The crystalline disordered phase obtained by mechanical alloying of elemental 75 at. % Ni and 25 at. % Al powders has been investigated. The stability of this phase with respect to the thermal reordering process leading to the L12 structure has been analyzed by means of x-ray diffractometry, scanning electron microscopy, and differential scanning calorimetry. Atomistic simulations on an Ni3Al model, reproduced via molecular dynamics using a many-body potential, have been used to interpret experimental data. The ordering transformation takes place in an extended range of temperature (from 320 to 600 °C) and occurs simultaneously with the release of internal strain. Numerical simulations performed under different conditions show that the activation energy of the Ni-vacancy migration mechanism responsible for the ordering process depends on the local state of strain, thus suggesting an explanation for the considerable lowering of this energy in samples obtained by ball milling.

Nanophase diamond films can be grown at room temperature with a laser plasma discharge source without the use of any catalyst. This technique produces films that adhere readily to materials for which there are important applications as protective coatings. Described here is a study of the bonding and properties realized with the direct deposition of nanophase diamond on the II-VI compound of zinc sulfide. It was shown that adhesion and mechanical properties of the films can be correlated with the amounts of defects and impurities in the zinc sulfide substrates. In all cases significant interfacial layers caused by the deep penetration of carbon atoms into the substrates were observed. Resistance to wear was estimated with a modified sand blaster, and results indicated that only 1 μm coating of nanophase diamond can increase lifetimes of the zinc sulfide samples by a factor better than 5. Protection afforded by the nanophase diamond under harsh environmental conditions of rain impacts was also described.

A small addition of boron is suggested to increase the ductility of the polycrystalline Ni3Al when the Al content is less than 25 at.%. Both metallographic and x-ray investigation have shown the alloys of Ni3Al (24 at.% Al) containing 0.20, 0.42, 0.79, 0.98, and 1.22 at.% B to be of single phase and that of 1.76 at.% B to be of two phase. With the addition of boron, the lattice parameter of the Ni3Al phase is found to increase. Microhardness measurements indicate that initially the hardness decreases for the alloy of 0.20 at.% B, while for the rest of the single phase alloys the hardness is found to increase with further addition of boron. The addition of boron increases the deformation stacking fault probability value except for the alloy with 0.20 at.% B.

Solid solution hardening effects are reported for Al-Co-based binary and ternary alloys as a function of off-stoichiometry and of the substitution of the solutes Mn and Re for Co, and of Ti, for Al. Microhardness and stepped flow stress are reported as a function of temperature. Hardening is of the magnitude expected to be caused by conventional elastic interactions between point defects and moving dislocations. For binary alloys the strengths of the interactions are in the proper relative strengths, but not for ternary alloys. In ternary alloys the effects of multiple types of defects are superimposed, but not yet adequately characterized.

Amorphous Bi2Sr2CaCu2O8 (Bi-2212) was crystallized under uniaxial loads (up to 1500 N) at temperatures up to 890 °C to induce texture. Well-textured samples (19 mm in diameter and 0.15 mm thick) were obtained for samples heated to 890 °C and quenched. Heated (10 °C/min) and quenched samples at 550, 750, 850, and 870 °C demonstrate that the crystallization path is Bi2Sr2CuO6 (Bi-2201) (550 °C) to Bi-2212 (850 °C), with texturing occurring during grain growth (T > 850 °C). Comparison of samples crystallized under load and no load demonstrates that the texturing is a result of the plane stress biasing the normal anisotropic grain growth normal to the applied stress direction.

Elemental Ti–Al powder blends were mechanically alloyed in order to study phase formation during the alloying process. In addition, the stability of intermetallic phases upon milling was investigated separately in order to determine the origins of phase selection during the milling process. It was found that by mechanical alloying of powder blends, as well as by ball milling of Ti-aluminides for long milling times, the same metastable phases were formed for corresponding compositions, i.e., the hep solid solution for Al concentrations up to 60 at. % and the fcc solid solution for 75 at. % Al. X-ray diffraction (XRD) analyses indicated that the process of mechanical alloying occurred via the diffusion of Al into Ti. By lowering the milling intensity, a two-phase mixture of the hcp solid solution and the amorphous phase was observed for Ti50Al50 and confirmed by transmission electron microscopy (TEM). The results show that phase selection in the final state during mechanical alloying of Ti–Al powder blends and milling of intermetallic compounds is mainly determined by the energetic destabilization of the competing phases caused by the milling process. The destabilization is most pronounced in the case of intermetallic compounds due to the decrease in long-range order upon milling. For the final milling stage, phase formation can be predicted by considering the relative stabilities of the respective phases calculated by the CALPHAD method using the available thermodynamic data for the Ti–Al system.

By adjusting the sputtering rate and gas pressure, it is possible to form nanoparticles of different sizes, phases, and materials. We have investigated the spatial distribution of sputtered particle formation using a vertical, linear arrangement of substrates. Collecting the particles soon after they are formed, before they have time to grow and agglomerate, allows one to obtain a narrow size distribution. In the case of molybdenum, a narrow distribution of cubic particles is formed at relatively large distances (8 cm) from the source. These cubic particles collide and self-assemble in the vapor into arrays of larger cubic particles. The particle size histograms are fitted to lognormal distribution functions. How supersaturation occurs is discussed qualitatively as a function of the distance from the substrate, sputtering rate, and the mean free path in the vapor. This method of nanocrystalline particle formation has potential use in magnetic and opto-electronic (quantum dot) applications, where a narrow size distribution is required.

The electron diffraction contrast of two types of rectangular dislocation networks in an Al70Co15Ni15 decagonal quasicrystal has been analyzed. One type of dislocation network consists of two dislocation sets whose Burgers vectors are parallel to the tenfold axis A10 and a twofold axis A2D. The other type of dislocation network consists of two dislocation sets whose Burgers vectors are parallel to the A10 and the other twofold axis of A2P. The characteristics of the diffraction contrast of the dislocation networks in the Al–Co–Ni decagonal phase are similar to those in conventional crystals.

A new method of measuring the mechanical strength of thin films is described. We prepare miniature arrays of four tensile specimens, each 0.25 mm wide, 1 mm long, and 2.2 μm thick, using deposition, patterning, and etching processes common to the semiconductor industry. Each array of four specimens is carried on and protected by a rectangular silicon frame. Thirty-six such specimens are produced on a single wafer. After a specimen frame is mounted, its vertical sides are severed without damaging the specimens. The load is applied by micrometers through a special tension spring. Tensile properties of a 2.2 μm thick Ti–Al–Ti film were determined.

We present evidence indicating that rapid, self-sustained, high-temperature reactions play an important role in the formation of tetragonal MoSi2 during room-temperature high-energy ball milling of elemental powders. Such reactions appear to be ignited by mechanical impact in an intimate, fine-grained, Mo–Si physical mixture formed after an initial milling period. Under certain conditions, limited propagation of self-sustained reactions in these uncompacted powder mixtures renders the compound formation seemingly gradual in bulk-averaged analysis. It is suggested that this type of reaction is an important mechanism in the mechanical alloying of highly exothermic systems. Results are discussed in comparison with similar reactions we observed in ball-milled Al–Ni powders, with self-sustained combustion synthesis previously reported for Mo–Si powders, and with interfacial diffusional reactions in Mo–Si powders or thin-film diffusion couples.

By a Plasma Enhanced Chemical Vapor Deposition process (PECVD), we are able to prepare nitrogenated amorphous carbon materials around room temperature from methane and nitrogen gas as precursors. We have also used chlorine gas as an additive to reduce the hydrogen content of our samples. Starting from the “as-deposited” materials, we have investigated their thermal stability by successive heat treatments up to 1400 °C. These compounds suffer a weight loss mostly due to the hydrogen departure. They become nonfusible and it turns out that nitrogen, chemically bound to sp2 hybridized carbons, induces some changes in the physical properties. In order to understand the relationship between the local structural organization and the physical characteristics, we have investigated different spectroscopic techniques such as Nuclear Magnetic Resonance, IR Absorption, and X-ray Photoelectron Spectroscopy. We have also investigated several transport properties: (i) The dc electrical conductivity shows a kind of metal/insulator transition around 700 °C. The temperature dependence for the conductive samples gives evidence for a pseudogap associated with the presence of localized states, (ii) The thermal conductivity exhibits, for the as-deposited compound, a very low value varying slowly with temperature; its magnitude as well as its temperature dependence, characteristic of noncrystalline materials, are modified by the annealing process. Finally, an electronic band model is proposed, explaining the structural evolution through a kind of Mott–Anderson pseudotransition.

This paper, the second of three parts, presents a detailed analysis of the crystallographic texture observed in lead lanthanum titanate (PLT) thin films deposited by multi-ion-beam reactive sputtering. Since films were deposited at a substrate temperature of 100 °C, they exhibit an amorphous structure that can be described by the structure zone model. The as-deposited microstructure is transformed via crystallization of the perovskite phase and PbO evaporation during postdeposition annealing. Transmission electron microscopy was used to determine that phase pure PLT films develop textured clusters (as large as 700 nm in diameter) consisting of 〈100〉 aligned 10 nm nanocrystals. As excess PbO is added to the PLT film, the textured cluster size decreases until only isolated PLT nanocrystals exist. Below a critical quantity of excess PbO in the as-deposited film (approximately 15 mole%), the textured cluster structure produces a 〈100〉 texture through the film thickness, which generates x-ray diffraction patterns typical of textured microstructures. At high PbO contents, the excess PbO causes the formation of clusters that are smaller than the film thickness, resulting in a polycrystalline-type x-ray diffraction pattern. A qualitative model describing microstructure development is presented.

Mechanical properties of TiAl base alloys were studied with three-point bending tests in vacuum, air, and gaseous hydrogen. The TiAl-Mn and TiAl (50:50) alloys homogenized at 1273 K exhibited relatively good bend ductility (about 2.5–3.0%) in vacuum, and poor ductility (about 1%) in air and hydrogen. The Ti-rich and Al-rich TiAl alloys, which exhibited little ductility even in vacuum, hardly showed any environmental effect on ductility. These facts may suggest that the mechanical properties of TiAl base alloys are affected at room temperature by the laboratory air and hydrogen, and that plastic deformation plays an important role in environmental embrittlement of TiAl base alloys at room temperature.

A model for the growth of a grain boundary crack in thermomechanical fatigue is derived for a single-phase or low alloy fcc metal at homologous temperatures near 0.5. Crack growth is hypothesized to proceed through vacancies binding in pairs at the crack tip to impurities or an oxide layer. This model is applied to 97Pb−3Sn, and the results compared with experiment. Good agreement is shown between the model and experiment, especially in predicting the effects of frequency and thermal-mechanical phasing. These effects do not appear to have been previously modeled successfully.

The structural transformations occurring during the mechanical alloying of mixtures of elemental oxides in the Bi–Sr–Ca–Cu–O system have been studied. Metastable cubic structures with complete chemical disorder and amorphous phases can be synthesized by strong mechanical deformations. With proper thermal treatments, these metastable phases transform into the high Tc superconducting structures. Discussions on the mechanisms of formation and on the thermal stability of the various metastable phases are presented.

The microstructural and morphological properties of thin (6000 Å) Al98.5wt. %Si1.0wt. %Cu0.5wt. % films on chemical-vapor-deposited tungsten-coated substrates have been characterized as functions of substrate temperature during deposition and a postdeposition sinter. Scanning electron and transmission electron microscopic investigations show these properties can be categorized with respect to the substrate temperature during deposition. The Al98.5wt. %Si1.0wt. %Cu0.5wt. % films deposited on substrates heated at temperatures ≤200 °C are rough and are comprised of rounded grains. For deposition on substrates heated at ≤300 °C, the films are smooth. Large voids and small precipitates (presumably Al2Cu) are present in the films deposited at 400 °C. The films retain their as-deposited texture during a 450 °C sinter. Precipitates and evidence of W interactions occur in the sintered films deposited on the lower temperature substrates. In addition, the shapes of thermal hillocks and mesa-like protrusions that form during the sintering process are influenced by the films' as-deposited morphologies.

The high temperature (T = 1083–1388 K, 0.65–0.84 Tm) creep behavior of single crystalline Ni3Al(Ta, B) was investigated. A change in the creep characteristics as a function of applied stress was observed at the uppermost testing temperatures of 1288 and 1388 K. At low applied stress levels the Norton law stress exponent is equal to 4.3; for higher stresses the stress exponent is equal to 3.2. Different creep curves were observed, depending on the value of the stress exponent. The change in stress exponent and nature of the creep curve correspond to a change in the controlling deformation mechanism from dislocation climb to viscous dislocation glide for Ni3Al(Ta, B). The experimentally observed transition stress values between climb and viscous glide are in good agreement with values predicted from theory, assuming that the major force retarding viscous dislocation glide in Ni3Al(Ta, B) is the antiphase boundary interaction.

Carbon aerogels are synthesized via the aqueous polycondensation of resorcinol with formaldehyde, followed by supercritical drying and subsequent pyrolysis at 1050 °C. As a result of their interconnected porosity, ultrafine cell/pore size, and high surface area, carbon aerogels have many potential applications such as supercapacitors, battery electrodes, catalyst supports, and gas filters. The performance of carbon aerogels in the latter two applications depends on the permeability or gas flow conductance in these materials. By measuring the pressure differential across a thin specimen and the nitrogen gas flow rate in the viscous regime, the permeability of carbon aerogels was calculated from equations based upon Darcy's law. Our measurements show that carbon aerogels have permeabilities on the order of 10−12 to 10−10 cm2 over the density range from 0.05–0.44 g/cm3. Like many other aerogel properties, the permeability of carbon aerogels follows a power law relationship with density, reflecting differences in the average mesopore size. Comparing the results from this study with the permeability of silica aerogels reported by other workers, we found that the permeability of aerogels is governed by a simple universal flow equation. This paper discusses the relationship among permeability, pore size, and density in carbon aerogels.

This paper, the third and final of a three part series, presents the electrical properties of postdeposition annealed, lead lanthanum titanate (PLT) thin films deposited by multi-ion-beam reactive sputtering (MIBERS). Also, a model is presented that explains the relations among composition, crystallographic structure, microstructure, and electrical properties of the PLT thin films. Thin films of PLT consisting of the perovskite phase exhibit 〈100〉 textured microstructures. Addition of a critical quantity of excess PbO results in the loss of this 〈100〉 texture, and continuity of the perovskite phase is disrupted while both excess PbO and porosity phases become continuous due to a percolation effect. Films with textured microstructures consisting of a continuous perovskite phase exhibit relatively high dc resistivities, high dielectric permittivities, and high remanent polarizations. At the transition between textured and nontextured microstructures, a discontinuous drop in the electrical properties occurs due to the ensuing continuity of the excess PbO and porosity. These composition-induced changes in the electrical properties were quantitatively modeled by applying a simple mixing rule model to the microstructure model developed in Part II of this series.

Because of their unusually large specific surface area (SSA), Activated Carbon Fibers (ACF's) have a huge density of micropores and defects. The Raman scattering technique and low-temperature dc electrical conductivity measurements were used as characterization tools to study the disorder in ACF's with SSA ranging from 1000 m2/g to 3000 m2/g. Two peaks were observed in every Raman spectrum for ACF's and they could be identified with the disorder-induced peak near ∼1360 cm−1 and the Breit–Wigner–Fano peak near ∼1610 cm−1 associated with the Raman-active E2g2 mode of graphite. The graphitic nature of the ACF's is shown by the presence of a well-defined graphitic structure with La values of 20–30 Å. We observed that the Raman scattering showed more sensitivity to the precursor materials than to the SSA of the ACF's. From 4 K to room temperature, the dc electrical resistivity in ACF's is observed to follow the exp [(T0/T)1/2] functional form and it can be accounted for by a charge-energy-limited tunneling conduction mechanism. Coulomb-gap conduction and n-dimensional (n ≤ 3) variable-range hopping conduction models were also considered but they were found to give unphysical values for their parameters.