Boundary Spacing Research Articles

An Analysis on Importance of CPTED Planning Factors to Make SAFETY Boundary Space at Condominium Complex in Republic of KOREA

An Analysis on Importance of CPTED Planning Factors to Make SAFETY Boundary Space at Condominium Complex in Republic of KOREA

Interaction of dislocation pile-up with a low-angle tilt boundary: a discrete dislocation dynamics study

The pile-up of dislocations between two low-angle tilt boundaries (LATB) in an fcc crystal was simulated using three-dimensional discrete dislocation dynamics. The LATB was constructed using glissile edge dislocations stacked on each other. The dislocations in the pile-up were chosen such that their reactions with the dislocations in the LATB resulted in glissile junctions. Parallel pairs of dislocations were inserted to a maximum allowable value estimated from theoretical expressions. A resolved shear stress was applied and increased in steps so as to move the dislocations in the pile-up towards the boundaries. The shear stress required to break the lead dislocation from the wall was determined for varying spacings between the two boundaries. The shear stress and boundary spacing followed the Hall–Petch type relation. Dislocation pile-ups without a LATB were also simulated. The spacing of the dislocations in the pile-up with LATB was found to be closer (ie higher dislocation density) than that without LATB. It was shown through analytical expressions that LATB exerts an attractive force on the dislocations in the pile-up thereby creating a denser pile-up.

Boundary spaces, objects and activities in mixed-actor knowledge production: making fishery management plans in collaboration

This paper investigates knowledge production in collaborations where the actors have different knowledge and interests. Building on boundary object theory, a conceptual framework is developed to analyse knowledge exchange in two stakeholder-led collaborations to make fishery management plans. The framework introduces boundary spaces to conceptualise the dynamic interaction between objects and activities. Within boundary spaces, actors can share, transfer and translate their knowledge, and common understanding can emerge. The collaborations analysed aimed to produce management plans for the Nephrops (Nephrops norvegicus) fishery in the North Sea and the boarfish (Capros aper) fishery in the Northeast Atlantic. Several boundary spaces were identified in each collaboration. During the production phase, the emerging management plans took on multiple representations as boundary objects that facilitated knowledge exchange. Activities were essential, as these created entry points for different actors to become part of the boundary spaces where they could contribute to knowledge production. Fishing industry representatives in the North Sea Advisory Council and the Pelagic Advisory Council played key roles in initiating and coordinating activities. The case studies demonstrate that Advisory Councils take on pro-active roles in initiatives that aim to expand the knowledge base for European fisheries management. Direct engagement was instrumental to create ownership of the problem addressed in the various collaborative settings that emerged during the management plan initiatives.

Effects of Grain Refinement by ECAP on the Deformation Resistance of Al Interpreted in Terms of Boundary-Mediated Processes

Results of a large set of tensile and compressive creep tests on pure Al were reanalyzed for the influence of low- and high-angle grain boundaries on the deformation resistance at the temperature T = 473 K = 0.51Tm where Tm is the melting point. Thermomechanical treatment by equal channel angular pressing followed by heating to T led to strong increase of areal fraction of high-angle boundaries in a structure of subgrains of ≈10−6 m in size, accompanied by significant reduction of subgrain strengthening and of the stress sensitivity of the deformation rate. (Sub)grain strengthening by low-angle boundaries is most effective; the strengthening effect virtually disappears during creep as the boundary spacings coarsen toward their stress-dependent, quasi-stationary size wqs. The same type of coarsening is found for (sub)grain structures with large fraction of high-angle boundaries; in the quasi-stationary state they lead to softening at low and strengthening at high stresses, and a significant increase in tensile fracture strain to values up to 0.8. The results are analogous to former results for Cu and are explained in the same way by the influence of boundaries on storage and recovery of crystal defects and the homogenization of glide.

Deformation behaviour of body centered cubic iron nanopillars containing coherent twin boundaries

Molecular dynamics simulations were performed to understand the role of twin boundaries on deformation behaviour of body-centred cubic (BCC) iron (Fe) nanopillars. The twin boundaries varying from 1 to 5 providing twin boundary spacing in the range 8.5–2.8 nm were introduced perpendicular to the loading direction. The simulation results indicated that the twin boundaries in BCC Fe play a contrasting role during deformation under tensile and compressive loadings. During tensile deformation, a large reduction in yield stress was observed in twinned nanopillars compared to perfect nanopillar. However, the yield stress exhibited only marginal variation with respect to twin boundary spacing. On the contrary, a decrease in yield stress with increase in twin boundary spacing was obtained during compressive deformation. This contrasting behaviour originates from difference in operating mechanisms during yielding and subsequent plastic deformation. It has been observed that the deformation under tensile loading was dominated mainly by twin growth mechanism. On the other hand, the deformation was dominated by nucleation and slip of full dislocations under compressive loading. The twin boundaries offer a strong repulsive force on full dislocations resulting in the yield stress dependence on twin boundary spacing. The occurrence of twin–twin interaction during tensile deformation and dislocation–twin interaction during compressive deformation has been discussed.

Microstructure and texture evolution in the cryorolled CuZr alloy

The microstructure and texture evolution of the room temperature and cryogenically rolled Cu-0.3%Zr alloy were investigated at a range of true strains (0.36, 0.69, 1.20, 2.30) by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The high angle boundary spacing of lamellar grains along the normal direction is mainly dependent on the geometrical effect, and cryorolling has a minor refining effect on the width of the lamellae. However, the deformation twins and shear bands within the lamellar structure of the cryorolled CuZr alloy contribute to the grain refinement and improve the mechanical properties. The cryorolling enhances the inhomogeneous deformation and forms brass-type shear bands. In contrast to the copper-type texture in the room temperature rolled CuZr alloy, the cryorolled CuZr alloy exhibits a typical brass-type texture. This texture discrepancy is mainly attributed to the closely spaced twin/matrix lamellae and brass-type shear bands induced by cryorolling. The twin/matrix lamellae stimulate the initial formation of brass-type texture, and the shear bands promote its development. Unlike the room temperature rolled CuZr alloy, the Goss component is observed in the remnant grains in the cryorolled CuZr alloy. The occurrence of the Goss component may be attributed to the suppression of cross slip during cryorolling.

Hadamard-type formulas via the Maslov form

Given a star-shaped bounded Lipschitz domain \({\Omega\subset{\mathbb{R}}^d}\), we consider the Schrodinger operator \({L_{\mathcal{G}}=-\Delta+V}\) on \({\Omega}\) and its restrictions \({L^{\Omega_t}_{\mathcal{G}}}\) on the subdomains \({\Omega_t}\), \({t\in[0,1]}\), obtained by shrinking \({\Omega}\) toward its center. We impose either the Dirichlet or quite general Robin-type boundary conditions determined by a subspace \({{\mathcal{G}}}\) of the boundary space \({H^{1/2}(\partial\Omega)\times H^{-1/2}(\partial\Omega)}\), and assume that the potential is smooth and takes values in the set of symmetric \({(N\times N)}\) matrices. Two main results are proved: First, for any \({t_0\in(0,1]}\) we give an asymptotic formula for the eigenvalues \({\lambda(t)}\) of the operator \({L^{\Omega_t}_{\mathcal{G}}}\) as \({t\to t_0}\) up to quadratic terms, that is, we explicitly compute the first and second \({t}\)-derivatives of the eigenvalues. This includes the case of the eigenvalues with arbitrary multiplicities. Second, we compute the first derivative of the eigenvalues via the (Maslov) crossing form utilized in symplectic topology to define the Arnold–Maslov–Keller index of a path in the set of Lagrangian subspaces of the boundary space. The path is obtained by taking the Dirichlet and Neumann traces of the weak solutions of the eigenvalue problems for \({L^{\Omega_t}_{\mathcal{G}}}\).

Twistor methods for AdS5

We consider the application of twistor theory to five-dimensional anti-de Sitter space. The twistor space of AdS$_5$ is the same as the ambitwistor space of the four-dimensional conformal boundary; the geometry of this correspondence is reviewed for both the bulk and boundary. A Penrose transform allows us to describe free bulk fields, with or without mass, in terms of data on twistor space. Explicit representatives for the bulk-to-boundary propagators of scalars and spinors are constructed, along with twistor action functionals for the free theories. Evaluating these twistor actions on bulk-to-boundary propagators is shown to produce the correct two-point functions.

Predicting microstructure and strength of maraging steels: Elemental optimisation

A physics–based modelling framework to describe microstructure and mechanical properties in maraging steels is presented. It is based on prescribing the hierarchical structure of the martensitic matrix, including dislocation density, and lath and high–angle grain boundary spacing. The evolution of lath–shaped reverted austenite is described using grain–boundary diffusion laws within a lath unit. The dislocation density provides the preferential nucleation sites for precipitation, whereas descriptions for particle nucleation, growth and coarsening evolution are identified for Ni3Ti, NiAl and its variants, and BCC–Cu clusters. These results are combined to describe the hardness at different ageing temperatures in several FeNi–, FeMn– and FeNiMn–based steels. A critical assessment on individual contributions of typical alloying elements is performed. Ni and Mn control the kinetics of austenite formation, where the latter shows stronger influence on the growth kinetics. Ti additions induce higher hardness by precipitating stronger Ni3Ti, whereas Cu clusters induce low strength. A relationship between the reverted austenite and the total elongation in overaging conditions is also found. This result allows to identify optimal process and alloy design scenarios to improve the ductility whilst preserving high hardness in commercial maraging steels.

Effects of chip conditions on the solid state recycling of Ti-6Al-4V machining chips

Five different types of Ti-6Al-4V machining chips were recycled in their solid state using equal channel angular pressing (ECAP). The chips were produced by either turning or milling with or without the application of a coolant. All the as-recycled materials achieved full density but their microstructures, including the prior chip boundary spacing and grain size, are dependent on the starting chip morphologies and dimensions. The differences, however, were largely removed in the subsequent mill-annealing processing consisting of β homogenisation, hot deformation and final annealing, leading to near-identical mill-annealed microstructures for all different types of chips consisting of equiaxed α+β grains with lamellar α, typical following such a treatment. The recycled and mill-annealed materials possess high yield strengths of >900MPa and satisfactory tensile ductility of >11%, comparable or superior to the original commercial mill-annealed Ti-6Al-4V and independent of the initial chip conditions.

Hedge mazes and landscape gardens as cultural boundary objects

Despite their obvious functional and stylistic differences, hedge mazes and English landscape gardens have salient symbolic and structural similarities which make them fruitful objects of comparative analysis. Both invert the norms expected of interior and exterior spaces, of human cultivation and “wilderness”, creating landscapes of semiotic uncertainty. Being at once natural and cultural, both types of space present a “problem to be solved” either by reaching a centre or understanding a layout. Both “play” with the notion of boundary by constructing uncrossable and at times oppressive walls from seemingly fragile plant matter or by hiding their boundaries. At the same time there are important differences which make this comparison of boundary spaces even more interesting: hedge mazes and landscape gardens are distinguishable by their respective structural levels, the presence or absence of a centre, their relation to other parts of gardens and connected human habitations.Using Juri Lotman’s notion of hybrid and transitional objects characteristic of boundary mechanisms, this paper explores the semiotically dense nature-culture boundary which these mazes and gardens both inhabit and create. The objects of our analysis are 17th-century English mazes and early English gardens dating from the beginning of the 18th century: mazes at Longleat and Hampton Court, and landscape gardens such as Rousham and Stowe.

Investigation of mechanical properties of twin gold crystal nanowires under uniaxial load by molecular dynamics method**Project supported by the National Science and Technology Pillar Program, China (Grant No. 2015BAK17B06), the Earthquake Industry Special Science Research Foundation Project, China (Grant No. 201508026-02), the Natural Science Foundation of Heilongjiang Province, China (Grant No. A201310), and the Scientific Research Starting Foundation for Post

Twin gold crystal nanowires, whose loading direction is parallel to the twin boundary orientation, are simulated. We calculate the nanowires under tensile or compressive loads, different length nanowires, and different twin boundary nanowires respectively. The Young modulus of nanowires under compressive load is about twice that under tensile load. The compressive properties of twin gold nanowires are superior to their tensile properties. For different length nanowires, there is a critical value of length with respect to the mechanical properties. When the length of nanowire is greater than the critical value, its mechanical properties are sensitive to length. The twin boundary spacing hardly affects the mechanical properties.

High thermal stability of submicron grained Cu processed by asymmetrical rolling

In this paper, we report the microstructure evolutions of symmetrically and asymmetrically rolled Cu at different temperatures, and reveal an important role of asymmetry induced microstructure homogeneity and removal of local severely distorted structures on the enhancement of thermal stability. With the same total reduction of 90%, two kinds of submicron grained (SMG) Cu were processed by symmetrical and asymmetrical rolling respectively. The temperature dependent boundary spacing curves show the asymmetrically rolled Cu has significantly higher thermal stability, with the onset temperature of obvious grain coarsening being 40°C higher than that for symmetrical rolled Cu. The enhanced thermal stability of asymmetrically rolled Cu can be attributed to the retardation of nucleation mainly realized by the improvement in microstructure homogeneity and the removal of local severely distorted structures. Our results demonstrate the feasibility of using asymmetrical rolling to improve the thermal stability of bulk SMG metals.

Ionic Strength Is a Barrier to the Habitability of Mars.

The thermodynamic availability of water (water activity) strictly limits microbial propagation on Earth, particularly in hypersaline environments. A considerable body of evidence indicates the existence of hypersaline surface waters throughout the history of Mars; therefore it is assumed that, as on Earth, water activity is a major limiting factor for martian habitability. However, the differing geological histories of Earth and Mars have driven variations in their respective aqueous geochemistry, with as-yet-unknown implications for habitability. Using a microbial community enrichment approach, we investigated microbial habitability for a suite of simulated martian brines. While the habitability of some martian brines was consistent with predictions made from water activity, others were uninhabitable even when the water activity was biologically permissive. We demonstrate experimentally that high ionic strength, driven to extremes on Mars by the ubiquitous occurrence of multivalent ions, renders these environments uninhabitable despite the presence of biologically available water. These findings show how the respective geological histories of Earth and Mars, which have produced differences in the planets' dominant water chemistries, have resulted in different physicochemical extremes which define the boundary space for microbial habitability. Habitability-Mars-Salts-Water activity-Life in extreme environments. Astrobiology 16, 427-442.

Boundary Crossing and the Professional Learning of Teacher Educators in New International Contexts

In this collaborative self-study, two teacher educators examine their experiences of working in new international contexts and the impact of those experiences on their professional learning and identities. Mandi moved from a major research university in one country to another, while Judy co-led a group of pre-service teachers on an international practicum for three weeks each year for three years. Using the concept of boundary crossing as a theoretical and analytical framework, each teacher educator identified a boundary-related critical incident or experience that occurred during her work in new international contexts. Through individual and collaborative analysis of their critical incidents, they found that working as a teacher educator in new international contexts involves the crossing and re-crossing of multiple personal, professional, linguistic and cultural boundaries. Working in these boundary spaces involved learning how to negotiate new kinds of relationships with colleagues and students, manage changed roles and responsibilities and, ultimately, a search for a renewed sense of self, as each sought to understand herself differently within a new professional context.

Grain growth in perovskites: What is the impact of boundary transitions?

Several perovskite ceramics are known to have non-Arrhenius type grain growth and to show grain growth transitions instead. These growth transitions are coupled to bimodal microstructures and seem to be caused by the existence, coexistence and transition of two different grain boundary types. This paper gives a review on the details of the grain growth transitions in perovskite ceramics. A simple mean field approach and standard grain growth experiments are used for quantification. However, this approach is far too simple to understand bimodal anisotropic grain growth completely as apparent in perovskites. Important parameters such as atomistic boundary structure, segregation effects, wetting transitions, faceting, grain boundary stoichiometry, space charge and interfacial drag effects are discussed with respect to their impact on grain growth in perovskites. A summary of open questions and future directions is given.

Boundary triples for Schrödinger operators with singular interactions on hypersurfaces

The self-adjoint Schrodinger operator Aδ,α with a δ-interaction of constant strength α supported on a compact smooth hypersurface C is viewed as a self-adjoint extension of a natural underlying symmetric operator S in L2(ℝn). The aim of this note is to construct a boundary triple for S* and a self-adjoint parameter Θδ,α in the boundary space L2(C) such that Aδ,α corresponds to the boundary condition induced by Θδ,α. As a consequence the well-developed theory of boundary triples and their Weyl functions can be applied. This leads, in particular, to a Krein-type resolvent formula and a description of the spectrum of Aδ,α in terms of the Weyl function and Θδ,α.

Microstructural evolution and mechanical properties of low SFE Cu-Al alloys processed by cryorolling followed by short-annealing

Cu-2% Al and Cu-4.5% Al alloys were developed by metal mould casting followed by homogenization at 800°C for cryorolling. The homogenized samples were cryorolled (CR) up to the maximum possible reduction in area (~75%). Activation energy for recrystallization of the CR samples was estimated by isoconversion methods using differential scanning calorimetry data; whereas, stored strain energy was determined by X-ray diffraction analysis. The activation energy for recrystallization correlated well with the stored energy of the corresponding sample. Microstructural evolution was analyzed by optical and transmission electron microscopy. The CR+annealed (225°C) sample showed an improved yield strength (YS) of 810MPa with a reasonable ductility of 5.1%. The YS found to be 10 times higher than that of the cast+homogenized sample (76MPa). This is attributed to the recovery of low angle grain boundaries, increasing grain boundary spacing, formation of nanotwins and decrease in the dislocation density without any recrystallization. The YS obtained by analytical modeling of the active strengthening mechanisms is highly corroborated with the experimental YS of the annealed sample. The Hall-Petch strengthening and dislocation strengthening found to play the pivotal role in the improvement of mechanical properties.

A polynomial chaos expansion based molecular dynamics study for probabilistic strength analysis of nano-twinned copper

Nano-twinned structures are mechanically stronger, ductile and stable than its non-twinned form. We have investigated the effect of varying twin spacing and twin boundary width (TBW) on the yield strength of the nano-twinned copper in a probabilistic framework. An efficient surrogate modelling approach based on polynomial chaos expansion has been proposed for the analysis. Effectively utilising 15 sets of expensive molecular dynamics simulations, thousands of outputs have been obtained corresponding to different sets of twin spacing and twin width using virtual experiments based on the surrogates. One of the major outcomes of this work is that there exists an optimal combination of twin boundary spacing and twin width until which the strength can be increased and after that critical point the nanowires weaken. This study also reveals that the yield strength of nano-twinned copper is more sensitive to TBW than twin spacing. Such robust inferences have been possible to be drawn only because of applying the surrogate modelling approach, which makes it feasible to obtain results corresponding to 40 000 combinations of different twin boundary spacing and twin width in a computationally efficient framework.

AdS/CFT and the geometry of an energy gap

We consider a CFT defined on a static metric that is the product of time with a smooth closed space of positive scalar curvature. We expect the theory to exhibit an energy gap and our aim is to investigate how that gap depends on the geometry of the space. For a free conformal scalar it is straightforward to show the gap normalised by the minimum value of the Ricci scalar of the space is minimized when the space is a sphere. Our main result is then to show using geometric arguments that precisely the same result holds for fluctuations of a scalar operator in any holographic CFT. We prove this under the assumption that the dual vacuum geometry is a smooth Einstein metric ending only on the conformal boundary, and then consider fluctuations of a minimally coupled massive scalar field about this. We also argue the holographic CFT will have states dual to small bulk black holes whose existence is related to the energy gap. We show the thermodynamic properties of these black holes obey a bound of a similar nature to that of the scalar fluctuations—the ratio of CFT energy to a power of entropy for the states dual to black holes in the ‘small’ limit is bounded from below when appropriately normalized by the minimum Ricci scalar of the boundary space. Again the bound is saturated for a sphere.