ZnSe layers have been grown by molecular beam epitaxy on high-purity, high-quality ZnSe wafers [(100) oriented] cut from ingots grown by the iodine vapor transport method. Photoluminescence (PL) analysis indicates the homoepitaxial ZnSe material to be of ultra-high purity as well as being strain-free relative to ZnSe/(100) GaAs layers which exhibit in-plane biaxial tension. The 10 K PL spectra recorded from homoepitaxial layers exhibit unsplit free- and donor-bound exciton transitions of comparable intensity together with a strong peak at 2.7768 eV believed to be the so-called Iv transition in relaxed ZnSc. The ultra-high purity nature of the homoepitaxial layers is attributed to the high purity of the substrate material in addition to the use of high purity Zn and Se source materials in this work.

The critical current density Jc of superconductors is strongly dependent on microstructure. The microstructure and crystallization of amorphous Nb75Ge24.5B0.5 melt spun ribbons were studied as a function of 24-h anneals between 680 and 860 °C. Metastable A15 was formed with a composition close to that of the starting amorphous material. As the temperature increased from 680 to 720°C, crystallization remained nonuniform and the average grain size decreased from 130 nm to 60 nm. Between 740 and 800°C crystallization was uniform and the average grain size was constant at 30 nm. Above 820°C grain growth occurred. In order to have high Jc's a uniformly crystallized small-grained material is desirable.

Polysynthetically twinned crystals of TiAl with a nearly stoichiometric composition have been grown and rolled at room temperature. The maximum total reduction in thickness which is attainable without fracture depends on lamellae orientation with respect to the rolling plane and rolling direction. When specimens are oriented such that shear deformation parallel to the lamellar planes is operative during rolling and its operation causes lengthening of specimens, such specimens can be rolled up to about 50% reduction in thickness. The (111) pole figures are determined for the surface of specimens rolled to several different amounts of reduction, and the formation of surface texture is briefly discussed.

The mechanisms of metallic glass formation and competing crystallization processes in mechanically-deformed Ni-Zr multilayered composites have been investigated by means of differential scanning calorimetry and x-ray diffraction. Our investigation of the heat of formation of amorphous NixZr1−x alloys shows a large negative heat of mixing (on the order of 30 kJ/mole) for compositions near Zr55Ni45 with a compositional dependence qualitatively similar to that predicted by mean field theory. We find that the products of solid state reactions in composites of Ni and Zr can be better understood in terms of the equilibrium phase diagram and the thermal stability of liquid quenched metallic glasses. We have determined the composition of the growing amorphous phase at the Zr interface in these Ni-Zr diffusion couples to be 55 ± 4% Zr. We investigated the kinetics of solid state reactions competing with the solid state amorphization reaction and found the value of the activation energy of the initial crystallization and growth of the growing amorphous phase to be 2.0 ± 0.1 eV, establishing an upper limit on the thermal stability of the growing amorphous phase.

The ordered fcc intermetallic compound Ni3Al was mechanically milled in a high energy ball mill. The severe plastic deformation produced by milling induced transformations with increasing milling time as follows: ordered fcc → 2; disordered fcc → 2; nanocrystalline fcc + amorphous. The milling time for complete disordering occurred at 5 h for stoichiometric Ni3Al milled at ambient temperature compared to 50 h for the first observation of an amorphous structure. The structural and microstructural evolution with milling time was followed by x-ray diffraction, TEM, hardness, and calorimetry. The major defect believed responsible for inducing the crystalline-to-amorphous transformation is the fine grain boundary structure with nanometer (∼2 nm diameter) dimensions. The calculated interfacial free energy of the grain boundaries is consistent with the estimated free energy difference between the fcc and amorphous phases in Ni3Al.

The preparation of multicomponent chalcogenide glassy thin films from bulk targets by laser ablation is described. The film stoichiometries are characterized by proton-induced x-ray emission (PIXE). Compared to single source thermal evaporation, laser ablation is found to preserve starting stoichiometries in the resulting thin films far more accurately. Thermally evaporated films were studied both by PIXE and by energy dispersed x rays (EDX) produced in a scanning electron microscope, and the results of these two analytical techniques compare well.

The effects of internal electrodes on the fracture properties of relaxor ferroelectric 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 multilayer structures were studied. It was observed that the position of the first electrode layer from the surface is critical to the strength and toughness of the multilayer structure. Positioning the electrode close to the surface was found to enhance interaction with cracks which initiate on the multilayer surface. This interaction limits the effective crack length and therefore increases the fracture strength and effective toughness of the composite. Samples indented parallel to the electrodes exhibited higher fracture strengths and toughness than those indented perpendicular to the conducting layers. Both parallel and perpendicular orientations gave higher strength and toughness than the control specimens (without electrodes).

This paper presents an atomic calculation of the wedging effect which occurs in a brittle crack when molecules of a chemisorbing species of molecules of sufficient size enter the crack mouth. A surface tension develops at the tip of the wedge caused by the difference between the covered and vacuum surface energies. This force draws the chemisorbing molecules toward the crack tip and distorts the crack faces, causing, in turn, a compensating elastic force on the molecules which tends to eject the molecules. We calculate the equilibrium penetration of the wedging molecules and the configuration of the crack and wedge by an atomistic calculation. We simulate mica/water chemistry by means of a simplification of the mica lattice and calculate interactions between the water and mica on the basis of Born–Mayer. Water is found to form a wedge tongue of two or three molecular thicknesses and a length of about 20 molecular distances, which penetrates into the crack tip cohesive zone. When strong wedging action occurs at a crack tip, crack advance near threshold loadings will be limited by molecular diffusion through the wedge tongue.

A model for investigating the influence of distributed disorder on the failure of brittle materials is introduced. The model assumes that microstructural features of a material can be represented by simple linear springs with a failure threshold, and that the entire material can be represented by a connected network of such springs. Distributed disorder is introduced by allowing spring-to-spring variations in spring characteristics such as the modulus and the failure strain. The conditions under which such a spring network model is valid for studying failure are discussed. The consequences of distributed residual stress disorder on macroscopic mechanical behavior are then studied using the network model, and a brittle to ductile-like transition in the stress-strain behavior is observed with increasing disorder. All the qualitative features of the network results can be described theoretically by a statistical analysis of this problem. Finally, notch tests are performed to evaluate the strength and toughness of the ductile-like materials as compared to the uniform (no disorder) material, and the ductile-like material is found to have (i) a larger work of fracture, (ii) comparable strength in the presence of processing flaws, and (iii) the possibility of larger toughness. Based on these results, the possibility of observing such ductile-like behavior in real composite materials is discussed.

A simple spring network model is utilized to investigate stress concentrations and toughness increases for a variety of traditional mechanics problems. The scaling of crack-tip stress with crack size and separation, the shielding due to a low modulus process zone, and the toughness increase from microcracking and transformation process zones are all in good agreement with available analytic or numerical mechanics results. Also, the, process zone growth and attendant R-curve behavior for microcracking and/or transformation toughening is simulated directly using the network model with only a few simple rules. The network model is thus expected to be useful for obtaining easy quantitative results for related problems which are not so easily solved by other means. In addition, the good agreement found here supports the use of the network model for studying problems involving distributed disorder, as discussed in the previous paper.

The diffusion-accommodated sliding of irregularly shaped grain boundaries in two-dimensional bicrystals is considered. The following assumptions are made: the grains adjoining the boundaries are rigid, the boundaries do not support any shear stresses, sliding displacements are infinitesimal, and sliding is accommodated only by grain boundary diffusion. The solution to this problem is illustrated for a bicrystal with a grain boundary consisting of three segments. The results of calculations involving up to 35 segments agree with Raj and Ashby's theory for the sliding of periodic boundaries. The influence of boundary conditions on the normal stress distributions along grain boundaries is examined. Zero-flux conditions at the intersection of a grain boundary with a free surface, which correspond to low surface diffusivities, can lead to high normal grain boundary stresses. The stress distributions and sliding rates of boundaries containing randomly spaced equisized bumps or equispaced bumps of random size are compared to the periodic case (i.e., equispaced equisized bumps). Substantial normal stresses can build up at such nonperiodic grain boundaries.

Four kinds of pitch precursor fibers possessing different mean sizes of mesophase domains in their transverse sections, but having the same chemical constituents, were prepared by controlling only the spinning conditions. The fibers were thermoset and heat-treated at various temperatures from 600°C to 3300 °C. Lc (002) or d002 vs THT curves showed that pitch precursor fibers with finer transverse microstructures have a lower degree of graphitizability. Further, these four kinds of fibers exhibited systematically different d002 vs Lc (002) curves. The resistivity at room temperature is dependent on d002, being independent of the mean size of the mesophase domains in the transverse section of the pitch precursor fiber. The transverse magnetoresistance at 77 K and 10 kG, (Δρ/ρ)t, also showed a universal dependence on d002 if (Δρ/ρ)t was positive. However, the (Δρ/ρ)t vs d002 curves seemed to be slightly split in the negative region of (Δρ/ρ)t for each of the four kinds of fibers.

In this paper two methods for measuring aluminum compositions in very thin (1.5–15 nm), individual AlxGa1–xAs layers are investigated. The transmission electron microscopy (TEM) thickness-fringe method uses the bright-field extinction fringes from a small, cleaved 90° wedge imaged in a [100] orientation. By comparing calculated and experimental extinction fringes, compositions are determined with a sensitivity in x of 0.03, in layers with a thickness of 3.5 nm. With secondary ion mass spectrometry (SIMS), compositions in AlxGa1–xAs layers with a thickness of 15 nm are measured with an accuracy in x between 0.02 and 0.05. Thicker layers (1 μm) with a composition known from x-ray diffraction measurements are used as a reference for both methods. Subsequently, TEM results are compared with SIMS and the reference measurements. The overall agreement is good, but for 0.25 < x < 0.65, the values of x found by TEM are systematically 0.05 too low. Using the SIMS and reference measurements as a calibration, compositions can be determined by TEM with an accuracy between 0.03 (low x) and 0.05 (high x) in layers as thin as 1.5 nm.

The structure of a near coincidence Ge tilt grain boundary, containing a step, has been derived from a high resolution electron micrograph. There are two possible interpretations of portions of this interface, one of which is the existence of a sheet of fivefold coordinated atoms between the Σ = 19 and Σ = 27 coincidence misorientations. This finding may represent the first experimental evidence that overcoordinated atoms are present at semiconductor grain boundaries free of a screw dislocation.

Growth of amorphous-titanium-silicidc and crystalline C49 TiSi2 in titanium/amorphous-silicon multilayer films was investigated using a combination of differential scanning calorimetry (DSC), thin film x-ray diffraction, Auger depth profiling, and cross-sectional transmission electron microscopy. The multilayer films had an atomic concentration ratio of 1Ti to 2Si and a modulation period of 30 nm. In the as-deposited condition, a thin amorphous-titanium-silicide layer was found to exist between the titanium and silicon layers. Heating the multilayer film from room temperature to 700 K caused the release of an exothermic heat over a broad temperature range and an endothermic heat over a narrow range. The exothermic hump was attributed to thickening of the amorphous-titanium silicide layer, and the endothermic step was attributed to the homogenization and/or densification of the amorphous-silicon and amorphous-titanium-silicide layers. An interpretation of previously reported data for growth of amorphous-titanium-silicide indicates an activation energy of 1.0 ± 0.1 eV and a pre-exponential coefficient of 1.9 × 10−7 cm2/s. Annealing at high temperatures caused formation of C49 TiSi2 at the amorphous-titanium-silicide/amorphous-silicon interfaces with an activation energy of 3.1 ± 0.1 eV. This activation energy was attributed to both the nucleation and the early stages of growth of C49 TiSi2. The heat of formation of C49 TiSi2 from a reaction of amorphous-titanium-silicide and crystalline titanium was found to be –25.8 ± 8.8 kJ/mol and the heat of formation of amorphous-titanium-silicide was estimated to be –130.6 kJ/mol.

Periodic structures are generated by solid state reaction between platinum (Pt) and silicon carbide (SiC). At temperatures above 900°C, periodic structures consisting of alternating layers of platinum silicides and carbon are produced in the diffusion zone. The composition profile across the diffusion zone and the chemistry of the periodic structures are investigated by scanning electron microscopy (SEM), scanning Auger microscopy (SAM), x-ray diffraction (XRD), and laser Raman microprobe. The formation of the platinum silicides causes an interfacial melting between Pt and SiC. X-ray diffraction indicates that Pt3Si is formed at 900°C, while Pt2Si is formed at 1000 °C. Laser Raman spectroscopy indicates that carbon is in either an amorphous state or a highly ordered graphitic state, depending upon its location from the reaction interface. The mechanism of formation of the periodic structure is discussed in terms of the solubility of carbon in platinum silicide during the solidification process.

The microstructural development of compacted nanocrystalline TiO2 powder was studied as a function of sintering temperature up to 1000°C. Grain growth was monitored using x-ray diffraction and scanning electron microscopy. The specific surface area and total porosity were determined quantitatively using the nitrogen adsorption BET. The density was measured by gravimetry using Archimedes principle. The green body density of compacted n-TiO2 with average grain size of 14 nm is as high as 75% of theoretical bulk density. Low temperature surface diffusion leads to the disappearance of small pores, while noticeable densification commences at 600°C and reaches near theoretical values at 900°C. Grain growth also begins at 600°C, accelerating rapidly by 1000°C. Hot isostatic pressing is observed to enhance densification while suppressing grain growth.

This article proposes a new scheme for fabricating homogeneous Al2O3–ZrO2 composite, in which a thermodynamic theory of interfacial electrochemical phenomena is applied. The theory predicts that a heterogeneous Al2O3 interface in colloidal dispersion can induce an enhanced concentration of the ionic species needed for a selective formation of the ZrO2 precursor at the Al2O3/aqueous solution interface. Based on this proposition, a homogeneous Al2O3–ZrO2 composite powder was fabricated by a surface-induced coating of the fine ZrO2 precursor on the kinetically stable colloid particles of Al2O3. The composite prepared by the surface-induced coating was characterized by a uniform spatial distribution of the dispersed ZrO2 phase and by the absence of large ZrO2 grains formed from hard ZrO2 agglomerates. The composite also showed highly uniform grain size distribution of both the dispersed ZrO2 and the matrix Al2O3 phases. The uniform grain size distribution of the matrix phase indicates that the homogeneous coating of the fine ZrO2 particles is effectively pinning the Al2O3 grain boundaries.

The ICECAP methodology is used to derive interionic potentials of some cation and anion impurities in MgO, namely, Li+, Na+, K+, Be2+, H−, S2−, and O2−. Analysis is given of the defect energies obtained by using the derived impurity potentials. Based on the available experimental data, comparison is made to justify the reliability of the derived impurity potential for Be2+. The calculated activation energy for Be2+ diffusion comes out to be 1.54 eV as compared to the experimental value of 1.60 eV, which is considered to be very satisfactory.

We have studied the weight loss during sintering and the resulting phase constituents and dielectric properties of a series of compositions in the lead-rich region of the PbO:MgO:Nb2O5 phase diagram near the stoichiometric composition of Pb(Mg1/3Nb2/3)O3 (PMN). While a ternary system of PbTiO3–Pb(CO1/3Nb2/3)O3–PMN was used, only the PMN component was varied in composition. The observed weight loss was ascribed to PbO evaporation from the nonstoichiometric PMN component. It was found that the sintered PMN compositions at about 1000°C correspond to the stoichiometric composition with varying amounts of excess MgO. These sintered compositions can be calculated by taking the Nb ion as the controlling cation in forming the PMN unit cell, from which the excess MgO can be determined. The excess PbO in the starting compositions, above the amount needed to form the sintered compositions of Pb(Mg1/3Nb2/3)O3 plus excess MgO, serves as a densification aid during the sintering process. Subsequently, the excess PbO is lost through evaporation. The peak dielectric constant of these compositions varies between 12000 and 18500, with the highest value attained in the sintered composition corresponding to stoichiometric PMN plus 0.229 mole of excess MgO. The optimum processing conditions for this composition are also identified.