Emília Illeková

FINEMET-type nanocrystallization kinetics

Abstract The primary nanostructure formation kinetics in a set of Fe–Cu–Nb–Si–B ribbons has been studied by the use of differential scanning calorimetry in both continuous-heating and isothermal measuring regimes. The nanostructure formation consists of two independent kinetic modes, namely the Johnson–Mehl–Avrami nucleation-and-growth mode and the mode which has been well described by the normal-grain-growth kinetic law. The nucleation-and-growth of the bcc α-Fe based phase is manifested as a rule in the early stages of the transformation. The normal-grain-growth-like mode is the major and principal kinetic characteristics of all FINEMETs independently of the content of Cu and Si. It is controlled by a specific rearrangement of niobium. After the start of the main nanostructure transformation, a long-range reordering of Si occurs during longer times in the FINEMETs containing Si. Fe 75.6 Cu 1 Si 14 B 9.4 is a conventional metallic glass. It does not contain niobium, does not exhibit the normal-grain-growth-like kinetics and does not form the nanocrystalline (FINEMET-type) structure.

Crystallization characterisics in the FeSiB glassy ribbon system

Abstract The kinetics of crystallization of Fe 80 Si x B 20 − x (with x = 2, 4, 6, 8, 10) and Fe 75 Si 15 B 10 glassy ribbons was examined using differential scanning calorimetry and high precision electrical resistivity measurements. The main crystallization parameters are presented. The corresponding structure of the materials was observed using transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. The mechanism of crystallization is described in terms of the local ordering (cluster) model.

Transformation kinetics of the Fe73.5Cu1Nb3Si13.5B9 ribbons to the nanocrystalline state

Abstract The transformation kinetics of the as-quenched Fe 73.5 Cu 1 NBb 3 Si 13.5 B 9 ribbons to the nanocrystalline structure has been studied. A differential scanning calorimeter in the continuous-heating and isothermal measuring regimes and a thermobalance with a permanent magnet have been used. The main differential scanning calorimetry (DSC) peak in the continuous-heating regime is wide and asymmetrical, having a long high temperature part and a small transformation enthalpy. The DSC signal in the isothermal regime is monotonically decreasing with a temperature-dependent amplitude, the total transformation enthalpy being slightly changed. Both as-quenched and nanocrystalline samples exhibit two magnetic transformations namely the glass-like and the nanophase-like transformations, the mutual proportion between them being changed with the proceeding transformation. All the observed phenomena are interpreted by the grain growth mechanism of the coarsening of the already existing fine-crystalline structure to the nanocrystalline structure. The experimental results can be interpreted also by the Johnson—Mehl—Avrami nucleation-and-growth crystallization mechanism with an abnormal exponent n ≈ 0.9.

The complex characteristics of crystallization of the Fe75Si15B10 glassy ribbon

The Fe75Si15B10 glassy ribbon was examined in the course of various isothermal and dynamic heat treatments. The number, type and mechanism of formation of the crystallization products formed during two crystallization stages in this alloy were determined by differential scanning calorimetry (DSC), thermomagnetometry, transmission electron microscopy and X-ray diffraction measurements. It is concluded that the first DSC peak is due to the crystallization of α-Fe(Si) or Fe3Si and the composite microcrystals containing Fe3B cores with α-Fe(Si) envelopes. The metastable Fe3B subsequently transforms into the stable Fe2B in the second DSC peak. The remaining amorphous matrix crystallizes by the eutectic reaction also in the second DSC peak forming the Fe3Si and Fe2B eutectic structures. The apparent activation energy E1⊛ decreases during the first crystallization stage from 480 kJ (g atom-1), characteristic of the Johnson–Mehl–Avrami (JMA) nucleation-and-growth kinetics of the as-quenched sample, to 350 kJ (g atom)-1 implying a dominance of the diffusion-controlled growth of iron in the already pre-nucleated sample. Simultaneously, the JMA exponent n1 decreases from 2.5 to 1.5. The apparent activation energy E2⊛ depends on the temperature of the first crystallization stage. It decreases with increasing Ta from 384 to 282 kJ (g atom)-1. This phenomenon was attributed to modification of the chemical composition of the remaining eutectic after the temperature dependent primary crystallization of Fe3B within the composites. The value of n2 is 3.

The crystallization kinetics of Fe80Si4B16 metallic glass

Abstract Differential scanning calorimetry has been used to study the kinetics of crystallization of Fe 80 Si 4 B 16 metallic ribbon. Continuous heating and isothermal measuring techniques were used. Two overlapping exothermal peaks were always observed. Both processes proceeded by Johnson-Mehl-Avrami crystallization kinetics having temperature- and time-dependent activation energies. In the first crystallization step E∗ 1 (755 K ) = 325 kJ ( g atom ) −1 continuously increased to E∗ 1 (794 K ) = 487 kJ ( g atom ) −1 and the exponent n 1 = 2.5 decreased to 1.5 for extended degrees of conversion. These observations were interpreted as crystallization of two types of primary product having quite different kinetics. In the second crystallization step the exponent n 2 = 4 and the activation energy E∗ 2 having the mean value of 340 kJ (g atom) −1 decreased slightly with increasing temperature. These observations were interpreted as the crystallization of the metastable and stable eutectic phases.

The complex DSC analysis of the first crystallization peak of Fe80 Si10 B10 metallic glass

Abstract The crystallization of Fe80 Si10 B10 glass was studied both by linear heating and isothermal differential scanning calorimetry (DSC). Two well separated crystallization peaks were always observed. On the basis of the classical isothermal Johnson-Mehl-Avrami(JMA) procedure both transient nucleation and transient growth kinetics with the complex exponent n ∼ 2.8 and mean activation energy E∗ JMA = 323 kJ mol −1 were determined for the first crystallization peak. Because for the degree of conversion α > 0.55 E∗(α) dependence was observed, the deconvolution of the isothermal peak into the subsequent JMA nucleation-and-growth and grain-growth effects was used to fit the measured data. Simple JMA kinetics could not be determined for the linear heating first crystallization peak. All these results correlate with structural analysis studies.

Complex behaviour of specific heat during structural relaxation of Fe73Co12B15 metallic glass

Abstract The non-isothermal DSC thermograms of the metallic glass Fe73Co12B15 were investigated. The complex analysis of the structural relaxation anomalies of the apparent specific heat of metallic glass samples was performed. The influence of ageing, pre-annealing and more complicated heat treatments, as well as of the heating rate, on the saturation of the structural relaxation enthalpy was demonstrated. The relaxation exotherm is principally connected with the rapid cooling of the sample. The relaxation endotherm represents the retarded approach to a certain heat treatment (non-isothermal crossover effect). All results can be generalized and interpreted by the DNLR (distribution of non-linear relaxation) model.

The relation between the bulk and ribbon Zr55Ni25Al20 metallic glasses

Abstract The planar-flow-casting technique and quenching in flowing water were used to produce the glassy Zr 55 Ni 25 Al 20 ribbon and bulk samples. The thermodynamic states of these two types of samples were compared using differential scanning calorimetry and analysis of the structural relaxation kinetics. The relaxation enthalpy is 3 times larger and the glass transition temperature is higher by 67.5 K in the case of the ribbon sample. These differences were theoretically confirmed by a kinetic model and were caused by the different quenching rates for the ribbon and bulk samples, namely 10 5 and 10 2 K s −1 , respectively. X-ray analysis has shown, that the main difference between the studied glassy structures consists in the different degree of the short-range order in the samples.

Local ordering model in Fe-Si-B amorphous alloys

Abstract The main kinetics parameters of the crystallization of Fe 80 Si x B 20− x (with x = 2, 4, 6, 8, 10) and Fe 75 Si 15 B 10 glassy ribbons have been determined using differential scanning calorimetry measurements. The structure of the materials at various stages has been observed using transmission electron microscopy, electron and X-ray diffraction analysis. The crystallization process in this system is found to consist of two distinct steps; individual steps represent complex processes from a structural point of view. The mechanism of crystallization is described in terms of a local ordering (cluster) model, based on the role of the Si atoms.

Physical Properties of Fluoride Glasses for Ionics

The ionic conductivity and permittivity of glasses based on ZrF4, BaF2, LaF3, AlF3 and NaF (ZBLAN) or PbF2, InF3, BaF2, AlF3 and LaF3 (PIBAL) are studied. The influence of the glass composition on the glass transition temperature (Tg) and on the crystallization temperature (Tx) is reported. For all ZBLAN glasses the temperature dependencies of the ionic conductivity are close one to another (s500 = 8(2)·10-6 S/cm) and their conduction activation enthalpies are equal to 0.82(1)eV. From the point of view of the ionic conductivity, the best glass compositions are the PIBAL50 (50 m/o PbF2) and PIB45 ( 45 m/o PbF2).