Effect of irradiation particle mass on crystallization of amorphous alloys

The crystallization temperature of amorphous alloys was found to be significantly lowered by heavy ion or electron irradiation during annealing. However, only heavy ion irradiation altered the mode of crystallization. Both a binary and multi-element amorphous alloy showed this type of response to irradiation. Radiation-enhanced diffusion processes in the amorphous state can explain the increased crystallization kinetics during irradiation. Heavy ion irradiation alters the crystallization mode by causing direct transformation to the final equilibrium phase as opposed to intermediate metastable phase formation during thermal annealing or electron irradiation. The equilibrium phase is believed to nucleate directly in the displacement cascades, which only form during heavy ion bombardment.     

Transformation studies and mechanical properties of melt-quenched amorphous titanium-silicon alloys

The formation, stability and decomposition characteristics of the amorphous phase in binary titanium-silicon alloys rapidly quenched from the molten state have been investigated. Electron microscopy and diffraction coupled with X-ray diffraction techniques suggested that the amorphous phase could be obtained in alloys containing 15 to 20 at% silicon. The transformation of the amorphous phase to the equilibrium phases took place in two stages. A metastable b c c titanium solid solution, containing silicon in excess of the equilibrium value, formed initially, followed by the precipitation of the Ti₅Si₃ intermetallic compound. Microstructural features at various stages of decomposition have been described and interpreted in terms of the constitution of the alloys. Mechanical properties of the amorphous alloys have also been investigated.     

Structural evolution during mechanical milling and subsequent annealing of Cu–Ni–Al–Co–Cr–Fe–Ti alloys

This study reports the structural evolution of high-entropy alloys from elemental materials to amorphous phases during mechanical alloying, and further, to equilibrium phases during subsequent thermal annealing. Four alloys from quaternary Cu_0.5NiAlCo to septenary Cu_0.5NiAlCoCrFeTi were analyzed. Microstructure examinations reveal that during mechanical alloying, Cu and Ni first formed a solid solution, and then other elements gradually dissolved into the solid solution which was finally transformed into amorphous structures after prolonged milling. During thermal annealing, recovery of the amorphous powders begins at 100 °C, crystallization occurs at 250–280 °C, and precipitation and grain growth of equilibrium phases occur at higher temperatures. The glass transition temperature usually observed in bulk amorphous alloys was not observed in the present amorphous phases. These structural evolution reveal three physical significances for high-entropy alloys: (1) the annealed state of amorphous powders produces simple equilibrium solid solution phases instead of complex phases, confirming the high-entropy effect; (2) amorphization caused by mechanical milling still meets the minimum criterion for amorphization based on topological instability proposed by Egami; and (3) the nonexistence of a glass transition temperature suggests that Inoue's rules for bulk amorphous alloys are still crucial for the existence of glass transition for a high-entropy amorphous alloy.     

Structure of the silicon–oxide interface

The interface between crystalline Si and its amorphous native oxide SiO₂ is the basis for most current computer technology; yet its structure remains poorly understood. Here, we summarize a recent computational approach to this problem, which explicitly averages over the ensemble of configurations of the amorphous oxide. The approach makes possible calculations of equilibrium interface structures and energies for an amorphous oxide interface. The resulting model for the Si–SiO₂ interface has low energy, and it reconciles a variety of puzzling experimental results.     

Oxygen-stabilised partial amorphisation in a Zr50Cu50 alloy

Structural transitions on heating a ZrCu martensite prepared by induction melting and subsequent rapid cooling have been investigated by means of differential scanning calorimetry (DSC), electrical resistivity measurements, X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The ZrCu martensite firstly transforms to the ZrCu compound with the B2 structure at around 300°C. Depending upon the annealing temperature, there are two routes for the ZrCu compound to reach equilibrium. By annealing at T 425°C, the equilibrium state, i.e. a eutectoid mixture of Zr₇Cu₁₀ and Zr₂Cu, is attained directly because the required chemical segregation for the equilibrium phases can be satisfied; alternatively, by annealing at 300°C < T < 425°C, the equilibrium state is reached via another metastable state, an amorphous phase. This is, however, present only locally and contains significant levels of oxygen. A qualitative explanation of the observed structural transitions is given in terms of thermodynamic and kinetic considerations.     

Structural evolution of the Ti70Ni15Al15 powders prepared by mechanical alloying

Amorphization and crystallization behaviors of Ti₇₀Ni₁₅Al₁₅ powders during mechanical alloying (MA) and subsequent heat treatments are studied. Amorphous phase that cannot be obtained in the rapidly quenched ribbon is formed in the powders after MA for 60 h. Upon continuous heating of the amorphous powders in DSC, two exothermic events are observed. The first exothermic event corresponds to the crystallization of the amorphous matrix into a supersaturated α-Ti phase of hexagonal close-packed structure. The growth kinetic of the α-Ti phase is sluggish, resulting in the formation of nanostructured α-Ti matrix. The second exothermic event corresponds to the solid state transformation of the meta-stable α-Ti into the equilibrium phases, Ti₂Ni and Ti₃Al. Using the amorphous powders, Ti-based bulk materials with novel microstructures can be developed for structural applications.     

Microstructure and enhanced atomic diffusion of friction stir welding aluminium/steel joints

Lap joints of friction stir welding between aluminium and stainless steel sheets were conducted using a welding tool with a cutting pin. The atomic diffusion of Fe–Al system during the severe plastic deformation was investigated. The interfacial microstructure and metallurgical reaction was analysed. The metallurgical reaction layers were identified as a compounds containing a phase of Al₃Fe, partial solid solution of Fe and Al, and amorphous with a thickness of 0.9–3.3?μm which depending on the process parameters. The interdiffusion coefficient between Al and Fe atoms is about 4 orders of magnitude compared with that under thermal equilibrium state. The nanocrystalline and partial amorphous were formed near the interface which may caused by the enhanced atomic diffusion.     

Phase separation in blends of oligo (caprolactone glycol)-based polyurethane with poly(vinyl chloride)

The structure of o!igo (caprolactone glycol)-based polyurethane-poly(vinyl chloride) (PVC) blends obtained from tetrahydrofuran solution was studied by X-ray diffraction and electron microscopy. The system is characterized by a oornp!ex phase diagram with two types of equilibria: (1) crystal-liquid equilibrium in the region of blend compositions up to 40 wt% PVC; (2) liquid-liquid equilibrium in the region of man compositions. The crystallization kinetics of oligo(caprolactone) polyurethane blocks from an initially compatible amorphous blend was investigated. Small-angle X-ray cattering (SAXS) studies provide evidence that the amorphous component (PVC) is incorporated in the interlamellar crystalline regions of the compositions. The amorphous blends are characterized by the presence of modulated structures at two scale levels, this being explained as e result of the process in which the bulk samples were obtained from a three-component polymer-polymer-solvent system.     

Highly stabilized amorphous 3-bis(4-methoxyphenyl)methylene-2-indolinone (TAS-301) in melt-adsorbed products with silicate compounds

3-Bis(4-Methoxyphenyl)methylene-2-indolinone (TAS-301) is a poorly water-soluble drug showing low oral bioavailability in rats and dogs. Previously, we reported that when a physical mixture of TAS-301 and a porous calcium silicate, Florite® RE (FLR), was heated at high temperature (250°C), the drug melted and was adsorbed by the FLR in an amorphous state, and that the preparation (melt-adsorbed product) showed a significantly increased solubility and dissolution rate, and a significantly enhanced oral bioavailability of the drug. The aim of the present study was to elucidate important factors for preparing a melt-adsorbed product showing greater stability of drug in an amorphous state. We examined the effects of the kind of adsorbent, drug/adsorbent ratio, heating conditions, and drug particle size on converting drug crystal into an amorphous state, the stability of amorphous state, and chemical stability of the drug in the melt-adsorbed products under a high temperature and high humidity condition (60°C/80% RH, open). FLR, light anhydrous silicic acid and two types of hydrated silicon dioxides were tested as adsorbents. For the batch method, TAS-301 was converted into an amorphous state by heating TAS-301/adsorbents physical mixtures above the melting point of TAS-301 for more than 2 min. The amorphous state was most stabilized when FLR was used as an adsorbent and drug/FLR ratio was 1:0.5 and more. For the continuous method using the twin screw extruder that enables significantly larger scale manufacturing than batch method, TAS-301 melt-adsorbed products were able to produce when only FLR was used as adsorbent. The heating temperature was needed to be set above the melting point of TAS-301 to convert it into an amorphous state as well as batch method. The amorphous state was stabilized when drug/FLR ratio was 1:2 and more. The micronization of the drug decreased the stability of the amorphous state. These results indicate the importance of optimizing the above factors in the preparation of melt-adsorbed product.     

Formation of metastable solid solutions in the Pb-Ge system

The formation and decomposition behaviours of metastable solid solutions in liquid-quenched and vapour-quenched Pb-Ge alloys were studied using X-ray diffraction, transmission electron microscopy (TEM), differential thermal analysis (DTA) and resistance measurement techniques. It is shown that the Pb-rich fcc phase can retain upto about 13 at% Ge on liquid-quenching and upto about 5 at% Ge on vapour-quenching. Decomposition of the Pb-rich fcc phase occurs in the temperature range 220 to 300° C and it is a temperature dependent nucleation and growth phenomenon. Upto about 7.5 at% Pb can be retained in Ge-rich compositions in an amorphous Ge matrix on vapour-quenching but there is no detectable solubility of Pb in crystalline Ge retained by liquid-quenching. On heating, the amorphous Ge-Pb films crystallize to a Ge-rich solid solution which decomposes to equilibrium constituents at higher temperatures. Stability of amorphous Ge-Pb films decreases on increasing metal concentration.     

The order and annealing of quench-condensed lead-based alloys studied by the phonon spectrum from electron tunnelling and by resistance measurements

Metal films consisting of Pb plus 10 at. % of one of eleven different impurities were quench-condensed on substrates at temperatures below 2 K. Ag, Cd, Ga, Ge, In, Mg, Na, Sb, Sn, Te and Zn were used as impurity elements. For these disordered films, the temperature for the transformation from an amorphous to a crystalline state varied qualitatively in accordance with the melting temperature of the impurity element. The degree of order in the disordered films was estimated from the phonon structure in superconductive tunnelling experiments. Structural changes after annealing at 30, 100 and 300 K were studied via altered phonon distributions. During the annealing the resistance as a function of temperature was also monitored. Three different transformations of the lattice order were registered. The most striking of these transformations was the one that took the film from an amorphous into a crystalline state. If the melting temperature of the impurity was greater than twice the value for pure lead, the film transformed directly from an amorphous to a long-range ordered crystalline film at a relatively high annealing temperature. A transformation within the amorphous state was also observed. In contrast to previously reported results, an atomic size difference of 10% between the constituents was not required in order to form an amorphous state in quench-condensed films for this type of alloy.     

Miscibility and melting properties of poly(ethylene 2,6-naphthalate)/poly(trimethylene terephthalate) blends

The miscibility and melting properties of binary crystalline blends of poly(ethylene 2,6-naphthalate)/poly(trimethylene terephthalate) (PEN/PTT) have been investigated with differential scanning calorimetry (DSC). The glass transition and cold crystallization behaviors indicated that in PEN/PTT blends, there are two different amorphous phases and the PEN/PTT blends are immiscible in the amorphous state. The polymer–polymer interaction parameter, , calculated from equilibrium melting temperature depression of the PEN component was −1.791 × 10⁻⁵ (300 °C), revealing miscibility of PEN/PTT blends in the melt state.     

Amorphization Behavior and Nanocrystalline Structure of Fe–P–Si–V Alloys

It is found that Fe–P–Si–V alloys tend to be in an amorphous state on cooling at a rate of 10⁵to 10⁶K/s. As compared to Fe–P–Si alloys, the crystallization behavior of the Fe–P–Si–V alloys is more complex owing to the formation of both metastable and equilibrium silicides. The metastable phases are nanocrystalline, as evidenced by transmission electron microscopy, which ensures a noticeable strength gain.     

Mechanical response of melt-spun amorphous filaments

High-speed melt spinning of a cyclo-olefin polymer (COP) and a copolyamide (CoPA) have been performed. Differential scanning calorimetry curves of the resulting monofilaments show that they remain in an amorphous state even after hot drawing. Wide angle x-ray diffraction patterns of undrawn and drawn COP filaments show that although the material remains in an amorphous state, a degree of orientation is induced in the polymer after drawing. The amorphous filaments show an enhanced bending recovery with respect to different semi-crystalline monofilaments commercially available. However, single fiber axial compressive testing indicates that the amorphous filaments exhibit a compressive modulus value which is 50% lower than what is observed for a reference semi-crystalline PET filament. Analysis of the compressive strains applied by the bending recovery test indicates that while the maximum applied strains remain well within the region of elastic deformation of the amorphous materials, the threshold between elastic and plastic deformation is reached for the semi-crystalline materials.     

Metastable defects in hydrogenated amorphous silicon

The electronic structure of hydrogenated amorphous silicon (a-Si:H) is in a state of metastable equilibrium and can change upon application of external stimuli. We study the effect of thermal quenching and light soaking in lithium-doped a-Si:H, on its conductivity and thermopower. We present evidence showing that the metastable state obtained after fast quenching is different than that obtained after light exposure. Experiments on chalcogenides show that they are not affected by thermal quenching although they change upon light soaking. This is in contrast with lithium doped a-Si:H in which both effects are observed. Our experiments suggest that hydrogen present in a-Si:H plays an important role by controlling heterogeneities and potential fluctuations in a-Si:H. Light soaking appears to enhance these potential flucutations, whereas fast cooling seems to have little effect on them.     

Study of structural free volumes in the supercooled liquid state of bulk amorphous ZrTiCuNiBe

Structural free volumes in amorphous alloys play an important role for a lot of solid state properties. In this paper, we report on the determination of both the size and the chemical environment of structural free volumes in bulk amorphous Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 and particularly in the supercooled liquid state of the alloy by making use of measurements of the positron annihilation radiation. The free volumes in the supercooled liquid state are detected to be surrounded dominantly by Be and Ti atoms together with less Zr atom. The present investigations are of interest for atomic-scale understanding of the atomic processes in bulk amorphous alloys.     

Investigation of Micro Formability of Bulk Amorphous Alloy in the Supercooled Liquid State Based on Fluid Flow and Finite Element Analysis

Research results on the viscous flow deformation behavior of bulk amorphous alloy in different systems are reviewed. The material exhibits an ideal Newtonian fluid at a high temperature. Analytical solution of lamellar fluid flow behavior is used to discuss the viscous flow behavior of the bulk amorphous alloy in the supercooled liquid state. A material model, which describes such deformation behavior of Mg60Cu30Y10 amorphous alloy,is introduced into the finite element method of microforming process. Surface feature size was investigated and found not sensitive to the micro formability. Bulk amorphous alloy may possibly be applied to microelectro-mechanical-systems (MEMS) fabrication.     

On the phase composition of polymethylsiloxane derived ceramics

The pyrolysis products of polymethylsiloxanes (PMS) have been studied in order to characterize their phase composition from the viewpoint of equilibrium thermodynamics. Dilatometry measurements and redox analyses with PbO have been conducted and the results analysed and compared with respect to the second law of thermodynamics. In agreement with equilibrium thermodynamics, which shows that there is no ternary compound in the Si–O–C system, the experimental results imply that the pyrolysis residue is an amorphous or nanocrystalline composite consisting of a SiO2-phase matrix with embedded SiC and C particles. These findings seem to contradict the accepted scientific opinion of ternary silicon-oxycarbide phases.     

Structural modification and phase transformation kinetics: crystallization of amorphous Fe40Ni40P14B6 eutectic alloy

The effect of processing histories (fluxing and pre-annealing) on the amorphous structure and the crystallization kinetics of amorphous Fe₄₀Ni₄₀P₁₄B₆ alloy prepared by melt spinning has been studied by differential scanning calorimetry, X-ray diffraction, transmission and scanning electron microscopy. For isothermal crystallization, an incubation time exists, and for isochronal crystallization, an abnormally sharp crystallization peak (with the transformed fraction corresponding to the transformation-rate maximum f _p as less than 0.632) occurs. Subjected to fluxing and pre-annealing, the incubation time (in isothermal crystallization) decreases, whereas the initial crystallization temperature (in isochronal crystallization) declines as well as the less sharp crystallization peak and f _p approaches 0.632. A kinetic model considering transient nucleation is proposed and analyzed, which could describe well the singular crystallization behavior of amorphous Fe₄₀Ni₄₀P₁₄B₆ alloy. A recipe based on the kinetic model is also proposed to obtain the kinetic parameters from experiment data. Via kinetic analysis and amorphous structural characterization, it is considered that pre-annealing and fluxing promote relaxation of the system close to the meta-stable equilibrium state; the atomic structure becomes more similar to the correspondingly crystallized phase, thus declining the amorphous stability and alleviating the transient effect on nucleation.     

Characterization of Commercial Lots of Erythromycin Base

The physical properties of five commercial lots of erythromycin base were evaluated with the object of characterizing the bulk chemical. The materials were investigated by thermal analyses, IR spectroscopy, Xray powder diffractions, equilibrium solubility in distilled water, and dissolution rates in phosphate buffer pH 7.5. Among the lots examined, three were found to be polymorphic variants of crystalline dihydrate, one a crystalline anhydrate and the other an amorphous solid. The different crystalline dihydrates displayed considerable variation in thermal properties but showed similar equilibrium solubilities in distilled water. At a given temperature, crystalline anhydrate has a higher aqueous solubility compared to the dihydrates. All crystalline forms of erythromycin showed rapid in vitro drug release from loosely-filled capsules. In contrast, the amorphous form exhibited a high equilibrium solubility but slower dissolution rate, a result attributable to its poor wettability. Contact angle measurements performed on powder compacts of the solid confirmed the hydrophobic nature of the amorphous form, an inference based on the high and stable contact angles formed. Under similar test conditions, low contact angles and rapid spreading of liquid film were noted for the crystalline dihydrates. The previously reported observations of lower solubility with increasing temperature was confirmed in the current studies for all forms of erythromycin base.