The microstructures and mechanical properties of single phase and multiphase alloys are reported for Al-rich intermetallic alloys containing up to 45 at. % Ti and as much as 68 at. % Cr. Among the individual phases formed in these alloys, TiAl and tau (Al67Cr8Ti25) had both the lowest hardnesses and the greatest resistance to cracking. Additionally, the presence of these two phases in multiphase alloys improved mechanical properties.

Calculations predict that carbon nanotubes may exist as either semimetals or semiconductors, depending on diameter and degree of helicity. This communication presents experimental evidence supporting the calculations. Scanning tunneling microscopy and spectroscopy (STM-S) data taken in air on nanotubes with outer diameters from 17 to 90 Å show evidence of one-dimensional behavior; the current-voltage (I-V) characteristics are consistent with a density of states containing Van Hove type singularities for which the energies vary linearly with inverse nanotube diameter.

Stable cubic ZrO2 up to a temperature of 1173 K has been synthesized by a chemical precipitation technique. Such a stability appears to be driven by particle size. The critical value below which cubic ZrO2 has been found to be stable is 17 nm. The x-ray diffraction pattern of such ultrafine cubic particles is similar to that obtained by stabilization of ZrO2 by the addition of 20 mole % CaO.

Pyrochlore-type Pb2FeWO6.5 is difficult to be sintered without applied pressure at temperatures lower than its decomposition temperature. Through hot pressing or hot isostatic pressing processes, densification of specimens is greatly enhanced; moreover, grain growth during sintering is effectively suppressed. Densified Pb2FeWO6.5 exhibits paraelectric characteristic from −135°to −2 °C with low dielectric constants and low dissipation factors. The dielectric constants show rather weak dependence of temperature and frequency.

The angular magnetic field dependence of the critical current density JC(θ) of epitaxial YBa2Cu3O7 thin films is presented in the temperature regime close to Tc. The high temperature behavior of Jc(θ) shows features that are significantly different from the earlier observations at lower temperatures. A comparison between the films on (100) LaAlO3 and (100) yttria stabilized zirconia (YSZ) indicates significant differences that may be attributed to the differences in the microstructure of the films on the two substrates. In particular, the observation of enhanced pinning in the films on YSZ may be due to the pinning effects of the low angle grain boundaries present in these films.

The YBa2Cu3O7−x formation kinetics from a spray-roasted precursor powder containing Y2O3, BaCO3, and CuO was followed via in situ, time-resolved x-ray diffraction as a function of gas atmosphere and temperature. In inert atmospheres, BaCO3 and CuO form BaCu2O2 which subsequently reacts with Y2O3 to form YBa2Cu3O6. However, YBa2Cu3O6 decomposes at temperatures exceeding 725 °C with Y2BaCuO5 being one of the decomposition products. In oxidizing atmospheres, YBa2Cu3O7−x formation involves the BaCuO2. At high temperatures (800–840 °C), oxygen increases the yield of YBa2Cu3O6. A nuclei growth model assuming two-dimensional, diffusion-controlled growth with second-order nucleation rate fits the experimental data.

Bi—(Pb)—Sr—Ca—Cu—O superconductors were synthesized by sol-gel processing from nitrates by complexation with citric acid. Their grain growth and sintering above 800 °C were studied by Scanning Electron Microscopy (SEM) and Brunauer, Emmett, Teller (BET) porosimetry. The sintering was limited by anisotropic grain growth, and microcracks in 2212 phase grains were created due to the formation of 2223 phase.

The introduction of more reactive precursors for Pb and Sr (oxalates), as well as Ca (citrate) and the use of a Bi nitrate decomposition route, has increased the percentage of the high-Tc (2223) phase in the Bi—Sr—Ca—Cu—O (BSCCO) system. Partial substitution of Bi(Pb) with Sb gives an almost single (2223) phase sample. In addition, a single (2212) phase sample is obtained when high purity Bi2O3 is used as a precursor, whereas Bi acetate leads to semiconducting behavior. The morphology of the samples is studied with a scanning electron microscope (SEM), the stoichiometry with energy-dispersive x-ray analysis (EDAX), and the structure with x-ray diffraction (XRD), while the superconducting properties are investigated by dc-resistivity, ac-susceptibility, and SQUID magnetometry techniques.

The microstructure and superconducting properties of Bi2Sr2CaCu2Ox (Bi-2212) during high-energy attrition milling were investigated in detail by a combination of x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and magnetization techniques. The starting superconducting powder was milled in a standard laboratory attritor using yttria-stabilized ZrO2 balls and a stainless steel tank. After selected time increments, the milling was interrupted and a small quantity of milled powder was removed for further analysis. It was found that the deformation process rapidly refines Bi-2212 into nanometer-size crystallites, increases atomic-level strains, and changes the plate-like morphology of Bi-2212 to granular submicron clusters. At short milling times, the deformation seems localized at weakly linked Bi-O double layers, leading to twist/cleavage fractures along the {001} planes. The Bi-2212 phase decomposes into several bismuth-based oxides and an amorphous phase after excessive deformation. The superconducting transition is depressed by about 10 K in the early stages of milling and completely vanishes upon prolonged deformation. A deformation mechanism is proposed and correlated with the evolution of superconducting properties. The practical implications of these results are presented and discussed.

The microstructure of Ba1−xKxBiO3 (BKBO) thin films from three different sources has been extensively compared by transmission electron microscopy studies. The three films were prepared independently in three different laboratories on three different substrates of (100) orientation and displayed excellent superconducting properties. The observed microstructure is remarkably similar in the three films. They are epitaxial with (100) orientation through all their extension and no cracks have been observed. Their defect density is similar and the resulting extension of defect-free regions is of the order of 50–80 nm.

Electrochemical and high pressure oxygenation experiments are described which extend the range of intercalated oxygen content beyond that accessible in 1 atm O2 treatments for the triangular planar cuprate delafossites YCuO2+x and LaCuO2+x, with the intention of increasing the hole doping, and filling vacancies in the intercalated oxygen lattice to decrease electrical resistivities. Stabilization of the 2H polytype of YCuO2 through 1% Ca substitution for Y and oxidation conditions for obtaining the pure orthorhombic intercalated oxygen superlattice form of YCuO2.50 are also described. Temperature dependent resistivities for some of the superoxygenated compositions are reported; metallic behavior was not observed.

The influence of finely dispersed, stable particles on the mechanical strength and microstructure of Al films on Si substrates has been studied. Aluminum oxide particles were produced in Al films by oxygen ion implantation, and the grain size was increased by a laser reflow treatment. Transmission electron microscopy (TEM) was employed to observe the oxide particles and the grain structure in the films after subsequent annealing, and the wafer curvature technique was used to study the deformation properties of the films as a function of temperature. Significant particle strengthening was obtained in the coarse-grained films in tension as well as in compression. In the as-deposited and ion-implanted films a very fine grain size of only 0.35 μm is stabilized after annealing which causes considerable softening of the film in compression at higher temperature because of the enhancement of grain boundary and volume diffusion controlled relaxation mechanisms. However, in tension at low temperature these films show high stresses comparable to those of the laser reflowed and ion-implanted films. The results are discussed in the light of TEM observations.

Grazing incidence x-ray scattering (GIXS) with a synchrotron source was used to measure elastic strain gradients as a function of temperature in aluminum and aluminum alloy thin films of different thicknesses on silicon. The stresses in the films are induced as a result of the difference in thermal expansion coefficient between film and substrate. Disregarding minor deviations at the surface, it is shown that there are no gross strain gradients in these films in the range of temperatures (between room temperature and 400 °C) considered. Significant x-ray line broadening effects were observed, suggesting an accumulation of dislocations on cooling the films and their annealing out as the films were being reheated. The variation of the dislocation density during thermal cycling compares well in nature with that of the concurrent variation in film stress, indicating that large strain hardening effects contribute toward the film flow stress.

Cu NMR spectra from cluster-assembled nanophase copper with an average grain size between 5 and 10 nm show a broadened peak, at the normal Knight-shifted frequency for copper metal, which arises from only the central 1/2 to −1/2 transition. The broadening of the central line is associated with a distribution of Knight shifts. A very broad background is observed on either side of that peak, associated with broadening due to internal electric field gradients. Pulsed NMR measurements of the central peak show that virtually all the copper signals are significantly broadened and have a spin-spin relaxation time longer than larger-grained copper samples. The strain within the grains is estimated to be 0.7%. Line shape measurements as a function of spin echo delay time show there are a number of copper sites with longer relaxation times which have a significantly larger broadening. Those sites are tentatively identified as being at or near a grain boundary or free surface. A small orientation effect is observed indicating an anisotropy within the samples. An isochronal anneal of one sample showed significant line narrowing after an anneal at 450 °C consistent with other nanophase metals which show grain growth above 40-50% of the absolute melting temperature. The dependence of NMR linewidth on average grain diameter is estimated.

The deformation characteristics of icosahedral Al70Pd21.5Mn8.5 have been investigated by high temperature creep experiments, and the resultant microstructures have been examined by transmission electron microscopy (TEM). From 730 to 780 °C, microstructural analysis revealed that the deformation is controlled by dislocation glide, with an activation energy of 210 ± 30 kJ/mole and a stress exponent of 1.2 ± 0.2. From 780 to 810 °C, microstructures were characteristic of deformation controlled by dislocation glide and climb. The activation energy and stress exponent were determined to be 1700 ± 80 kJ/mole and 2.9 ± 0.3, respectively. Hardness measurements also reflected an increase in dislocation density, as the hardness of the deformed samples was approximately 10% higher than the as-cast sample.

A kinetic theory of ordering based on the path probability method was implemented in the pair (Bethe) approximation and used to study the kinetics of short- and long-range ordering in alloys with equilibrium states of B2, DO3, or B32 order. The theory was developed in a superposition approximation for a vacancy mechanism on a bcc lattice with first- (1nn) and second-nearest neighbor (2nn) pair interactions. Chained 1nn conditional probabilities were used to account for the entropy of 2nn pair configurations. Monte Carlo simulations of ordering were also performed and their results compared to predictions of the pair approximation. Comparisons are also made with predictions from an earlier kinetic theory implemented in the point (Bragg-Williams) approximation. For all three calculations (point, pair, and Monte Carlo), critical temperatures for B2 and DO3 ordering are reported for different 1nn and 2nn interaction strengths. The influence of annealing temperature on the kinetic paths through the space of B2, DO3, and B32 order parameters was found to be strong when the thermodynamic preferences for the ordered states were of similar strengths. Transient states of intermediate order were also studied. A transient formation of B32 order in an AB3 alloy was found when 2nn interactions were strong, even when B32 order was neither a Richards-Allen-Cahn ground state nor a stable equilibrium state at that temperature. The formation of this transient B32 order can be argued consistently from a thermodynamic perspective. However, a second example of transient B2 order in an AB alloy with equilibrium B32 order cannot be explained by the same thermodynamic argument, and we believe that its origin is primarily kinetic.

A reverse martensitic phase transformation was observed in Nb-enriched Nb-Co multilayers induced by room temperature 200 ke V xenon ion mixing. Further experiments revealed that this bcc-fcc transition proceeds in two steps, i.e., bcc-hcp and hcp-fcc. A crystallographic model is proposed to explain the two-step transition through shearing and sliding, which are mediated by irradiation-induced defects and strain in the films. In addition, the existence of the hcp and fcc metastable states in the Nb-Co system was confirmed by high-temperature solid state interdiffusion of the corresponding multilayers.

Monolithic CoSi and TiB2 reinforced CoSi materials were produced by spray atomization and co-deposition. The creep behavior of both materials at elevated temperature was investigated. The unreinforced material of grain size ≍25 μm exhibited a stress exponent of three, activation energy for creep of 320 kJ/mole, dislocation substructure of homogeneously distributed dislocations, and inverse creep transients upon stress increases. These results suggest that the creep behavior of CoSi is controlled by a dislocation glide mechanism. In contrast, the reinforced material of a finer grain size (≍10 μm) exhibited a stress exponent of unity, activation energy for creep of 240 kJ/mole, and negligible creep transients upon stress increases, suggesting that the creep behavior of the reinforced material is controlled by a diffusional creep mechanism. The creep resistance of the reinforced material was lower than that for the unreinforced material. This is a result of the finer grain size and higher porosity in the reinforced material.

In the process of crystallization of amorphous melt-spun Nd15Fe77Bx (x = 6–14) alloys, it was found that the crystallization behavior strongly depended upon the boron content even in the same equilibrium three-phase coexistence range. The Nd2Fe14B phase was crystallized directly from the amorphous state in the amorphous Nd-Fe-B materials with relatively lower boron content (Nd15Fe77B6 and Nd15Fe77B8 alloys). In contrast, the metastable body-centered-cubic (bcc) α-iron phase crystallized from the amorphous state prior to the crystallization of the Nd2Fe14B phase in the amorphous Nd-Fe-B materials with relatively higher boron content (Nd15Fe77B10, Nd15Fe77B12, and Nd15Fe77B14 alloys). The existence of the metastable bcc α-iron phase in the amorphous matrix was confirmed by TEM studies, magnetic measurements, and in situ observation by x-ray diffraction. The formation of the metastable phase stabilized the amorphous state, increasing the crystallization temperature of the Nd2Fe14B phase.

Stages 4-6 FeCl3-graphite intercalation compounds (GIC's) have been prepared by an ordinary two-bulb method, and FeCl3-IBr-graphite bi-intercalation compounds (GBC's) are synthesized by holding the FeCl3-GIC's in the saturated vapor of IBr. The x-ray diffraction patterns of the FeCl3-IBr-GBC's obtained from stages 4, 5, and 6 FeCl3-GIC's give the stacking sequences as G(FeCl3)GG(IBr)GG(FeCl3)G, G(FeCl3)GG(IBr)GGG(FeCl3)G, and G(FeCl3)GG(IBr)GG(IBr)GG(FeCl3)G, respectively, where G, (FeCl3), and (IBr) refer to the graphite, FeCl3, and IBr layers, respectively. The multi-intercalation of H2SO4 into the FeCl3-IBr-GBC's synthesized from stages 4 and 6 FeCl3-GIC's occurs at all the vacant galleries of the GBC's at the same time. In contrast, the multi-intercalation of H2SO4 into the FeCl3-IBr-GBC obtained from the stage 5 FeCl3-GIC takes place in two processes. The first multi-intercalation occurs at the gallery adjacent to the bi-intercalated IBr layer, and the stacking sequence of the resulting graphite multi-intercalation compound is determined to be G(FeCl3)GG(IBr)G(H2SO4)GG(FeCl3)G, where (H2SO4) refers to the H2SO4 layer. The second multi-intercalation occurs at all the rest of the vacant galleries.