating agent, such as acetylacetone (AcAcH) and ethylacetoacetate (EAA) are used to avoid the undersirable precipitation of gelatinuous aluminum hydroxides during hydrolysis of aluminum-sec-butoxides, Al(OBus). This procedure leads to alumina sols with particle size distributions in the range of 1 to 15 nm which depend on the type of chelating agent as well as on the molar Al(OBus)3/chelating agent ratio. Generally, EAA leads to particles smaller than AcAcH. Because of the small particle size, gel powders are X-ray amorphous and do not crystallize at temperature < 750°C. Crystallization of α-alumina starts between 950 and 1000°C. Complete transformation to α-alumina is achieved between 1050 and 1100°C. Coating experiments with different sols on silica glass lead to nearly crack-free dried gel films with AcAcH system. An ultrafine, dense microstructure is obtained.

Effects of particles on development of microstructure and texture during rolling and recrystallisation in fcc alloys

Abstract: Precipitation causes changes in the modes of plastic deformation and recrystallisation, as well as in the development of rolling and recrystallisation textures. The nature of these changes varies strongly with the size, shape, and density of the particles and, in particular, on whether the particles are cut by dislocations during deformation. This leads to numerous phenomena, some rather surprising, which are systematised, established through many new experimental results (mainly on aluminium alloys), and discussed.MST/1293

Aluminium composites reinforced with a new aluminium borate whisker

Abstract: This is the first report on the metal matrix composite reinforced with a new aluminium borate whisker