N. Clavaguera

Crystallisation kinetics and microstructure development in metallic systems

The primary crystallisation of a highly undercooled/supersaturated liquid is considered, and the application to nanocrystallisation by heat treatment of metallic glasses is studied from the thermodynamic, kinetic and microstructural point of view. The thermodynamic evolution is modelled assuming transformation rates low enough to ensure thermal equilibrium to be almost achieved. A mean field approximation is used, which allows us to determine the time evolution of the kinetic variables governing the transformation. The interplay between interface and diffusion controlled growth rate is studied, and both nucleation and crystal growth changes within the transformation are considered as soft mechanisms. The kinetics of the transformation is described in the framework of the Kolmogorov, Johnson and Mehl and Avrami (KJMA) model, which is adequately generalized for primary transformations. The microstructural evolution is described by a populational model, also based on KJMA. The predicted kinetic evolution results are compared to the experimental data on the primary nanocrystallisation of a FINEMET alloy.

Diffusion controlled grain growth in primary crystallization: Avrami exponents revisited

The anomalous behaviour of the Avrami exponents found in the primary crystallization of amorphous alloys leading to nanostructured materials is considered. A kinetic model able to adequately treat such phase transformation has been formulated by means of the implementation of a soft-impingement diffusion mechanism after a transient interface controlled growth. A decrease in the nucleation rate as crystallization proceeds has also been considered. Comparison of the model with experimental data is performed, giving excellent agreement. The soft-impingement diffusion mechanism is demonstrated to be responsible for the anomalous behaviour of the Avrami exponent, the decreasing nucleation rate being a second-order effect.

Glass formation and crystallization in the GeSe2-Sb2Te3 system

The glass formation and crystallization of liquid-quenched (GeSe2)100-y/(Sb2Te3)y alloys was investigated by means of differential scanning calorimetry, X-ray diffraction and optical and scanning electron microscopy. By water quenching glasses are obtained from compositions in the range 5≲y≲30. Qualitative parametrization of glass-forming tendency gives, as best glass formers, alloys with y≅20. Crystallization on heating proceeds in one stage for glasses withy≲20 and in two stages for those with greater Sb2Te3 content. For compositions lying in the GeSe2 primary crystallization region crystals appear preferentially at the surface of the sample, but for the other compositions (24≲y≲30) the crystals emerge in the bulk and often develop in spherulitic or axialitic form.

Thermodynamic assessment of the AlNi system

Abstract An optimal set of thermodynamic functions for the AlNi system is obtained by means of the CALPHAD (CALculation of PHAse Diagrams) technique applied to almost all the experimental phase diagram and thermodynamic data available. The phases are modeled with the association model (liquid), substitutional solution model (solid solution based on f.c.c. AlAl and f.c.c. AlNi), as a stoichiometric compound (Al 3 Ni), and with the sublattice model (Al 3 Ni 2 , AlNi, Al 3 Ni 5 and AlNi 3 ). The magnetic contributions to the Gibbs energies are introduced for AlNi, AlNi 3 and f.c.c. Al (Al,Ni). Comparison between the calculated and measured phase diagrams and thermodynamic quantities show that most of the experimental information is satisfactorily accounted for by the thermodynamic calculation.

Non-equilibrium crystallization, critical cooling rates and transformation diagrams

Abstract Two different approaches are currently used to study non-equilibrium crystallization. One is based on crystallization kinetic theories and the other is based on nucleation and crystal growth theories. The second, exact, treatment of the kinetics of crystallization under a non-isothermal regime is presented, based on models of nucleation and growth, to establish the temperature versus heating rate-transformation (THRT) and temperature versus cooling rate-transformation (TCRT) diagrams. The usefulness of the method is illustrated by analysis of the crystallization process in three model materials: an oxide glass (SiO2), a chalcogenide glass (Se61.5Ge15.4Sb23.1), and a metallic glass (Nd2Fe14B). The calculations also yield the justification and limitations both for the additivity assumption, often used to describe the relationship between isothermal and continuous heating/cooling crystallization studies, and for the empirical method of constructing the low temperature part of the time-temperature-transformation (TTT) and THRT diagrams, assuming an Arrhenius form of the crystallization rate constant. By using an exact method to construct the TCRT diagram, the crystallization temperatures predicted by the model are established. The critical cooling rate calculated is between two and four times lower than that obtained by use of the additivity assumption and between six and eight times lower than that resulting from the TTT diagram. By extension of the model, the THRT diagram is obtained and a critical heating rate is defined. In all cases, the exact heating rate is about two orders of magnitude higher than that obtained by use of the additivity assumption.

Crystallization kinetics of a Se–Ge–Sb alloy glass

Abstract The glass-forming ability of a Se0.615Ge0.154Sb0.231 alloy is analysed by computing the critical cooling rate and the time-temperature-transformation curve, at various temperatures. The calculation is based on some experimentally determined portions of the time-temperature-transformation curve, from which the rate of increase of viscosity with falling temperature below the melting point is deduced, using classical crystallization kinetics treatment.

Eutectic mixtures for pharmaceutical applications: A thermodynamic and kinetic study

The thermodynamics and kinetics of the solidification process of several mixtures of SR 33557 and PEG 6000 have been analyzed by differential scanning calorimetry. The calculated phase diagram showed a negative interaction energy between the constituents in the liquid phase. A unified description for solidification accounting for isothermal and continuous cooling is presented. The onset of solidification shifts to higher temperatures on decreasing the cooling rate and to longer times on decreasing the annealing temperature under continuous cooling and isothermal holding, respectively. The analysis is based on the fact that nuclei have to be created prior to any crystal growth. The driving force for nucleation is considered proportional to the undercooling, ΔT (= TL − T). By coupling the isothermal and continuous cooling experiments, the high temperature part of the time-temperature transformation and temperature-cooling rate transformation diagrams are constructed under a wide range of conditions.

Accuracy in the experimental calorimetric study of the crystallization kinetics and predictive transformation diagrams: Application to a Ga–Te amorphous alloy

The uncertainties inherent to experimental differential scanning calorimetric data are evaluated. A new procedure is developed to perform the kinetic analysis of continuous heating calorimetric data when the heat capacity of the sample changes during the crystallization. The accuracy of isothermal calorimetric data is analyzed in terms of the peak-to-peak noise of the calorimetric signal and base line drift typical of differential scanning calorimetry equipment. Their influence in the evaluation of the kinetic parameter is discussed. An empirical construction of the time-temperature and temperature-heating rate transformation diagrams, grounded on the kinetic parameters, is presented. The method is applied to the kinetic study of the primary crystallization of Te in an amorphous alloy of nominal composition Ga20Te80, obtained by rapid solidification.

KINETICS OF MICROSTRUCTURAL DEVELOPMENT IN NANOCRYSTALLINE MATERIALS

Abstract A quantitative determination of the microstructural evolution resulting from nucleation and growth of a primary phase is yielded. Recently developed kinetic models constitute a powerful tool when an appropriate physical description of the growth mechanisms which drive the process is implemented. To account for the high grain density developing in nanocrystalline materials, diffusion controlled growth with soft impingement has been modeled. Comparison between computed microstructures obtained with hard and soft impingement shows that the latter explains the delay of the transformation observed experimentally. Application to the nanocrystallization of a Fe-Si-B-Cu-Nb amorphous alloy (FINEMET) is presented. The observed kinetic evolution of both transformed fraction and microstructural quantities agree with computed values when soft impingement is considered. The influence of the annealing temperature on the resulting volume and surface grain size distributions is discussed.

Amorphization of soft magnetic alloys by the mechanical alloying technique

Abstract The progress of amorphization by mechanical alloying on FeB and FeBSi powdersis studied by X-ray diffraction and differential scanning calorimetry. Apart from the well known broad exothermic effect, a well defined endothermic effect is present after milling times typically of 300 h. The enthalpy and activation energy of this endothermic peak are obtained and related to the crystallization ones. The results exclude diffusion as involved in the endothermic process.