On the structural unit model of grain boundary structure

Abstract

A critique of the structural unit model is presented. It is shown that the predictive capacity of the model is severely limited when it is applied to boundaries with relatively-high-index rotation axes. Recent computer simulations suggest that ‘high index’ means 〈221〉 and above. A new approach to grain boundary structure is needed for mixed tilt and twist boundaries.     

Dynamic balance between grain refinement and grain growth during high-pressure torsion of Cu powders

High-pressure torsion (HPT) was applied to unmilled coarse-grained (CG) Cu powders with low initial dislocation density and cryomilled nanocrystalline (nc) Cu powders with high initial dislocation density, with identical processing parameters. HPT of unmilled CG Cu powders resulted in exceptional grain refinement and increase in dislocation density, whereas significant grain growth and decrease in dislocation density occurred during HPT of cryomilled nc Cu powders. Equilibrium structures were achieved under both conditions, with very similar stable grain sizes and dislocation densities, suggesting dynamic balances between deformation-induced grain refinement and grain growth, and between deformation-induced dislocation accumulation and dislocation annihilation. The equilibrium structures are governed by these two dynamic balances.     

Fracture mechanisms of symmetrical tilt grain boundaries

The mechanisms of failure of symmetrical tilt boundaries have been investigated using an atomistic technique. The fracture process advances in steps that are related to the structural unit of the symmetrical tilt grain boundary. These boundaries are shown to fail through a combined mechanism involving dislocation emission in certain regions of the structural unit, followed by microcleavage, and again dislocation emission. The overall toughness of the boundaries is given by the combination of the toughness of these two processes.     

Strategic regulation of grain size in memory reporting

To increase their report accuracy, rememberers may either withhold information that they feel unsure about or provide relatively coarse information that is unlikely to be wrong. In previous work (A. Koriat & M. Goldsmith, 1996c), the authors delineated the metacognitive monitoring and control processes underlying the decision to volunteer or withhold particular items of information (report option) and examined how these processes are used in the strategic regulation of memory accuracy. This article adapts that framework to address control over the grain size (precision-coarseness) of the information that people report. Results show that rememberers strategically regulate the grain of their answers to accommodate the competing goals of accuracy and informativeness. The metacognitive processes underlying this regulation are elucidated.     

Nanocracks at grain boundaries in nanocrystalline materials

A theoretical model is proposed to describe the generation and evolution of nanoscale cracks (nanocracks) in strained nanocrystalline materials. In the framework of the model, the nanocracks are nucleated and grow along grain boundaries, changing the direction of their growth at triple junctions.     

Anisotropic grain growth kinetics in nanocrystalline nickel

ABSTRACT

Intriguing properties exhibited by nanocrystalline metals, including a high level of mechanical strength, arise from their nanometer-scale grain sizes. It is critical to determine the evolution of grain size of nanocrystalline materials at elevated temperature, as this process can drastically change the mechanical properties. In this work, a nanocrystalline Ni foil with grain size ～ 25?nm was annealed in situ in an X-ray diffractometer. X-ray diffraction peaks were analysed to determine the grain growth kinetics. The grain growth exponents obtained were ～ 2–4 depending upon the crystallographic direction, indicating the anisotropic nature of the grain growth kinetics.     

Pinning of grain boundary migration by a coherent particle

We studied single-particle pinning of grain boundary (GB) migration during grain growth. A phase-field model was formulated to simulate the pinning by a coherent particle and validated quantitatively by comparison with analytical prediction. A study of GB migration velocity using this model revealed that second-phase coherent particles have a previously unknown restraining effect over the whole of the GB-particle interaction range, which is qualitatively different from the interaction between GB and incoherent particles.     

Nanoindentation creep behavior of grain boundary in pure magnesium

Nanoindentation creep tests were performed at the grain boundary and grain interior in pure magnesium. The grain boundary showed a high strain rate sensitivity exponent and was dominated by grain boundary sliding due to the high diffusion rate at the grain boundary. The grain boundary affected the deformation behavior of the area at a distance of 2 µm into the grain interior. On the other hand, the grain interior had a low strain rate sensitivity exponent, because its matrix was too large to be influenced by the grain boundary. The deformation mechanism in the grain interior was determined to be dislocation slip.     

Effect of varying the duration of grain presentation on automaintenance

In a series of three experiments the effects of variation in grain duration on automaintenance were evaluated. In the first experiment, key illumination was followed by grain only when pigeons did not peck the key. Each subject was exposed to 2-, 4-, and 8-second feeder durations in blocks of 10 sessions. Subjects pecked on a high percentage of trials at all feeder durations. The mean peck latency was shorter in the 8-second condition than in the two other conditions in five of six subjects. The conditional probability of pecking given successive keylight-grain pairings did not increase as the number of pairings increased. The second experiment was identical to the first, except that key pecking had no scheduled consequence. Under these conditions, all three subjects showed substantial responding. The recorded measures showed no systematic relationship to feeder duration in this study. In the third experiment, two different stimuli were followed by feeder presentations of either identical (2- or 8-second) or different (2- and 8-second) durations within each session. Subjects tended to respond sooner and with a higher overall rate in the presence of the stimulus associated with the longer feeder duration only when different feeder durations were presented within the same session. This result was confirmed by direct observation of the pigeons. The results of these experiments suggest that the effects of varying grain duration may be small, compared to the effects of varying other variables. The results also suggest that the location as well as the frequency of pecking may be an important measure in the analysis of factors controlling the pigeon's key peck.     

Odor magnitude: Coarse vs. fine grain

Despite a dismal reputation, olfaction’s resolving power actually appears to be keen, especially when the contaminating influence of noise in the stimulus is discounted from psychophysical performance. In a previous investigation, subjects were able to resolve a change of 4% in the concentration of butyl alcohol. In the present experiment, these same subjects resolved a change of 9% in the concentration of amyl butyrate. The data imply that resolving power varies among odorants. The growth of odor intensity assessed by magnitude estimations of easily discriminable (i.e., grossly different) concentrations also varies among odorants. These observations prompt the question: Does an odorant for which resolution is relatively difficult lead to slower growth of perceived intensity with concentration than an odorant for which resolution is easy? In a scaling experiment, amyl butyrate indeed led to slower growth of intensity than butyl alcohol. Nevertheless, amyl alcohol, which permitted poorer resolution than butyl alcohol, led to approximately the same rate of growth as butyl alcohol. Accordingly, growth of odor intensity with concentration may correlate only weakly with resolving power.     

Kinetics of interstitial segregation in Cottrell atmospheres and grain boundaries

Trapping of interstitial (e.g. carbon) atoms is driven by the reduction in energy in the system. Diffusion of interstitials, together with their trapping in dislocation cores and/or grain boundaries, is studied by the thermodynamic extremal principle. In addition to the total Gibbs energy, a well-established formulation of the total dissipation is applied. Dimension-free evolution equations are derived, whose solution is well approximated by an easy to handle kinetic equation. Cottrell’s power law can be verified in the initial stage.     

Micro-mechanical measurements of fracture toughness of bismuth embrittled copper grain boundaries

Measuring the fracture properties of single grain boundaries has until now required macroscopic bi-crystals which are expensive and not always available. We describe a method for fracture testing using micro-cantilevers, manufactured using focussed ion beam machining and tested using a nanoindenter. We have used the method to measure the fracture toughness of selected grain boundaries in bismuth-embrittled copper. This technique is applicable to grain boundaries in other brittle polycrystalline samples for which large bi-crystals cannot be produced for conventional testing.     

Stimulus correlates of visual pattern discrimination and the problem of grain

Attention, Perception, & Psychophysics - A recent paper by V. J. Polidora (1966) is criticized on the basis that the experimental design did not provide an adequate test of the usefulness of 15...     

Experimental and simulated grain boundary groove profiles in tungsten

Grain-boundary grooving has been studied on polished surfaces of polycrystalline tungsten annealed at 1350°C. Atomic force microscopy images were taken in the same area for each groove after different annealing times. Secondary oscillations next to the main groove maxima (predicted for grooving by surface diffusion) were observed, to our knowledge for the first time. The agreement between experimental and calculated groove profiles (using the surface diffusion model of Mullins (1957, J. appl. Phys., 28, 333)) improved when grain-boundary fluxes were introduced.     

Accumulation of geometrically necessary dislocations near grain boundaries in deformed copper

Cross-correlation-based analysis of electron backscatter diffraction patterns has been used to map the distribution of geometrically necessary dislocation (GND) density in deformed polycrystalline copper. Patterning of the dislocations into high-density cell walls and low-density cell interiors was readily observed at the micron scale. Patterning at the longer length scale of the grain size was also evident with high-density regions (GND hot spots) tending to be in clusters, often found close to some but not all grain boundaries and triple junctions.