Materials research does not necessarily need to eliminate defects, but rather to characterize them, and to understand and control their effects. In most cases chacterization of defects means making structural or dynamic measurements of their properties. To understand these measurements in order to predict material and defect properties outside the range of the measurements is a much harder problem. Ideally a theory is required. However in the materials examples considered in this review, point defects in uranium oxide, copper clusters in steel, grain boundary aggregations, and stress concentrations, a true analytic theory is beyond our capabilities. Here computer modelling is often able to make the progress needed. This review considers the complementary nature of experimental characterization and computer simulation in our understanding of defects in materials.

A significant fraction of the atoms in nanophase materials are associated with grain boundaries. Hence, the nature of the grain boundary structures in these materials is interesting in its own right as well as ultimately important to the properties of nanophase materials. Considerable uncertainty has surrounded the atomic structure of nanophase boundaries, but recent experiments have shed some light on these interesting structures. These investigations have included direct HREM observations combined with image simulations, as well as Raman spectroscopy and SANS. The emerging picture is one of nanophase boundaries that are not dissimilar in structure to those normally encountered in conventional polycrystals. The connections between this picture, of nanophase materials as ultrafine-grained polycrystals, and experimental observations on these materials are discussed.

It is well established from studies of bicrystals that the properties of a grain boundary depend on the atomic structure of the boundary. However, constitutive relations for the properties of polycrystalline materials do not currently take into account this boundary-toboundary variability. Instead, such relations depend on a single length scale, typically the average grain diameter. We extend the traditional viewpoint by proposing that boundaries may be divided into two distinct categories, depending on their misorientation angle. The relevant length scale in constitutive relations for polycrystals is then the average cluster size, where clusters consist of grains connected by boundaries in the same misorientation category. A brief discussion of this additional length scale and how it may be reflected in various constitutive relations for physical and mechanical properties of polycrystals is given.

Grain boundary chemical diffusivities for a series of symmetric [001] tilt boundaries in the Au/Ag system were measured by the surface accumulation method using newly developed thin-film multi-crystal specimens, in which the grain boundaries feeding the accumulation surface were all of the same type. Possible effects due to segregation at the grain boundaries and surfaces were avoided. CSL boundaries of low-Σ ( i.e., 5, 13, 17, 25) and also more general boundaries with tilt angles between the low-Σ orientations were selected. The diffusivities were found to vary monotonically with tilt angle ( i.e., no cusps at low-Σ's were found) in a manner consistent with the Structural Unit model.

Recent atomistic simulations using interatomic potentials for Nb developed employing the embedded atom method (EAM) and the model generalized pseudopotential theory (MGPT) have indicated a possible cusp at the Σ5(310) orientation in the energy vs tilt angle curves for<001> symmetric tilt grain boundaries. In addition, the most stable structure predicted using EAM exhibits shifts of one crystal relative to the other along the tilt axis and along the direction perpendicular to the tilt axis lying in the boundary plane. The structure predicted using the MGPT was mirror symmetric across the plane of the grain boundary. This boundary has been prepared for experimental study using the ultra high vacuum diffusion bonding method. A segment of this boundary has been studied using high resolution electron microscopy.

The structural characteristics of [111] twist boundaries in gold are investigated using a combination of x-ray diffraction and computer modeling techniques. Comparison of the measured scattering effects with those generated from EAM computer models reveals that the [111] boundary displacement field is weak, rotational in form and centered on ‘O’ lattice sites. Furthermore, the measured intensities of the strong ‘O’ lattice reflections decrease smoothly with increasing boundary angle up to 30°, as calculated from the model. The effect of double positioning on the diffraction pattern and the structural analysis is discussed.

LMTO-ASA calculations were performed on a 26-atom supercell model of a Σ3(111) grain boundary (GB) in bcc Fe. The supercell emulated two GB's with 11 (111)planes of Fe atoms between the GB planes. One of the GB's was clean, with a structural vacancy at the GB core in the center of a trigonal prism of Fe atoms, while on the other GB this site was occupied by a H atom. The interplanar spacings of the supercell were relaxed using a modified embedded atom method. As in the case of P and S in a similar GB environment in Fe there is only a weak interaction between H and nearest Fe atoms. Almost all the Fe d-states are nonbonding. A very weak covalent bond exists between H and Fe due to s-d hybridization, the hybrid bonding part located far below the Fermi energy. This bond is mostly of σ-type, connecting H with the Fe atoms in the GB plane; the δ-component of this bond across the GB is weaker. A weak electrostatic interaction attracts Fe-atoms across the clean GB, but results in repulsion if a H atom is present. The magnetic contribution to intergranular cohesion is decreased when H is present due the suppression of the magnetic moments of the nearest Fe atoms both in the GB plane and directly across the GB.

Using the embedded atom method we atomistically model the accommodation of the lattice nismatch and study the propertiesof the misfit dislocations in parallel oriented bicrystals. We calculate and analyze in detail the excess interfacial energies on the (100) and (111) boundary planes. The Ag/Ni system is chosen as a model system for metal/metal interfaces with a large lattice mismatch.

Among the possible boundary planes in parallel oriented fcc bicrystals, the ones with terminating {111} planes arc energetically most favourable and are most often observed in experiments. This can be explained by a detailed analysis of the elastic strain fields in the interfaces, which correspond to networks of misfit dislocations. While the misfit dislocations on the (100) and (011) planes have a ½[01ī] Burgers vector, those on (111) can dissociate into misfit partials. The elastic strains connected with the misfit partials are, of course, much smaller than those for other types of misfit dislocations. The misfit partials form a triangular network within the boundary plane.

Two classes of interatomic interactions, the embedded atom method and the model generalized pseudopotential theory are used to calculate the structure of tilt boundaries in bcc metals.These interactions differ in the inclusion of explicitlyangular dependent interactions. The results show that these different models of the interactions can lead to qualitatively different predictions for the atomic structure of the boundary. The applicability of the embedded atom method to bcc transition metals is also discussed.

A model based on the embedded atom method [1] has been used to calculate antiphase boundary (APB) energies of three low-index planes for alloys having the L12 structure. The lattice constant, cohesive energy, unrelaxed vacancy formation energy, bulk modulus, and average shear modulus for each element are used as inputs into the model. Effects of the APB orientation and of the range of interaction in the model are examined. Both unrelaxed and relaxed APB energies are compared with available experimental values and earlier theoretical results. A strong anisotropy was found in six of the seven alloys studied. The {111} APB energy was consistently smaller than that for the {110} APB, while the {100} APB energy was found to be very close to zero with very little difference between the unrelaxed and relaxed values. For both energy and relaxation amounts, the results did not vary much with the range of interaction, so that 3rd nearest-neighbor calculations were found to be satisfactory approximations.

Calculations by Krivogtaz [1] dealing with quasiline formation in highly distorted lattices undergoing phase separation have been extended for randomly arranged particles. Qualitative experimental evidence from powder patterns, already in the literature for Cu-Be,Ni-Be,Cu-Ti and Nimonic alloys[2-5], have demonstrated the existence of quasilines. This extended calculation deals with ellipsoids of revolution and allows one to examine different shapes and transformation strains in an anisotropic medium. it is shown that the precipitate transformation strains play a very important role in shaping the Bragg-like profiles. This is most obvious in the intermediate stage which includes Bragg scattering from the lattice, regular static diffuse scattering and the quasiline. For precipitate sizes associated with maximum age hardening, all three normally become scrambled into a broad assymetrically shaped Bragg-like peak. However, a comparison of the theoretical calculations with experimental data from an age hardened Cu-Be alloy shows qualitative agreement, which we believe is due to the non-random nature of precipitation in this system.

The techniques employed for X-ray diffraction analysis of dislocation substructures in hexagonal close-packed metals are descibed and assessed by comparison with direct observations using transmission electron microscopy.

The phases and defects produced in a binary alloy, with nominal composition Nb-18at.% Al, have been studied using transmission electron microscopy (TEM). In addition to the equilibrium A15 phase (Nb3Al) a previously unreported Nb-rich B2 phase is present. A significant number of grow-in dislocations were observed in the A15 phase, these have b= a<100> and are predominantly screw on character. They are dissociated into two partial dislocations which bound a stacking fault according to the reaction;

<100> → > 1/2 <100> + SF + 1/2 <100>

The habit plane of the fault is shown to be {012}. These observations are compared with previous studies performed on the isostructual compound V3Si. The consequences of the observed dissociation for the plastic deformation of Nb3Al are discussed.

35 CrMoV steel is widely used in industries. To select the best technology for improving the mechanical properties, a series of heat treatments were tested. Metallographic examination indicates that the microstructure is a kind of non-typical bainite and, to a certain extent, similar to tempered troostites. TEM observation shows that rod-like alloying cementite distributed in ferrite grains laid parallel to each other and ultra-fine alloying carbides also dispersed in the lath-like ferrite grains. Moreover, there exists high density of dislocation tangles and microtwins. Detailed discussions are made on the solid solution strengthening, the work hardening process and the crack propagation by view of microstructure, defects and stress concentration.

In this paper we will present several new theoretical results on the properties of oxygen atoms in bulk crystalline silicon. Specifically, these properties will include (1) oxygen migration - where we will suggest that the conventional adiabatic-barrier model for oxygen migration may not be valid for this system; (2) oxygen catalysis - where we will demonstrate that certain oxygen configurations can act as “catalysts” to reactions that form silicon broken bond defects; and (3) oxygen aggregation - where we will introduce a new mechanism for the initial stages of aggregation and oxidation within the bulk of crystalline silicon.

The total energies and structures of a number of Be-induced defects in Si are investigated using ab-initio local density calculations. Our primary results are: 1) The geometry of the isoelectronic center is found to correspond to a [111] substitutionalinterstitial pair (SIP); 2) The low energy defect spectrum includes large Be complexes containing at least one substitutional atom; and 3) Simple bonding rules exist for the stability of the different types of bonds in the material. Thus the Si-Be bond is found to be stable for all defect configurations while the Be-Be bond is metastable.

The efficacy of three modern empirical potentials in the molecular dynamic simulation of the configurations, energies and mobility of dislocation cores and their excitations is assessed in the light of recent literature. The results are found to be inconsistent both between different potentials and with experimental evidence. It is argued that the discrepancies are rooted in the limited databases which have been used in the construction of empirical potentials. The reason for the discrepancies is demonstrated by comparing empirical and density functional calculations of the generalized stacking fault energy.

We present first-principles calculations of the electronic structure of Nb grain boundaries. These are the first such calculations for a bcc metal using the real-space multiple-scattering theory (RSMST). Local densities of states near a Σ5 twist grain boundary are compared to those for bulk Nb.

The growth of semiconductor crystals by molecular beam epitaxy often involves the motion of distinct steps, which are the boundaries of incomplete atomic layers. We review some of the crystal growth mechanisms based on step generation and motion. Ising models have been widely used to study equilibrium faceting and crystal growth. We discuss more general models of steps which are based on molecular dynamics calculations of atomic motion and empirical interatomic potentials. These models include the possibility of surface and step reconstructions, and here we discuss their influence on the step energy and motion. We find that certain types of steps have a structure with drastically reduced energy compared to unreconstructed steps. We have also examined the effect of stress resulting from misfit in epitaxial systems. We find that 1% misfit can completely change the nature of a step, since its excess energy may change sign from negative to positive, or vice versa. Simulations of molecular beam epitaxy give direct information on the conditions under which step growth mechanisms play a role.

Defects in polymers were observed by high resolution electron microscopy and inferred from their mechanical and dielectrical behavior. The details of their generation was not known, however, in the past. During the last few years we have been able to extend the molecular dynamics simulation of polyethylene to crystals containing up to 6100 atoms and to times as long as 100 ps. The major observation was that single bond rotations of more than 90° become possible already more than 100 K below the melting temperature. These defects have lifetimes of only a few ps. By coupling to kinks (2g1) they can extend their lifetime considerably. Addition of a thermal, mechanical or dielectric free energy gradient to the thermally created defects seems to be able to account for the microscopic motion needed to explain the macroscopically observed annealing, deformations and relaxation effects. Key to the mechanical and dielectric properties is thus the existence of conformational disorder (condis crystal).