The various types of configurations formed in face-centered cubic (fcc) and body-centered cubic (bcc) structures by two interacting, non-coplanar, dislocation segments of various orientations are examined and discussed. The focus is on junction formation and on a particular interaction, the collinear interaction, which deserves much more attention than paid up to now.

The deformation microstructure under spherical microindents in single crystal Cu has been investigated with submicron spatial resolution using x-ray structural microscopy. A polychromatic, submicron diameter (∼ 0.5 μm) microbeam was used in combination with micronresolution depth profiling to make direct, nondestructive measurements of plastic deformation induced lattice rotations under an indent made with a 69 μm radius spherical indenter and 200 mN maximum load. Lattice orientations relative to the undeformed crystal were determined as a function of position under the indent using differential-aperture x-ray structural microscopy (DAXM). Rotation-axes and misorientation-angles were determined for micron steps along selected microbeam penetration directions.

PTFE-based coatings have been widely used, for example, on aluminum molds for molding of polyolefin packaging materials for its non-stick property. However, these coatings do not meet the users' scratch resistance and durability requirements. This paper describes a preliminary study on the synthesis and characterization of a PTFE/sol-gel composite coating material which combines the required non-stick and low friction properties of PTFE filler with the high scratch resistance and durability of a silica-based sol-gel matrix. The non-stick and low friction properties were achieved by using both the PTFE filler and the lubricious compound resulting from the reaction of a solvent with siloxane. The high scratch resistance was attributed to the enhanced adhesion to the electro-chemically pre-treated surface and the optimized silica and alumina filler contents in the sol-gel material. FE-SEM/EDX, FTIR, contact angle goniometry, scratch testing and a pin-on-disc tribometry were used to evaluate the coating properties.

The extremely high melting point and excellent resistance to oxidation and corrosion offered by iridium suggest numerous applications of this transition metal in static components at high temperatures and in aggressive environments. However, the mechanical and physical properties of f.c.c. Ir exhibit numerous anomalies when compared to other metals that crystallize in the f.c.c. structure. Notable examples include a negative Cauchy pressure, 1/2 (C12 – C44), brittle transgranular cleavage after a period of plastic flow even in pure single crystals and anomalous [ΆΆ0] branches in the phonon spectra. Atomistic studies of extended defects are needed to elucidate the origin of anomalous mechanical properties, such as brittleness. For this purpose we developed a Bond-Order Potential (BOP), an O(N) tight-binding formalism, employing physically transparent parameterizations that use experimental and ab initio data, generated in this study using the Full Potential Augmented Plane Wave plus Local Orbitals (APW+lo) method. The constructed BOP reproduces then both equilibrium as well as a variety of nonequilibrium properties of Ir and represents an excellent description of cohesion in f.c.c. Ir. This description of interatomic interactions is imminently suitable for studies of defects, such as dislocations and grain boundaries, that control plastic deformation and fracture.

The interaction between solute atoms and dislocations is known to lead to negative strain rate sensitivity and poor formability. The negative rate sensitivity causes inhomogeneous flow and the Portevin-LeChatelier (PLC) effect. These observations motivate the present study of the solute-dislocation interaction in binary Al-Mg alloys. We report here on three issues. First, we determine the size and shape of stable Mg clusters at stationary dislocations of edge, 60° and pure screw type. We then evaluate the accuracy with which clustering is predicted by the classical pressure field-solute interaction formula, i.e. within the assumption that solute do not interact and the pressure field of the dislocation is unperturbed by the solute. Second, we investigate to what extent the presence of the cluster perturbs the far field of the dislocation. Finally, the effect of the solute on the stacking fault energy is investigated.

The paper focuses on the formation and on the role of prismatic loops during deformation. The analysis is restricted to non-diffusive processes in fcc-related crystals. Double cross-slip and cross-slip dipolar annihilation yield strings of loops such that one loop extremity is aligned in the screw direction with the extremity of its nearest neighbour. Reactions between prismatic loops and mobile dislocations are at the origin of a number of microstructural reactions including recovery, dislocation entanglements and patterning in single slip, dislocation multiplication and there is indication that they may nucleate twins.

The role of grain boundary constraint in strain localization, slip system activation, slip transmission, and the concomitant constitutive response was examined performing a series of uniaxial compression tests on tantalum bicrystals. Tantalum single crystals were diffusion bonded to form a (011) twist boundary and compressed along the [011] direction. The resulting threedimensional deformation was analyzed via volume reconstruction. With this technique, both the effective states of stress and strain over the cross-sectional area could be measured as a function of distance from the twist boundary, revealing a highly constrained grain boundary region. Post-test metallurgical characterization was performed using Electron Back-Scattered-Diffraction (EBSD) maps. The results, a spatial distribution of slip patterning and mapping of crystal rotation around the twist-boundary, were analyzed and compared to the known behavior of the individual single crystals. A rather large area near the grain boundary revealed no crystal rotation. Instead, patterns of alternating crystal rotation similar to single crystal experiments were found to be some distance away (~1mm) from the immediate grain boundary region, indicating the large length scale of the rotation free region.

A constitutive model was developed to bridge the cyclic plasticity behavior of single crystals and the corresponding characteristic dislocation structures. Yield and flow were built on the individual slip systems. The Armstrong-Frederick kinematic hardening rule was invoked to capture the Bauschinger effect. A material memory parameter was introduced to consider the amplitude dependence of cyclic hardening. Latent hardening considering the interactions among the slip systems was used to describe the anisotropic cyclic behavior. The experimental results of copper single crystals were used to validate the model developed. It was found that the model was able to adequately describe the well-known three distinctive regions in the cyclic stress-strain curve of the FCC single crystal oriented for single slip and the associated dislocation substructures. The model was capable of capturing the enhanced hardening observed in copper single crystals in multi-slip orientations. For a given loading history, the model can predict not only the saturated stress-strain response but also the detailed evolution of the transient cyclic behavior. The characteristic dislocation structures can be featured with the slip evolution.

The intensity distribution of Laue diffraction is analyzed as a function of local misorientation. We show how unpaired dislocations alter the white beam Laue patterns for isolated dislocations, for dislocation walls, and for a combination of both. We consider the effect of different statistically and geometrically necessary dislocation densities on the intensity distribution along and perpendicular to the Laue streak. A 3D x-ray crystal microscope is used to analyze the complicated plastic-elastic field in a grain of a Ni polycrystalline sample during in-situ uniaxial pulling. A change of dislocation activity with depth is demonstrated. The dislocation slip systems and their densities are determined at various depths. The model parameters are used to simulate the whole Laue pattern including details about the contours for specific Laue spots; good agreement is found between simulated and experimental contours.

Tension compression fatigue tests and subsequent TEM observations were conducted on single crystalline silicon in a temperature and strain rate domain where lattice friction is still effective: 800-900°C and 1.5 to 6x10-4s-1. Samples oriented for single slip conditions were cyclically loaded under plastic strain amplitude control. For amplitudes ranging from 6x10-4 to 10-2, cyclic stress-strain curves exhibit two different stages of hardening and pass through a maximum before saturation is reached. TEM observations suggest that strain localization takes place near the maximum cyclic stress and beyond. Before mechanical saturation, edge dislocation dipoles sit mainly in thick rectilinear walls. Once the maximum stress is reached, these thick walls “condense” in much thinner walls that seem to carry out the imposed deformation while other regions become inactive. In this case, the dislocation structure anneals out and a loop structure is created from the dipolar walls.

An analytical model for strain relaxation by misfit dislocation arrays in thin films is presented that takes into account all components of the strain tensor, including shear strains. The model is developed for (001) films and applied to strain relaxation in (011) oriented FCC metal films. Our results show that shear strains strongly influence the total strain energy of the film. Since both the critical strain for dislocation formation, and the equilibrium spacing of dislocations in arrays depend on the minimum energy values, these quantities are found to be different from those predicted by previous models. This model is useful for understanding both critical strain data and strain relaxation in films.

In situ straining in the transmission electron microscope has been combined with molecular dynamics computer simulations to investigate the nature of the interaction of glissile dislocations with radiation-produced defects (loops, stacking-fault tetrahedra, and He bubbles), and to determine the mechanisms by which the dislocation loops and stacking-fault tetrahedra are annihilated and defect-free channels are created. The defect pinning strength depends on the defect and on the interaction geometry. The experiments and simulations show that a single interaction is not always sufficient to annihilate a dislocation loop or a stacking-fault tetrahedra and that the nature of the defect may be changed because of the interaction. The edge/screw character of the dislocation is also important as they have different efficiencies for annihilating a defect. The dislocations responsible for creating the defect-free channels are not the preexisting dislocations but originate from grain boundaries and other stress concentrators. Cross-slip of dislocations within the channels is important for clearing and widening the channel and can create new channels. Based on these observations a dispersed-barrier hardening model in which the influence of the radiation defects and dislocation density are combined. The resulting model predicts the observed behavior, including the apparent yield drop at high defect densities.

This paper presents a study of compressively loaded single crystals of tantalum. Strain gauges, interferometry, optical microscopy, and micrometers were employed to measure the deformation caused by loading. All of the measures indicated non-uniform deformation at plastic strains of less than 0.01. By using these measurements we were able to show that surface creation and shape changes of the sample introduce additional components which affected the measurement recorded by the strain gauge.

A modification of the jogged-screw model has been adopted recently by the authors to explain observations of 1/2[110]-type jogged-screw dislocations in equiaxed Ti-48Al under creep conditions. The aim of this study has been to verify and validate the parameters and functional dependencies that have been assumed in this previous work. The original solution has been reformulated to take into account the finite length of the moving jog. This is a better approximation of the tall jog. The substructural model parameters have been further investigated in light of the Finite Length Moving Line (FLML) source approximation. The original model assumes that the critical jog height (beyond which the jog is not dragged) is inversely proportional to the applied stress. By accounting for the fact that there are three competing mechanisms (jog dragging, dipole dragging, dipole bypass) possible, we can arrive at a modified critical jog height. The critical jog height was found to be more strongly stress dependent than assumed previously. The original model assumes the jog spacing to be invariant over the stress range. However, dynamic simulation using a line tension model has shown that the jog spacing is inversely proportional to the applied stress. This has also been confirmed by TEM measurements of jog spacings over a range of stresses. Taylor's expression assumed previously to provide the dependence of dislocation density on the applied stress, has now been confirmed by actual dislocation density measurements. Combining all of these parameters and dependencies, derived both from experiment and theory, leads to an excellent prediction of creep rates and stress exponents. The further application of this model to other materials, and the important role of atomistic and dislocation dynamics simulations in its continued development is also discussed.

We report on tensile tests performed on thin sheet samples of tantalum that have a nearcolumnar structure. By annealing sheet of approximately 1 mm thickness at high temperatures, a columnar structure is generated with grain boundaries nearly perpendicular to the flat surfaces. The purpose of the experiments is to investigate the mechanical behavior in terms of crystal plasticity in a geometry that is readily amenable to both experimental characterization and computer simulation. Automated electron backscatter diffraction (EBSD) has been used to obtain orientation maps both before and after deformation. For the small strains used in this study (<10%), the grains deform rather uniformly with little evidence of cell formation. Analysis of the lattice rotations shows considerable scatter in the rotation axis and demonstrates significant disagreement with polycrystal plasticity calculations.

The plastic deformation of polycrystalline fcc metal thin .lms with thicknesses of 1 μm and less is investigated by simulating the dynamics of discrete dislocations in a representative columnar grain. The simulations are based on the assumption that dislocation sources or multiplication sites are rare and that sources have to operate several times to generate appreciable plastic deformation. This model is thoroughly tested by calculating the response of randomly distributed dislocation sources to an applied stress and comparing the results with experimental data. Stress–strain curves, the influence of boundary conditions, dislocation densities, work hardening rates and their dependence on the film thickness as well as the dependence on grain orientation are studied. The agreement between simulation and experiment is good and many aspects of thin film plasticity can be understood with the assumption that small-scale plastic deformation is source controlled rather than mobility controlled.

The atomic scale structures of the partial dislocation cores in GaAs are explored using ab initio electronic structure total energy techniques. The structure of the 30° partial dislocations are expected to be period doubled along the core. The structure of the 90° partial dislocations remains more uncertain, and here, an effort is made to predict which of two proposed reconstruction, double period or single period, is more stable. The relative energies of the two core structures are found to be equal, within the accuracy of the present calculations. It is suggested that at temperature, both core reconstructions will be present.

When confronted by severe geometric constraints, dislocations may respond in unforeseen ways. One example of such unexpected behavior is parallel glide in unpassivated, ultrathin (200 nm and thinner) metal films. This involves the glide of dislocations parallel to and very near the film/substrate interface, following their emission from grain boundaries. In situ transmission electron microscopy reveals that this mechanism dominates the thermomechanical behavior of ultrathin, unpassivated copper films. However, according to Schmid's law, the biaxial film stress that evolves during thermal cycling does not generate a resolved shear stress parallel to the film/substrate interface and therefore should not drive such motion. Instead, it is proposed that the observed dislocations are generated as a result of atomic diffusion into the grain boundaries. This provides experimental support for the constrained diffusional creep model of Gao et al.[1], in which they described the diffusional exchange of atoms between the unpassivated film surface and grain boundaries at high temperatures, a process that can locally relax the film stress near those boundaries. In the grains where it is observed, parallel glide can account for the plastic strain generated within a film during thermal cycling. One feature of this mechanism at the nanoscale is that, as grain size decreases, eventually a single dislocation suffices to mediate plasticity in an entire grain during thermal cycling. Parallel glide is a new example of the interactions between dislocations and the surface/interface, which are likely to increase in importance during the persistent miniaturization of thin film geometries.

A formalism of the quasicontinuum method suitable for atomistic-continuum modeling of oxide crystals is presented. Multiple interacting quasicontinua, one per sublattice, which overlap in the physical crystal space, are used to model the oxide crystals. The method is implemented with the shell model for atomic interactions in ionic crystals, along with the Wolf's method for treating the long-range forces. Results are presented for the structural relaxation of strained and unstrained Fe2O3 crystal under periodic boundary conditions.

An X-ray method was developed to determine the dislocation density in metal materials as a distribution depending on the orientation of Burgers vector. The method includes registration of X-ray line profiles by each successive position of the sample in the course of diffractometric texture measurement using reflections of two orders, the following determination of coherent domain size and lattice distortion by means of the Warren-Averbach method for each orientation of reflecting planes, separate calculation of the density of c- and a-dislocations with all possible orientations of Burgers vector and presentation of results in the generalized pole figures. The method was used to determine the dislocation density in tubes of Zr-based alloys for nuclear industry. Obtained data show, that the dislocation density varies within very wide interval of several orders of magnitude depending on the grain orientation both in as-rolled and annealed tubes. Features of the dislocation distribution in tubes are closely related to their crystallographic texture.