Phase transformation and crystallization kinetics of melt-spun Ni60Nb20Zr20 amorphous alloy

The glass transition and crystallization kinetics of melt-spun Ni₆₀Nb₂₀Zr₂₀ amorphous alloy ribbons have been studied under non-isothermal and isothermal conditions using differential scanning calorimetry (DSC). The dependence of glass transition and crystallization temperatures on heating rates was analyzed by Lasocka's relationship. The activation energies of crystallization, E_x, were determined to be 499.5 kJ/mol and 488.6 kJ/mol using the Kissinger and Ozawa equations, respectively. The Johnson–Mehl–Avrami equation has also been applied to the isothermal kinetics and the Avrami exponents are in the range of 1.92–2.47 indicating a diffusion-controlled three-dimensional growth mechanism. The activation energy obtained from the Arrhenius equation in the isothermal process was calculated to be E_x = 419.5 kJ/mol. The corresponding three dimensional (3D) time–temperature–transformation (TTT) diagram of crystallization for the alloy has been drawn which provides the information about transformation at a particular temperature. In addition, the intermetallic phases and morphology after thermal treatment have been identified by X-ray diffraction (XRD) and scanning electron microscope (SEM).     

Crystallization kinetics and magnetic properties of Fe63.5Co10Si13.5B9Cu1Nb3 nanocrystalline powder cores

Amorphous ribbons of the alloy Fe_63.5Co₁₀Si_13.5B₉Cu₁Nb₃ were prepared by the standard single copper wheel melt spinning technique in air and their crystallization kinetics was analyzed by non-isothermal differential scanning calorimetry (DSC) measurements. The crystallization activation energies (E_x, E_a1 and E_a2) of amorphous ribbons calculated from Kissinger model were 448, 385 and 396 kJ/mol for the first and the second crystallizations, respectively. The Avrami exponent n was calculated from the Johnson–Mehl–Avrami (JMA) equation and was used to identify the crystallization mechanism for the amorphous ribbons. The ribbons were milled into different sized flakes, which were molded subsequently to cores using 3 wt.% epoxy as a binder. The effective permeability of the cores showed high frequency stability and increased with the size of the flakes. For the cores made from small sized flakes (? 75 μm), the quality factor was increased at high frequencies, which was attributed to the reduction in the eddy current loss.     

Kinetics of non-isothermal crystallization of chalcogenide glasses from the Sb32As5S48I15 system

The work considers the kinetics of non-isothermal crystallization, especially from the point of view of the possibility of applying the Augis–Bennett method for the determination of activation energy. The necessity of introducing the usual approximation T ≫ T₀ was analyzed. It was shown that the method can be applied not only in the cases when T ≫ T₀, but also when T > T₀ that is, when the two temperatures differ only by about 10%. We show also that the application of the Augis–Bennett formula in such cases is more accurate the higher the value of the Avrami index n is. The method was applied to study the crystallization kinetics of the glass Sb₃₂As₅S₄₈I₁₅. The results obtained by differential scanning calorimetry (DSC) under non-isothermal conditions were also analyzed using the Kissinger and Matusita method. In the process of glass heating, a single peak is observed for glass transformation and a double peak for glass crystallization. The activation energies of glass transition (E_g) and crystallization (E_c) were determined, as well as the value of the Avrami index n for the second crystallization process. The obtained values for n and m indicate that the crystallization phase involves three-dimensional nucleation and growth. On the other hand, in agreement with the obtained results of the theoretical analysis, such n value justifies the application of the Augis–Bennett method for the investigated glass for which T and T₀ do not differ significantly.     

Kinetics of non-isothermal crystallization and glass transition phenomena in Ga10Se87Pb3 and Ga10Se84Pb6 chalcogenide glasses by DSC

The crystallization process affects solid properties through the crystal structure and morphology established during the transition process. An important aspect of the crystallization process is its kinetics, both from the fundamental point of view of amorphous material as well as the modeling and phase transition. In the present research work, non-isothermal crystallization data in the form of heat flow vs. temperature curves has been studied by using some well known models for amorphous Ga₁₀Se₈₇Pb₃ and Ga₁₀Se₈₄Pb₆ chalcogenide glasses, prepared by the melt quenching technique. The glass transition phenomena and crystallization of these glasses have been studied by using non-isothermal differential scanning calorimetery (DSC) measurements at constant heating rates of 5, 10, 15, 20, 25 and 30 K/min. The glass transition temperature (T_g), crystallization temperature (T_c), and melting temperature (T_m) were determined from DSC thermograms. The dependence of T_g and T_c on the heating rate was used to determine different crystallization parameters such as the order parameter (n), the glass transition energy (ΔE_g) and the crystallization activation energy (ΔE_c). The results of crystallization were discussed on the basis of different models such as Kissinger's approach and the modification for non-isothermal crystallization in addition to Johnson, Mehl, Ozawa and Avrami.     

The effect of the underlying substrate on crystallization kinetics of TiNi thin films

The crystallization kinetics of amorphous thin TiNi films deposited on SiO₂ (or NaCl)/Al foils substrates were investigated. A dramatic acceleration of the crystallization rate was observed for amorphous attached-substrate films. The acceleration originated from the presence of the thin film/middle-wafer interface which served as a two-dimensional nucleus for the growth of the crystalline phase. In the process of non-isothermal annealing by DSC, apparent activation energies for two kinds of underlying thin TiNi films were determined to be 352.96 and 403.69 kJ/mol, respectively, which was lower than those free-standing films studied in previous works. For the process of isothermal annealing, the crystallization kinetics parameters had remarked drop, reflected from the lower Avrami exponent n (the range of 1.35–2.11) and shorter incubation time τ (the range of 0.1–0.4 min) between 758 and 775 K.     

Crystallisation kinetics,glass forming ability and thermal stability in glassy Se100 − xInx chalcogenide alloys

Differential scanning calorimetry (DSC) studies have been done under non-isothermal conditions at different heating rates for glassy Se_100 ? xIn_x (5 ≤ x ≤ 20) alloys. DSC traces with well-defined endothermic and exothermic troughs and peaks at glass transition (T_g), crystallisation (T_c) and melting (T_m) temperatures were observed. The crystallisation kinetics parameters, Avrami index (n), activation energy for crystallisation (E_c) and frequency factor (K_o), have been calculated on the basis of the classical Johnson–Mehl–Avrami (JMA) model and related methods derived by Kissinger, Augis–Bennett and Mahedevan. Activation energy for glass transformation (E_t) has been evaluated on the usual two different non-isothermal methods developed by Moynihan and Kissinger. An extension of the Augis–Bennett method well known for evaluating E_c to calculate E_t has been explored with satisfactory results. Results obtained from these methods are in close agreement with each other. Close correlation between E_t, E_c and heating rate (β) was observed. The glass forming ability (GFA) and thermal stability parameters have been calculated for each glass system. It was found that the proportion of indium additive changed significantly the values of glass/crystal transformation, GFA and thermal stability of the studied system.     

Effect of arsenic atom substitute with antimony on crystallization processes and thermal stability of the (Sb,As)-S-I system

Non-isothermal differential scanning calorimetry measurements of Sb_xAs_37-xS₄₈I₁₅ chalcogenide glasses for x = 0, 7, 12, 22, 32 and 37 at.% at different heating rates were applied for the analysis of the crystallization processes and thermal stability. The activation energies of glass transition (E_g) and crystallization (E_c) were obtained. Methods based on the temperature of peak and methods based on the shape of peak of crystallization were used for E_c calculations. The activation energies of glass crystallization were determined for the Sb₃₇S₄₈I₁₅ glass, as well as the value of the Avrami index n using the Matusita–Sakka theory. The analysis showed that the detected crystallization processes are characterized by the value n which indicates that volume nucleation and three-dimensional growth are involved. Depending on the model used, the values of this parameter for SbSI ranged from 122(14) to 133(14) kJ/mol, whereas for the structural unit Sb₂S₃, the values were in the range from 160(18) to 176(50) kJ/mol. Values of E_c for Sb₂S₃ structural unit, correspond to values which can be found in the literature. E_g values are in the range from 222(40) kJ/mol to 302(8) kJ/mol, depending on Sb content. Reducing of thermal stability against crystallization can be explained by the exchange of the elements from the same group of the Periodic Table (As and Sb) and similar nature of the analogous structural units, involving no change in the coordination number, whereas the share of the ionic bond in them is changed.     

Structural characterization and phase transformation kinetics of Se58Ge42−xPbx (x = 9, 12) chalcogenide glasses

The glass transition behavior and crystallization kinetics of Se₅₈Ge_42?xPb_x (x = 9, 12) have been investigated using Differential Scanning Calorimetry (DSC) at five different heating rates under non-isothermal conditions. It has been observed that these glassy systems exhibit single glass transition and double crystallization on heating. The XRD pattern revealed that the considered glasses get crystallized into GeSe₂ and PbSe/Se phases after annealing at 633–643 K for 2 h. The GeSe₂ and Se phases were found to crystallize in monoclinic structure while, PbSe phase crystallizes in cubic structure. Besides this, a mixed phase was also observed in DSC thermograms after annealing. The kinetic studies include determination of various parameters such as Avrami exponent (n), frequency factor (K_o), dimensionality of growth (m), the activation energy for glass transition (E_t) and for crystallization (E_c). The values of E_t increases while that of E_c decreases after annealing. Also, dimensionality of growth decreases to one dimension from two and three dimensions after annealing.     

Crystallization kinetics of a-Se75Te25 − xGax (x = 0, 5, 10 and 15 at wt %) glassy system

Bulk glasses of a-Se₇₅Te_25 ? xGa_x (x = 0, 5, 10 and 15 at wt %) have been prepared by melt quenching technique. These samples were structurally characterized by using X-ray diffraction. Kinetic of crystallization in these glasses was studied under non-isothermal conditions using differential thermal analysis (DTA). DTA is performed at different heating rates of 5, 10, 15, 20 and 30 °C/min. The values of glass transition (T_g) and crystallization peak temperature (T_p) are found to be composition and heating-rate dependent. The obtained results have been analyzed in terms of activation energy of glass transition (E_g) using Kissinger's and Mahadevan et al. relations. Values of E_g obtained by the two relations are in agreement with each other. The results indicate that the crystallization process is a three-dimensional growth.     

Calorimetric study and Kissinger analysis of melt-spun DyMn6−xGe6−xFexAlx (1 ⩽ x ⩽ 2.5) alloys

Thermal properties of rapidly quenched alloys from the DyMn_6?xGe_6?xFe_xAl_x (1 ? x ? 2.5) series produced by melt-spinning have been investigated by differential scanning calorimetry (DSC). The DSC curves show two exothermic effects connected with crystallization processes. Crystallization temperatures and enthalpies ΔH have been estimated. The systematic changes in these parameters allow concluding that the crystallization exothermic events are independent. Effective activation energies E have been determined using the Kissinger analysis and relatively high values up to 480 ± 20 kJ/mol for DyMn₄Ge₄Fe₂Al₂ have been found indicating high thermal stability of the amorphous state in this alloy series.     

Kinetic theory of crystallization of amorphous materials

Careful analysis of the Avrami equation [x = 1 ?exp(?Atⁿ)] shows that an activation energy for crystallization (E_c) for amorphous materials can be defined over a selected range of temperatures. This activation energy can be determined experimentally using non-isothermal differential scanning calorimetry (DSC) by determining the crystallization temperature (T_c) as a function of heating rate (φ). A plot of (ln(φ/T_c) has a slope equal to E_c/n. The activation energy for the crystallization of amorphous arsenic obtained by this non-isothermal method is found to be in fair agreement with that obtained from an isothermal DSC experiment.     

Kinetics of crystallization process of Mg–Cu–Gd based bulk metallic glasses

The crystallization behavior of Mg₆₁Cu₂₈Gd₁₁ and (Mg₆₁Cu₂₈Gd₁₁)₉₈Cd₂ bulk metallic glasses was studied using DSC in the mode of continuous and isothermal heating, and its crystallization process and microstructure were confirmed by XRD and TEM. In continuous heating, the activation energies of glass transition, onset and peak crystallization were determined by the Kissinger method, which yields 110 ± 12, 77 ± 9 and 79 ± 10 kJ/mol, respectively, for Mg₆₁Cu₂₈Gd₁₁ glassy alloy, and 144 ± 10, 126 ± 6 and 131 ± 5 kJ/mol, respectively, for (Mg₆₁Cu₂₈Gd₁₁)₉₈Cd₂ glassy alloy. The isothermal kinetics was modeled by the Johnson-Mehl-Avrami equation. The Avrami exponent of the base alloy was in the range from 1.98 to 2.56 (± 0.01), which indicated a decreasing nucleation rate and a diffusion-controlled growth. For Cd-added glassy alloy, the Avrami exponent was in the range from 3.26 to 4.08, which indicated an increasing nucleation rate. The activation energies in isothermal process were calculated to be 88 ± 2 and 132 ± 2 kJ/mol, respectively, for the base and Cd-added glassy alloys. It was found that Mg₂Cu phase was the primary phase in the initial crystallization and the strong affinity between Cd and Mg/Gd tended to impose resistance to the formation of Mg₂Cu phase and thus improves the thermal stability.     

Magnetic susceptibility and atomic structure of paramagnetic Zr–(Co, Ni, Cu) amorphous alloys

The experimental results for the magnetic susceptibility and superconducting transition temperature of amorphous Zr–(Co, Ni, Cu) alloys extending over a wide composition range are analyzed in some detail. By combining the results of this analysis with the literature results for the electronic density of states at E_F for the same alloy systems we obtained a set of parameters associated with the electronic structure of the amorphous Zr. The comparison of these parameters with the results of the band structure calculations for different crystalline phases of Zr and with the results of the atomic structure and crystallization studies of the same alloy systems indicates an fcc-like local atomic structure for amorphous Zr.     

Crystallization kinetics in Se–Te glassy system

Differential scanning calorimetry measurements were used to study crystallization in the Se_yTe_(1 ? y) glassy system under non-isothermal conditions. The examined compositions were y = 0.1, 0.2 and 0.3. The crystallization kinetics was described in terms of the Johnson–Mehl–Avrami nucleation–growth model. It was found that with the addition of tellurium into the selenium matrix the apparent activation energy of the crystallization process monotonically increases and, more importantly, the parameter of the JMA model m decreases. This change in the crystallization mechanism was interpreted as a gradual crossover from surface to bulk process. Both processes are competitive and proceed simultaneously in glasses with low tellurium content, while in the case of increased Te content the surface crystallization mechanism recedes. The transition between mechanisms was discussed in terms of changes in molecular structure of the material.     

Effect of laser irradiation on thermal and optical properties of selenium–tellurium alloy

The crystallization parameters such as glass transition temperature (T_g), onset crystallization temperature (T_c), peak crystallization temperature (T_p) and enthalpy released (ΔH_C) of the bulk Se–Te chalcogenide glass has been studied by using Differential Scanning Calorimeter (DSC), under non-isothermal condition at a heating rate of 20 K/min. The values of T_g, T_c, T_p and ΔH_C with and without laser irradiation for different exposure time have been studied. The optical absorption of pristine and laser irradiated thermally evaporated Se–Te films has been measured. The films shows indirect allowed interband transition that is influenced by the laser irradiation. The optical energy gap has been found to decrease from 1.61 to 1.38 eV with increasing irradiation time from 5 to 20 min. The results have been analyzed on the basis of laser irradiation-induced defects in the film.     

Variation of Avrami parameter during non-isothermal surface crystallization of glass powders with different sizes

The influence of grain size on the surface crystallization kinetics is investigated by non-isothermal Differential Thermal Analysis and Scanning Electron Microscopy. For this purpose, different powder fractions of a model glass forming diopside via surface crystallization and crystallization induced porosity, are used. The activation energy of crystallization, E_C, is determined from Kissinger plot, and the Avrami parameter, m, is estimated by both Ozawa and of Augis–Bennett methods. The values of E_C decrease from 530 to 300 kJ/mol when grain size increases. Simultaneously, the m value drops from 2.5 to 1.3. This result contradicts to the widespread fallacy that the reaction order should be equal to one in the case of surface crystallization. During the initial stage of surface crystallization three dimensional growth takes place, a fact confirmed by the SEM investigations. Later on, one dimensional growth inwards the grain is observed, leading to the reaction order changes from 3 to 1.     

Kinetics of crystal growth of germanium disulfide in Ge0.38S0.62 chalcogenide glass

The crystal growth kinetics of GeS₂ in Ge_0.38S_0.62 glass has been studied by Differential Scanning Calorimetry (DSC) and microsopy. The linear crystal growth kinetics of both high temperature α-GeS₂ and low temperature β-GeS₂ polymorphs has been observed over a relatively broad range of temperatures, i.e. 420 < T < 494 °C that correspond to viscosity of supercooled melt: 3 × 10⁹ > η > 8 × 10⁵ Pa s. It seems that 2D nucleated growth is the most probable mechanism of crystallization for high temperature α-GeS₂ under these conditions. However, there are significant deviations for this model for the crystallization of low-temperature β-GeS₂. This might indicate some changes in crystal-melt interfacial energy or break down of Stokes–Einstein relation in that particular case. At temperatures below 500 °C the temperature range of directly observed crystal growth overlaps with isothermal DSC measurements. In this case overall crystallization kinetics can be described by the Johnson–Mehl–Avrami (JMA) nucleation-growth model for kinetic exponent n ≅ 4. The value of activation energy of nucleation estimated from these experiments E_N = 434 kJ mol⁻¹ is comparable with the activation energy of viscous flow in supercooled Ge_0.38S_0.62 melt (E_η = 478 kJ mol⁻¹). A more complex eutectic crystallization involving both GeS₂ and GeS phases has been observed at higher temperatures. This process is probably associated with secondary nucleation and cannot be described by a simple JMA model.     

Crystallization behavior and microstructure of (CaO·ZrO2·SiO2)–Cr2O3 based glasses

The effects of chromium oxide on the crystallization behavior of glass compositions in the calcium, zirconium and silicon oxides system were investigated by differential thermal analysis, X-ray diffraction and scanning electron microscopic. Results indicate that crystallization is predominantly controlled by a surface nucleation mechanism, even though a partial bulk nucleation has been encountered in compositions containing more than 1.0 mol% of doping oxide. The effect of heating rate on differential thermal analysis curves was studied in order to investigate nucleation mechanisms and to extract the corresponding crystal growth activation energies E_c for the different crystalline phases. Activation energy (E_c) was found to be 490 ± 5 kJ/mol for 5.0 mol% chromium oxide in glasses. The most suitable nucleation temperature was determined as 810 °C for the above mentioned glass. The results of this study have highlighted that a small percentage of chromium oxide strongly affects the crystal formation thereby reducing the time and temperature of the thermal treatment and enhancing the degree of crystallization of calcium, zirconium and silicon oxides glasses.     

Crystallization kinetic studies on Bi1.75Pb0.25Sr2Ca2Cu3-xSnxOδ glass-ceramic by using non-isothermal technique

The Sn substituted Bi_1.75Pb_0.25Sr₂Ca₂Cu_3-xSnxO_δ glass ceramic (where x = 0, 0.1, 0.3, and 0.5) samples were prepared by the melt-quenching method. Crystallization kinetic studies of the samples were conducted using the differential thermal analysis (DTA). The oxidation behavior of the samples was also analyzed using the thermogravimetry analysis (TG). The DTA curves were registered with different heating rates (5, 10, 15, and 20 Kmin^? 1) up to 1200 ± 0.5 K. The crystallization results were analyzed, and activation energy of crystallization process as well as the crystallization mechanisms and the effect of Sn substitution on powder glass ceramic were characterized. The glass transition temperature (T_g), the first crystallization peak temperature (T_x1) and the second crystallization peak temperature (T_x2) values were obtained as 713.0 ± 0.5–746.6 ± 0.5, 731.0 ± 0.5–760.8 ± 0.5 and 789.0 ± 0.5–820.1 ± 0.5 K, respectively. The activation energy (E_a) of crystallization was estimated from DTA results to be about 332.8 ± 0.1, 358.0 ± 0.1, 353.1 ± 0.1 and 348.9 ± 0.1 kJ/mol for x = 0, 0.1, 0.3 and 0.5, respectively, by using the Kissinger method. The Avrami parameter (n) values calculated at different Sn ratio from DTA results were found to be between 1.70 ± 0.01 and 2.57 ± 0.01, results reflect the growth of small particle with a decreasing nucleation rate.