Nanocrystallization and structure of Fe78.5Ni1.0Mo0.5Si6.0B14.0 amorphous alloy

The effect of Ni and Mo alloying additions on crystallization of an Fe–Si–B based amorphous alloy was studied by applying various experimental techniques – DSC, XRD and TEM. It was shown that both alloying additions Ni and Mo change the crystallization temperature as well as the activation energy of primary crystallization. The phases formed during primary crystallization for the Fe₈₀Si₆B₁₄ and Fe_78.5Ni₁Mo_0.5Si₆B₁₄ alloys were the same, however the morphologies were significantly different. The addition of 1.0 at.% of Ni and 0.5 at.% Mo changed the crystallization mechanism and the type of formed phases. Such additions also resulted in formation of nanocrystals. The kinetic and thermodynamic characteristics of annealed specimens of amorphous metallic Fe₈₀Si₆B₁₄ and Fe_78.5Ni₁Mo_0.5Si₆B₁₄ alloys were established. These characteristics were determined based on measurements of instantaneous changes of electrical strength. It was shown that the method of electromotive force measurements was more sensitive to structural changes and the phase composition of amorphous metallic electrodes in comparison with the X-ray method.     

Stability and crystallization of Fe-Co-Nb-B amorphous alloys

New soft magnetic amorphous alloys with a wide supercooled liquid region were obtained by increasing the boron content (up to 30 at.%) and by addition of cobalt (composition range between 5 and 42 at.%) in FeNbB based materials. Upon thermal treatments, the stability and the crystallization behavior of Fe₅₂Co₁₀Nb₈B₃₀ and Fe₂₂Co₄₀Nb₈B₃₀ have been studied by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). On the other hand, the Curie temperature change, T_c, of the remaining amorphous phases has been determined by thermomagnetic analysis (TMG). Sub-micrometric fcc-(Co,Fe) and fcc-(Fe,Co)₂₁Nb₂B₆ grains were observed to crystallize at a relatively high temperature (950-970 K) providing an excellent stability of the amorphous states. Due to the partitioning behavior, occurring during crystallization, the Curie temperature of the remaining amorphous structure is observed to increase in the case of the Fe₅₂Co₁₀Nb₈B₃₀ composition while a decrease of T_c to the lower temperatures is observed with the Fe₂₂Co₄₀Nb₈B₃₀ composition.     

The formation and crystallization of amorphous Al65Fe20Zr15

Amorphous Al₆₅Fe₂₀Zr₁₅ alloy has been prepared by mechanical alloying. The as-milled powders were analyzed by XRD and DTA. The fully amorphous alloy was identified by the hallo rings of XRD pattern. The effective activation energy of crystallization was estimated with modified Kissinger’s plot and it is 321.0 kJ/mol evaluated from the peak temperature of crystallization. The product phases of crystallization composed of Al₃Zr, Al₈₂Fe₁₈, AlFeZr and an unidentified phase. The crystallization kinetics was also discussed by using the isochronal DTA scans and the values of corresponding kinetic parameters were determined.     

Thermal behavior of substituted FeCo-based metallic glasses

Three compositions of metallic glasses, Fe₅₈Co₂₄Nb₃Ta₁Mo₁B₁₃, Fe_61.5Co_20.5Nb₃Ta₁Mo₁B₁₃ and Fe₆₆Co₁₈Si₁B₁₅ were prepared by rapid quenching from the melt and subsequently annealed for 1 hour at 450, 650 and 750 °C. The samples were analyzed by X-ray diffraction (XRD) and Mössbauer spectroscopy. All Mössbauer spectra were fitted with a six-line pattern corresponding to the crystalline α-(FeCo) phase and a hyperfine magnetic field distribution representing the amorphous component. The Mössbauer spectra of annealed Fe₆₆Co₁₈Si₁B₁₅ revealed the presence of a secondary crystalline phase, namely (FeCo)₃(BSi). Moreover, the last Mössbauer spectrum in the set was fitted with an additional sextet, corresponding to the appearance of hematite in the system. It is inferred that the addition of Nb, Ta and Mo considerably delays the onset of bulk crystallization and iron oxidation in these systems. The XRD patterns are in qualitative agreement with the Mössbauer results and are consistent with a surface layer of hematite nanoparticles in the system. The activation energy of 4.34 eV for oxidation was estimated from the Mössbauer results.     

Non-isothermal crystallization kinetics of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>–CaO–SiO<Subscript>2</Subscript> glass containing nucleation agent P<Subscript>2</Subscript>O<Subscript>5</Subscript>/TiO<Subscript>2</Subscript>

Fe₂O₃–CaO–SiO₂ glass ceramics containing nucleation agent P₂O₅/TiO₂ were prepared by sol-gel method. The samples were characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The activation energy and kinetic parameters for crystallization of the samples were calculated by the Johnson-Mehi-Avrami (JMA) model and Augis-Bennett method according to the results of DSC. The results showed that the crystallization mechanism of Fe₂O₃–CaO–SiO₂ glass, whose non-isothermal kinetic parameter n = 2.3, was consistent with surface crystallization of the JMA model. The kinetics model function of Fe₂O₃–CaO–SiO₂ glass, f(α) = 2.3(1–α)[–ln(1–α)]^0.57, was also obtained. The addition of nucleation agent P₂O₅/TiO₂ could reduce the activation energy, which made the crystal growth modes change from onedimensional to three-dimensional.     

Kinetics of crystallization of FeB-based amorphous alloys studied by neutron thermo-diffractometry

Kinetics of crystallization of two amorphous alloys, Fe₇₀Cr₁₀B₂₀ and Fe₈₀Zr₁₀B₁₀, have been followed up by neutron thermo-diffractometry experiments performed in the two axis diffractometer D20 (Institut Laue-Langevin, Grenoble). The structural changes are directly correlated with the temperature dependence of the magnetization. Fe₇₀Cr₁₀B₂₀ crystallizes following a two-step process: an eutectic crystallization of α-Fe (BCC) and the metastable tetragonal phase (Fe_0.8Cr_0.2)₃B followed by another eutectic transformation to the stable phase (Fe_0.75Cr_0.25)₂B and more segregation of α-Fe. These tetragonal phases are magnetically anisotropic, giving rise to a large increase of the coercivity. This behavior is similar to that of Fe₈₀B₂₀ alloys, with Cr atoms replacing the Fe positions in both crystalline phases. Fe₈₀Zr₁₀B₁₀ shows also a two-step process in which two polymorphic transformations take place.     

Microstructure evolution and grain size distribution in nanocrystalline FeNbBCu from synchrotron XRD and TEM analysis

The microstructure evolution during nanocrystallization of an Fe₇₇Nb₇B₁₅Cu₁ amorphous alloy is investigated using in situ synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructure of the nanocrystallized alloy consists in dispersion of bcc-Fe nanocrystals of 4–6 nm of diameter embedded in a stabilized amorphous remaining matrix. The grain size distribution of the nanocrystalline Fe₇₇Nb₇B₁₅Cu₁ alloy was obtained using three different methodologies: statistical analysis of TEM images, the Warren–Averbach and Langford methods to analyse the XRD patterns and modelling of the diffraction pattern from the Debye equation. A lognormal distribution function has been assumed in all three methods in order to obtain comparable results. A good agreement is found in the calculated average radius and dispersion although some deviations are found with the Langford approach. The microstructure evolution during crystallization was obtained from the XRD patterns during heating (5.0 · 10^?3 K s^?1) at temperatures between 700 and 900 K. A decrease and prompt saturation of the growth rate is obtained, indicative of the diffusion barrier caused to the overlap between the concentration gradients at the interface of growing grains (soft impingement). A simple model assuming nucleation and initial fast growth of the crystalline grains followed by reduced growth capable of predicting microstructural evolution is presented. The modelling results agree with the experimental observations.     

Glass-to-crystalline transformation in rapidly quenched Fe78B9Si13 ferromagnetic alloy

Calorimetric studies of the ferromagnetic Curie temperature T_c, and the crystallization kinetics of the metallic glass Fe₇₈B₉Si₁₃ have been performed in an attempt to elucidate the possibility of reversible relaxation processes near T_c and the crystallization mechanisms taking place. From the change of T_c with heating rate and on annealing it appears that ageing irreversibly increases the Curie temperature. Crystallization is thermally activated following an Arrhenius behaviour and proceeds in two stages, the best fit to the experimental data for each stage of crystallization has been obtained by use of a Johnson-Mehl-Avrami-Erofe'ev equation. The effective activation energy and the kinetic exponent are respectively E = (4.7 ± 0.1) eV, n = 2.0 ± 0.2 for the first and E = (4.5 ± 0.1) eV, n = 4.0 ± 0.2 for the second stage of crystallization. From these results it appears that the mechanisms of crystallization are quite different in both stages.     

Crystallization kinetics of (Ni0.75Fe0.25)78Si10B12 amorphous alloy

Amorphous ribbon specimen of (Ni_0.75Fe_0.25)₇₈Si₁₀B₁₂ has been prepared by a single roller melt-spinning technique in the air atmosphere. The crystallization kinetics of the alloy has been investigated using different thermal analysis by means of continuous heating and isothermal heating. The activation energy of the alloy has been calculated by using Kissinger plot method and Ozawa plot method based on differential thermal analysis data, respectively. The products of crystallization have been analyzed by X-ray diffraction. A single phase of γ-(Fe, Ni) solid solution with grain size of about 10.3 and 18.5 nm precipitates in the amorphous matrix after annealing at temperatures 715 and 745 K, respectively. The crystallized phases are γ-(Fe, Ni) solid solution, Fe₂Si, Ni₂Si, Fe₃B and unidentified phase after annealing at 765 K. The details of nucleation and growth during the isothermal crystallization are expatiated in terms of local Avrami exponent and local activation energy.     

Preparation of thin ribbon and bulk glassy alloys in CoFeBSiNb(Ga) using planar flow casting and suction casting methods

Co-based ferromagnetic bulk glassy alloy (BGA) system is promising for future applications as new structural and functional materials. In the present paper as-cast Co₄₇Fe_20.9B_21.2Si_4.6Nb_6.3 bilayer, ribbon, rods with diameter up to 5 mm and [Co₄₇Fe_20.9B_21.2Si_4.6Nb_6.3]₉₈Ga₂ as-cast ribbon as well as rod with 4 mm diameter were investigated. Co based master ingots with the composition Co₄₇Fe_20.9B_21.2Si_4.6Nb_6.3 have been prepared in a vacuum furnace. The as-prepared master ingots were then purified by fluxing. Small amount of gallium (2 at.%) was added into one part of the purified master alloy. Ribbons, bilayer and bulk samples in form of rods were prepared by subsequent planar flow casting and suction casting method, respectively. Glassy structure of as-cast samples was examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thermal stability associated with glass transition temperature (T_g), crystallization temperature (T_x) and supercooled liquid region (?T_x = T_x – T_g) were examined by differential scanning calorimetry (DSC). The Curie temperature of the investigated ribbons and rods was determined by magnetic thermogravimetry analysis (TGA). Using special disc-shaped samples field dependencies of magnetostriction in parallel and perpendicular directions of the applied magnetic field were obtained by direct measurement.     

Crystallization behaviours ofCo100−1(x+y)NbxBy amorphous alloys

The crystallization behaviours of Co_100?1(x+y)Nb_xB_y amorphous alloys were investigated by means of differential thermal analysis and a conventional X-ray diffractometer. The crystallization sequences are discussed in terms of the equilibrium phase diagram of the ternary alloy system.Assuming crystallization occurs as a result of nucleation and growth, the stabilizing effect of eutectic phase separation on the crystallization is shown by introducing the effective free energy of the critical nucleus.     

Engineering of the magnetic properties of Finemet based nanocrystalline glass-coated microwires

The magnetic properties of Finemet nanocrystallized ribbons have been extensively studied and well understood. However, results obtained on microwires of such compositions are ambiguous considering the form of the hysteresis loop after the crystallization thermal treatment. In this paper, we show results on the static characterization of annealed Finemet based microwires (Fe_73.2Si_15.8B_6.9Cu_1.2Nb_2.9) of different geometries and illustrate how the magnetic anisotropy of Finemet based nanocrystalline microwires depends on the annealing temperature and the geometric characteristics of the microwires.     

Influence of rare earth elements on crystallization of Fe82Nb2B14RE2 (RE = Y, Gd, Tb, and Dy) amorphous alloys

The work concerns influence of rare earth elements on crystallization of Fe₈₂Nb₂B₁₄RE₂ (RE = Y, Gd, Tb, and Dy) group of amorphous alloys. The samples were obtained by typical melt spinning technique. The crystallization studies were carried out with the use of (i) differential scanning calorimetry (DSC) in the temperature range from 300 K to 850 K with different heating rates and (ii) standard magnetic balance (Faraday type). The crystalline structure before and after the first stage of crystallization were checked by XRD and HRTEM techniques. The measurements allow the determining crystallization temperatures, activation energies of crystallization, average size of nanograins formed during crystallization and the Curie temperatures. In the paper all the obtained results are widely discussed in the context of different rare earth alloying additions.     

High‐strength bimodal ultrafine Ti‐based alloys with enhanced ductility

Bulk Ti₆₅Fe₃₅ and (Ti₆₅Fe₃₅)_96.15‐xSn_3.85Nb_x (x = 0, 3, 5 and 7 at.%) alloys were prepared by cold crucible levitation melting, and tested in compression at room temperature. The Sn and Nb modified hypereutectic Ti‐Fe alloys exhibited improved mechanical properties under compression. (Ti₆₅Fe₃₅)_93.15Sn_3.85Nb₃ alloy displayed an ultimate compressive strength of 2.7 GPa and a compressive plastic strain of 15%. Electron microscope observations revealed Ti‐Fe(Sn,Nb) alloys having a bimodal microstructure with micrometer‐scale primary TiFe dendrites distributed in an ultrafine eutectic (β‐Ti+TiFe) matrix. The orientation relationship of β‐Ti with TiFe phases is TiFe (011)[100] ∥ β‐Ti (011)[100]. The improved mechanical properties are attributed to the morphology of the phase constituents and the larger lattice mismatches between β‐Ti and TiFe phases due to the additions of Sn and Nb.     

Nanocrystallization effects on the specific heat of Fe–Co–Nb–B amorphous alloy

Specific heat at constant pressure, C_P, was measured on amorphous, nanocrystalline and fully crystalline samples of Fe₆₀Co₁₈Nb₆B₁₆ alloy. Magnetic and calorimetric measurements agree, describing a continuously decreasing Curie temperature of the amorphous phase. A clear enhancement of C_P over the Dulong–Pettit limit has been observed (from 14% to 25 %). Part of the enhancement is due to magnetic (mainly amorphous phase) and electronic contributions, although an excess volume can be inferred from the high value of the slope of C_P versus temperature.     

Influence of composition in the crystallization process of Fe75−xNb10B15+x metallic glasses

The devitrification process of Fe_75−xNb₁₀B_15+x (x = 0, 5, 10) metallic glasses produced by melt-spinning has been analyzed by calorimetric, microstructural and magnetic measurements. The experimental results show large differences in the behavior of these alloys as a function of composition. The alloy x = 0 undergoes a primary crystallization process separated in two stages while only one is observed for alloys with x = 5 and x = 10. This difference, observed by DSC, is correlated with a microstructural change in the phases that precipitate. For alloys x = 5 and 10, Fe₂₃B₆ and bcc-Fe are identified after the first calorimetric peak. In the sample x = 0, bcc-Fe and an unidentified phase precipitate in the first peak but massive crystallization of bcc-Fe is observed after the second stage. Finally the thermal dependence of magnetization has been measured and the Curie temperatures for the metallic glasses are determined. The change of these quantities with heat treatment and composition is discussed.     

Effect of Nb in the nanocrystallization and magnetic properties of FeNbBCu amorphous alloys

The crystallization of melt-spun Fe_79?xNb_5+xB₁₅Cu₁ (x = 0, 2, 4) ribbons has been studied by differential scanning calorimetry and X-ray diffraction. A primary crystallization of bcc-Fe nanoparticles embedded in an amorphous matrix, followed by the precipitation of metastable borides from the residual matrix at higher temperatures is observed. The characteristic temperatures of crystallization events change with Nb concentration. The results obtained from thermal and structural characterization are related to the magnetic properties of the sample. A dependence of the magnetic behavior with the Fe/Nb content in the alloy is also unveiled. The decrease of Nb content in the alloy leads to an enhancement of both the saturation polarization and the Curie temperature due to variations in the exchange coupling between Fe atoms. However, the maximum values of magnetic entropy change do not vary appreciably among the three amorphous alloys. In nanocrystalline samples the amount of the nanocrystalline transformed fraction seems to be the main reason for the change in the saturation polarization of the sample.     

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.     

Crystallization of CoFeSiB metallic glass induced by long-time ball milling

Milling up to 800 h causes amorphous Co_70.3Fe_4.7Si₁₀B₁₅ alloy, prepared in the form of thin ribbon, to partially crystallize thus forming a powder material consisting of an amorphous phase and fcc-Co nanocrystals with an average grain size of about 10 nm. A gradual increase of the nanocrystalline fcc-Co fraction, produced by ball milling, was detected. Prolonged milling results in destabilization of the fcc-Co phase and oxidation of the powder material (presence of CoO phase after 1500 h of milling). The thermal stability studies of as-quenched and milled Co_70.3Fe_4.7Si₁₀B₁₅ alloy emphasized a two step crystallization behavior. During the first crystallization event, cobalt rich phases, i.e., fcc-Co and hcp-Co crystallize, whereas after the second crystallization event, Co₂B and Co₂Si are formed.