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  • Relaxation of rapidly quenched metallic glasses: Effect of the relaxation state on the slow low temperature dynamics

     Pineda Soler, Eloy; Bruna Escuer, Pere; Ruta, Beatrice; Gonzalez Silveira, Marta; Crespo Artiaga, Daniel
    Acta materialia
    Date of publication: 2013-05
    Journal article

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    The relaxation spectrum of rapidly quenched Mg65Cu25Y10 metallic glass ribbons is studied by mechanical spectroscopy at temperatures below and around the glass transition. The comparison between hyper-quenched and relaxed samples is used to examine the origin of the low temperature ¿excess wing¿ of internal friction commonly observed in mechanical spectroscopy of metallic glasses. The results show that the excess wing can be attributed to access of the system to the broad a-relaxation process while evidence of secondary relaxations is not found. This suggests that in this glassy system the activation energies of structural relaxation and low temperature deformation are directly related to the activation energy of the main relaxation process of the glassy state.

  • Relaxation dynamics and aging in structural glasses

     Ruta, Beatrice; Chushkin, Y.; Monaco, Giulio; Cipelletti, L.; Giordano, V.; Pineda Soler, Eloy; Bruna Escuer, Pere
    International Symposium on Slow Dynamics in Complex Systems
    Presentation's date: 2012-12
    Presentation of work at congresses

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  • Atomic-scale relaxation dynamics and aging in a metallic glass probed by X-ray photon correlation spectroscopy

     Ruta, Beatrice; Chushkin, Y.; Monaco, Giulio; Cipelletti, L.; Pineda Soler, Eloy; Bruna Escuer, Pere; Giordano, V.; Gonzalez Silveira, Marta
    Physical review letters
    Date of publication: 2012-10-17
    Journal article

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  • Influence of a magnetic field applied during the quenching process on the spin density and nanoscale structure of an amorphous Fe-B ribbon

     Bonastre, J.; Suñol Martinez, Juan Jose; Bruna Escuer, Pere; Sato, K.; Santos, J.D.; Hernando, B.
    Materials letters
    Date of publication: 2012-11-15
    Journal article

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    The application of a magnetic field to the melt in the transverse direction of the wheel rotation during the solidification process induces magnetic anisotropy in amorphous magnetic ribbons. This procedure is here applied to a FeB amorphous alloy in order to check their influence on local electronic and nanoscale structure by comparing two samples (as quenched without field and field quenched). The existence of magnetic domains was showed via the Bitter technique. Mössbauer spectra analysis confirms an increase in the spin density (from 28 to 48%) in the applied field direction for the field quenched ribbon. Positron lifetime spectra analysis determines that at room temperature there is no influence on the nanoscale structure, but the annealing at 300 °C provokes the beginning of the nanocrystallization process in the field quenched sample while the samples as quenched without field remain fully amorphous.

  • Inelastic X-ray scattering in metallic glasses

     Bruna Escuer, Pere; Serrano, Jorge; Pineda Soler, Eloy; Duarte, Maria Jazmin; Zhao, Kun; Wang, Wei Hua; Crespo Artiaga, Daniel
    Intermetallics
    Date of publication: 2012-11
    Journal article

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  • Communication: Are metallic glasses different from other glasses? A closer look at their high frequency dynamics

     Bruna Escuer, Pere; Baldi, Giacomo; Pineda Soler, Eloy; Serrano Gutierrez, Jorge; Suck, J.B.; Crespo Artiaga, Daniel; Monaco, Giulio
    Journal of chemical physics
    Date of publication: 2011-09-14
    Journal article

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  • Polyamorphic transitions in Ce-based metallic glasses by synchrotron radiation

     Duarte Correa, Maria Jazmin; Bruna Escuer, Pere; Pineda Soler, Eloy; Crespo Artiaga, Daniel; Garbarino, Gaston; Verbeni, Roberto; Zhao, Kun; Wang, Wei Hua; Romero, Aldo H; Serrano Gutierrez, Jorge
    Physical review B: condensed matter and materials physics
    Date of publication: 2011-12-01
    Journal article

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  • Acoustic properties of metallic glasses in the mesoscopic regime by inelastic X-ray scattering

     Bruna Escuer, Pere; Baldi, Giacomo; Pineda Soler, Eloy; Serrano Gutierrez, Jorge; Crespo Artiaga, Daniel; Monaco, Giulio
    Journal of alloys and compounds
    Date of publication: 2011-06
    Journal article

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  • Role of Mo in the local configuration and structure stabilization of amorphous steels, a Synchrotron X-ray diffraction and Mossbauer study

     Pineda Soler, Eloy; Bruna Escuer, Pere; Serrano, Jordi; Torrens Serra, Joan; Crespo Artiaga, Daniel
    Journal of alloys and compounds
    Date of publication: 2011-07
    Journal article

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    Amorphous steels are promising materials with potential structural applications. The glass-forming ability (GFA) and mechanical properties of metallic glasses are intimately related to the local structure. In Fe-based alloys, Cr andMocontentseemto play a key role in stabilizing the amorphous atomic-level structure. Here we present a study on the effects of changing Mo content in Fe72−xC7Si3.3B5.5P8.7Cr2.3Al2Mox amorphous steels.Westudy the local structure of these alloys by Synchrotron X-ray diffraction and Mössbauer spectroscopy. The results show how the amorphous phase evolves from a ferromagnetic Fe-rich structure to a structure with predominance of paramagnetic environments with the increase of Mo content. The changes in the distribution of magnetic environments cannot be attributed only to the Fe–Mo substitution but to a change of local configuration in the amorphous phase.

  • Producción de vidrios metálicos masivos. Estudio de la estructura local y estabilidad para la mejora de propiedades mecánicas 2

     Pineda Soler, Eloy; Bruna Escuer, Pere; Rojas Gregorio, Jose Ignacio; Jove Casulleras, Roger; Vallés Sales, Araceli; Renner, Frank U.; Madinehei, Seyed Milad; Crespo Artiaga, Daniel
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  • Effect of minor additions on the glass forming ability and magnetic properties of Fe-Nb-B based metallic glasses

     Torrens Serra, Joan; Bruna Escuer, Pere; Rodriguez Viejo, J.; Roth, S.; Clavaguera-Mora, M.T
    Intermetallics
    Date of publication: 2010-05
    Journal article

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  • Fragility measurement of Pd-based metallic glass by dynamic mechanical analysis

     Pineda Soler, Eloy; Serrano Gutierrez, Jorge; Bruna Escuer, Pere; Crespo Artiaga, Daniel
    Journal of alloys and compounds
    Date of publication: 2010-08
    Journal article

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    The measurement of fragility of glass-forming liquids involves either the determination of viscosity at different temperatures above the glass transition or the measurement of the dynamic glass–liquid transition by calorimetry or dilatometry at different heating rates. Significant data dispersion is observed when comparing fragility parameters obtained by different techniques. Dynamic mechanical analysis (DMA) is widely used for characterizing glass transition in polymer science while it is still seldom used in metallic glasses. In this work we determine the relaxation time ( ) vs temperature of a Pd77.5Si16.5Cu6 glass by means ofDMAmeasurements. The measured range of relaxation times cover the whole region from glass transition ( > 100 s) to times of the order of 10−2 s. The fragility parameter obtained for this alloy is found significantly higher than the values obtained from previous viscosity and calorimetric techniques. The application range, limitations and correctness of the technique are discussed.

  • Thermal and structural study of nanocrystalline Fe(Co)NiZrB alloys prepared by mechanical alloying

     Gonzalez, A; Escoda, L; Suñol Martinez, Juan Jose; Shao, G.; Arcondo, B.; Bruna Escuer, Pere
    Journal of materials science
    Date of publication: 2010-01
    Journal article

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    Three nanocrystalline alloys, Fe75-xCox(Ni70 Zr30)15B10 (x = 0, 10, and 20), were synthesized from elemental powders in a planetary high-energy ball mill. Their microstructure, magnetic properties, and thermal stability were characterized by X-ray diffraction, transmission Mössbauer spectroscopy, transmission electron microscopy, scanning electron microscopy, induction coupled plasma, vibrating sample magnetometry, and differential scanning calorimetry. After 80 h ofmilling, the nanocrystallites size of alloys is in the range 6–10 ± 1 nm. The lattice parameter decreases when increasing (decreasing) milling time (Fe content). Furthermore, the thermal stability of the nanocrystalline phase increaseswhen increasingCo concentration. The activation energy of themain crystallization process, between 275 ± 8 and 311 ± 10 kJ mol-1, is associated with grain growth. Slight contamination from milling tools and milling atmosphere was detected. Minor differences were detected after Mössbauer analysis.

  • Effect of minor Co additions on the crystallization and magnetic properties of Fe(Co)NbBCu alloys

     Torrens Serra, Joan; Bruna Escuer, Pere; Roth, S; Rodríguez Viejo, Javier; Clavaguera Mora, Maria Teresa
    Journal of alloys and compounds
    Date of publication: 2010
    Journal article

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  • High frequency dynamics of BMG determined by synchrotron radiation: A microscopic picture

     Serrano Gutierrez, Jorge; Pineda Soler, Eloy; Bruna Escuer, Pere; Labrado, Anna; Le Tacon, Mathieu; Krisch, M.; Monaco, Giulio; Crespo Artiaga, Daniel
    Journal of alloys and compounds
    Date of publication: 2010-04-16
    Journal article

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  • Bulk soft magnetic materials from ball-milled Fe77Nb7B15Cu1 amorphous ribbons

     Torrens-Serra, J; Bruna Escuer, Pere; Roth, S; Rodríguez-Viejo, J; Clavaguera-Mora, M.T
    Intermetallics
    Date of publication: 2009-01
    Journal article

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  • Structural and thermal changes induced by mechanical alloying in a Fe-Ni based amorphous melt-spun alloy

     Suñol Martinez, Juan Jose; Saurina, J & P Bruna; Bruna Escuer, Pere
    Materials chemistry and physics
    Date of publication: 2009
    Journal article

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  • Structural and magnetic characterization of FeNbBCu alloys as a function of Nb content

     Torrens-Serra, J; Bruna Escuer, Pere; Roth, S; Rodriguez-Viejo, J; Clavaguera-Mora, Mt
    Journal of physics D. Applied physics
    Date of publication: 2009-01
    Journal article

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  • Structural study of conventional and bulk metallic glasses during annealing

     Pineda Soler, Eloy; Hidalgo, I; Bruna Escuer, Pere; Pradell Cara, Trinitat; Labrador, A.; Crespo Artiaga, Daniel
    Journal of alloys and compounds
    Date of publication: 2009-01
    Journal article

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  • Phase-field modelling of microstructural evolution in primary crystallization

     Bruna Escuer, Pere; Pineda Soler, Eloy; Crespo Artiaga, Daniel; Rojas Gregorio, Jose Ignacio
    Journal of alloys and compounds
    Date of publication: 2009-08-26
    Journal article

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  • Single phase MnFe2O4 powders obtained by the polymerized complex method

     Popa, M; Bruna Escuer, Pere; Crespo Artiaga, Daniel; Moreno, Jose Maria
    Journal of the American Ceramic Society
    Date of publication: 2008-08
    Journal article

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  • Microstructural characterization and kinetics modelling of vermicular cast irons

     Karina, Laneri; Bruna Escuer, Pere; Crespo Artiaga, Daniel
    Materials science and technology
    Date of publication: 2008-10
    Journal article

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  • Structural evolution of metallic glasses during annealing through in-situ Synchrotron X-ray diffraction

     Pineda Soler, Eloy; Boneu, F; Bruna Escuer, Pere; Pradell Cara, Trinitat; Labrador, A; Crespo Artiaga, Daniel
    Journal of non-crystalline solids
    Date of publication: 2008-05
    Journal article

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  • Socio-economic benefits of using space technologies to monitor and respond to earthquakes

     Christensen, Ian A; Bruna Escuer, Pere; Rojas Gregorio, Jose Ignacio; Crespo Artiaga, Daniel
    59th International Astronautical Congress
    Presentation's date: 2008-09-29
    Presentation of work at congresses

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  • Mössbauer characterization of an amorphous steel with optimal Mo content

     Bruna Escuer, Pere
    International Workshop on Non-Crystalline Solids
    Presentation's date: 2008-04-27
    Presentation of work at congresses

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  • Producción de vidrios metálicos masivos. Estudio de la estructura local y estabilidad para la mejora de propiedades mecánicas

     Crespo Artiaga, Daniel; Bruna Escuer, Pere; Rojas Gregorio, Jose Ignacio; Pineda Soler, Eloy; Pradell Cara, Trinitat; Serrano Gutierrez, Jorge; Labrador, A.; Garrido Galera, Victor; Torres Heredia, Victor Elias; Flores Garcia, Juan Carlos
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  • Structural characterization of Fe-based BMGs by Mössbauer spectroscopy

     Pineda Soler, Eloy; Facchini, L; Bruna Escuer, Pere; Serrano, J.; Crespo Artiaga, Daniel
    The 13th International Conference on Rapidly Quenched & Metastable Materials
    Presentation of work at congresses

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  • Análisis de la aplicación de nuevas metodologias docentes en ingenieria

     Bruna Escuer, Pere; Calvete Manrique, Daniel; Crespo Artiaga, Daniel; Pilar, Gil; Gutierrez, Jordi; Torres Gil, Santiago; Alonso Maleta, Maria Aranzazu
    VI Taller Iberoamericano de Educación en Ciencia e Ingeniería de Materiales (TIECIM)
    Presentation's date: 2008-12-02
    Presentation of work at congresses

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  • Structural evolution of metallic glasses during annealing through in-situ synchroton X.-ray diffraction

     Bruna Escuer, Pere
    International Workshop on Non-Crystalline Solids
    Presentation's date: 2008-04-27
    Presentation of work at congresses

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  • Mechanical properties of BMG determined by Inelastic X-Ray Scattering

     Serrano, J.; Pineda Soler, Eloy; Bruna Escuer, Pere; Labrador, A; Tacon, M Le; Zhao, K; Krisch, M; Monaco, G; Crespo Artiaga, Daniel
    The 13th International Conference on Rapidly Quenched & Metastable Materials
    Presentation of work at congresses

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  • Mechanical properties of BMG determined by Synchrotron Radiation: a microscopic picture

     Serrano, J.; Pineda Soler, Eloy; Bruna Escuer, Pere; Labrador, A; Tacon, M Le; Krisch, M; Monaco, Giulio; Crespo Artiaga, Daniel
    ISMANAM 2008 International Symposium on Metastable and Nano Materials
    Presentation of work at congresses

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  • Mechanical properties of BMG determined by Inelastic X-Ray Scattering

     Serrano, J.; Pineda Soler, Eloy; Bruna Escuer, Pere; Labrador, A; Tacon, M Le; Zhao, K; Krisch, M; Monaco, G; Crespo Artiaga, Daniel
    NOCLAMS 2008 New opportunities and challenges for liquid and amorphous materials science
    Presentation of work at congresses

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  • Study of crystallization process of Fe65Nb10B25 and Fe70Nb10B20 glassy metals

     Torrens-Serra, J; Bruna Escuer, Pere; Rodrguez-Viejo, J; Pradell Cara, Trinitat; Clavaguera-Mora, T Pradell & M T
    Reviews on advanced materials science
    Date of publication: 2008-05
    Journal article

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  • Microstructure description from the differential Avrami formulation

     Crespo Artiaga, Daniel; Bruna Escuer, Pere; Pineda Soler, Eloy; Pradell Cara, Trinitat
    Reviews on advanced materials science
    Date of publication: 2008-05
    Journal article

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  • Mössbauer characterization of an amorphous steel with optimal Mo content

     Facchini, L; Bruna Escuer, Pere; Pineda Soler, Eloy; Crespo Artiaga, Daniel
    Journal of non-crystalline solids
    Date of publication: 2008-05
    Journal article

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  • Mechanical properties of BMG determined by Synchrotron Radiation: a microscopic picture

     Serrano, J.; Pineda Soler, Eloy; Bruna Escuer, Pere; Labrador, A; Tacon, M; Krisch, M; Monaco, Giulio; Crespo Artiaga, Daniel
    Journal of alloys and compounds
    Date of publication: 2008-01
    Journal article

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  • Phase-field modeling of glass crystallization: Change of the transport properties and crystallization kinetic

     Bruna Escuer, Pere; Pineda Soler, Eloy; Crespo Artiaga, Daniel
    Journal of non-crystalline solids
    Date of publication: 2007-04
    Journal article

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  • MICROSTRUCTURAL CHARACTERIZATION AND MODELLING IN PRIMARY CRYSTALLIZATION  Open access

     Bruna Escuer, Pere
    Defense's date: 2007-11-15
    Department of Applied Physics, Universitat Politècnica de Catalunya
    Theses

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    L'objectiu de la tesi és estudiar la cinètica de les cristal·litzacions primàries en vidres metàl·lics mitjançant simulacions de tipus phase field. Una cristal·lització primària és una transició de fase sòlid-sòlid on la fase que cristal·litza (fase transformada o fase secundaria) té una composició química diferent de la fase precursora (fase no transformada o fase primària).Les dades experimentals obtingudes a partir de l'estudi calorimètric de cristal·litzacions primàries s'analitzen generalment en el marc del model KJMA (Kolmogorov, Johnson & Mehl, Avrami). Aquest model proporciona l'evolució temporal de la fracció transformada basant-se en tres hipòtesis: - Els nuclis de la fase secundaria estan distribuïts aleatòriament en tot l'espai.- El creixement d'aquests nuclis és isotròpic.- El creixement s'atura únicament per xoc directe (hard impingement).En la cristal·lizació de vidres metàl·lics s'ha observat experimentalment un alentiment de la cinètica respecte del comportament calculat emprant la citada cinètica KJMA. Aquest alentiment s'explica a la literatura en base a que en aquest tipus de transformacions, controlades per difusió, la interacció entre els cristalls no és directa sinó que es produeix a través dels perfils de concentració (soft impingement) i, a més, l'evolució d'aquests perfils de concentració causa canvis en la concentració de la matriu amorfa, estabilitzant la i per tant fent que la nucleació de nous cristalls esdevingui no aleatòria. Diversos autors han proposat modificacions del model KJMA per tal d'intentar superar aquestes limitacions, basats bé en consideracions geomètriques, bé en aproximacions de camp mitjà. A pesar de tot, cap d'aquests models és capaç d'explicar satisfactòriament la cinètica observada en cristal·litzacions primàries. L'objectiu d'aquest treball ha estat la simulació realista de la cinètica de les transformacions primàries per trobar una explicació consistent a les diferències observades entre les dades experimentals i els models teòrics disponibles.Per tal de poder descriure de forma realista el procés de cristal·lització primària s'ha d'estudiar el procés de nucleació i creixement de la fase secundaria alhora que es resol l'equació de difusió en la fase primària. En aquest treball s'ha emprat un model de simulació phase field que permet estudiar aquest sistema introduint una nova variable lligada al camp de concentració que pren dos valors diferents segons es tracti de fase transformada o no transformada. Amb aquest tipus de models també es poden introduir diferents protocols de nucleació i per tant estudiar independentment els efectes de la nucleació en la cinètica. D'aquesta manera s'han realitzat simulacions en 2 i 3 dimensions de cristal·litzacions primàries amb diferents graus de fracció transformada final). Els resultats de les simulacions s'ha comparat amb el model KJMA i, contra el que es preveia, s'ha obtingut un bon acord entre les fraccions transformades del model KJMA i de les simulacions. Donat que el model KJMA no reprodueix satisfactòriament el comportament experimental d'aquest resultat es dedueix que ni el soft impingement ni la nucleació no aleatòria son les responsables de l'alentiment de la cinètica obtingut en cristal·litzacions primàries. Per tal de trobar una explicació físicament convincent del comportament observat experimentalment s'ha aprofundit en l'estudi teòric de les cristali·litzaciones primàries, incloent-hi l'efecte dels canvis composicionals que tenen lloc en la matriu a mesura que la transformació es produeix. Aquest fet, tot i ser conegut a la bibliografia, ha estat sistemàticament ignorat en l'elaboració de models cinètics. En concret, s'ha fet palès que canvis en la composició química de la fase primària han d'afectar de forma radical a la viscositat, que varia fortament a prop de la transició vitrea, i han de produir canvis en les propietats de transport atòmic. Això s'ha modelat a través de l'assumpció d'un coeficient de difusió depenent de la concentració, en base a la relació modificada d'Stokes-Einstein entre la viscositat i el coeficient de difusió. Les simulacions phase-field amb un coeficient de difusió d'aquest tipus donen lloc a una cinètica més lenta i que mostra un acord excel·lent amb la cinètica experimentalment observada en cristal·litzacions primàries de vidres metàl·lics. Per tant, les simulacions phase field confirmen que la cinètica de les cristal·litzacions primàries està controlada fonamentalment pel canvi en les propietats de transport atòmic, mentre que els efectes de soft impingement i nucleació no aleatoria, tot i estar presents, son secundaris.

    El objetivo de la tesi es estudiar la cinética de las cristalizaciones primarias en vidrios metálicos mediante simulaciones de tipo phase field. Una cristalización primaria es una transición de fase sólido-sólido donde la fase que cristaliza (fase transformada o fase secundaria) tiene una composición química diferente a la fase precursora (fase no transformada o fase primaria).Los datos experimentales obtenidos a partir del estudio calorimétrico de cristalizaciones primarias se analizan generalmente en el marco del modelo KJMA (Kolmogorov, Johnson & Mehl, Avrami). Este modelo proporciona la evolución temporal de la fracción transformada basándose en tres hipótesis: - Los núcleos de la fase secundaria están distribuidos aleatoriamente en todo el espacio- El crecimiento de estos núcleos es isotrópico- El crecimiento se detiene únicamente por choque directo (hard impingement).En la cristalización de vidrios metálicos se ha observado experimentalmente un retardo de la cinética respecto del comportamiento calculado usando la cinética KJMA. Este retardo se explica en la literatura en base a que en este tipo de transformaciones, controladas por difusión, la interacción entre los cristales no es directa sino que se produce a través de los perfiles de concentración (soft impingement) y, además, la evolución de estos perfiles de concentración causa cambios en la concentración de la matriz amorfa, estabilizándola y por tanto haciendo que la nucleación de nuevos cristales sea no aleatoria. Varios autores han propuesto modificaciones del modelo KJMA para intentar superar estas limitaciones, basados bien en consideraciones geométricas, bien en aproximaciones de campo medio. A pesar de todo, ninguno de estos modelos es capaz de explicar satisfactoriamente la cinética observada en cristalizaciones primarias. El objetivo de este trabajo ha sido la simulación realista de la cinética de las transformaciones primarias para hallar una explicación consistente a las diferencias entre los datos experimentales y los modelos teóricos disponibles.Para describir de manera realista el proceso de cristalización primaria se tiene que estudiar el proceso de nucleación y crecimiento de la fase secundaria a la vez que se resuelve la ecuación de difusión en la fase primaria. En este trabajo se ha usado un modelo de simulación phase-field que permite estudiar este sistema introduciendo una nueva variable ligada al campo de concentración que toma dos valores diferentes según se trate de fase transformada o no transformada. Con este tipo de modelos también se pueden introducir diferentes protocolos de nucleación y por tanto estudiar independientemente los efectos de la nucleación en la cinética. De esta manera se han realizado simulaciones en 2 y 3 dimensiones de cristalizaciones primarias con diferentes grados de fracción transformada final. Los resultados de la simulaciones se han comparado con el modelo KJMA y, en contra de lo que se preveía, se ha obtenido un buen acuerdo entre las fracciones transformadas del modelo KJMA y de las simulaciones. Dado que el modelo KJMA no reproduce satisfactoriamente el comportamiento experimental, de este resultado se deduce que ni el soft impingement ni la nucleación no aleatoria son las responsables del retardo en la cinética obtenido en cristalizaciones primarias.Para encontrar una explicación físicamente convincente del comportamiento observado experimentalmente se ha profundizado en el estudio teórico de las cristalizaciones primarias, incluyendo el efecto de los cambios composicionales que tienen lugar en la matriz a medida que la transformación se produce. Este hecho, aún y ser conocido en la bibliografía, ha sido sistemáticamente ignorado en la elaboración de modelos cinéticos. En concreto, se ha hecho patente que cambios en la composición química de la fase primaria tienen que afectar de forma radical a la viscosidad, que varía fuertemente cerca de la transición vítrea, y tienen que producirse cambios en las propiedades de transporte atómico. Esto se ha modelado a través de la asunción de un coeficiente de difusión dependiente de la concentración, en base a la relación de Stokes-Einstein modificada entre la viscosidad y el coeficiente de difusión. Las simulaciones phsae-field con un coeficiente de difusión de este tipo dan lugar a una cinética más lenta y que muestra un acuerdo excelente con la cinética experimentalmente observada en cristalizaciones primarias de vidrios metálicos. Por tanto, las simulaciones phase-field confirman que la cinética de las cristalizaciones primarias está controlada fundamentalmente por los cambios en las propiedades de transporte atómico, mientras que los efectos de soft-impingement y nucleación no aleatoria, aún y estar presentes, son secundarios.

    The aim of this thesis is to study the kinetics of primary crystallization in metallic glasses by means of phase-field simulations. A primary crystallization is a solid-solid phase transformation where the crystallized phase (transformed phase or secondary phase) has a chemical composition different than the precursor phase (untransformed phase or primary phase).Experimental data from calorimetric studies of primary crystallization are usually studied in the framework of the KJMA model (Kolmogorov, Johnson & Mehl, Avrami). This model yields the temporal evolution of the transformed fraction on the basis of three main assumptions: - A random distribution of particle nuclei of the secondary phase- The growth of these nuclei is isotropic- The growth is only halted by direct collisions (hard impingement).In the crystallization of metallic glasses, a slowing down of the kinetics respect the behavior calculated with the KJMA kinetics has been observed. This delay is explained in the literature by the fact that in this kind of transformations, that are diffusion controlled, the interaction between the crystals is not direct but through the concentration profiles (soft impingement) and moreover, the evolution of these profiles causes changes in the concentration of the amorphous matrix, stabilizing it and thus, the nucleation of new nuclei become non random. Several authors had proposed modifications to the KJMA model to try to overcome these limitations, based either on geometrical considerations or in mean field approaches. However, none of these models is able to explain the observed kinetics in primary crystallizations. The aim of this work has been the realistic simulation of the kinetics of primary crystallization to find a explanation to the differences between the experimental data and the available theoretical models.In order to describe in a realistic way the process of a primary crystallization, the nucleation and growth process of the secondary phase has to be studied at the same time that the diffusion equation is solved in the primary phase. In this work, it has been used a phase field model for the simulations that allows to study this system introducing a new variable, coupled to the concentration field, that takes two different values in each of the existing phases. With these kinds of models, different nucleation protocols can also be introduced and thus, independently study the effects of the nucleation in the kinetics. Therefore, 2 and 3 dimensional simulations of primary crystallization have been performed with several degrees of final transformed fraction. The simulation results have been compared with the KJMA model and, unexpectedly, a good agreement between the simulations and the KJMA model has been obtained. As the KJMA model does not reproduce satisfactorily the experimental behavior, from this result can be deduced that neither the soft impingement nor the non random nucleation are the responsible of the slowing down observed in the kinetics of primary crystallization.In order to find a physical convincing explanation of the observed experimental behavior, the theoretical study of primary crystallization has been extended, including the effects of the compositional changes that take place in the matrix as the transformation proceed. This fact, notwithstanding being known in the literature, has been systematically ignored in the development of the kinetics models. In particular, it has become clear that changes in the chemical composition of the primary phase have to radically affect the viscosity, that strongly varies near the glass transition, and some changes in the atomic transport properties must occur. This has been modeled through the assumption of a compositional dependent diffusion coefficient, on the basis of a modified Stokes-Einstein relation between viscosity and diffusion coefficient. Phase field simulations with a diffusion coefficient of this type yield a slower kinetics and show an excellent agreement with the kinetics experimentally observed in primary crystallization of metallic glasses. Thus, phase field simulations confirm that the kinetics of primary crystallization is fundamentally controlled by the changes in the atomic transport properties, while the soft impingement and non random effects, although being present, are secondary.

  • Phase-field modelling of microstructural evolution in primary crystallization

     Bruna Escuer, Pere
    14th International Symposium on Metastable and Nano Materials (ISMANAM 2007)
    Presentation's date: 2007-08-26
    Presentation of work at congresses

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  • Thermal and structural study of nanocrystalline Fe(Co)NiZrB alloys prepared by mechanical alloying

     Bruna Escuer, Pere
    14th International Symposium on Metastable and Nano Materials (ISMANAM 2007)
    Presentation's date: 2007-08-26
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  • A New Approach for Modelling Recrystallization Kinetics

     Bruna Escuer, Pere
    EUROMAT 2007. European Congress and Exhibition of Advanced Materials and Processes
    Presentation's date: 2007-09-10
    Presentation of work at congresses

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  • Magnetic and structural analysis of soft magnetic alloys as quenched and as milled

     Bruna Escuer, Pere
    Soft Magnetic Materials Conference
    Presentation's date: 2007-09-02
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  • A new approach for modelling recrystallization kinetics

     Pineda Soler, Eloy; Bruna Escuer, Pere; Rojas Gregorio, Jose Ignacio; Crespo Artiaga, Daniel
    EUROMAT 2007. European Congress and Exhibition of Advanced Materials and Processes
    Presentation's date: 2007-09-10
    Presentation of work at congresses

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  • Erratum: Cell size distribution in random tessellations of space [Phys. Rev. E 70, 066119 (2004)]

     Pineda Soler, Eloy; Bruna Escuer, Pere; Crespo Artiaga, Daniel
    Physical review E, statistical physics, plasmas, fluids, and related interdisciplinary topics
    Date of publication: 2007-11
    Journal article

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  • On the validity of Avrami formalism in primary crystallization

     Bruna Escuer, Pere; Crespo Artiaga, Daniel; Gonzalez Cinca, Ricardo; Pineda Soler, Eloy
    Journal of applied physics
    Date of publication: 2006-09
    Journal article

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  • Phase-field modelling of glass crystallization: change of the transport properties and crystallization kinetic

     Bruna Escuer, Pere
    International Workshop on Non-Crystalline Solids
    Presentation's date: 2006-06-20
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  • Influence of Ni content in the nanocrystalline behaviour of several Fe based alloys

     Bruna Escuer, Pere
    9th European Symposium on Thermal Analysis and Calorimetry
    Presentation's date: 2006-08-27
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  • Aanalitzador termo-mecànic

     Crespo Artiaga, Daniel; Bruna Escuer, Pere; Pineda Soler, Eloy; Rojas Gregorio, Jose Ignacio
    Participation in a competitive project

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  • Microstructure description from the differential Avrami formulation

     Bruna Escuer, Pere
    13th International Symposium on Metastable and Nano Materials. ISMANAM 2006
    Presentation's date: 2006-08-27
    Presentation of work at congresses

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