We present a computer simulation study of thin crystalline multilayers constructed from two fcc solids with differing lattice constants and binding energies. Initially the two solids have the same orientation, and the interface is perpendicular to the common [100] direction. We then minimize the energy of the system at zero temperature or equilibrate it at a finite temperature. Both materials are described by Lennard-Jones interatomic potentials. A novel technique for analyzing local atomic ordering, common neighbor analysis, is used to identify structural characteristics in these systems. As we gradually vary the lattice mismatch between the two solids, several structural changes are observed in the layers of smaller atoms after energy minimization. At a mismatch larger than 14%, the layers transform into the hep structure, while at smaller mismatches extended structural defects are generated. At elevated temperatures, the hcp structure is transformed back to fcc, and the structure defects disappear.

One of the main problems found in the nuclear applications of graphite is its dimensional instability under irradiation, involving both swelling and shape changes. In order to understand better the mechanisms that give rise to these changes, highly oriented pyrolytic graphite was irradiated with 300 keV electrons at temperatures between 25 and 657 °C in a transmission electron microscope (TEM). Microscopic dimensional changes and structural disordering were studied in directions parallel and perpendicular to the graphite basal plane. Changes in the specimen length were investigated by measuring the distance between two markers on the specimen surface in TEM images. Changes in the lattice parameter and the crystalline structure were studied by a TEM diffraction technique. In agreement with reported results, large increases in the specimen length and the lattice parameter were observed along the c-axis direction, whereas a relatively small decrease was observed along the a-axis. In irradiation studies conducted at room temperature, it was found that the dimensional change saturates at high dose, at an elongation along the c-axis direction of about 300%. High resolution microscopy revealed that the microstructure had become nanocrystalline. Electron energy loss spectroscopy results showed that the volume change was recovered at this stage. These observations are discussed in terms of point defect evolution and its effects on the microstructure of irradiated graphite.

The mechanical properties of the interfaces in an Al2O3 fiber reinforced β-21S Ti alloy have been evaluated by using fiber pushout tests. The Al2O3 fibers were coated with a refractory metal and Y2O3 which served as a diffusion barrier during the HIPing used to produce the metal matrix composites. By doing fiber pushout tests, the interfacial fracture was found to occur at the interface between the refractory metal and the Y2O3. The interfacial shear strength and interfacial frictional stress were measured to be 323 and 312 ± 2 MPa, respectively. The interfacial frictional stress, which is due to asperity interlocking during the fiber sliding, was correlated to the surface roughness of the coated Al2O3 fiber obtained with the aid of an atomic force microscope. The measured surface roughness of 18.8 ± 2.2 nm was related to the frictional stress through Hutchinson's model.9 The frictional coefficient between the Al2O3 fiber and the Ti matrix is calculated to be 0.32 ± 0.02.

The structural and electrocatalytic properties of metastable Ni-Mo alloys have been investigated for the hydrogen evolution reaction in alkaline solutions. Amorphous and nanocrystalline phases have been prepared by mechanically alloying the elemental components under various milling conditions. Fcc nanocrystals are formed when the Mo concentration is smaller than 30 at. %. The nanocrystalline state becomes unstable with respect to the amorphous phase when the Mo content in the solid solution exceeds 30 at. %. The electroactive phase for the hydrogen evolution reaction in alkaline solutions is the nanocrystalline supersaturated solid solution. The presence of oxygen during the milling process improves the properties of the alloys.

A micro-wear testing technique has been developed by incorporating a piezoelectric pusher into an existing microindenter system. The pusher and its associated servo-control circuitry were designed to generate a precise reciprocating horizontal motion at the indenter tip for implementing a microscaled wear test. The information acquired from the test includes the wear loading curve, i.e., the normal applied load versus wear penetration depth, the friction force, and in turn, the apparent wear friction coefficient. Measuring the electrical resistance across the coating thickness is also possible if an electrical conducting indenter is utilized. Furthermore, in conjunction with the surface characterization tools, the wear morphology revealed useful information regarding the coating failure mechanism(s) and shed some light toward understanding coating tribology. The tester design concepts, operating procedure, data acquisition, and analysis will be examined. Experimental results on ultrathin carbon coatings with various thicknesses will be employed to illustrate the capabilities of the micro-wear tester.

“Morphology of TiSi2 and ZrSi2 on Si(100) and (111) surfaces” [J. Mater. Res. 9, 1214–1227 (1994)]

This paper investigates the criterion for a ductile-to-brittle transition in materials, due to nonlocal shielding effects at the crack tip when the dislocation free zone (DFZ) size is small. It is found that both cleavage and emission criteria are altered by nonlocal shielding, but that the emission shift is dominant, and is always in the direction to increase the local critical stress intensity for emission, kIIe. The nonlocal shift varies with the sum, Σ(γusdj)−3/2, over each dislocation (j), where γus is the unstable stacking fault energy, and dj is the distance from each dislocation to the crack tip. When there is a pileup of many shielding dislocations against a barrier near the crack tip, the total shift for the pileup varies as (γusd)−1. The most likely candidates for a brittle transition induced by the nonlocal shift are materials where barriers to dislocation motion exist within 10–100 nanometers of the crack tip, such as in thin films, multilayers, or ultrafine grain materials.

A unified kinetic and thermodynamic description of the glass transition in undercooled liquids at normal pressure is established. The following results are obtained for the first time: (1) The glass transition temperature Tg is determined to be in the range of Ts < Tg < Tn. Both Ts and Tn are material-dependent and each of them is characterized by a different Ω(T) = TΔslc(T)/Δslc(T) with Δhlc as the excess enthalpy and Δslc the excess entropy. (2) Being above Kauzmann's isentropic temperature, the lowest limit Ts is determined by Ω(Ts) = 1 −2/(3γ) with γ being the ratio between the total energy and the free energy of the liquid-crystal interface. (3) Although a glass preserves the entropy and enthalpy values of the liquid at Tg, the ratio Ω(Tg) is found to be bound by a Tg-independent material constant 1 −2/(3γ). (4) Tg increases linearly with the logarithm of the cooling rate and such a linear relationship is found to be not always valid. (5) The observed cooling-rate dependent glass transition at Tg is the kinetically modified reflection of an underlying cooling-rate independent transition at Ts, and the underlying transition at Ts is kinetically equivalent to the sudden and strong divergence of the structure relaxation time of the liquid. (6) It is shown that if the cooling rate exceeds a minimum value determined here as a function of temperature, the atoms of an undercooled liquid will not have sufficient time to rearrange themselves into the corresponding crystalline configuration; consequently, crystalline nucleation can be prevented. The results are supported by the available experimental evidence. A systematic test of the results on different systems is possible since the results are in terms of experimentally accessible quantities.

Effects of thermal cycling in a reducing (deuterium) atmosphere on the structure and chemistry of Cr/Cu/Cr/polyimide (PI)/Si and Au/Cu/Ti/polyimide (PI)/Si multilayer systems have been studied. In the Cr/PI system the interface between the Cr and PI was sharp and distinct in the as-deposited state, and after the anneal in the reducing atmosphere. Tensile cracks through the Cr/Cu/Cr layers were found after annealing and are the result of thermal stresses. No evidence for significant diffusion of Cr into the PI was found. In the Ti/PI system, the interface between the Ti and PI was sharp in the as-deposited state. After annealing in vacuum and in the reducing environment, regions of the interface between the Ti layer and the PI were converted to an oxide, Ti5O9. Annealing in the deuterium environment also caused delamination of the Ti film from the PI and blistering of the metal in the sample interior. No significant diffusion of the Ti into the PI was detected. In both systems, the metal in contact with the PI acted as a barrier to the diffusion of Cu into the PI.

The statistics of random cellular patterns are analyzed in cross sections of low-density microcellular materials. Agreement is found with a variety of topological relations previously found for other networks, namely Lewis's law and Aboav's law. To investigate three-dimensional packing effects, experiments are performed on compressed polystyrene shot material, the resulting networks of which are subsequently analyzed in cross section. Implications for material properties and stability are discussed.