Boundary Spacing Research Articles

A Continuous Model of Cortical Connectivity.

We present a continuous model for structural brain connectivity based on the Poisson point process. The model treats each stream-line curve in a tractography as an observed event in connectome space, here a product space of cortical white matter boundaries. We approximate the model parameter via kernel density estimation. To deal with the heavy computational burden, we develop a fast parameter estimation method by pre-computing associated Legendre products of the data, leveraging properties of the spherical heat kernel. We show how our approach can be used to assess the quality of cortical parcellations with respect to connectivty. We further present empirical results that suggest the "discrete" connectomes derived from our model have substantially higher test-retest reliability compared to standard methods.

From boundaries to boundary work: middle managers creating inter-organizational change.

Purpose In healthcare, organizational boundaries are often viewed as barriers to change. The purpose of this paper is to show how middle managers create inter-organizational change by doing boundary work: the dual act of redrawing boundaries and coordinating work in new ways. Design/methodology/approach Theoretically, the paper draws on the concept of boundary work from Science and Technology Studies. Empirically, the paper is based on an ethnographic investigation of middle managers that participate in a Dutch reform program across health, social care, and housing. Findings The findings show how middle managers create a sense of urgency for inter-organizational change by emphasizing "fragmented" service provision due to professional, sectoral, financial, and geographical boundaries. Rather than eradicating these boundaries, middle managers change the status quo gradually by redrawing composite boundaries. They use boundary objects and a boundary-transcending vocabulary emphasizing the need for societal gains that go beyond production targets of individual organizations. As a result, work is coordinated in new ways in neighborhood teams and professional expertise is being reconfigured. Research limitations/implications Since boundary workers create incremental change, it is necessary to follow their work for a longer period to assess whether boundary work contributes to paradigm change. Practical implications Organizations should pay attention to conditions for boundary work, such as legitimacy of boundary workers and the availability of boundary spaces that function as communities of practice. Originality/value By shifting the focus from boundaries to boundary work, this paper gives valuable insights into "how" boundaries are redrawn and embodied in objects and language.

Characterization of Defect Structure in Electrodeposited Nanocrystalline Ni Films

The microstructure of electrodeposited Ni films produced without and with organic additives (formic acid and saccharin) was investigated by X-ray diffraction (XRD) line profile analysis and cross-sectional transmission electron microscopy (TEM). Whereas the general effect of these additives on the microstructure (elimination of columnar growth as well as grain refinement) was reproduced, the pronounced intention of this study was to compare the results of various seldom-used high-performance structural characterization methods on identical electrodeposited specimens in order to reveal fine details of structural changes qualitatively not very common in this field. In the film deposited without additives, a columnar structure was observed showing similarities to the T-zone of structure zone models. Both formic acid and saccharin additives resulted in equiaxed grains with reduced size, as well as increased dislocation and twin fault densities in the nanocrystalline films. Moreover, the structure became homogeneous and free of texture within the total film thickness due to the additives. Saccharin yielded smaller grain size and larger defect density than formic acid. A detailed analysis of the grain size and twin boundary spacing distributions was carried out with the complementary application of TEM and XRD, by carefully distinguishing between the TEM and XRD grain sizes.

Formation of a dominant Dillamore orientation in a multilayered aluminum by accumulative roll bonding

In the present work, we report for the first time the formation of a dominant Dillamore orientation in a multilayered high-purity aluminum (99.99 %) subjected to an ultra-high von Mises strain of 11.2 by accumulative roll bonding (ARB). This multilayered aluminum with an average layer thickness of 180 nm was produced from an original aluminum sheet thickness of 100 μm. Microstructure and texture evolutions during ARB processes were investigated in detail and the formation of a dominant orientation is explained. The grain boundary spacing was observed to reduce with increasing number of ARB cycles. However, after 8 ARB cycles, the average grain boundary spacing was measured to be larger than the theoretical layer thickness. Also, a single and strong Cu orientation formed after an intermediate number (8–12) of ARB cycles, which rotated to a dominant Dillamore orientation with further ARB cycles. This is rationalized on the basis of three mechanisms: First, a homogeneous deformation causing lamella structure formation; second, the enhanced dynamic recovery of this pure aluminum through the movement of Y-type triple junction boundaries, resulting in grain coarsening; and third, the coarsening process eliminating the original few and small Brass and S lamellae and simultaneously enhancing the Cu and Dillamore orientations.

Minimum thermal conductivity in periodically twinned SrTiO3

In this work we present non-equilibrium molecular dynamics simulation results of the thermal conductivity of periodically twinned SrTiO3 as a function of twin boundary spacing and specimen size. We show that the thermal conductivity of SrTiO3 can be significantly reduced through incorporating twin boundaries. Simulation results demonstrate that the periodically twinned SrTiO3 exhibits a minimum in thermal conductivity at a critical twin spacing, and the critical twin spacing is found to increase with the size of the specimen. The mechanism underlying the minimum thermal conductivity is identified. In addition, the results of the effect of simulation cell size reveal an appreciable contribution from long wavelength or long mean free path phonons to the thermal conductivity of twinned SrTiO3.

Structural/textural changes and strengthening of an advanced high-Mn steel subjected to cold rolling

The microstructure and texture evolution during cold rolling with total thickness reductions of 20–80% (nominal strains of 0.22–1.61) and their effect on the tensile behavior of an Fe–17Mn–1.5Al–0.3C steel were studied. The cold rolling to total strains of 0.5 was accompanied by multiple deformation twinning, resulting in the formation of twin/matrix lamellae with the boundary spacing of 35nm. Correspondingly, the textural changes were characterized by the rapid evolution of rather strong Brass and S texture components. An increase in the rolling strain led to further reduction in the boundary spacing down to about 20nm and the development of shear banding, which promoted the formation of γ-fiber (〈111〉||ND) at large strains. Before cold rolling, the investigated steel exhibited a large total elongation above 90% because of the TWIP (twinning-induced plasticity) effect and a relatively low yield strength of 245MPa. The cold rolling resulted in significant increase in the yield strength up to 1440MPa after rolling to a total strain of 1.61, whereas the total elongation decreased to 5%. The strengthening of the present steel during cold rolling was attributed to increased dislocation density and reduced twin/grain boundary spacing.

Grain Boundary Plane Orientation Fundamental Zones and Structure-Property Relationships.

Grain boundary plane orientation is a profoundly important determinant of character in polycrystalline materials that is not well understood. This work demonstrates how boundary plane orientation fundamental zones, which capture the natural crystallographic symmetries of a grain boundary, can be used to establish structure-property relationships. Using the fundamental zone representation, trends in computed energy, excess volume at the grain boundary, and temperature-dependent mobility naturally emerge and show a strong dependence on the boundary plane orientation. Analysis of common misorientation axes even suggests broader trends of grain boundary energy as a function of misorientation angle and plane orientation. Due to the strong structure-property relationships that naturally emerge from this work, boundary plane fundamental zones are expected to simplify analysis of both computational and experimental data. This standardized representation has the potential to significantly accelerate research in the topologically complex and vast five-dimensional phase space of grain boundaries.

Plasticity in nanotwinned polycrystalline Ni nanowires under uniaxial compression

Molecular dynamic simulation (MD) was used to study the compression behavior of nanotwinned polycrystalline Ni nanowires (NWs). An inverse Hall–Petch (HP) relation as a function of twin boundary spacing (TBS) is observed, which can be mainly understood in terms of a transition of the dominant intra-granular deformation mechanism. Through the simulations, we provide an atomic scale view of the entire process of compression deformation.

Atomistic study of crack growth behavior in crystalline Mg/amorphous Mg–Al nanocomposites

The effects of crystalline boundary (CB) spacing and crystal orientation on the crack propagation behavior in crystalline/amorphous (C/A) Mg/Mg–Al nanocomposites under tensile loading are investigated using molecular dynamics simulation method. Three samples with different crystal orientations of initial crack are considered. The results show that although both samples with cracks exhibit plasticity, the deformation mechanisms are drastically different. For samples A and C with larger CB spacing, the deformation twinning and the nucleation and growth of new grain dominate the crack plastic deformation. For samples A and B with smaller CB spacing, the dislocation and interfacial slip are important plastic deformation mode at crack top. However, the crack plastic deformation of sample C with small CB spacing is mainly induced by the nucleation and growth of new grain in neighbor crystalline phase sections. The above mentioned crack deformation behaviors of C/A Mg/Mg–Al nanocomposites are also disclosed and analyzed in present work. The results here provide a strategy for the design of high-performance hexagonal-close-packed metal and alloy materials.

Molecular dynamics study of creep mechanisms in nanotwinned metals

Nanotwinned structures have shown great promise as optimal motifs for evading the strength–ductility trade-off. In this paper, we present a study of high temperature creep in polycrystalline nanotwinned face-centered cubic metals using molecular dynamics. The simulations reveal that the nanotwinned metals exhibit greater creep resistance with decreasing twin boundary spacing over a large range of applied stresses. The findings also indicate that the presence of twin boundaries entails higher stress for the onset of power-law creep compared to the nanocrystalline counterparts. Nanotwinned metals with very high density of twin boundaries exhibit a new creep deformation mechanism at high stresses governed by twin boundary migration. This is in contrast to nanocrystalline and nanotwinned metals with larger twin spacing, which exhibit a more conventional transition from grain boundary diffusion and sliding to dislocation nucleation.

Father involvement in young children’s care and education: exploring boundaries and starting conversations

This paper analyses the impact of a UK local authority initiative – Fathers’ Friday – aiming to involve fathers in their children’s education, which took place in 20 early years’ and primary school settings. Whilst the study involved a range of methods, in this paper interview data associated with practitioners’ perspectives of the initiative are used. Theoretically, Bernstein’s concept of boundary and related notions of classification and framing provide a framework for exploring fathers’ positioning, and Bernstein’s concept of ‘voice’ helps to explain how relations are repositioned, as fathers engage with settings through their participation in the initiative. The authors’ contention is that whilst an existing structure of social relations positions how settings and individuals can relate to one another, experiencing the Fathers’ Friday initiative under changed conditions produces a weakened boundary space for fathers, from which ‘conversations can start’. Children themselves form part of this process of redrawing weakened boundaries.

Dislocation-Governed Plastic Deformation and Fracture Toughness of Nanotwinned Magnesium.

In this work, the plastic deformation mechanisms responsible for mechanical properties and fracture toughness in nanotwinned (NT). magnesium is studied by molecular dynamics (MD) simulation. The influence of twin boundary (TBs) spacing and crack position on deformation behaviors are investigated. The microstructure evolution at the crack tip are not exactly the same for the left edge crack (LEC) and the right edge crack (REC) models according to calculations of the energy release rate for dislocation nucleation at the crack tip. The LEC growth initiates in a ductile pattern and then turns into a brittle cleavage. In the REC model, the atomic decohesion occurs at the crack tip to create a new free surface which directly induces a brittle cleavage. A ductile to brittle transition is observed which mainly depends on the competition between dislocation motion and crack growth. This competition mechanism is found to be correlated with the TB spacing. The critical values are 10 nm and 13.5 nm for this transition in LEC and REC models, respectively. Essentially, the dislocation densities affected by the TB spacing play a crucial role in the ductile to brittle transition.

Atomistic mechanisms of fatigue in nanotwinned metals

We investigate the fatigue behavior of nanotwinned Cu using a combination of molecular statics and molecular dynamics simulations. The presence of nanoscale twins is found to enhance fatigue crack growth resistance. For the twin-free nanocrystalline samples, the fatigue crack propagates by linking the nanovoids that are formed ahead of the crack tip. In the case of the nanotwinned samples, however, it advances as the crack tip alternately blunts and re-sharpens due to dislocation emission and slip. Both detwinning and crack closure are observed in the path of the fatigue crack in nanotwinned samples with a high density of twin boundaries. As the twin number per grain (quantified by the ratio of the mean grain size to the twin boundary spacing d/λ) increases, detwinning increases the dissipated energy of fatigue cracking, leading to enhanced fatigue resistance. The atomistic simulations show that fatigue crack growth in nanotwinned Cu conforms to Paris’ law. In conjunction with the experimental results, we obtain a quantitative estimation of the Paris’ law exponent (∼4.0), which is in agreement with the theoretical predictions from the damage accumulation model.

Influence of Substrate on Conductivity of Sr and Mg Doped Lanthanum Gallate Thin Film Electrolyte for Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) consisting of ion conductive ceramics has various advantages such as the high generating efficiency, the utilization of hydrocarbon-containing fuel and the non-necessity of expensive rare metal catalyst. Yttria-stabilized Zirconia (YSZ) or Gadolinium-doped Ceria (GDC) are mainly used as the electrolyte for SOFC. Strontium- and Magnesium-doped Lanthanum Gallate (La0.9Sr0.1Ga0.8Mg0.2O3-δ, LSGM) has higher conductivity than YSZ, negligible electronic conduction and high chemical stability in a wide oxygen partial pressure range compared to GDC.Recently, it was reported that YSZ and GDC thin films have different values of ion conductivity from those of the bulk. LSGM should have the similar phenomenon. The reason for this change of the conductivity is assumed to be the lattice strain in the thin film, the difference between the bulk resistivity and the grain boundary resistivity or space charge layers at homo- or hetero-junctions. However, the mechanism of the conductivity change of LSGM thin film from the bulk is not revealed.In this study, the radio frequency magnetron sputtering, which is a reliable method for preparing thin film, was used for obtaining various thickness films on sapphire and quartz substrate. However, the deposited film was amorphous. To obtain the crystallized LSGM film, we investigated the influence of the annealing temperature. By proper post-annealing, crystallized LSGM film was obtained. We evaluated the crystallinity by X-ray diffraction, observed the microstructure with a scanning electron microscope and measured the conductivity by electrochemical impedance spectroscopy. In this paper, we report the effect on the electrical conductivity of the difference of thermal expansion co efficiency between LSGM thin film and the substrate.

Chemical reactivity of twin-modified copper nanowire surfaces

Effect of twin boundary (TB) spacing on atomic surface structure and chemical reactivity of nanotwinned Cu nanowires (NWs) is investigated. Post-etching surface structure and wire diameter of Cu NWs were examined by transmission electron microscopy. When TB spacing is less than 10 nm, the Cu NWs remain almost intact after chemical attack and show faceted surface structures with low atomic step density. A mechanism based on surface tension torque acting on TB/surface triple junctions is proposed to explain the faceted structure formation and enhanced corrosion resistance of nanotwinned Cu NWs.

Characterization of Si particles and their effects on and recrystallization in a nanostructured cold rolled Al-1% Si alloy

An Al-1.08 vol.% Si alloy was cold rolled to a reduction of 98% (εvM = 4.5) and then annealed at different temperatures up to 210°C (0.52 Tm) for different times. The deformed structure is characterized by a nanoscale lamellar structure with the presence of Si particles of coarse (> 1 μm), medium (100 nm - 1 μm) and fine (< 100 nm) sizes in the microstructure. Deformation zones are formed around the coarse Si particles and the boundary spacing is finer in the deformation zone than in the matrix. The medium Si particles have little effect on the morphology and boundary spacing. The fine Si particles are aligned along the lamellar boundaries indicating a stabilizing effect on the structural refinement during cold rolling. After annealing, enhanced recovery occurs in the deformation zones around the coarse Si particles. However the reduction in stored energy during recovery and the pinning effect of fine Si particles on the boundary migration prevent the advantage of particle stimulated nucleation (PSN) of coarse Si particles in the nanoscale lamellar structure. This study also demonstrates an important effect of the fine particles in delaying both recovery and recrystallization processes. This effect diminishes with increasing annealing temperature and coarsening the fine particles especially at triple junctions.

Non-Monotonous Mechanical Behavior at the Nanoscale: Elastic and Plastic Properties

The non-monotonous behavior of strength, activation volume, and pressure sensitivity parameter, as the grain size enters the nanoscale regime, is interpreted on the basis of a “rule of mixtures” argument, commonly used for composites. Grain interior and grain boundary spaces are treated as two independent “phases” with distinct mechanical properties. When the simple “rule of mixtures” argument is extended to incorporate concepts from the continuum theory of mixtures, the Laplacian of strain is generated in the local constitutive equation for each phase. A simple one-dimensional configuration for a nanopolycrystal is assumed and a related boundary value problem is solved to nterpret the dependence of the overall elastic modulus on the grain size.

Orientation-dependent recrystallization in an oxide dispersion strengthened steel after dynamic plastic deformation

The microstructure of the oxide dispersion strengthened ferritic steel PM2000 has been investigated after compression by dynamic plastic deformation to a strain of 2.1 and after subsequent annealing at 715 °C. Nanoscale lamellae, exhibiting a strong 〈100〉 + 〈111〉 duplex fibre texture, form during dynamic plastic deformation. Different boundary spacings and different stored energy densities for regions belonging to either of the two fibre texture components result in a quite heterogeneous deformation microstructure. Upon annealing, preferential recovery and preferential nucleation of recrystallization are found in the 〈111〉- oriented lamellae, which had a higher stored energy density in the as-deformed condition. In the course of recrystallization, the initial duplex fibre texture is replaced by a strong 〈111〉 fibre recrystallization texture.

Gradient nanostructured surface of a Cu plate processed by incremental frictional sliding

The flat surface of a Cu plate was processed by incremental frictional sliding at liquid nitrogen temperature. The surface treatment results in a hardened gradient surface layer as thick as 1 mm in the Cu plate, which contains a nanostructured layer on the top with a boundary spacing of the order of 100 nm. The boundary spacing increases with increasing distance from the surface, and is accompanied with a decrease in hardness from 2179±31MPa in the topmost surface layer to 568±10 MPa in the undeformed matrix.

A Plastic Deformation Mechanism by Necklace Dislocations Near Crack-like Defects in Nanotwinned Metals

Nanotwinned metals are a class of hierarchically structured materials that appear to transcend the limits of conventional material systems by exhibiting an exceptional combination of superior strength, ductility and resistance to fracture, fatigue, and wear. Recently, we reported a type of necklace dislocations in nanotwinned metals which become operative when the twin boundary (TB) spacing falls below a few nanometers. Here, we show that the presence of a cracklike defect as the dominant dislocation source could allow the same mechanism to operate at much larger twin spacings. This finding calls for further theoretical and experimental investigations of a new type of TB related dislocation mechanism which may play particularly important roles in crack-tip deformation in nanotwinned metals.