Semi-coherent interfaces without length misfit accommodation in a lamellar Al–Al2Cu eutectic

Abstract

The interfacial structure of a lamellar Al(α)-Al₂Cu(θ) eutectic obtained by directional solidification is investigated using conventional transmission electron microscopy (TEM) and high-resolution TEM. The average lamellar habit plane is close to (2 3 3)α and lie 10° from the atomically densest planes (1 1 1)α//(2 –1 1)θ. Networks of linear contrasted features are observed along the interfaces, the lines being separated each other’s in a wide range of spacings, typically 6–500 nm. These features are identified as interfacial dislocations with 1/2<1 1 0>α Burgers vectors from image contrast analysis. According to previous works, they are associated with ledge-like defects, the heights of which can reach 3 nm. The high-resolution TEM images do not confirm the presence of atomic terraces parallel to the atomically dense plane (1 1 1)α or the habit plane (2 3 3)α. The interface ensures the quasi-continuity of atomically dense planes, which is a configuration corresponding to the plane-matching model. It is suggested that α/θ interfaces can be considered as semi-coherent but in a particular sense since, according to our analysis, the theoretical length misfits between the fcc and bct lattices are too large to explain the presence of some loose dislocation networks. Their general irregular geometry suggests that these dislocation networks behave like non-equilibrated low-angle grain boundaries superposed on the α–θ interfaces.     

Collaborative testing of eddy structure identification methods in free turbulent shear flows

 The thrust of this paper is to validate, test and compare several Coherent Structure eduction methods utilizing the same data base. The flow chosen was that of an experimental study of a plane, incompressible, fully developed turbulent two-stream mixing layer. The mixing layer was chosen as the data base because it has been studied extensively from a coherent structures point of view. In addition, its characteristics (similarity, convection velocities, etc.) are well documented. There are also no wall effects so that comparisons between techniques are simplified. The data was collected from hot wire rakes with good spatial resolution thus allowing the contributors to apply and test different structure eduction techniques. The techniques chosen for discussion and used here have found wide utilization over the past decade, and all hold forth the promise of extensive application in the future. These include: Conditional Sampling (Vorticity-based and other methods); Wavelets; Pattern Recognition Analysis; Proper Orthogonal Decomposition; Stochastic Estimation; Topological Concept-based methods; Full Field Methods (e.g., pseudo flow visualization). All are illustrated by application to the mixing layer data base, and comparisons made between the results. This common study has shown that direct comparisons between results of several methods are now possible. Good quantitive and qualitative agreement between the different methods have been observed as well as some differences noted. As an example, the size of the averaged structures computed from the various methods compare to within 6 percent. Received: 15 December 1994/Accepted: 18 December 1997     

On the crystallization of syndiotactic polypropylene (sPP)/high density polyethylene (HDPE) blends

 The crystallization kinetics of syndiotactic polypropylene (sPP)/high-density polyethylene (HDPE) blends were investigated by thermal and microscopical methods. Isothermal crystallization obtained by differential scanning calorimetry (DSC) was used to study the nucleation ability of HDPE on sPP crystallization. Morphological studies with polarized light microscopy (PLM) of solution cast films resulted in a transcrystalline-like overgrowth of the sPP on HDPE spherulites. The arrangement of the HDPE lamellar crystals in the blend were observed in transmission electron microscopy (TEM). But from these results, no straightforward conclusion about the transcrystalline nature of the sPP crystals can be given. Received: 24 November 1997 Accepted: 23 February 1998     

Further characterization of large-scale coherent structure signatures in a turbulent-plane mixing layer

In order to complement a previous collaborative testing of eddy structure identification methods in free turbulent shear flows, a study about the influence of the transverse location of these large-scale vortical structures on their signatures for the same referenced data base is carried out. Using vorticity-based conditional sampling, events which are located on the same transverse locations are selected and averaged. Results show that a coherent structure has its proper identity contained in its core and keeps it whatever its trajectory. In this way, only its signature in comparison with the surrounding environment changes. The comparison with vorticity-based conditional sampling without this transverse selection brings to the fore a bias on the signature of coherent structures. This result throws back into question previous conclusions about the morphology of structures and related energy transfer scenarios. Received: 24 July 2000 / Accepted: 25 June 2001 Published online: 29 November 2001     

On the key factors of angular correlations in complex-forming elementary reactions

In the mid-seventies, Case and Herschbach argued that for complex-forming three-atom reactions governed by long-range forces and performed in supersonic molecular beam experiments, vectorial properties are determined by a single parameter Λ' = 〈L'/(L' + j')〉, L' and j' being respectively the moduli of the orbital and rotational angular momenta of the products. A simple mathematical relation between vectorial properties and Λ' was then proposed. However, Λ' must be determined beforehand by phase space theory calculations. Besides, we have recently shown that scalar properties are mainly controled by two factors ρ'₁ and ρ'₂ respectively called angular excitation and diatomic inertial contribution. We show here that these factors control also vectorial properties. Moreover, the way they control them is summarized in a set of four figures. The advantage of our method is that ρ'₁ and ρ'₂ are related to the mechanical parameters of the reaction by very simple formulas, contrary to Λ'. Last by not least, our parameters appear to be mostly independent, so that vectorial properties cannot be said to strictly depend on Λ'. Nevertheless, it turns out that the rule proposed by Case and Herschbach is reasonable in many realistic situations.     

On Poset Boolean Algebras

Let (P,≤) be a partially ordered set. The poset Boolean algebra of P, denoted F(P), is defined as follows: The set of generators of F(P) is {x _p  : p∈P}, and the set of relations is {x _p ⋅x _q =x _p  : p≤q}. We say that a Boolean algebra B is well-generated, if B has a sublattice G such that G generates B and (G,≤ ^B |G) is well-founded. A well-generated algebra is superatomic. THEOREM 1. Let (P,≤) be a partially ordered set. The following are equivalent. (i) P does not contain an infinite set of pairwise incomparable elements, and P does not contain a subset isomorphic to the chain of rational numbers, (ii) F(P) is superatomic, (iii) F(P) is well-generated. The equivalence (i) ⇔ (ii) is due to M. Pouzet. A partially ordered set W is well-ordered, if W does not contain a strictly decreasing infinite sequence, and W does not contain an infinite set of pairwise incomparable elements. THEOREM 2. Let F(P) be a superatomic poset algebra. Then there are a well-ordered set W and a subalgebra B of F(W), such that F(P) is a homomorphic image of B. This is similar but weaker than the fact that every interval algebra of a scattered chain is embeddable in an ordinal algebra. Remember that an interval algebra is a special case of a poset algebra. This revised version was published online in June 2006 with corrections to the Cover Date.     

Experimental 3D Analysis of the Large Scale Behaviour of a Plane Turbulent Mixing Layer

A new structure education method is proposed for representing the three dimensional large scale structures from hot wires measurement data. Due to the limited number of hot wire probes that can be used simultaneously, we firstly have to optimise the spatial location of sensors in order to detect large scale coherent structures. For such a purpose a detailed analysis of the 3D space-time flow organisation is given with particular emphasis on the full two point correlation tensor of the velocity field. Secondly, several procedures are developed in order to reconstruct instantaneous 3D velocity field on any finer and larger mesh than the experimental one from the limited available experimental information. The particular case of a spatially developing turbulent plane mixing layer is investigated. We then show that Stochastic Estimation and Proper Orthogonal Decomposition lead to a satisfactory reconstruction preserving the 3D coherent character of the flow. Finally, this reconstructed velocity field provides a 3D space-time representation of the large scale structures in a streamwise section of the flow and can then be used as realistic turbulent inflow condition for Large Eddy Simulations.     

Donor-Stabilized Silylene/Phosphine-Supported Carbon(0) Center with High Electron Density

An isolable donor-stabilized silavinylidene phosphorane was synthesized. This molecule, which can also be regarded as a new carbon(0) complex featuring a phosphine and a donor-stabilized silylene ligand, presents a central carbon atom with a remarkably high electron density (−1.82). Furthermore, the experimental electron-density study of this compound demonstrates the delocalization of the σ-lone pair at the central carbon atom toward the silicon center, a feature which is remarkably different from electronic situation of other bent-allene-type molecules. This result clearly demonstrates the powerful electron-donating ability of donor-stabilized silylene ligands, as well as their excellent electron-acceptor properties.     

A FEM-based topological sensitivity approach for fast qualitative identification of buried cavities from elastodynamic overdetermined boundary data

A time-domain topological sensitivity (TS) approach is developed for elastic-wave imaging of media of arbitrary geometry. The TS, which quantifies the sensitivity of the misfit cost functional to the creation at a specified location of an infinitesimal hole, is expressed in terms of the time convolution of the free field and a supplementary adjoint field as a function of that specified location. Following previous studies performed under (mostly) static or time-harmonic conditions, the TS field is here considered as a natural and computationally efficient approach for defining a defect indicator function. This study emphasizes the implementation and exploitation of TS fields using standard displacement-based FEM approaches, a straightforward task once the correct sensitivity formulation is available. A comprehensive set of numerical experiments on 3D and 2D elastodynamic and acoustic configurations is reported, allowing to assess and highlight many features of the proposed TS-based fast qualitative identification such as its ability to identify multiple defects and its robustness against data noise.     

Experimental and computational investigation of the unexpected formation of β-substituted polyoxygenated furans from conveniently functionalized carbohydrates

In the course of our current studies on the reactivity of alkylidenecarbenes, prepared with the trimethylsilylazide/Bu₂SnO method, on conveniently functionalized α-cyanomesylates derived from d-allose, d-arabinose, and d-threose, we have observed the unexpected, but mechanistically interesting formation of enantiomerically β-substituted polyhydroxylated furans. These compounds are the result of a series of cascade fragmentation reactions on unstable intermediates obtained during 1,5C-H insertion reactions from alkylidenecarbenes. The mechanism of the reaction has been investigated by computational methods using DFT analysis.