Showing papers on "Microstructure published in 1994"

The surface science of metal oxides

Abstract: 1. Introduction 2. Geometric structure of metal-oxide surfaces 3. Surface lattice vibrations 4. Electronic structure of non-transition metal-oxide surfaces 5. Electronic structure of transition metal-oxide surfaces 6. Molecular adsorption on oxides 7. Interfaces of metal oxides with metals and other oxides References Index.

The gyroid: A new equilibrium morphology in weakly segregated diblock copolymers

Abstract: The authors report the identification of a new equilibrium microdomain morphology in an intermediate to weakly segregated diblock copolymer melt. A polystyrene-polyisoprene (SI) diblock copolymer consisting of 37 wt% styrene and of total M[sub w] = 27,400 was observed to transform from the lamellar morphology (in equilibrium at low annealing temperatures) to a new morphology at annealing temperatures approximately 50 C below the order-disorder transition (ODT). The transformation was observed to be thermoreversible. Investigation of the new morphology via small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) revealed the new structure to have remarkable three-dimensional long-range order, to belong to the cubic space group Ia3d, and to possess a bicontinuous cubic microstructure. From computer simulations of model structures and comparison with microscopy results, the authors propose models for the new morphology based on the triply periodic G minimal surface (gyroid) discovered by Schoen; similar morphologies have been observed in a variety of microphase-separated surfactant-water systems. Blends of this diblock with various short-chain homopolymers were used to investigate the compositional extent of the region of Ia3d stability on the phase diagram; the results indicate that the Ia3d phase is stable over a wide range of minority component volume fractions.

Anisotropic effective‐medium modeling of the elastic properties of shales

Abstract: Shales are complex porous materials, normally consisting of percolating and interpenetrating fluid and solid phases. The solid phase is generally comprised of several mineral components and forms an intricate and anisotropic microstructure. The shape, orientation, and connection of the two phases control the anisotropic elastic properties of the composite solid. We develop a theoretical framework that allows us to predict the effective elastic properties of shales. Its usefulness is demonstrated with numerical modeling and by comparison with established ultrasonic laboratory experiments. The theory is based on a combination of anisotropic formulations of the self‐consistent (SCA) and differential effective‐medium (DEM) approximations. This combination guarantees that both the fluid and solid phases percolate at all porosities. Our modeling of the elastic properties of shales proceeds in four steps. First, we consider the case of an aligned biconnected clay‐fluid composite composed of ellipsoidal inclusion...

Microemulsion microstructure and interfacial curvature

Abstract: The typical phase behavior of microemulsion systems undergoing phase inversion is briefly reviewed. As a model system H2O-n-octane-C12E5 is studied with various experimental techniques. The occurring microstructures are visualized by freeze fracture electron microscopy and the corresponding domain sizes are quantified by small-angle neutron scattering. From the variations of the domain sizes the mean and Gaussian curvatures of the interfacial film with temperature are determined. It is found that the mean interfacial curvatureH changes gradually and nearly linearly with temperature from positive (Winsor I) to negative (Winsor II), passing through zero for bicontinuous microemulsions where these contain exactly equal volume fractions of water and oil. There the interfacial tension between bulk water-and oil-rich phases passes through an extreme minimum. Quantitative knowledge of the curvatures permits the measurements of interfacial tensions between the bulk phases to be discussed in terms of the relative contributions of bending energy and entropy of dispersion.

Stable and unstable growth in molecular beam epitaxy

Abstract: We consider the growth of films by molecular beam epitaxy in the presence of step-edge (Schwoebel) barriers using numerical simulation and experiments We show that the growth of a singular surface is unstable, but that a miscut above a certain critical slope (which depends on growth conditions) leads to stable growth in a step-flow mode For singular surfaces the instability gives rise to the formation of large mounded structures on the surface for which the slope is in the stable regime We identify these in GaAs epitaxy using atomic force and scanning tunneling microscopy We propose a continuum equation which exhibits these features

IC-processed microneedles

Abstract: A method of fabricating a microstructure is disclosed. The method includes providing a substrate for forming an interface region and an elongated portion extending away from the interface region. A patterned, non-planar etchable structure is formed on one side of the elongated portion of the substrate. An unetchable membrane layer is deposited atop the etchable structure. At least one etching hole is formed in the membrane layer. The etchable structure is etched by placing an etchant into the etching hole to form a cavity underneath the membrane layer, thereby producing a shaft.

Method for the simultaneous determination of anisotropic residual stresses and texture by x‐ray diffraction

Abstract: A method is developed that yields the residual stress, the orientation distribution coefficients, the average crystallite dimension, the microstrain, and the crystal structure parameters from x‐ray diffraction data in a single‐step procedure. To this end, a general approach is introduced that combines the equations of micromechanics with the harmonic description of texture. All relationships are cast into a Rietveld‐like format, which incorporates a microstructure model derived from line‐broadening methods. In this manner, data collected over the whole x‐ray‐diffraction pattern at different tilting of the sample can be fitted directly. The associated fitting parameters are the crystal structure and microstructure, the texture coefficients, and the micromechanical properties and fields.

Effect of microstructure (particulate size and volume fraction) and counterface material on the sliding wear resistance of particulate-reinforced aluminum matrix composites

Abstract: The effects of microstructure (namely, particulate volume fraction and particulate size) and the counterface materials on the dry-sliding wear resistance of the aluminum matrix composites 2014A1-SiC and 6061Al-Al2O3 were studied. Experiments were performed within a load range of 0.9 to 350 N at a constant sliding velocity of 0.2 ms-1. Two types of counterface materials, SAE 52100 bearing steel and mullite, were used. At low loads, where particles act as loadbearing constituents, the wear resistance of the 2014A1 reinforced with 15.8 µm diameter SiC was superior to that of the alloy with the same volume fraction of SiC but with 2.4 µm diameter. The wear rates of the composites worn against a steel slider were lower compared with those worn against a mullite slider because of the formation of iron-rich layers that act asin situ solid lubricants in the former case. With increasing the applied load, SiC and A12O3 particles fractured and the wear rates of the composites increased to levels comparable to those of unreinforced matrix alloys. The transition to this regime was delayed to higher loads in the composites with a higher volume percentage of particles. Concurrent with particle fracture, large strains and strain gradients were generated within the aluminum layers adjacent to contact surfaces. This led to the subsurface crack growth and delamination. Because the particles and interfaces provided preferential sites for subsurface crack initiation and growth and because of the propensity of the broken particles to act as third-body abrasive elements at the contact surfaces, no improvement of the wear resistance was observed in the composites in this regime relative to unreinforced aluminum alloys. A second transition, to severe wear, occurred at higher loads when the contact surface temperature exceeded a critical value. The transition loads (and temperatures) were higher in the composites. The alloys with higher volume fraction of reinforcement provided better resistance to severe wear. Wearing the materials against a mullite counterface, which has a smaller thermal conductivity than a counterface made of steel, led to the occurrence of severe wear at lower loads.

Microstructure evolution of eutectic Sn-Ag solder joints

Abstract: Laser and infrared reflow soldering methods were used to make Sn-Ag eutectic solder joints for surface-mount components on printed wiring boards. The microstructures of the joints were evaluated and related to process parameters. Aging tests were conducted on these joints for times up to 300 days and at temperature up to 190°C. The evolution of microstructure during aging was examined. The results showed that Sn-Ag solder microstructure is unstable at high temperature, and microstructural evolution can cause solder joint failure. Cu-Sn intermetallics in the solder and at copper-solder interfaces played an important role in both the microstructure evolution and failure of solder joints. Void and crack formation in the aged joints was also observed.

Role of Eta-carbide Precipitations in the Wear Resistance Improvements of Fe-12Cr-Mo-V-1.4C Tool Steel by Cryogenic Treatment

Abstract: The wear resistance of an Fe-12.2wt%Cr-0.84wt%Mo-0.43wt%V-1.44wt%C alloy tool steel after cold treatment at 223K (subzero treatment) and after cryogenic treatment 93K (ultra-subzero treatment) has been investigated. The wear resistance of steels after cryogenic treatment is superior to that after cold treatment. The effects of cryogenic treatment on the microstructure were also studied by means of X-ray diffraction and transmission electron microscopy methods. Unlike cold treatment, cryogenic treatment improves the preferential precipitation of fine η-carbides instead of e-carbides. These fine carbide particles enhance the strength and thoughness of the martensite matrix and then increase the wear resistance. The formation mechanism of fine η-carbide is discussed.

Preparation of Pb(Zr,Ti)O3 Thin Films on Ir and IrO2 Electrodes

Abstract: Pb(Zrx Ti1-x )O3 (PZT) thin films were prepared on Ir and IrO2 electrodes. Ir has very similar properties to Pt, and IrO2 is a conductive oxide. Perovskite single-phase PZT thin films were obtained on their electrodes. PZT thin films were grown by the conventional sol-gel method with rapid thermal annealing (RTA) at 700° C. When Pt thin films were deposited directly on poly-Si, PtSi layers were formed, and PZT thin films on the Pt had very poor crystallinity. When an IrO2 layer was formed between PZT and poly-Si, a high-quality PZT thin film was obtained. Moreover, when electrodes including the IrO2 layer were used, fatigue properties of PZT thin films were drastically improved.

Toughness Properties of a Silicon Carbide with an in Situ Induced Heterogeneous Grain Structure

Abstract: Toughness characteristics of a heterogeneous silicon carbide with a coarsened and elongated grain structure and an intergranular second phase are evaluated relative to a homogeneous, fine-grain control using indentation–strength data. The heterogeneous material exhibits a distinctive flaw tolerance, indicative of a pronounced toughness curve. Quantitative evaluation of the data reveals an enhanced toughness in the long-crack region, with the implication of degraded toughness in the short-crack region. The enhanced long-crack toughness is identified with crack-interface bridging. The degraded short-crack toughness is attributed to weakened grain or interface boundaries and to internal residual stresses from thermal expansion mismatch. A profound manifestation of the toughness-curve behavior is a transition in the nature of mechanical damage in Hertzian contacts, from classical single-crack cone fracture in the homogeneous control to distributed subsurface damage in the heterogeneous material.

Experimental characterization of the local strain field in a heterogeneous elastoplastic material

Abstract: A new technique, which allows to characterize the local strain field over a domain representative of the microstructure of a heterogeneous material, is described. It is based on scanning electron microscopy, microelectrolithography, image analysis and in situ tensile tests. The in-plane components of the local strain field are characterized by their averages per phase and their distribution functions. The results are accurate for global strains between 5 and 15%. It is also possible to get contour plots of these components of the local strain field over the considered domain. The obtained strain maps give a powerful qualitative information on the strain localization modes during the deformation. This technique has basically been developed for two-phase elastoplastic materials, namely iron/silver and iron/copper blends, submitted to uniaxial tensile tests; it could also be used for polycrystals or other composite materials and for other mechanical tests.

Microstructure Control of Silicon Nitride by Seeding with Rodlike β‐Silicon Nitride Particles

Abstract: The effect of seeding on microstructure development and mechanical properties of silicon nitride was investigated by the use of morphologically regulated rodlike β-Si3N4 singlecrystal particles with a diameter of 1 μm and a length of 4 μm as seed crystals. Silicon nitride with a bimodal microstructure was fabricated under a relatively low nitrogen gas pressure of 0.9 MPa owing to the epitaxial growth of β-silicon nitride from the seed particles. Grain growth from seeds followed the empirical equation Dn–D0n=kt, with growth exponents of 3 and 5 for the c-axis direction and the a-axis direction, respectively, being analogous to the kinetics of matrix grain growth. By seeding morphologically regulated particles, fracture toughness of silicon nitride was improved from 6.3 to 8.4–8.7 MPa·m1/2, retaining high strength levels of about 1 GPa.

Processing of molybdenum disilicide

Abstract: Inspection of the scientific literature reveals that intermetallic compounds have, in recent years, attracted considerable interest as a result of their unique elevated temperature characteristics. Among the wide range of intermetallic compounds that are actively being studied, MoSi2 has been singled out as a result of its unique combination of properties, which include an excellent oxidation resistance, a high modulus of elasticity, and an elevated melting point (2030°C). In view of this interest, the present work was undertaken with the objective of providing the reader with a comprehensive review of the mechanical and oxidation behaviour of MoSi2, paying particular attention to the synergism between processing and microstructure. Accordingly, synthesis techniques, including powder metallurgy, self-propagating hightemperature synthesis, spray processing, solid-state displacement reactions, and exothermic dispersion, are critically reviewed and discussed. In addition, recent efforts aimed at using MoSi2 as a matrix material in metal-matrix composites are also critically reviewed and discussed.

Stiction of surface micromachined structures after rinsing and drying: model and investigation of adhesion mechanisms

Abstract: The mechanisms causing stiction of polysilicon structures fabricated by surface micromachining techniques have been investigated. It is found that during drying from rinse liquids attractive dynamic capillary forces are responsible for bringing micromechanical structures into contact with the underlying substrate. Measured adhesion energies of sticking microbridges indicate that van der Waals forces are responsible for the stiction of hydrophobic surfaces and that hydrogen bridging is an additional adhesion mechanism for hydrophilic surfaces. Methods to reduce the stiction problem are indicated.

Thermoplastic-toughened epoxy polymers

Abstract: The microstructure and properties of two epoxy-resin systems which have been modified with varying amounts of a thermoplastic to improve the toughness of the thermosetting epoxy polymers, have been studied. The curing agent was 4,4′ diaminodiphenylsulphone and the thermoplastic was a reactively terminated poly (ether sulphone) copolymer. Different microstructures were found to occur as the concentration of the thermoplastic component was steadily increased. In particular, the relationships between the microstructures and values of stress-intensity factor, KIc, and fracture energy, GIc, were explored.

Size effects on tensile fracture properties: a unified explanation based on disorder and fractality of concrete microstructure

Abstract: Tests were carried out using three independent jacks orthogonally disposed, making it possible to apply a purely tensile force, so that the secondary flexural stresses, if kept under control, constitute a degree of error comparable with the values allowed for normal testing apparatus. The method enables a stress versus strain curve to be plotted with the descending (softening) branch up to the point where the cross-section of the tensile specimen breaks away. The principal purpose is to avoid any spurious effect that might provide a fallacious explanation of the recurring size effects on apparent tensile strength and fictitious fracture energy. Once the secondary effects have been excluded, only the disorder and fractality of the concrete microstructure remain to explain such fundamental trends. In the case of tensile strength, the dimensional decrement represents self-similar weakening of the material ligament, due to pores, voids, defects, cracks, aggregates, inclusions, etc. Analogously, in the case of fracture energy, the dimensional increment represents self-similar tortuosity of the fracture surface, as well as self-similar overlapping and distribution of microcracks in the direction orthogonal to that of the forming macrocrack.

Strength of mixtures of bainite and martensite

Abstract: Recently published experimental data demonstrate that the strength of mixed microstructures of tempered bainite and martensite can peak at an intermediate volume fraction of martensite. In the present work, a quantitative interpretation of these observations is achieved by modelling the mechanical properties of bainite and martensite in their tempered states. It is found that the peak in the curve of the strength as a function of the volume fraction of martensite can be attributed to two factors. When bainite forms it enriches the residual austenite with carbon, so that the strength of the subsequent martensite increases. In addition, during its deformation, the strength of the bainite is enhanced via plastic constraint by the surrounding stronger martensite. Taking these effects into account, it is possible to predict accurately both the trends and the absolute values of published experimental data on the strength of mixed microstructures.MST/1901

Constitutive models for porous materials with evolving microstructure

Abstract: A constitutive model is developed for the effective behavior of nonlinear porous materials which is capable of accounting, approximately, for the evolution of the material's microstructure under large quasi-static deformations. The model is formulated in terms of an effective potential function for the porous material, which depends on appropriate variables characterizing the state of the microstructure, together with evolution equations for these state variables. For the special case of triaxial loading of an initially isotropic porous material, the appropriate state variables are the porosity and the aspect ratios of the typical void; they serve respectively to characterize the evolution of the size and shape of the pores. The implications of the model are studied in the context of two specific examples: axisymmetric and plane strain loading conditions. It is found that the porosity acts as a hardening mechanism when the material is subjected to boundary conditions resulting in overall hydrostatic compression, and as a softening mechanism for overall hydrostatic tension. On the other hand, the change in shape of the voids is found to have a more subtle influence on the overall behavior of the porous material. Thus the change in shape of the voids has a direct effect which may range from strong softening during void collapse to slight hardening during void elongation, but it also has an indirect effect through its concomitant effect on the evolution of the porosity, which may actually be quite significant. Because of the complex interplay between these hardening-softening mechanisms, the new model is found to yield significantly different predictions, in particular for the onset of localization, than the well-known model of Gurson, which neglects the change in shape of the voids, especially, for low-triaxiality loading conditions. The model, in its present form, is not meant to be used for high-triaxialities for which the Gurson and other associated models are considered to be quite accurate.

Effect of Notch‐Root Radius on the Fracture Toughness of a Fine‐Grained Alumina

Abstract: The dependence of K IC on the notch-root radius has been examined for a notch radius as small as a few micrometers in a dense, fine-grained, polycrystalline alumina ceramic. The notch radius can be systematically varied by using a semimanual procedure in a special jig which polishes out rather than cuts the specimen. K IC is independent of the notch sharpness for notch-root radii <10 μm. The results are critically compared with those obtained by other standard techniques and discussed in terms of residual compressive stresses introduced during the notching procedure

Improved mechanical properties in new, Pb-free solder alloys

Abstract: The mechanical properties of solders benefit from uniform dispersion of fine precipitates and small effective grain sizes. Metallurgical methods of attaining such a beneficial microstructure have been investigated in two new, near-eutectic, Pb-free solder alloys systems—Sn-Zn-In (m.p. ∼188°C) and Sn-Ag-Zn (m.p.∼217°C). It has been found that small alloying additions of Ag dramatically improve the mechanical properties of the ternary Sn-8Zn-5In alloy. The improvement is attributed to the elimination of the coarse and nonuniform distribution of plate-like dendrites and refining the effective grain size in the solidified microstructure. Also, small amounts of Cu dramatically improve the ductility in the ternary Sn-3.5Ag-lZn alloy. The quaternary Sn-3.5Ag-lZn-0.5Cu has better mechanical properties than the binary Sn-3.5Ag alloy because it has a uniform fine dispersion of precipitates and a small effective grain size. The combination of high mechanical strength and high ductility is likely to yield improved fatigue resistance properties in the interconnection of electronic components.