Crystallization Kinetics Of Amorphous Alloy Research Articles

Influence of the preparation cooling rate on crystallization kinetics of Fe74Mo6P13C7 amorphous alloys

Fe74Mo6P13C7 amorphous alloys have been prepared by three techniques with different preparation cooling rates, namely the melt-spinning, copper-mold casting, and J-quenching techniques. Differential scanning calorimetry (DSC) was used to investigate the thermal behavior and non-isothermal crystallization kinetics of the amorphous alloy samples. Through this, the effect of the preparation cooling rate on the amorphous alloy thermal parameters, apparent activation energy (Eapp), and first crystallization event kinetics were investigated. The DSC tests show that there are no significant differences in the glass transition temperature (Tg) and the initial crystallization temperature (Tx) among the Fe74Mo6P13C7 amorphous alloy samples prepared by the different techniques, but the total crystallization heat increases with the preparation cooling rate. The Eapp values corresponding to the first crystallization events of the three amorphous alloy samples are determined by the Kissinger and Ozawa methods, and the results show that Eapp increases with a decreasing preparation cooling rate. This indicates that the amorphous alloy sample prepared at the lower cooling rate has a larger crystallization resistance, which may be ascribed to the lesser free volume. The non-isothermal kinetics analyses of the first crystallization events show that the local Avrami exponents (n) of the three amorphous alloy samples are <1.5 during the initial stages, implying a crystallization mechanism based on the direct growth of pre-existing nuclei with a zero nucleation rate. This is then <2.5 for the two bulk samples and in the range of 2.5–3.0 for the melt-spun ribbon sample in the intermediate crystallization stages. These values indicate the diffusion-controlled three-dimensional growth of small particles with decreasing (n<2.5) and increasing (n>2.5) nucleation rates. Meanwhile, the n values increase with the preparation cooling rate, indicating more short-range order in the amorphous alloy sample prepared at the lowest cooling rate.

Effect of ion bombardment on the crystallization kinetics of FeSiNbBCu amorphous alloys

Amorphous FeSiNbBCu alloy ribbons were bombarded by Ar ion with different time, the phase structure and crystallization kinetics of amorphous alloys after ion bombardment were evaluated by XRD and differential scanning calorimetry (DSC), respectively. The apparent activation energy of the first crystallization process dropped from 378.6 kJ/mol for the raw amorphous ribbon to 305.1 kJ/mol for the sample after ion-bombardment, ion-bombardment has no obvious effect on the apparent activation energy of the second crystallization process. Ion-bombardment has an important effect on the crystallization mechanism of the first crystallization of amorphous alloys. Ion-bombardment will lead to the structure relaxation and induce crystallization of amorphous alloys, which then has an important effect on the nucleation and growth mechanism of the initial stage of the first crystallization process.

Applying Vogel–Fulcher–Tammann relationship in crystallization kinetics of amorphous alloys

The Vogel–Fulcher–Tammann relationship is a generic equation for describing the transport properties of supercooled fragile liquids. Coupling this reasonable equation, a kinetic model is derived in the present work for describing the crystallization kinetics of metallic glass in supercooled liquid region. In the modeling, a constant value for apparent activation energy is substituted by a temperature-dependent one, so that a model is carried forward to describe more general solid-state phase transformation kinetics. Then, recipes of kinetic analysis are proposed, and information about liquid fragility is obtained. Applying the newly proposed model, the evolution of apparent activation energy during the crystallization of many amorphous alloys can be explained. Finally, the crystallization kinetics of Zr55Cu30Al10Ni5 bulk metallic glass, respectively measured by isothermal and non-isothermal differential scanning calorimetry, is studied and interpreted using the present model.

Crystallization kinetics of Ti20Zr20Cu60 metallic glass by isoconversional methods using modulated differential scanning calorimetry

The study of crystallization kinetics of amorphous alloys has been a matter of great interest for material researchers for past few decades, since it provides information about the kinetic parameters i.e., activation energy of crystallization and the frequency factor. These kinetic parameters can be calculated by model-free isoconversional methods. Isoconversional methods allow calculating the activation energy as a function of degree of conversion, α. Hence, these methods provide accurate results for multistep processes like crystallization. Model-free methods are categorized as linear and non-linear isoconversional methods. Linear methods are further classified as linear differential and linear integral isoconversional methods. In present work, we have used these isoconversional methods to study the effect of non-linear heating rate, employed by modulated differential scanning calorimetry (MDSC), on the non-isothermal crystallization kinetics of Ti20Zr20Cu60 metallic glass. For Ti20Zr20Cu60, MDSC curves clearly indicate a two-step crystallization process. Both crystallization peaks were studied based on the modified expressions for isoconversional methods by non-linear heating rate. The term corresponding to non-linearity comes out to be (A T ω/2β)2. The effect of non-linear heating rate on measurement of kinetic parameters by isoconversional methods is studied. The activation energy of crystallization is calculated for Ti20Zr20Cu60 metallic glass for various degrees of conversion by linear integral isoconversional methods i.e., Ozawa–Flynn–Wall, Kissinger–Akahira–Sunose, and also with Friedman method which is a linear differential isoconversional method.

The Kinetic Description for Solid State Phase Transformation

The progress of solid-state phase transformation can be subdivided into three overlapping mechanisms: nucleation, growth, and impingement. On the basis of an analytical phase transformation model, the maximum in the transformation rate of an isothermal solid-state transformation has been evaluated. Then, the mode of nucleation, growth and impingement, and the separate activation energies for nucleation and growth can be determined. Finally, application in the crystallization kinetics of amorphous alloy was described.

Accurate electrical resistance measurement of the crystallization kinetics of amorphous alloys

An accurate four-line ac electrical resistance measurement (ERM) apparatus was developed. By using the ERM the crystallization kinetics of amorphous Ni80P20, FeZr2, Fe86B14 alloys were investigated. The experimental results show that the ERM can identify the early stage of crystallization in amorphous alloys. The ERM detects a crystallization temperature range obviously wider than the DSC does, indicating that the ERM is more sensitive to the structure evolution in crystallization. For the eutectic or polymorphic crystallization, three distinct processes can be identified from the measured resistance variation: (i) crystal nucleation, (ii) subsequent growth of crystal nuclei, and (iii) coarsening of the crystallites. In the early stage of the primary crystallization, the ERM results reflect the nucleation information as well.

The crystallization kinetics of amorphous alloys

The crystallization kinetics of Fe-based amorphous alloys was investigated by differential thermal analysis (DTA) and differential scanning calorimetry (DSC). Rather high values of activation energies were explained as a result of simultaneously correlated displacements of groups of atoms during the course of crystallization and allowed the correlation of amorphous alloys regarding their different composition. Connections between the temperatures, activation energies and thermodynamic driving forces were considered. Special attention was paid to the formation of nanocrystalline structures (Finemet).

Time-Scaling Properties and Crystallization Kinetics of Amorphous Alloys

We have investigated the time-scaling properties of the isothermal crystallization kinetics for amorphous Co 75 B 25 , Fe 8O B 2O , Fe 40 Ni 40 B 20 , Fe 81 B 17 Si 2 and Fe 60 Ni 20 B 10 Si 10 alloys by differential scanning calorimetry. All the alloys exhibit a single crystallization stage. The time scale was defined as the time when the crystallization has reached half completion. All the time-scaled crystallization curves of each alloy fall onto a single muster curve of the Johnson-Mehl-Avrami form in each case. The Avrami exponent indicates that the crystallization process of the Fe 60 Ni 20 B 10 Si 20 is governed by interface-controlled growth with a constant nucleation rate, whereas those of the other alloys are by interface-controlled growth of a fixed number of nuclei. The Williams-Landel-Ferry equation based on a free-volume consideration gives a universal function of time-scaling parameter as a function of temperature for all the alloys. This suggests that we have to take into account the relaxation occurring in the amorphous phase outside the crystallite in the crystallization process.

Comparison of the two Mössbauer methods of transformation kinetics measurements

The in situ Mossbauer scanning of the crystallization kinetics of amorphous alloys is compared to the commonly used interrupted-annealing procedure. In contrast to this, little information is gained on the hyperfine structure of the Moss-bauer spectra, yet the time gain by scanning the outermost crystalline phase Zeeman line intensity is substantial. Results of both methods on the Fe−Co−B alloys are shown and kinetic parameters of crystallization briefly discussed.

On the crystallization kinetics of glassy alloys in the Cu-As-Se system

A study of the crystallization kinetics of amorphous alloys in the Cu-As-Se system was made using a method in which the crystallization rate is deduced bearing in mind the dependence of the reaction rate constant on time, through temperature. The method was applied to the experimental data obtained by differential scanning calorimetry, using continuous-heating techniques. The kinetic parameters determined have made it possible to discuss the glass-forming ability, as well as the different types of nucleation and crystal growth exhibited by the alloys studied.