Dislocation core field. I. Modeling in anisotropic linear elasticity theory

Aside from the Volterra field, dislocations create a core field, which can be modeled in linear anisotropic elasticity theory with force and dislocation dipoles. We derive an expression of the elastic energy of a dislocation taking full account of its core field and show that no cross term exists between the Volterra and the core fields. We also obtain the contribution of the core field to the dislocation interaction energy with an external stress, thus showing that dislocation can interact with a pressure. The additional force that derives from this core field contribution is proportional to the gradient of the applied stress. Such a supplementary force on dislocations may be important in high stress gradient regions, such as close to a crack tip or in a dislocation pile-up

Services rendus par les foraminifères benthiques dans l’étude de l’influence des forçages naturels (e.g. changement climatique) et anthropiques sur l’écosystème estuarien. Exemple de la Loire.

Due to its intermediate position between ocean and continent, estuary is located in the heart of the economic, social and cultural activities. Awareness of the need to manage this vulnerable environment, has led in recent years, to an increase in surveillance activities of the environmental quality. Physico-chemical methods, although dominant and indispensable, reach their limits as a tool for environmental management. This is why other ways are being explored, such as evaluating the environmental quality by bio-indicators. It is in this perspective that SEMHABEL project is subscribed (Suivi Environnemental des Micro-HAbitats Benthiques de l’Estuaire de la Loire - Plan Loire Grandeur Nature 2007-2013 - FEDER). This is the first study in the Loire incorporating the use of benthic foraminifera as a new biological approach. Following a sampling cruise in September 2012, 320 samples of surface sediments were collected from Nantes to de Saint-Gildas. The data obtained allowed to represent and analyze the spatial distribution of communities of benthic foraminifera, along the upstream-downstream continuum and following geochemical and sedimentary evolutions. These results are the first support for the establishment of a database that will allow a better understanding of the actual functioning of this ecosystem. Ultimately, we hope to assess the evolution of the estuarine ecosystem according to climate change and catchment area management and to develop a biotic index for routine monitoring of the health of the estuary

Nucleation of Al3Zr and Al3Sc in aluminum alloys: from kinetic Monte Carlo simulations to classical theory

Zr and Sc precipitate in aluminum alloys to form the compounds Al3Zr and Al3Sc which for low supersaturations of the solid solution have the L12 structure. The aim of the present study is to model at an atomic scale this kinetics of precipitation and to build a mesoscopic model based on classical nucleation theory so as to extend the field of supersaturations and annealing times that can be simulated. We use some ab-initio calculations and experimental data to fit an Ising model describing thermodynamics of the Al-Zr and Al-Sc systems. Kinetic behavior is described by means of an atom-vacancy exchange mechanism. This allows us to simulate with a kinetic Monte Carlo algorithm kinetics of precipitation of Al3Zr and Al3Sc. These kinetics are then used to test the classical nucleation theory. In this purpose, we deduce from our atomic model an isotropic interface free energy which is consistent with the one deduced from experimental kinetics and a nucleation free energy. We test di erent mean-field approximations (Bragg-Williams approximation as well as Cluster Variation Method) for these parameters. The classical nucleation theory is coherent with the kinetic Monte Carlo simulations only when CVM is used: it manages to reproduce the cluster size distribution in the metastable solid solution and its evolution as well as the steady-state nucleation rate. We also find that the capillary approximation used in the classical nucleation theory works surprisingly well when compared to a direct calculation of the free energy of formation for small L12 clusters.Comment: submitted to Physical Review B (2004

Complex Precipitation Pathways in Multi-Component Alloys

One usual way to strengthen a metal is to add alloying elements and to control the size and the density of the precipitates obtained. However, precipitation in multicomponent alloys can take complex pathways depending on the relative diffusivity of solute atoms and on the relative driving forces involved. In Al-Zr-Sc alloys, atomic simulations based on first-principle calculations combined with various complementary experimental approaches working at different scales reveal a strongly inhomogeneous structure of the precipitates: owing to the much faster diffusivity of Sc compared with Zr in the solid solution, and to the absence of Zr and Sc diffusion inside the precipitates, the precipitate core is mostly Sc-rich, whereas the external shell is Zr-rich. This explains previous observations of an enhanced nucleation rate in Al-Zr-Sc alloys compared with binary Al-Sc alloys, along with much higher resistance to Ostwald ripening, two features of the utmost importance in the field of light high-strength materials

Time reversal methods in acousto-elastodynamics

The aim of the article is to solve an inverse problem in order to determine the presence and some properties of an elastic “inclusion” (an unknown object, characterized by elastic properties discriminant from the surrounding medium) from partial observa- tions of acoustic waves, scattered by the inclusion. The method will require developing techniques based on Time Reversal methods. A finite element method based on varia- tional acousto-elastodynamics formulation will be derived and used to solve to solve the forward, and then, the time reversed problem. A criterion, derived from the reverse time migration framework, is introduced, to help use to construct images of the inclusions to be determined. Our approach will be applied to configurations modeling breast cancer detection, using simulated ultrasound waves

La cultura como motor de la innovación social corporativa: descifrando sus factores clave

This work studies the organizational culture as the motor to promote the corporate social innovation in companies. Through a systematic literature review, the main academic contribution is the design of a theoretical framework that allows defining the most appropriate characteristics of the organizational culture for the development of CSI activities and the role of the key factors to promote it. This proposal aims to become a guideline to managers and professionals in order to develop organizational cultures prone to social innovation inside firms.Este trabajo estudia la cultura organizacional (CO) como motor para impulsar la innovación social corporativa (ISC) en las empresas. Con una metodología basada en la revisión sistemática de la literatura, la principal contribución académica es el diseño de un marco teórico que permite dilucidar qué tipo de CO es la más adecuada para albergar la ISC, así como una propuesta original de factores clave para promover dicha cultura. Esta propuesta pretende también ser una guía para directivos y profesionales a fin de implantar culturas organizacionales proclives a la innovación social en sus empresas

The Influence of the effect of solute on the thermodynamic driving force on grain refinement of Al alloys

Grain refinement is known to be strongly affected by the solute in cast alloys. Addition of some solute can reduce grain size considerably while others have a limited effect. This is usually attributed to the constitutional supercooling which is quantified by the growth restriction factor, Q. However, one factor that has not been considered is whether different solutes have differing effects on the thermodynamic driving force for solidification. This paper reveals that addition of solute reduces the driving force for solidification for a given undercooling, and that for a particular Q value, it is reduced more substantially when adding eutectic-forming solutes than peritectic-forming elements. Therefore, compared with the eutectic-forming solutes, addition of peritectic-forming solutes into Al alloys not only possesses a higher initial nucleation rate resulted from the larger thermodynamic driving force for solidification, but also promotes nucleation within the constitutionally supercooled zone during growth. As subsequent nucleation can occur at smaller constitutional supercoolings for peritectic-forming elements, a smaller grain size is thus produced. The very small constitutional supercooling required to trigger subsequent nucleation in alloys containing Ti is considered as a major contributor to its extraordinary grain refining efficiency in cast Al alloys even without the deliberate addition of inoculants.The Australian Research Council (ARC DP10955737)