Nearby Dislocation Research Articles

Cascade Diffusion Theory of Displacement-Induced Point Defect Concentration and Flux Fluctuations

A theoretical approach, which has been developed to assess the fluctuations in point defect concentrations and fluxes to sinks that are induced by the production of point defects in spatially and temporally discrete cascades, is summarized. Solutions for cascade dissipation are outlined for the case where cascades occur in a homogeneous lossy medium as well as for the more involved geometries where cascades occur in the presence of a nearby dislocation or cavity. By superposition of solutions representing discrete cascades under conditions of interest, time profiles of point defect concentrations and fluxes are generated. The profiles exhibit extreme fluctuations. Continuum rate theory results arise by applying limits and approximations to cascade diffusion theory. Application of the theory to microstructural processes shows that property changes, especially creep are affected by the fluctuations.

The Climbing Motion of Faulted Dislocation Loops Induced by Elastic Interaction

The movement of faulted dislocation loops during annealing has been studied with transmission electron microscopy using the usual stereotechnique. It is revealed that the distance between two nearby faulted dislocation loops in a thin foil varies during annealing at 95°C for 2 hours without their sizes changing. This variation in distance is considered to occur by pipe diffusion due to the elastic interaction between the loops. The vacancy concentration and the vacancy flow along the perimeter of the loops are calculated by considering the interaction force between their segments. It is shown that the vacancy flow that moves loops exists when the distance between loops is less than 200 Å. The activation energy for pipe diffusion is estimated to be about 0.90 eV.

The initial development of a yield point due to carbon in nickel 200 during static strain aging

A model is proposed for the development of the first stage of the yield point return in Nickel 200. It is assumed that immediately after straining and unloading, vacancies cre-ated during deformation become trapped by interstitial carbon atoms. The carbon-va-cancy pairs thus formed then reorient quickly in the stress fields of nearby dislocations thus reducing the dislocation line energy so that dislocation pinning occurs. This model is based on a hypothesis proposed by Rose and Glover in 1966 for austenitic stainless steel.

Studies on the Yield Phenomena : Preparatory Slip and Propagation of Slip at Lower Yield Stress

In the present paper, slip behavior in α-brass before and after yielding is investigated by means of etch-pits technique which can reveal the faint slip lines as dislocation rows. By this method, it has been found that the stress concentrations at the inclusions and the edge corner of the test piece activate nearby dislocation sources to cause the preparatory slips, and that the degree of the constraint for slip deformations imposed by the neighbouring crystals is one of the important factors determining whether the preparatory slip can propagate into undeformed regions or not. The slip deformations at the lower yield stress propagate over the test piece under these conditions.

Tinted films for sound positives

More than 200 samples of diamonds from the Snap Lake dyke system in Canada were studied in details using optical and EPR spectroscopy. About 30% of them were colorless or slightly greenish tinted and were related to IaB type with a high aggregation level of defects: N3 (N3V), B nitrogen centers (N4V0) and clusters of interstitials (B′ centers). Traces of heavy plastic deformation in the crystals were found by X-ray techniques. Visual examination of photoluminescence (PL) of another batch of samples shows one type emission: a blue core surrounded by a green shell. As a rule these diamonds were colored in brownish to greenish tints and belong to the IaA type containing predominantly N–N nitrogen aggregates.Specific features of type IaB diamonds are a dominant broad line in the P2 EPR spectrum and an unusually high concentration of nitrogen in a single substitutional form, NS0. We suggest that this uncommon combination was produced naturally by the destruction of B-centers (N4V0) during plastic deformation. When IaA and IaB diamonds are heated, movement of dislocations throughout the crystal causes further decomposition of the complicated nitrogen defects (A, B), producing simpler nitrogen forms: P1, P2 and H3 centers. Analysis of light illumination effects on the IaB type diamonds revealed that some of these defects are in different charge states and the color of plastically deformed crystals is believed to be due to the charge transfer between the nitrogen defects and the dislocation core. Results of the current study suggest that a line broadening in P2/N3 and H3 systems and thus unusual ratio between strengths of the zero phonon lines and vibronic bands in PL spectra are related to the lattice distortion because of nearby dislocations. The sharp 1332 cm− 1 IR peak in these samples is due to NS+ rather than to B-centers.The PL 490.7 nm center, most manifesting itself in plastically deformed IaA diamonds, is first shown to be always accompanied by PL blue systems at 406 and 423 nm and to correlate with EPR signal from dangling bonds in the dislocation cores.