Novel sintering technique for oxide dispersion strengthened steels

Los aceros reforzados por dispersión de óxidos (ODS steels, del inglés Oxide Dispersion Strengthened) forman parte de una clase de materiales con interesantes propiedades mecánicas a alta temperatura, como así también adecuada estabilidad en ambientes sometidos a distintos tipos de radiaciones. Estas características transforman a estas aleaciones en candidatas para su uso en la industria de generación de energía, en particular en la industria nuclear. Sin embargo, estos materiales no pueden ser preparados en hornos de fusión, por las altas temperaturas involucradas y por la necesidad de tener una dispersión fina de las partículas del óxido. Por esta razón, la forma de síntesis más usada es mediante molienda mecánica seguida por algún proceso de conformado, que normalmente incluye el sinterizado de las partículas y la reacción de los elementos involucrados. En esta etapa se pondrá a punto un nuevo método de sinterizado, similar al conocido en inglés como Spark Plasma Sintering, usando pulsos de corriente eléctrica de alta intensidad.Oxide Dispersion Strengthened (ODS) steels are members of a class of materials with interesting high-temperature mechanical properties, as well as an adequately stability in environments with different kinds of radiations. These characteristics allow to consider these alloys as candidates for its use in the energy-generating industry, in particular in the nuclear sector. However, these materials cannot be prepared by conventional furnace melting, due to the very high temperatures involved and the need of a fine dispersion of the oxide particles. For this reason, the commonly used synthesis method is mechanical milling followed by some forming process, which normally includes particle sintering and reaction of the elements in play. At this stage a new sintering method, similar to Spark Plasma Sintering, will be developed using high intensity electric current pulses

Efectos psicosociales de los videojuegos

Desde su aparición en los años 70, los posibles efectos psicosociales de los videojuegos, positivos o negativos, han sido objeto de una considerable polémica. La mayoría de los argumentos en uno u otro sentido se han basado más en opiniones personales y prejuicios que en los datos de investigación. En este artículo revisamos el estado de la cuestión en relación con estos argumentos, tanto negativos (adicción, agresividad, aislamiento social, rendimiento escolar, desplazamiento de otras actividades, conducta delictiva o antisocial, juego patológico, consumo de sustancias, trastornos médicos) como positivos (entrenamiento y mejora de habilidades, utilidades terapéuticas, uso como medio didáctico).The possible positive or negative psycho-social effects of the video games have been subjected to polemics since they appeared back in the 1970s. Most of the arguments in one or the other direction have been supported more in personal opinions and prejudices than in research data. This article reviews the state of the art with regards to these arguments, both negative (addiction, aggression, social isolation, school performance, displacement of other activities, criminal or antisocial behaviour, pathological gambling, use of substances, medical conditions) and positive (training and improvement of skills, therapeutic uses, use as didactic tools)

A simple model to predict long-range atomic ordering temperatures in Cu-based shape memory alloys

The order–disorder and order–order phase transition temperatures in the austenitic phase of Cu-based shape memory alloys were used to obtain a set of first- and second-neighbour pair interchange energies. To this end, a mean field model was postulated. Then, the applicability to different alloys of this simple model was analysed. It was found that a good agreement with the experimental phase diagram is obtained for Cu–Zn–Al, Cu–Al–Ni and Cu–Al–Be alloys using composition-independent parameters. It was also found that for Cu–Al–Mn alloys, composition-dependent pair interchange energies need to be employed.Fil: Pelegrina, Jorge Luis. Comision Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Física de Metales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

The “critical limits for crystallinity” in nanoparticles of the elements: A combined thermodynamic and crystallographic critique

The theme of the present work is the procedure for evaluating the minimum size for the stability of a crystalline particle with respect to the same group of atoms but in the amorphous state. A key goal of the study is the critical analysis of an extensively quoted paper by F.G. Shi [J. Mater. Res. 9 (1994) 1307–1313], who presented a criterion for evaluating a “crystallinity distance” (h) through its relation with the “critical diameter” (dC) of a particle, i.e., the diameter below which no particles with the crystalline structure are expected to exist at finite temperatures. Key assumptions of Shi's model are a direct proportionality relation between h and dC, and a prescription for estimating h from crystallographic information. In the present work the accuracy of the Shi model is assessed with particular reference to nanoparticles of the elements. To this end, an alternative way to obtain h, that better realizes Shi's idea of this quantity as “the height of a monolayer of atoms on the bulk crystal surface”, is explored. Moreover, a thermodynamic calculation of dC, which involves a description of the bulk- and the surface contributions to the crystalline/amorphous relative phase stability for nanoparticles, is performed. It is shown that the Shi equation does not account for the key features of the h vs. dC relation established in the current work. Consequently, it is concluded that the parameter h obtained only from information about the structure of the crystalline phase, does not provide an accurate route to estimate the quantity dC. In fact, a key result of the current study is that dC crucially depends on the relation between bulk- and surface contributions to the crystalline/amorphous relative thermodynamic stability.Fil: Pelegrina, Jorge Luis. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Fernandez Guillermet, Armando Jorge. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Assessment of entropy differences from critical stress versus temperature martensitic transformation data in Cu-based shape-memory alloys

The entropy differences per unit volume (ΔStrans) between the close-packed phases in a martensitic transformation (MT) in Cu-based shape-memory alloys are obtained from mechanical tests by measuring, as a function of temperature (T), the critical resolved stress (τ). Specifically, Δ Strans values are obtained from the slope of τ versus T plots by invoking a relation which is straightforwardly derived from the classical Clausius–Clapeyron equation, viz., dτdT=-ΔStransγ, where γ is the transformation shear strain. Motivated by the significant scatter of the so obtained Δ Strans values, the thermodynamic bases of such evaluation procedure have been revised, by accounting for the nucleation step of a martensite plate. The interface, elastic strain, and chemical contributions to the Gibbs energy of nucleation have been considered. A new expression of the type dτdT=Ω-ΔStransγ is obtained, where the Ω term involves the elastic properties and their temperature dependence. The new τ- T- Δ Strans relation is used to assess the Δ Strans values corresponding to the 2H/18R and 18R/6R MTs in Cu–Al–Ni and Cu–Zn–Al alloys. The ΔStrans values obtained by the present approach fall on a scatter band centered around the zero value.Fil: Pelegrina, Jorge Luis. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Condo, Adriana Maria. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Fernandez Guillermet, Armando Jorge. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin

Comment on: Isothermal nature of the B2-B19 martensitic transformation in a Ti51.2 Ni (at.%) alloy

Fukuda et al. published a paper (Scripta Mater. 68 (2013) 984-987) in which they claimed to be able to determine the isothermal nature of the martensitic transformation in a Ti–Ni alloy. They analyzed the evolution of electrical resistance during isothermal holding stages and concluded that the variations were a consequence of the isothermal transition between austenite and martensite. In the following we will demonstrate that the discussed data do not allow the interpretation presented and that the isothermal nature is not proved.Fil: Pelegrina, Jorge Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Yawny, Alejandro Andres. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Olbricht, J.. Bundesanstalt für Materialforschung und -prüfung; Alemani

The limitations of electrical resistance for accurate positioning of shape-memory actuators: The case of well-oriented CuZnAl single-crystals under uniaxial loading

The possibility of using the electrical resistance as a feedback signal for precise positioning in the design of compact shape-memory actuators was revised. The analysis was performed on well-oriented CuZnAl single crystalline shape-memory alloys subjected to uniaxial tensile loadings, combining direct strain measurements determined with an extensometer and the four-lead electrical resistance technique. In that way the relationship between strain and electrical resistance could be evaluated by formulating a physics based model that can be easily contrasted with the experimental characterization of a simple benchmark system. It was found that the model here developed provides general guidelines for the selection of a shape-memory material for positioning actuators and that the correlation between strain and electrical resistance is of rather limited precision, even for the simplest material case and loading condition analyzed. The effects of temperature, deformation rate, temperature dependence of the electrical resistivity of austenite and martensite phases and temperature dependence of the strain associated with the stress induced transformation were carefully characterized. Results suggest that the direct use of feedback from electrical resistance measurements for precise positioning is arguable unless materials with low hysteresis and low latent heat of transformation are used in combination with quasi-static operating conditions.Fil: Bertolino, Graciela Mabel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Yawny, Alejandro Andres. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Pelegrina, Jorge Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentin

Temperature-induced transformations and martensitic reorientation processes in ultra-fine-grained Ni rich pseudoelastic NiTi wires studied by electrical resistance

Temperature-induced, stress-induced martensitic phase transitions and martensite reorientation process in Ni rich (50.9 at.%) NiTi pseudoelastic NiTi wires with ultra-fine grained (UFG) microstructure were studied by electrical resistance measurements. Measurements of the electrical resistance as a function of temperature at different constant mechanical loads accompanied by complementary experiments with variable loads at constant temperature were performed. Results show that the transformation mechanisms in UFG microstructures exhibit a higher level of complexity when compared with those characterizing the behavior of other microstructures (e.g., recrystallized or larger grains size). It was found that a threshold stress level below 150 MPa delimits the transition from a homogeneous (low stress) to localized but reversible Lüders type transformation (high stress) when the transformations are induced under constant applied stress and that reorientation processes require stresses of 100 MPa in the present UFG wires. Even though the strain evolutions do not always show two distinct yielding events during cooling or heating, electrical resistance measurements proved that a two-step transformation involving R-phase and B19′ martensite was always present in the extended range of temperatures and stresses investigated here.Fil: Pelegrina, Jorge Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comision Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Física de Metales; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Olbricht, J.. Ruhr Universität Bochum; AlemaniaFil: Yawny, Alejandro Andres. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comision Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Física de Metales; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Eggeler, G.. Ruhr Universität Bochum; Alemani

Diffusive Phenomena and the Austenite/Martensite Relative Stability in Cu-Based Shape-Memory Alloys

The main characteristic of martensitic phase transitions is the coordinate movement of the atoms which takes place athermally, without the contribution of diffusion during its occurrence. However, the impacts of diffusive phenomena on the relative stability between the phases involved and, consequently, on the associated transformation temperatures and functional properties can be significant. This is particularly evident in the case of Cu-based shape-memory alloys where atomic diffusion in both austenite and martensite metastable phases might occur even at room-temperature levels, giving rise to a variety of intensively studied phenomena. In the present study, the progresses made in the understanding of three selected diffusion-related effects of importance in Cu–Zn–Al and Cu–Al–Be alloys are reviewed. They are the after-quench retained disorder in the austenitic structure and its subsequent reordering, the stabilization of the martensite, and the effect of applied stress on the austenitic order. It is shown how the experimental results obtained from tests performed on single crystal material can be rationalized under the shed of a model developed to evaluate the variation of the relative stability between the phases in terms of atom pairs interchanges.Fil: Pelegrina, Jorge Luis. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comision Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Física de Metales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Yawny, Alejandro Andres. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comision Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Física de Metales; ArgentinaFil: Sade Lichtmann, Marcos Leonel. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comision Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Física de Metales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin

Transformation activity in ultrafine grained pseudoelastic NiTi wires during small amplitude loading/unloading experiments

Martensitic phase transformations were studied in ultrafine grained Ni-rich pseudoelastic NiTi wires during cyclic deformation under small imposed strain amplitudes. Small strain variation tests were complemented by a thermographic analysis of the emerging temperature distributions in the specimens. The characteristics of the observed thermal profiles result from the specific phase transitions which take place at different stress levels. Homogeneous temperature changes, corresponding to a non-localized transformation activity along the specimen length, were observed throughout the whole range of applied stresses, starting from values as low as 100 MPa. This behavior is in line with previous literature reports for the stress-induced transformation from B2 austenite to R-phase as long as the critical stress for B19′ martensite formation is not reached. In the present study, similar type of transformation activity could also be demonstrated at higher strains/stresses, even after the stress induced transformation to B19′ was apparently completed. These findings suggest that transformation activity involving the B2 phase is present throughout the whole pseudoelastic stress-strain cycle; i.e., it is not restricted to the initial loading portion. Finally, non-localized transformation to or from B19′ was identified during small amplitude strain variations in the plateau-like coexistence ranges of the pseudoelastic cycle.Fil: Pelegrina, Jorge Luis. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Yawny, Alejandro Andres. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Olbricht, J.. Ruhr University Bochum; AlemaniaFil: Eggeler, G.. Ruhr University Bochum; Alemani