In this study, Monte Carlo simulations have been combined with Embedded Atom Method (EAM) calculations to study hydrogen segregation at the atomic level in an ideal nickel lattice with a Σ9 tilt boundary. The calculations indicate that trap binding energies exceed 0.5 eV on the tilt boundary, but decrease rapidly with distance. Furthermore, the calculations show that trap site occupancy increases with trap site binding energy and hydrogen activity, and reach saturation at high hydrogen concentrations. Most importantly, significant rearrangements in tilt boundary structure are predicted to occur as hydrogen concentration increases. The results are consistent with observations that show significant hydrogen concentration enhancement at grain boundaries in nickel and palladium. They also parallel the effect of hydrogen concentration on crack growth susceptibility in nickel and iron-rich alloys. However, the change in boundary structure as hydrogen concentration increases challenges our understanding of hydrogen-induced fracture.

High-resolution electron-microscopy (HREM) and computer simulations of <110> tilt grain boundaries (GBs) in Au are used to investigate correlations between atomic-scale GB structure and energy. The energies calculated for a variety of symmetric and asymmetric GBs suggest that asymmetric GB-plane orientations are often preferred over symmetric ones. The experimentally observed faceting behavior agrees with the computed energies. Computer simulations indicate general interrelations between GB energy and (i) volume expansion and (ii) the number of broken bonds per unit area of GB. These atomic-scale microstructural GB parameters, as evaluated from HREM observations, are compared to simulation results.

We have simulated the atomic structures of the Σ 5 (210)/[001 ] symmetric tilt grain boundary using interatomic potentials for Nb developed employing the embedded atom method (EAM) and the model generalized pseudopotential theory (MGPT). These potentials do not predict the same lowest energy structure for the Σ 5 (210)/[001]. Using the ultra high vacuum diffusion bonding process, we have fabricated Σ 5 (210)/[001] bicrystals. The samples have been observed using high resolution electron microscopy and the observed images have been compared with those simulated based on the structures predicted theoretically. The experimental result for the Σ 5 (210)/[001] is in close agreement with the structure predicted using the EAM.

Nanocrystalline copper has been produced by two different techniques -- gas phase condensation and mechanical wear. Both nanocrystalline coppers have hardnesses and strain rate sensitivities that are greater than those of conventional, large-grained copper. These properties differences are tentatively explained on the basis of the two routes by which grain boundaries traditionally influence plastic deformation: by acting as barriers to dislocation glide, and by providing the vacancy sources and sinks necessary to facilitate diffusional flow. Additionally, there are notable differences in the mechanical properties of the two nanocrystalline coppers. Although the materials appear quite similar microstructurally, nanoindentation experiments show that the wear debris is 50% harder and 3 times more strain rate sensitive than the gas phase condensation-produced copper. These differences indicate that the microstructures are probably not as similar as first perceived, and that a previously overlooked microstructural feature may be influencing deformation in one or both of the materials.

Two types of special grain boundaries were examined in Ti-49Al-3Nb deformed at elevated temperatures. The first can be described as a Σ=2 order-changing boundary. This boundary is faceted and results from the nucleation and growth of variants of the γ phase from the parent α phase. The second type of grain boundary is a γ/α2 interphase boundary, and transmission of slip through this type of boundary was examined. Slip transmission from γ into α2 did not produce distinct slip bands in α2,. Slip transmission from γ through α2, and into the next γ lamella was possible even when the next γ lamella was in an orientation rotated relative to the first γ lamella.

Crack growth rates during intergranular stress corrosion, in materials such as Fe in Ca(NO3)2 at +750 mV (SCE), appear to exceed rates which can occur by dissolution and ion transport within the crack. Brittle crack jumps induced by a corrosion or stress corrosion process, and which magnify the dissolution supported crack velocity by a factor of up to 103, could explain this behavior. Hydrogen can induce these very fast crack growth rates, but these rates have been obtained at electrochemical potentials where H reduction is not thermodynamically possible. Other possible mechanisms for this accelerated crack growth include: 1) intergranularcorrosion altered tearing modulus, 2) grain boundary dealloying, or 3) adsorption induced brittle fracture. Examination of each of these mechanisms suggests that intergranular corrosion could reduce the tearing modulus through changes in the crack shape. However, crack opening analysis clearly shows that the crack tip is shielded from the applied stress intensity and that an embrittlement process which reduces the interface toughness is need for brittle intergranular crack jumps to occur. Further research is in progress to identify the source of this embrittlement.

A two dimensional phase transition from one adlayer structure to another is an inherent part of the thermal desorption of one monolayer of Hg on Cu(100). The energetics of this phase transition have been studied using thermal desorption spectroscopy (TDS). The TDS spectra reflect the coexistence of the two structural phases for a range of Hg exposures. The TDS spectra have been analyzed within a Polanyi-Wigner framework modified to account for the phase transition.

We present calculations of dislocation spacing for grain boundaries in zinc, and show that the spacings may be large enough to be resolved in conventional transmission electron microscopy even at extreme deviations from ideal coincidence misorientations. This effect is a result of the need for the dislocation arrays to accommodate slight differences between the ideal and actual axial ratios in addition to the difference between the ideal and actual misorientations. Recent observations of sliding behavior as a function of misorientation may also be rationalized within the framework of our structure calculations.

A preliminary investigation of the defect structure of the monoclinic zeta phase, within the interfacial region of Fe-Zn couples, has been performed using cross-section transmission electron microscopy (TEM). Twin boundaries and dislocations have been unambiguously identified, however, examples of defects which, as of yet are unknown, are also presented. The monoclinic zeta phase was found to twin by a rotation of 180° about the normal to the (110) plane.

This work reports on the microstructure of a particulate reinforced nickel aluminide composite synthesized by spray atomization and co-deposition. This synthesis methodology involved using high energy inert gas to disintegrate the nickel aluminide matrix (IC-396) into a Gaussian distribution of micrometer-sized, semi-solid droplets while simultaneously co-injecting SiC and TiB2 particulates, followed by deposition onto a water cooled substrate. The microstructure of the as-spray deposited composite material consisted of a mixture of the ordered γ (Ni3Al) and the disordered γ (Ni-rich) phase with a homogeneous distribution of SiC and TiB2 particulates. Scanning electron microscopy, combined with Energy Dispersive X-ray Spectroscopy were used to provide insight into the reactions taking place in the interfacial zone. The results of the present study show that a well defined reaction zone was formed at the Ni3Al/TiB2 interface; whereas extensive reaction occurred between the SiC particulates and matrix, resulting in complete transformation of almost all SiC particulates into other carbide phases.

The Hall effect on Ag/Cr superlattice system has been investigated. When the Cr layer thickness is changed from 1 monolayer (ML) to 6ML for the same Ag layer thickness of 10 ML, the Hall coefficient (RH) varies anomalously. For 1 ML Cr sample, we obtained RH value close to that of pure Ag as expected. The absolute value of RH is enhanced with increasing Cr layer thickness. After having a negative peak near 3 ML Cr, it decreases at larger Cr thicknesses. The field dependence of the Hall resistivity is linear for all the samples, and there is no sign of long range magnetic order down to 1.5K.

Rayleigh surface wave velocities in Ag/Cr and Au/Cr metallic superlattices have been measured by Brillouin scattering spectroscopy. These metallic superlattices were prepared by the metal-MBE method. X-ray diffraction measurements indicated that the structures of Ag/Cr and Au/Cr superlattices were almost the same. In the Au/Cr superlattices a maximum in a plot of surface wave velocities as a function of the bilayer thickness was observed. On the contrary, a distinct dip was shown in Ag/Cr systems. This anomalous behavior of the sound velocity could not be attributed to the expansion and the contraction of the lattices. Differences in atomic scale structure in the interface seem to play a major role in these systems.

Internal friction measurements have been performed on Ni-20 wt% Cr alloys containing trace additions of Ce ranging from 0 to 180 at ppm, in order to determine grain boundary sliding kinetics and associated stress relaxation phenomena. Two anelastic relaxation peaks have been observed corresponding to intrinsic grain boundary sliding between carbide precipitates and to macroscopic sliding with elastic accommodation at triple points. The effects of Ce on these grain boundary phenomena and resulting alloy ductilities are also presented.

Changes in electrical resistance measured by the four probe method was adopted as a structure sensitive physical property used to trace changes in the atomic microstructure below and above the δ'-solvus line in the Al-Li binary alloys. The kinetics of the growth and dissolution of δ' phase (Al3Li) and the formation of δ phase (AlLi), below and above δ'-solvus line, during the coarsening process were studied by isochronal and isothermal investigations. The diffusional growth of δ'-phase, below the δ'-solvus line, was found to be activated by 0.24 eV. Interpretation of the results showed that this energy involved two events during the coarsening process. The first event seemed to be a breakdown of the bonding between the vacancy-Lithium pair and the second event involved the transport of Li atoms to Al-particles to coarsen δ'; (Al3Li) particles. Furthermore, the study of the temperature dependence of electrical resistance of the stable phases δ' and δ of Al-Li alloys was found to be accompanied by difference in the temperature coefficient of the electrical resistance and was attributed to the change in structure of the two phases considered.

Alloy carbide precipitation associated with the transformation interface between austenite and ferrite, and austenite and pearlite in steels can result in a fine particle dispersion that can contribute significantly to strength in a wide range of commercial carbon steels. Studies of this precipitation reaction are hindered by the fact that the transformation interface is lost upon cooling to room temperature, either by further transformation or by decomposition of the residual austenite phase to martensite. In the present work this has been avoided by developing an alloy in which the precipitation reaction occurs, but in which the austenite is stabilised to room temperature, thus allowing a detailed examination by TEM and APFIM techniques of the interfacial region.

The factors affecting the second layer grain structure of a two-aluminum-layer sputtered film were investigated with the use of a computer-aided experimental design and analysis program. A cold aluminum layer followed by a hotter aluminum layer has been described in the literature as a means of improving metal step coverage. Several factors were screened to determine the most important parameters by which the first layer influences the second. The two-layer interactions were determined by using scanning electron microscope (SEM) analysis to measure final grain structures. A model was generated from the experimental results that shows the influence of the first layer grain size on the second layer if the second sputter deposition is made above ∼300°C. The influence increases at higher temperatures. The results will be compared to grain growth models found in the literature.

The stability of semicoherent interfaces between two solids with small lattice mismatch is examined. I consider the cases in which the rotation θ between the two solids is both zero and small. The interface orientation, characterized by a single angle φ, is arbitrary. The interfaces are composed of regularly spaced misfit dislocations, steps, and lattice dislocations. For a wide range of φ, many flat interfaces are unstable with respect to breaking up into two interfaces of distinct orientations, one of which contains only one of the two types of dislocations possible.

The morphology of directionally solidified CuAl-Pb monotectic alloys has been studied. The structure consisted of a hexagonal array of Pb rods in a Cu-based matrix. In addition, elongated grain boundaries in the Cu-based matrix with lens-shaped Pb fibers on the boundary and a “denuded zone”, depleted of Pb rods, were observed. Existence of these boundaries is shown to reduce the overall surface energy of the system leading to the formation of the elongated grain boundaries.

Al with additions of Cu is commonly used as the conductor metallizations for integrated circuits (ICs). As the packing density of ICs increases, interconnect lines are required to carry ever higher current densities. Consequently, reliability due to electromigration failure becomes an increasing concern. Cu has been found to increase the lifetimes of these conductors, but the mechanism by which electromigration is improved is not yet fully understood. In order to evaluate certain theories of electromigration it is necessary to have a detailed description of the Cu distribution in the Al microstructure, with emphasis on the distribution of Cu at the grain boundaries. In this study analytical electron microscopy (AEM) has been used to characterize grain boundary regions in an Al-2 wt.% Cu thin film metallization on Si after a variety of thermal treatments. The results of this study indicate that the Cu distribution is dependent on the thermal annealing conditions. At temperatures near the θ phase (CuAl2) solvus, the Cu distribution may be modelled by the collector plate mechanism, in which the grain boundary is depleted in Cu relative to the matrix. At lower temperatures, Cu enrichment of the boundaries occurs, perhaps as a precursor to second phase formation. Natural cooling from the single phase field produces only grain boundary depletion of Cu consistent with the collector-plate mechanism. The kinetic details of the elemental segregation behavior derived from this study can be used to describe microstructural evolution in actual interconnect alloys.

The effect of deposition temperature and the addition of Si to sputter deposited Al-Cu thin-film microstructure was studied with transmission electron microscopy. Films were studied in the as-deposited and annealed condition. The effects of thermal treatment were studied with in-situ hot stage microscopy. Al2Cu (θ) precipitated at the grain boundaries and the sublayer interface. At higher deposition temperatures, with alloy composition in single phase region (Al-1.5 wt.%Cu), Al2Cu precipitated during cooldown. At lower temperatures, in the two phase Al-0 region, Al2Cu precipitated during deposition. The addition of Si caused formation of Si precipitates and retarded Al2Cu precipitation during deposition or cooldown.