Crystallization acceleration of amorphous Fe40Ni40P14B6 alloy by fluxing technique

The effect of fluxing on the structure and the crystallization of amorphous Fe₄₀Ni₄₀P₁₄B₆ alloy has been studied. Subjected to fluxing, the incubation time upon isothermal crystallization decreases, whereas, the onset crystallization temperature upon non-isothermal crystallization (with constant heating rate) decreases, and crystallization peaks become less sharp. via structural characterization, it is considered that fluxing promotes relaxation of the system; the atomic structure becomes more similar to the corresponding crystallized phase, thus alleviating the transient effect on nucleation and accelerates the crystallization.     

Correlation between heat- and deformation-induced crystallization of amorphous Al alloys

Deformation-induced crystallization is correlated with thermal-induced crystallization in alloys with different compositions in a single amorphous alloy system. In Al₈₇Y₆Ni₅Co₂ and Al₈₅Y₈Ni₅Co₂ alloys, which undergo primary crystallization during heating, deformation-induced crystallization of fcc-Al has been observed. In Al₈₃Y₁₀Ni₅Co₂ alloy, which undergoes eutectic-like crystallization, no deformation-induced crystallization was observed. These observations can be explained by the presence or absence of quenched-in nuclei and the work required for the creation of an amorphous/crystalline interface under compressive stress.     

Effect of crystallization on the surface area of porous Ni-based metallic glass foams

The change of the specific surface area in porous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ metallic glass (MG) upon partial crystallization was investigated. The observed increase in the surface area of the annealed Ni-based MG foams is due to the formation of homogeneously distributed Ni₁₀(Zr,Ti)₇ rod-shape intermetallic phases with nominal diameters around 250?nm and ～800?nm length on the surface of MG struts during the crystallization. For longer annealing, the specific surface area decreases again due to a change of the morphology of the crystals from rod-like to disc-like appearance, thus suggesting an optimum regime for increasing the specific surface area upon isothermal annealing at a given temperature.     

Thermomagnetic study of devitrification in Fe-Si-B-Cu-Nb(-X) alloys

Thermomagnetic measurements have been used to study the magnetic and structural changes occurring at the two steps of the crystallization process of Fe_73.5Si_13.5B₉Cu₁Nb₁X₂ (X = Zr, Nb, Mo and V) alloys. Alloying raises the thermal stability of the amorphous phase against nanocrystallization in the order V < Mo < Nb < Zr and some differences in the final crystalline phases are found. The Curie temperature of the amorphous phase increases (about 15K) during structural relaxation. In the course of nanocrystallization a further increase of about 30K in the Curie temperature of the amorphous intergranular is observed for samples with X = Zr, Nb and Mo, but only of about 15K for samples containing V. The observed increase in the Curie temperature of the Fe-Si phase between the end of the first crystallization process and the end of the second crystallization process is associated with a reduction in the Si content, in agreement with X-ray diffraction results.     

Thermal stability and glass-forming ability of Se80− x Te20Ag x (x = 0, 3, 5, 7 and 9) chalcogenide glasses

The thermal stability and glass-forming ability (GFA) of Se_{80? x} Te₂₀Ag _x (x?=?0,?3,?5,?7 and 9) chalcogenide glasses have been investigated using differential scanning calorimetry (DSC). The DSC runs have been taken at five different heating rates (10, 20, 30, 40 and 50?K/min) under non-isothermal condition. The thermal stability and GFA are monitored through determination of the temperature difference ΔT?=?T _c???T _g, where T _c is the onset crystallization temperature, T _g is the glass transition temperature, H ^l is the stability parameter, ΔH _c is the enthalpy released during crystallization and F _i is the fragility index. The activation energy of crystallization E _c and crystallization rate factor K have also been determined as indicators of the thermal stability of the above-mentioned samples. It is found that Se₇₁Te₂₀Ag₉ is the most stable among all the samples of the series.     

A thermodynamic model for nanocrystallization in bulk metallic glasses

The structural complexity of glass-forming alloys, which generally contain more than three components, can lead by partial crystallization during annealing to a dispersion of nanocrystals in an amorphous matrix, giving the material a very high mechanical strength. In the present study, the evolution of the driving force for crystallization is expressed as a function of the composition and the chemical potentials of the components. Application to Zr₆₀Al₁₀Cu₃₀ and Zr₆₀Al₁₀Cu₂₀Pd₁₀ bulk metallic glasses shows that the first crystallization step leads to a metastable equilibrium between nanocrystals of an intermetallic and a percolating amorphous phase. The effects of the number of components and of chemical bonding on the fraction crystallized is analysed and discussed.     

Evolution of a sharp {110} texture in microcrystalline Fe78Si9B13 during magnetic crystallization from the amorphous phase

The effect of magnetic crystallization on texture evolution and control in nanocrystalline materials has been studied using a melt-spun amorphous Fe₇₈Si₉B₁₃ alloy. The magnetic crystallization was conducted at temperatures ranging from 653 to 853?K in a magnetic field up to 6?T. The temperatures used for magnetic crystallization were chosen on the basis of the Curie and crystallization temperatures of the amorphous phase, and the Curie temperature of the crystallized phase. The resultant microstructure was characterized by X-ray diffraction and FE-SEM/EBSP/OIM techniques. It was found that a sharp {110} texture developed when the amorphous precursor was crystallized at 853?K in a magnetic field of 6?T applied in a direction parallel to the ribbon surface.     

Determination of glass-forming ability of Ge1− x Sn x Se2.5 (0 ≤ x ≤ 0.5) alloys using differential scanning calorimetry

The glass-forming ability of Ge_{1? x} Sn _x Se_2.5 (0 ≤ x ≤ 0.5) alloys was studied using differential scanning calorimetry. Samples were scanned at different heating rates under non-isothermal conditions. Various simple quantitative methods were employed to assess the stability of the glassy materials in the above-mentioned system. All of these methods are based on characteristic temperatures, such as the glass-transition temperature, T _g, the onset-of-crystallization temperature, T _c, and the peak crystallization temperature, T _p. A crystallization rate factor, K, has also been used as a measure of the thermal stability of the glasses. It was found that Ge_0.7Sn_0.3Se_2.5 was the least stable among all the samples.     

Crystallization behaviour of Al-based amorphous alloy and nanocomposites by rapid quenching

The microstructure and crystallization behaviour of melt-spun Al₈₈Ni₉Ce₂Fe₁ amorphous alloy and nanophase composites have been studied by means of X-ray diffraction, transmission electron microscopy and scanning and isothermal calorimetry. The diffraction patterns from Al₈₈Ni₉Ce₂Fe₁ amorphous alloys are diffuse, indicating a basically amorphous structure but contain two rings presumed to be associated with quenched-in nuclei. In the cases of Al₈₈Ni₉Ce₂Fe₁ nanophase composites, nanoscale precipitated particles are homogeneously dispersed in an amorphous matrix, and the crystallite diameter and volume fraction are sensitive to quenching conditions. During thermal crystallization, a two-step phase transformation occurs in the amorphous alloy and nanocomposites, which is characterized by a diffusion-controlled precipitation of nanoscale Al particles and the growth of a Al₃(Ni, Fe) nanophase prior to a Al₁₁Ce₃ nanophase. This study gives insight into structure-control for obtaining nanophases dispersed in an amorphous matrix by rapid quenching.     

Microstructural features of phase transformation in a binary Pd–Si metallic glass

Glassy ribbons of Pd–Si alloys were prepared by a combination of melt spinning and flux treatment. The crystallization behaviour of a Pd₈₁Si₁₉ glassy alloy was studied through isothermal annealing at temperatures ranging lower than the glass-transition temperature T _g to around the onset of crystallization. The evolution of microstructures arising from isothermal annealing was investigated by X-ray diffraction (XRD) and (high-resolution) transmission electron microscopy ((HR)TEM). XRD spectra showed that, after the sample was annealed at a sub-T _g temperature, its first diffraction peak was split into two overlapping broad peaks. TEM analysis revealed the formation of a spherical, particle-like glassy phase embedded in the glassy matrix together with a finely connected network morphology within both. Combining these observations with compositional analysis suggested that phase separation had taken place during sub-T _g annealing. When the glassy alloy was annealed at temperatures higher than T _g, nanocrystalline structures, composed of Pd₃Si and Pd phases plus a Pd₉Si₂ phase with a lamellar structure, was formed.     

Difference in microstructure of Zr41Ti14Cu12.5Ni10Be22.5 glasses prepared in a 52 m drop tube and by water quenching

A 52 m drop tube has been used to solidify bulk-glass-forming Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 alloy. Glassy balls with different sizes solidified from the droplets whose structural features, glass-transition behaviour and crystallization kinetics have been investigated. The results indicate that the apparent activation energies of the glass transition and main crystallization reaction are significantly different from those of samples prepared by water quenching. The structural difference between the two types of glassy specimen is revealed by compression studies and in situ energy-dispersive X-ray diffraction. The results are important for understanding the structural features of bulk-forming glasses.     

Precipitation of an icosahedral quasicrystalline phase in Hf59Ni8Cu20Al10Ti3 metallic glass

The crystallization process of a Hf₅₉Ni₈Cu₂₀Al₁₀Ti₃     

Identification of crystal nucleation and growth in an amorphous FeZr2 alloy by means of electrical resistance measurements

A polymorphous crystallization process in an amorphous FeZr₂ alloy has been investigated by means of accurate electrical resistance measurements (ERMs) at elevated temperatures. It was found that, upon crystallization of the amorphous alloy, the electrical resistance increased with increasing temperature, exhibiting three distinct stages. Quantitative microscopy observations revealed that the three stages originated from crystal nucleation, from subsequent growth of crystal nuclei and from coarsening of the crystallites respectively. The activation energies for the crystal nucleation and growth determined from the ERM data agree satisfactorily with the data in the literature. The success in identification of the crystal nucleation and growth processes by means of ERMs may originate from differences in electrical resistance changes due to the crystal formation and the crystalline-amorphous interface formation processes from the amorphous phase.     

Laser-induced synthesis of selenium,silver and silver–selenide nanocrystallites in amorphous Se98Ag2 alloy

We report high-yield synthesis of selenium, silver and silver–selenide nanocrystallites (Ag₂Se) in an amorphous matrix of Se₉₈Ag₂ alloy. The method avoids the necessity of exotic chemicals, high temperatures and high pressures, and requires only a few seconds of laser exposure (～30?s). The amorphous Se₉₈Ag₂ alloy is first synthesized by the conventional and cost-effective melt-quenching technique. Then, laser-induced crystallization was performed using a Nd:YVO₄ laser. The morphology and microstructure of the obtained nanocrystallites have been analysed by DSC, XRD and SEM.     

Formation of ductile Al-based metallic glasses without rare-earth elements

Al₇₅Cu₁₇Mg₈ is a eutectic composition according to the ternary phase diagram, which can be quenched into a fully amorphous phase by adding 2-8at.% Ni, but the addition of a similar percentage of Gd failed to form the amorphous phase. The amorphous alloys obtained exhibit two broad diffuse peaks in the X-ray diffraction curves and, correspondingly, two halo rings in the electron diffraction patterns, implying that two types of local atom configuration exist. Thermal analysis of the amorphous alloys indicates that the primary crystallization peak shifts to higher temperatures with increasing Ni content. The occurrence of a nucleation and crystal growth peak during isothermal crystallization reveals the amorphous nature of the quenched ribbon alloys. The quenched amorphous ribbons do not break after bending by 180°. Mechanical testing yielded a tensile strength of 810MPa for (Al₇₅Cu₁₇Mg₈)₉₅Ni₅, and a vein structure, characteristic of amorphous fracture, is apparent in scanning electron micrographs. The different effects of Ni and Gd on the glass formation indicate that the large atomic size of Gd is not critical to the glass formation.     

Microwave-induced heating of a single glassy phase and a two-phase material consisting of a metallic glass and Fe powder

Metallic glasses exhibit low viscosity in a temperature range between the glass transition and the crystallization temperature, a feature that allows successful sintering of glassy powders. Microwave heating, being volumetric, has significant advantages over conventional heating in materials processing, such as substantial energy savings, high heating rates and process cleanliness. In the present study, we investigate the stability of Cu₅₀Zr₄₅Al₅ glassy powders and the formation of a bulk two-component metallic glassy-crystal sample by microwave heating in a single-mode cavity (915 MHz) in an alternating magnetic field.     

An explanation for the low-temperature H evolution peak in hydrogenated amorphous silicon films deposited 'on the edge of crystallinity'

The puzzling existence of a sharp low-temperature (T = 400°C) H evolution peak in compact hydrogenated amorphous silicon (a-Si : H) films deposited 'on the edge of crystallinity' is examined. From infrared absorption and X-ray diffraction (XRD) measurements, we show that none of the standard methods used to explain the existence of this peak in a-Si : H materials is applicable to the present films. From the Si-H wag-mode peak frequency, we postulate the existence of very small Si crystallites contained within the amorphous matrix. While the crystallite volume fraction is too small to be detected by XRD in the as-grown films, crystallization is observed for this material at anneal temperatures as low as 500°C. It is proposed that these crystallites catalyse the crystallization of the remainder of the amorphous matrix upon moderate annealing, enabling H surface desorption and H₂ out-diffusion to the sample surface along newly formed grain boundaries at low anneal temperatures.     

Structural investigations on Nd-doped silica nanocomposites: effect of sintering temperature and dopant concentration

Neodymium-doped silica nanocomposites were prepared from an acid-catalysed sol–gel solution followed by heat treatment. The structural and microstructural properties of the prepared samples as a function of sintering temperature and Nd concentration are reported. Fourier transform infrared spectra show that phase separation occurs during heat treatment. The presence of Nd₂O₃ and α-Nd₂Si₂O₇ phases in the samples was established by X-ray diffraction (XRD), and transmission electron microscopy (TEM) micrographs revealed the microstructure of the nanocomposites. From XRD patterns, the crystallite size was determined using the Debye–Scherrer formula, while the particle size was estimated from TEM micrographs. The results suggest that sintering at high temperature enhances the crystallinity and density of Nd₂O₃–SiO₂ nanocomposites, while the high concentration of neodymium prevents the crystallization of SiO₂.     

Formation of the icosahedral quasicrystalline phase in Zr70Pd30 binary glassy alloy

It is found that a single icosahedral quasicrystalline phase is formed as a primary precipitation phase in the melt-spun Zr₇₀Pd₃₀ binary glassy alloy with a two-stage crystallization process. The onset temperature of the transformation from the amorphous to the icosahedral phase is 701 K at the heating rate of 0.67 K s-1. The size of the icosahedral particles lies in the diameter range below 10 nm and the particles are distributed homogeneously. The second-stage crystallization reaction results in the formation of a Zr₂Pd phase through a single exothermic reaction. The formation of the nanoscale icosahedral phase indicates the possibility that icosahedral short-range order exists in the Zr-Pd binary glassy state. Comparison with the thermal stability of an icosahedral phase in the Zr-Ni-Pd system shows that the icosahedral phase is stabilized by the addition of Ni. The stabilization is due to the restraint of the long-range rearrangement of the constitutional elements resulting from the strong chemical affinity between Zr and Ni.     

Thermomagnetic gravimetric study of the crystallization process in amorphous Fe-Si-B-Cu alloys

The thermal properties of amorphous Fe_84-xSi₆B₁₀Cu_x and Fe_77.5-xSi_13.5B₉Cu_x alloys have been investigated using thermomagnetic gravimetry (TMG). After electronically cancelling the sample weight, an apparent weight (AW) was applied by placing a magnet beneath the sample pan. Heating was then carried out to detect the formation of ferromagnetic crystallization-induced phases. In both Cu-free Fe₈₄Si₆B₁₀ and Fe_77:7Si_13:5B₉ alloys, a two-stage increase in AW was observed on the TMG curve. The addition of Cu to these alloys caused a decrease in the crystallization temperatures as well as a marked increase in AW in the above two-stage process. It is presumed that such behaviour originates from the lack of metalloid elements in the amorphous matrix, which is caused by their distribution to Cu-enriched clusters.