Primary Transformation Rate Measurements Through Differential Scanning Calorimetry

The primary crystallization of molten alloy systems at high undercooling is studied by a precise quantitative analysis of the calorimetric signal obtained during the transformation in terms of the reaction rate under isothermal and continuous heating regimes. It is shown that, under specific conditions, namely, stoechiometric primary precipitates, generalized relationships for the crystallization enthalpy and the reaction rate may be obtained.     

Residual entropy of glasses and their relation with the crystallization behaviour

For the alloy of eutectic composition in the quasibinary GeSe₂-Sb₂Se₃ system the heat capacities of the glass, undercooled and stable liquid and crystalline phases have been measured using differential scanning calorimetry. Enthalpy, entropy and Gibbs free energy differences of vitreous and crystalline forms have been deduced for this alloy. The kinetic parameters of crystallization for glasses of nearby composition have been obtained by continuous heating experiments. The effective activation energy obtained has a value that accounts for the activation energy for nucleation calculated using simple nucleation theory formulae.

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Zusammenfassung Die Wärmekapazitäten von Glas, unterkühlter und stabiler Flüssigkeit und kristallinen Phasen von Mischungen der eutektischen Zusammensetzung des quasibinären Systems GeSe₂-Sb₂Se₃ werden mittels DSC gemessen. Aus den Ergebnissen werden Enthalpie, Entropie und freie Enthalpie der glasartigen und kristallinen Formen dieser Mischung abgeleitet. Die kinetischen Parameter der Kristallisation von Gläsern benachbarter Zusammensetzungen wurden durch temperaturprogrammierte Untersuchungen ermittelt. Die erhaltenen effektiven Aktivierungsenergien entsprechen dem für die Keimbildung nach einfachen theoretischen Formeln berechneten Wert.

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, , GeSe₂-Sb₂Se₃. , . . , .

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This research was supported by the Comisión Asesora de Investigatión Cientifica y Técnica (CAICYT) project no. 759-84 which is gratefully acknowledged.     

Crystallization kinetic studies: A means to evaluate time-temperature-transformation curves. Application to metallic glasses

It is shown that the study of the kinetics of crystallization of an amorphous alloy may be used to obtain the correct form of the low temperature part of the time-temperature-transformation (T-T-T) curves. A unified review of the current kinetics of crystallization studies is presented and the general kinetic equation which gives the reaction rate as a function of temperature and crystalline fraction is shown to contain information related to theT-T-T-curves. Finally examples of application to two metallic glasses, Nd₃Fe₇₇B₂₀ and Fe_67.5Co₁₅Nb_1.5B₁₆, obtained by rapid solidification techniques are presented. The comparison between theory and experiment is satisfactory.

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Zusammenfassung Es wird gezeigt, daß die Untersuchung der Kristallisationskinetik von amorphen Legierungen genutzt werden kann, die korrekte Form des Niedertemperaturbereiches der Zeit-Temperatur-Transformation (T-T-T) Kurven zu erhalten. Es wird ein einheitlicher Überblick über die jüngsten Untersuchungen von Kristallisationskinetiken gegeben und an der allgemeinen Gleichung der Reaktionsgeschwindigkeit als eine Funktion von Temperatur und Kristallanteil wird gezeigt, daß sie Informationen bezüglich der T-T-T-Kurven beinhaltet. Letztlich wird eine Anwendung an zwei, durch schnelles Erstarren gewonnenen metallischen Gläsern Nd₃Fe₇₇B₂₀ und Fe_67.5Co₁₅Nb_1.5B₁₆ dargelegt. Zwischen Theorie und Praxis konnte eine ausreichende Übereinstimmung festgestellt werden.

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Dedicated to Professor Dr. H. J. Seifert on the occasion of his 60^th birthday

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This work was supported by the Comision Interdepartamental de Cienca y Tecnologia, CICYT (project No. MAT88-0439) which are acknowledged.     

Glass Transition Behavior in Se-Rich Ge-Se Alloys

Differential scanning calorimetry (DSC) was utilized to study the behavior of six Ge-Se glasses containing 0, 5, 10, 15, 17.5 and 20 at.% Ge during the glass transition. These alloys readily form glasses and can be prepared by quenching in air. Moreover, their behavior depends greatly on the composition. This work reveals that two additional properties must be considered: the variation in the glass transition temperature and different structural relaxation. The quantity used to quantify the relaxation was the enthalpy relaxation as this measures the heat lost by the glass during annealing. Given the complexity of the relaxation process, the experimental results were analysed by means of the empirical Kohlrausch-Williams-Watts relaxation model. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal Stability Study of Fe-Ni-Based Alloys Determination of T-HR-T and T-T-T diagrams

The low-temperature parts of the temperature-heating rate-transformation (T-HR-T) and temperature-time transformation (T-T-T) diagrams were obtained for crystallization processes. A knowledge of the kinetic model governing crystallization is not needed because both transformation curves can be obtained from non-isothermal calorimetric experiments. The calorimetric study was performed by means of differential scanning calorimetry. The method was applied to analyse crystallization processes of Fe-Ni-based amorphous alloys prepared by melt spinning. The compositions studied were Fe₄₀Ni₄₀P₁₄Si₆, Fe₄₀Ni₄₀P₁₀Si₁₀ and Fe₄₀Ni₄₀P₆Si₁₄. A good concordance was observed between the experimental T-HR-T curves obtained by calculation and the experimental data, which verifies the reliability of the method. In the T-T-T diagrams, the agreement was good in process B1, while in processes A1 and C1 there are small differences that could be related to different crystallization products obtained in isothermal/non-isothermal experiments. This revised version was published online in July 2006 with corrections to the Cover Date.