A review of the relationship between elastic moduli and interatomic force constants precedes the description of a method for determining the composition dependence of single crystal elastic moduli of disordered alloys having the face-centered cubic structure. The method treats the alloy as a virtual crystal, characterized by an effective pair potential between atoms. Results of calculations are presented using experimental data on Cu-rich Cu-Al alloys.

The results of a systematic investigation by transmission electron microscopy (TEM), cathodoluminescence (CL), Rutherford backscattering (RBS), X-ray diffraction and topography and scanning force microscopy (SFM) techniques on several InGaAs/InP compressive and tensile strained layers covering the misfit range from −2.3 to 1.5×10−2 and grown by the metal organic vapor phase epitaxy (MOVPE) technique are reported. In compressively strained films the same dependence for the residual strain vs the film thickness as for the InGaAs/GaAs is found whereas a different strain release rate and different extended defects are found in tensile stressed InGaAs alloy. In particular in tensile stressed samples, grooves, planar defects and cracks are present in addition to the interfacial network of misfit dislocations. The correlation between the observed planar defects and the mechanisms of strain relaxation in the case of tensile strained layers is discussed.

Cyclic loading of a plane strain mode I crack under small scale yielding is analyzed using discrete dislocation dynamics. The dislocations are all of edge character, and are modeled as line singularities in an elastic solid. At each stage of loading, superposition is used to represent the solution in terms of solutions for edge dislocations in a half-space and a nonsingular complementary solution that enforces the boundary conditions, which is obtained from a linear elastic, finite element solution. The lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation are incorporated into the formulation through a set of constitutive rules. An elastic relation between the opening traction and the displacement jump across a cohesive surface ahead of the initial crack tip is also specified, which permits crack initiation and crack growth to emerge naturally. It is found that crack growth can occur under cyclic loading conditions even when the peak stress intensity factor is smaller than the stress intensity required for crack growth under monotonic loading conditions.

Polychromatic synchrotron x-ray microbeam diffraction allows one to record Laue patterns simultaneously from all of the grains within the column of material irradiated. This paper describes 3-D texture mapping in polycrystalline Al and Cu samples. Transmission methods for determining the depths of diffracting volumes are outlined. The focus is on how the texture scale between micro- (i.e., individual grain orientations) and macro-texture defines fatigue crack paths in Al-Li 2090 T8E41: very large volumes of near single crystal material lead to large asperity formation, and at least forty volume percent of the material consists of these volumes. Results on grain subdivision in Cu after large deformations are also briefly reviewed.

A single slip plane dislocation dynamics based model for the brittle to ductile transition has been extended to have multiple slip planes around the crack-tip. The crack-tip plastic behaviour is studied for a variety of dislocation source configurations. The results are presented for the case of iron. The effect of modelling the plastic-zone as a single slip plane and as an array of parallel slip planes are compared.

A quantitative analysis of the morphology of damage cavities in metallic materials is performed. At larger length scales, the self-affine correlation length of fracture surfaces is shown to be correlated to the grain size. These observations suggest a new scenario for the origin of scaling laws observed on fracture surfaces. It is argued that they reflect strong correlations in damage created prior to fracture during crack propagation.

Crack propagation in columnar saline and freshwater ice has been investigated with high-speed photography, acoustic emission detection and the resistance method. High-speed photography was found to be a single reliable technique. The resistance method proved effective for freshwater ice samples, but not for saline ice samples due to the presence of conductive fluid inclusions. Acoustic emissions pinpointed the moment of crack initiation, but did not correspond to the crack propagation time. Crack velocity has been characterized over a temperature range of -5°C to -30 °C for freshwater and saline ice. Freshwater ice exhibited an overall average velocity of 198 m/s, and did not vary with temperature. Crack velocity in saline ice demonstrated temperature dependence, increasing from an average of 86 m/s in the -5°C to -20°C range, to 131 m/s at -30°C. The crack velocity was also shown to have a general dependence on fracture toughness K' of the material, however, the microstructural variation between samples is also shown to influence significantly the crack behavior in both saline and freshwater ice. Nonuniform crack tip advance and crack reorientation were observed as crack slowing mechanisms in freshwater ice, while in saline ice fracture crack tip blunting on voids greatly reduced average crack velocities.

An extended Gurson model incorporating the effects of the void shape and distribution on the growth and coalescence is proposed. The emphasis is placed on void coalescence, which is modeled as a transition from diffuse plasticity around the void to transverse localized plastic yielding in the intervoid ligament. Selected results showing the importance of correctly accounting for the void coalescence stage, as well as for the void shape and distribution effects, are presented and discussed.

Finite strain rate atomistic simulations were conducted on Ni and Ni+H lattices containing voids to better understand the dislocation-scale mechanisms of void growth and coalescence and how hydrogen affects these damage processes. Void growth is governed by dislocations that nucleate at the void surface to transport mass away from the void as well as dislocations arriving at the void from the lattice exterior to deposit vacancies and accommodate void-surface expansion. Hydrogen can retard void growth when large local hydrogen concentrations impede dislocation nucleation and propagation at the void surface. The formation of hydrogen gas molecules in the void interior does not necessarily aid void growth. Pressure in small voids may be mitigated by the mutual interaction of hydrogen molecules and the interaction of molecules with the void surface.

Dislocation-density based multiple-slip constitutive formulations and specialized computational schemes are introduced to account for large-strain ductile deformation modes in polycrystalline aggregates. Furthermore, new kinematically based interfacial grain-boundary regions and formulations are introduced to account for dislocation-density transmission, absorption, and pile-ups that may occur due to grain-boundary misorientations and properties.

The indentation response of a mica-containing glass-ceramic that exhibits yield in an indentation test is interpreted in terms of events occurring on the microstructural scale. This work is unique in that it links the macroscopic and microstructural properties of the specimen material in the indentation process.

The long distance roughness of the fracture surface of a dendritic aluminum alloy is studied over a wide range of length scales. Self-affinity analysis was performed over samples broken in Charpy impact tests. Simultaneous use of Atomic Force Microscopy, SEM and stylus profilometry allowed us to cover a wide spectrum of length scales, spanning over seven decades, from a few nanometers up to one centimeter. The roughness exponent and correlation length were obtained using the variable bandwidth method. For the roughness exponent, a value of 0.8 was obtained, corresponding to the reported universal exponent. Correlation length was found to correspond well to the characteristic length of the largest heterogeneities in the complex microstructure. Our results provide information that can help to improve our understanding of the role of microstructural parameters on crack propagation mechanisms.

The self-aff'me behavior of fracture surfaces of polymeric materials was qualitatively and quantitatively studied. SEM images of fracture surfaces of both polypropylene and polystyrene show Chevron marks at several magnifications. In addition, for polystyrene the mirror and Hackle zones were also observed. For quantitative analysis, the average roughness exponent, ζ, of height profiles generated by AFM images, was estimated by applying the variable bandwidth method. Values of ζ=0.788 and ζ=0.810 were obtained for polypropylene and polystyrene, respectively. These results are in very good agreement with the claimed universal exponent of 0.8 reported in the literature for other non-polymeric materials. By choosing the AFM appropriate operating conditions, measurements of roughness on plastic material surfaces could be performed.

A trinitrotoluene (TNT) fracture surface image is characterized in terms of a self-affine fractal structure. The fracture surface was produced by high acceleration in an ultracentrifuge when the TNT strength was exceeded. An atomic force microscope (AFM) captured the topography of a 4 micron square region on the fracture surface. The present analysis supports a self-affine description of the TNT fracture surface (wavelengths of 0.016 micron to 4.0 micron) and provides a new prespective on fracture processes in TNT. An essential step in self-affine fractal characterization of surfaces is the determination of reference surfaces. A self-affine fracture surface can be described in terms of a single-valued height function. In the TNT fracture surface, single-valued height functions, which describe surface texture can only be defined with respect to curved reference surfaces. By employing curved reference surfaces, we have demonstrated that self-affine fractal scaling can be used to characterize the TNT fracture surface. This provides important information that is not evident in the analysis of individual surface scans.

Atomic-scale computer simulation has been used to investigate the interaction of crystal dislocations with two interfaces in hexagonal-close-packed (HCP) metals, namely the {1012} twin boundary and a <1210>/90° tilt boundary that is incommensurate in the direction perpendicular to the tilt axis. Crystal dislocations are absorbed in the tilt boundary with concomitant reconstruction of their cores. In the twin boundary, a broader range of interactions is observed, including defect transmission from matrix to twin and decomposition in the interface into discrete defects. The role of crystallographic features and interfacial structure is elucidated by comparing interaction processes in the two interfaces. The core structure of interfacial defects can be complex and contributes significantly to total defect energy.

The interactions of dislocations asscociated with deformation modes active in hard orientation compressed polysynthetically-twinned (PST) TiAl with lamellar boundaries have been studied by transmission electron microscopy (TEM). The deformation modes observed in the γ-phase involved soft superdislocation and hard ordinary dislocation slip, and hard twin systems. The transfer of these latter hard twin shears occurred across all types of lamellar (γ/γ)- and frequently across (γ/α2)-boundaries. Slip of superdislocations in the minority U2-phase lamellae has been shown to be associated with both direct transfer of piled-up twinning dislocation shears across the lamellar boundaries and the activation of interfacial sources due to pile-up stresses. The active deformation modes were consistent with macroscopic shape changes of the PST-TiAI when the various defect interactions with the lamellar boundaries and the proposed shear trasnfer processes were considered.

Multilayered Cu-Ni has a peak yield strength four orders of magnitude higher than either Cu or Ni because the multitude of interfaces obstruct glissile dislocations. The barrier strengths of the interfaces may be traced to four mismatches across an interface: modulus, lattice parameter, chemical and slip geometry. This paper describes sample embedded atom method (EAM) simulations of dislocations crossing interfaces, designed to separate the effects of the four mismatches. The results confirm some classical calculations and emphasize the importance of three new effects (i) an interface-chemical effect in which dislocations are trapped by core spreading in the interface, (ii) a coherency-chemical effect caused by coherency strains changing effective stacking fault energies and (iii) a coherency-modulus effect in which coherency strains change elastic moduli (and hence the Koehler stress) significantly.

The Three Dimensional Atom Probe produces a real space map of the elemental identities and positions of atoms field-evaporated from a sharply pointed specimen. The analyzed volume is on the order of 20 nm × 20 nm × 100 nm. This is large enough to enclose microstructural features such as grain- or heterophase boundaries. Correlation of the measured atomic positions with such features results in an atom-by-atom description of the chemical environment of these crystallographic defects. We describe here a method for identifying these interfaces and profiling the composition in the vicinity of the interfaces without any assumptions about the interface geometry. This approach is applied to quantitative determination of interfacial segregation of Ag at a MgO/Cu(Ag) heterophase interface. We discuss the implications of our technique with respect to classical treatments of segregation at interfaces.

Atomistic simulation employing many-body central-force potentials was performed to elucidate the diffusion mechanisms in the bulk and at lamellar interfaces assuming a vacancy mechanism. First the self- diffusion of Ti and Al in stoichiometric structures was studied. It was found that the diffusion was faster along lamellar interfaces than in the bulk; the effective activation energy for the diffusion coefficient is about ∼15% lower. The simulations were then extended to investigate diffusion along lamellar boundaries with segregated Ti which is likely in Ti rich alloys. The surprising result is that diffusion remains practically unchanged when compared with the stoichiometric case. The reason is that while the path controlling the diffusion is different, the corresponding effective formation and migration energies are practically the same as in the stoichiometric case.

The segregation of interstitial impurities to symmetrical tilt grain boundaries (STGB) in bodycentered cubic transition metals is studied by means of ab-initio electronic-structure calculations based on the local density functional theory (LDFT). Segregation energies as well as changes in atomic and electronic structures at the ΣE5 (310) [001] STGB in Mo caused by segregated interstitial C atoms are investigated. The results are compared to LDFT data obtained previously for the pure Σ5 (310) [001] STGB in Mo. Energetic stabilities and structural parameters calculated ab initio for several crystalline Molybdenum Carbide phases with cubic, tetragonal or hexagonal symmetries and different compositions, MoCx, are reported and compared to recent high-resolution transmission electron microscopy (HRTEM) observations of MoCx, intergranular films and precipitates formed by C segregation to a Σ5 (310) [001] STGB in a Mo bicrystal.