Effect of residual stress on recrystallization behavior of mechanically alloyed steels

This paper presents a finite element modeling analysis of deformation on iron-base mechanically alloyed oxide dispersion strengthened alloy by spherical indentations (Brinell test). Results of the model are used to interpret the role of residual shear stresses on the development of recrystallized grain structure and the temperature at which recrystallization occurs.I. Toda Caraballo acknowledges the Spanish Ministerio de Ciencia e Inovacio´n for financial support in the form of Ph.D. research grant (FPI). The authors acknowledge financial support for this investigation from the Spanish Ministerio de Educacio´n y Ciencia through the Plan Nacional 2006 ENE2006-15170-C02- 01/ALT.Peer reviewe

Simulaciones de crecimiento y movilidad de fronteras de grano en el sistema Fe-Cr

La presente memoria constituye la Tesis Doctoral de D. Isaac Toda Caraballo, presentada en el Departamento de Física de Materiales de la Facultad de Ciencias Físicas de la Universidad Complutense de Madrid, para optar al grado de Doctor en Ciencias Físicas. Este trabajo ha sido realizado en el Grupo de Investigación MATERALIA del Departamento de Metalurgia Física del Centro Nacional de Investigaciones Metalúrgicas (CENIM-CSIC), bajo la dirección de los Doctores Carlos Capdevila Montes, Investigador Científico del CSIC y Carlos García de Andrés, Profesor de Investigación del CSIC.Los aceros ferrítico/martensíticos con alto contenido en cromo han tenido gran aplicabilidad en industrias petroquímica y energética debido a sus excelentes propiedades de resistencia a la corrosión y fluencia a altas temperaturas [1], como en centrales eléctricas [2] o en la industria nuclear [3]. Sus primeras aplicaciones en calderas y turbinas [4] contaban con un contenido en cromo en torno al 10-15%, aleado con pequeñas adiciones de molibdeno. La necesidad de reducir costes en plantas de producción eléctrica se tradujo en un aumento en eficiencia con el consecuente incremento en la temperatura de servicio. Esto requiere aceros más aleados con mayor resistencia a la fluencia. Por ello, pequeñas cantidades de vanadio, niobio, níquel y tungsteno, además de la presencia de una dispersión de óxidos ofrecen gran aplicabilidad, por ejemplo, en generadores de vapor en centrales nucleares. Su resistencia frente a bombardeo de neutrones de alta energía en servicios de alta temperatura (550-600 ºC) les hace también apropiados para su uso en los núcleos de reactores de fusión y fisión [4]. La segunda generación de este tipo de aceros son los denominados aceros o aleaciones ODS (endurecidas por dispersión de óxidos – oxide dispersion strengthened). Estos materiales son creados por aleación mecánica [2], una técnica en la que diferentes elementos de aleación en polvos (en estado sólido) son mezclados y deformados en un molino de alta energía hasta obtener una solución sólida en la que es posible la adición de óxidos estables a elevadas temperaturas. El tratamiento finaliza con un proceso de extrusión en caliente. El resultado es un material con una gran mejora en la resistencia y fluencia a temperaturas más altas que sus predecesores. Esto es debido a la gran estabilidad de los óxidos a esas temperaturas, por lo que el movimiento de dislocaciones y vacantes queda impedido por dichas partículas de tamaño nanométrico.Peer reviewe

Neural network design with combined backpropagation and creeping-random- search learning algorithms applied to the determination of retained austenite in TRIP steels

[ES]A partir de los años noventa, el interés que los aceros TRIP despiertan en la industria ha producido un incremento considerable de su estudio y aplicación. En este trabajo, aprovechando la flexibilidad que las redes neuronales proporcionan para la modelización de propiedades complejas, se ha abordado el problema de la determinación de la austenita retenida en los aceros TRIP. Una combinación de dos algoritmos de aprendizaje (retropropagación y búsqueda aleatoria progresiva) de la red neuronal ha permitido crear un modelo que predice la cantidad de austenita retenida en aceros multifase con bajo contenido en aluminio y silicio en función de los parámetros de procesado.[EN]At the beginning of the decade of the nineties, the industrial interest for TRIP steels leads to a significant increase of the investigation and application in this field. In this work, the flexibility of neural networks for the modelling of complex properties is used to tackle the problem of determining the retained austenite content in TRIP-steel. Applying a combination of two learning algorithms (backpropagation and creeping-random-search) for the neural network, a model has been created that enables the prediction of retained austenite in low-Si / low-Al multiphase steels as a function of processing parameters.Peer reviewe

Analgesia obstétrica peridural continua para el parto vaginal en el HEODRA

Tesis (Dr. Especialista en Anestesiología)-Universidad Nacional Autónoma de Nicaragua, León.UNAN-Leó

Chemical banding revealed by chemical etching in a cold‐rolled metastable stainless steel

The current work describes themetallographic characterization of the initialmicrostructure of a cold rolled precipitation hardening semi-austenitic stainless steel (12Cr–9Ni–4Mo–2Cu–1Ti, in wt.%). The use of the Lichtenegger and Blöch (L–B) color etching solution has been shown to reveal not only the phases present in the microstructure, but also the existence of chemical banding along the rolling direction. The L–B reagent has beenfound to color the microstructure in bands depending on what alloying elements have segregated to each band. Twodimensional electron probe microanalysis (EPMA) maps have shown that Ni, Cu and Ti segregate together in the bands, while Cr has an opposite behavior. Mo has a mixed segregation behavior although much weaker than the other elements and more prompt to segregate likeNi does. A direct comparison of light opticalmicrographs with the EPMAmaps of the same area of the microstructure has enable to establish a direct relationship between the alloying element band concentration and the resulting etching color contrast obtained with the L–B reagent. Thermodynamic calculations predict that solidification in this steel takes place with ferrite as the primary phase. Equilibriumpartitioning coefficient calculations seem to support the observed segregation patternsfinancial support from the Ministerio de Economía y Competitividad (Project No. MAT2010-19522). Consejo Superior de Investigaciones Científicas (CSIC) in the form of a JAE-predoc grant under the program “Junta para la Amplianción de Estudios”, co-funded by the European Social Fund. Dr. Niels H. van Dijk from TUDelft (The Netherlands)Peer reviewe

Experimental and computational analysis of abnormal grain growth

© 2015 Institute of Materials, Minerals and Mining. The mechanisms involved in the abnormal grain growth of the iron based oxide dispersion strengthened alloys are analysed in the present work. Its microstructural evolution takes place at high temperatures (0.9Tm) and is characterised by an initial submicrometre size microstructure and a strong || rolling direction (RD) texture that evolves into a few extremely coarse grains (mm sizes) with ||RD orientation. The analysis of the observed grain boundaries has been completed by molecular dynamics simulations. Microstructure evolution consists of an extended recovery process, followed by an abnormal grain growth stage, consequence of the orientation pinning mechanism and the proximity to a symmetric tilt boundary family between the ||RD and ||RD grains.PM 2000 is a trademark of Plansee GmbH Inc. CC and GP acknowledge the Ministry of Economy and Competitiveness (MINECO) for the financial support through the National project no. ENE2009-13766-C04-01. GP acknowledges the MINECO for supporting her research under a FPI Grant (grant no. BES-2010-032747). The use of the computational facilities provided by CTI (Trueno cluster) is also gratefully acknowledged.Peer Reviewe

Generalized universal equation of states for magnetic materials: A novel formulation for an interatomic potential in Fe

The development of quantitative models for understanding physical properties of alloys requires a proper treatment of magnetic interactions, which is of paramount importance for the microstructural stability, especially in steels and high-entropy alloys containing magnetic elements. These magnetic interactions also control the defects behavior which affects the mechanical properties and the response under irradiation. Current interatomic potentials for molecular dynamics (MD) simulations still lack an adequate formulation to include magnetism into the simulations. In this paper, the universal equation of states (UES) is revisited and generalized by including ferromagnetic (FM) and antiferromagnetic (AFM) configurations with the aim of proposing a new formulation to develop interatomic potentials with magnetic contribution. For the case of Fe, given a fixed magnetic configuration and magnitude of the magnetic moment, the energy of the system is calculated by means of three parameters, namely, the energy, volume, and corresponding scaling volume (directly related to the bulk modulus) at the local ground state of the corresponding lattice. These parameters depend on three terms: firstly, a distance-dependent function, which gathers the nonmagnetic influence of the surrounding atoms; secondly, a magnetically dependent function, contributing to the energy by means of the magnetic nature of the atom, irrespective of the magnetic moment magnitudes of its surrounding atoms; and finally, a term which is magnetically and distance dependent simultaneously, which describes the influence of the magnetic state of the surrounding atoms on the energy considering their interatomic distance. This latter term is built via two functions which cannot be disconnected: one dependent on the distance between two atoms (a decreasing function with the distance in absolute value) multiplied by another function which is dependent on the magnetic moment of these two atoms. In this way, the magnetic influence of a distant atom scales with its distance. The new formulation is tested for magnetic iron, where 18 240 spin polarized density functional theory (DFT) calculated energies for different lattices, volumes, and magnetic moments in FM and AFM configurations showed that the generalized UES (GUES) accurately describes the energy of the system. The root-mean-square error of the GUES is in the range of 5.9×10-3eV over all DFT calculated energies, showing good accuracy and allowing us to propose a formulation for developing a magnetic interatomic potential (MIP) in Fe. The potential is developed for FM configuration in iron, aiming at studying the stability of the ferritic and austenitic phases but also defects and other configurations of special relevance as, for instance, in irradiation conditions for fusion or fission applications. The distance and magnetic functions of the GUES are tabulated to obtain a MIP, which describes the DFT calculated energies for different lattices, volumes, and magnetic moments in FM configuration. Further, the MIP has been validated in other crystal lattices (A15 and C15), elastic constants, stresses in the lattice, vacancies, interstitials, forces at different temperatures, transformation paths between body-centered cubic, face-centered cubic, hexagonal close-packed, and simple cubic structures as well as γ surfaces. We conclude that the results in this paper pave the way to develop MIPs with accuracy and predictability beyond the state of the art in MD

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A molecular dynamics study of grain boundary free energies, migration mechanisms and mobilities in a bcc Fe-20Cr alloy

Curvature driven migration of a series of 〈1 1 0〉 tilt grain boundaries in a bcc Fe-20Cr alloy is simulated using molecular dynamics to investigate the relationship between the atomic migration mechanism and mobility at medium to high temperatures. The boundaries studied include low angle boundaries (LAGB), high angle boundaries (HAGB) and singular boundaries, such as coherent twins. The steady-state boundary shape and curvature are compared with a simple analytical model which incorporates the dependence of absolute mobility and free energy on boundary inclination. The comparison indicates that the 109.5° (11̄2) Σ3 coherent twin boundary will have relatively low energy but high mobility. This result is attributed to a particularly effective repeated shuffle mechanism which occurs on the twinning plane. Two other migration mechanisms are observed, one involving the motion of 〈1 1 1〉 glissile dislocations in LAGB and the other involving uncorrelated atomic shuffles in HAGB, sometimes associated with interfacial steps. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.Peer Reviewe

Modelling solid solution hardening in high entropy alloys

Solid solution hardening (SSH) is one of the major contributions to the excellent mechanical properties displayed by high entropy alloys (HEAs). SSH is first analysed for binary systems in face-centred cubic and body-centred cubic alloys with different elemental additions in the temperature range 5-623 K. The prediction of the SSH has been possible by using Labush's approach for SSH modelling, where the necessary parameters have been incorporated without fitting to experimental data. Among these parameters, elastic misfit is shown to be prominent; experimental evidence suggests it has a dominant effect with respect to other misfit forms. Nevertheless, Labush's approach cannot be directly applied to model SSH in HEAs, since it is based on the misfit produced in the lattice of a solvent/reference atom, which does not exist in HEAs. Its extension to HEAs has been performed by using Mooren's approach for the computation of interatomic spacing in multicomponent alloys, allowing the creation of a model for elastic misfit in HEAs. This has led to a methodology for computing SSH effect in HEAs, where the results have successfully been compared with a collection of experimental data from the literature. The explanation of how different atoms can modify the yield strength can be formulated in terms of this approach