Mechanically driven grain boundary relaxation: a mechanism for cyclic hardening in nanocrystalline Ni

Molecular dynamics simulations are used to show that cyclic mechanical loading can relax the non-equilibrium grain boundary (GB) structures of nanocrystalline metals by dissipating energy and reducing the average atomic energy of the system, leading to higher strengths. The GB processes that dominate deformation in these materials allow low-energy boundary configurations to be found through kinematically irreversible structural changes during cycling, which increases the subsequent resistance to plastic deformation.     

Positron lifetime study of an Al-1.7at.% Mg-1.1at.% Cu alloy

The behaviour of vacancies during isothermal ageing following quenching of an Al-1.7at.% Mg-1.1at.% Cu alloy has been studied by positron annihilation lifetime spectroscopy. The positron lifetime parameters vary in parallel with the Vickers hardness of the alloy, suggesting simultaneous migration of vacancies and changes in the size and concentration of vacancies containing clusters and dislocation loops. The results also explain the long hardness plateau observed previously in ageing experiments carried out between 100 and 240°C in terms of a continuous growth of Mg-Cu-vacancy clusters during ageing. The vacancy concentration of the clusters increases gradually until the setting in of the cluster Guinier-Preston-Bagaryatsky zone transformation.     

Grain boundary phase observed in Al–5 at.% Zn alloy by using HREM

The nature and behaviour of grain boundary (GB) phases is very important since they can control strength, plasticity, resistivity, grain growth, corrosion resistance, etc, especially in nanocrystalline materials. For nanocrystalline Al-based light alloys, extremely high plasticity has been observed in restricted temperature and concentration intervals close to the solidus line. This phenomenon is not fully understood. It can be explained by formation of GB phases not included in the bulk phase diagram. Therefore, the structure of GB phases, as well as thermodynamic conditions for their existence, has to be carefully studied. In this work the structure and composition of GBs and GB triple junctions in Al–5 at.% Zn polycrystals annealed in the temperature region above and below the bulk solidus line were studied by high-resolution electron microscopy and analytical transmission electron microscopy. Evidence has been obtained that a thin layer of a liquid-like phase exists in GBs and GB triple junctions slightly below the bulk solidus line.     

Accelerating heterogeneous nucleation to increase hardness and electrical conductivity by deformation prior to ageing for Cu-4 at.% Ti alloy

ABSTRACT

Deformation bands, especially shear bands were intentionally formed in supersaturated Cu-4 at.% Ti alloy by groove-rolling with 75% area reduction. Cold deformation prior to the ageing process changed the precipitation behaviour of the alloy. The deformed structure with shear bands was maintained, without showing recovery and recrystallisation, even after prolonged ageing at 450°C for more than one day. The thermally stable shear bands act as nucleation initiation sites and prevent the expansion of discontinuous cellular precipitates. The hardness reached a value of 305?Hv and an electrical conductivity of 15% IACS (IACS?=?International Annealed Copper Standard. 100% IACS is defined as the conductivity corresponding to a volume resistivity at 20°C of 17.241?nΩ), compared to 283?Hv and 13% IACS for conventional solid-solutioned and peak-hardened alloy. In addition, the ageing time to reach the highest hardness was shortened from 720 to 180?min at 450°C.     

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.     

Characterization of grain-boundary dislocation networks by a combination of large-angle convergent-beam electron diffraction and weak-beam techniques

The Burgers vector of very close intrinsic dislocations in a near-Σ = 11,{311} grain-boundary in nickel is identified using a geometrical method based on local and accurate measurements of the angular deviation from perfect coincidence Σ = 11 by large-angle convergent-beam electron diffraction and of the dislocation spacing from weak-beam images of the grain boundary.     

Fragmentation testing for ductile thin films on polymer substrates

Fragmentation testing of thin films on substrates has in the past been mostly performed in situ using optical and scanning electron microscopy. While these techniques work well for brittle materials, they cannot always discern the difference between through-thickness cracks (TTCs) and localized plastic deformation of ductile films. Here, we describe fragmentation testing with atomic force microscopy and present criterion to distinguish TTCs and localized deformation for ductile films.     

A derived transfer of simple discrimination and self-reported arousal functions in spider fearful and non-spider-fearful participants

Two experiments investigated the derived transfer of functions through equivalence relations established using a stimulus pairing observation procedure. In Experiment 1, participants were trained on a simple discrimination (A1+/A2-) and then a stimulus pairing observation procedure was used to establish 4 stimulus pairings (A1-B1, A2-B2, B1-C1, B2-C2). Subsequently, a transfer of the simple discrimination functions through equivalence relations was observed (e.g., C1+/C2-). These procedures were modified in Experiment 2, which demonstrated that spider-fearful and non-spider-fearful participants show differing levels of a transfer of self-reported arousal functions for stimuli used in equivalence relations with video-based material depicting scenes with spiders. The results demonstrate that the stimulus pairing observation procedure provides a viable alternative to matching-to-sample, and also offer tentative support for a derived-relations model of the acquisition of anxiety responses in at least one sub-clinical population.     

Quartic rotation criteria and algorithms

Most of the currently used analytic rotation criteria for simple structure in factor analysis are summarized and identified as members of a general symmetric family of quartic criteria. A unified development of algorithms for orthogonal and direct oblique rotation using arbitrary criteria from this family is given. These algorithms represent fairly straightforward extensions of present methodology, and appear to be the best methods currently available.The research done by R. I. Jennrich was supported by NSF Grant MCS-8301587.