Time-resolved studies of Ti34−xCu47Zr11Ni8Six metallic glass devitrification using high temperature X-ray powder diffraction

Time-resolved devitrification studies of Ti_34−xCu₄₇Zr₁₁Ni₈Si_x metallic glasses were performed using a recently developed high temperature furnace in a Debye–Scherrer geometry. Samples included powders produced by high pressure gas atomization and surface coatings deposited by air plasma spraying. Synchrotron radiation at the Advanced Photon Source at Argonne National Laboratory was used to follow the devitrification of samples during heating at 40 K min⁻¹ between 623 and 1073 K. The crystallization behavior observed with structural diffraction data compare well with results from thermal analysis using differential scanning calorimetery. At 1073 K, these amorphous alloys evolve to a four phase microstructure which includes phases that appear to be closely related to Cu₅₁Zr₁₄, CuTi and Cu₂TiZr.     

Calorimetric investigation of the structural relaxation and crystallization of amorphous Ge16Te84 alloys

Differential scanning calorimetric measurements have been performed on amorphous Ge₁₆Te₈₄ alloys; the study of the dependence of the cooling rate on the reciprocal fictive temperature yields the activation energy of the relaxation process; this value fits well that deduced from shear viscosity measurements. The enthalpy difference between the supercooled liquid and the crystal has been investigated by drop calorimetry on samples annealed at different temperatures below T_g, during different times. These results, combined with the former obtained by DSC lead to the enthalpy values of the metastable liquid system from 390 to 510 K.     

Crystallization processes of Ag–Ge–Se superionic glasses

The interest in superionic systems has increased in recent years because of the potential application of these materials as solid electrolytes. In this field, amorphous materials present important advantages when compared to the crystalline solids: larger conductivity, isotropy and absence of grain boundaries. In this work, amorphous alloys of compositions (Ge₂₅Se₇₅)_100−yAg_y with y=10, 15, 20 and 25 at.% have been studied. Amorphous samples in bulk were obtained from the liquid by water quenching (melt-quenching technique). The crystallization kinetics of the amorphous alloys have been studied under continuous heating and isothermal conditions by means of differential scanning calorimetry. A glass transition and two exothermic transformations were observed in all the samples. The quenched samples and the crystallization products have been characterized by X-ray diffraction. The primary crystallization of the ternary phase Ag₈GeSe₆ and the secondary phase GeSe₂ was observed. The glass and crystallization temperatures, the activation energy and the crystallization enthalpy are reported. The first step of the crystallization of the Ag₈GeSe₆ phase in all the (Ge₂₅Se₇₅)_100−yAg_y samples is modelled taking into account the Johnson–Mehl–Avrami–Kolmogorov theory and considering that the changes in the composition only modify the viscosity of the undercooled liquid. The transformation diagrams (TTT and THRT) are calculated and the glass forming ability is analyzed. The experimental results are discussed and correlated with the structures proposed for the glass. The present results and conclusions are also compared with those reported by other authors.     

Thermal expansion of amorphous metals

The thermal expansion of three commercial amorphous metals has been measured from room temperature to above the crystallization temperature. The results clearly demonstrated the glassy nature of these materials and show the reduction in volume which results from crystallization. The glass transformation temperature is more evident here than on the usual differential scanning calorimeter curves. The activation energies associated with the glass transformation and crystallization are derived from data taken as a function of heating rate. The measured temperatures for the glass transformation and crystallization from the thermal expansion measurements are the same as those obtained from DSC measurements at the same heating rate.     

Crystallization kinetics of electro-deposited amorphous CoP alloys

The crystallization characteristics of amorphous Co-15.2 at.% P and Co-18.4 at.% P alloys were studied by means of a differential scanning calorimeter (DSC) and a transmission electron microscope (TEM). The analysis of DSC data obtained in the isothermal operation was performed by means of the Johnson-Mehl-Avrami (J-M-A) rate equation. The values of the J-M-A time index, n, for the crystallization of Co-15.2 at.% P and Co-18.4 at.% P alloys were 3.3 ± 0.1 and 2.2±0.1, respectively. It has been revealed through these experimental results and TEM observation during the crystallization process of the amorphous alloys that the crystallization of the former alloy results in interface-controlled three-dimensional spherical growth (n = 3) of Co₂P crystal, whereas the crystallization of the latter alloy results in two-dimensional growth (n = 2) of Co₂P disks having constant thickness. It has also been found that the apparent activation energy for the crystallization is 195 kJ/mol. and 190 kJ/mol. for Co-15.2 at.% P and Co-18.4 at.% P alloys, respectively.     

Theoretical prediction of bulk glass forming ability (BGFA) of Ti-Cu based multicomponent alloys

The bulk glass forming ability (BGFA) of Ti-Cu based multicomponent alloys has been evaluated via theoretical modeling and computer simulation studies based on a combination of electronic theory of alloys in the pseudopotential approximation and the statistical thermodynamical theory of liquid alloys. The magnitude of atomic ordering energies, calculated by means of the electronic theory of alloys in the pseudopotential approximation, was subsequently used for calculation of the key thermodynamic parameters such as enthalpy, entropy and Gibbs free energy of mixing, viscosity, and critical cooling rate of the binary Ti-Cu and ternary Ti-Cu-X alloys. The potential alloying elements (X) can be divided in two groups defined by their effect on the variation of the negative heat of mixing and their influence on the critical cooling rate. Most of the predicted candidate alloying elements from either X_I (Al, Si, Ag) or X_II (Co, Ni, Fe, Sn, Be) and/or both groups have already been used successfully for the fabrication of new Ti-Cu based bulk metallic glasses. It was also shown that the critical cooling rate appears to be a more important parameter rather than the change in the negative heat of mixing for the prediction of candidate alloying elements improving BGFA.     

Microstructure evolution and thermal properties in FeMoPCB alloy during mechanical alloying

Fe_79.3Mo_4.5P_8.1C_6.75B_1.35 amorphous alloy composite powder from respective element powders of Fe, Mo, C, B, and Fe–P intermediate compound, was synthesized by mechanical alloying. Microstructure evolution analysis indicates that the synthesized amorphous alloy composite powder after a milling time of 70 h encompasses predominately amorphous matrix embedded by nanocrystalline α-Fe with a grain size of about 5.5 nm. However, unlike other Fe-based amorphous alloys, the synthesized amorphous alloy composite powder exhibits no obvious supercooled liquid region with only crystallization temperature. The corresponding crystallization onset temperature and exothermic enthalpy measured from DSC curves are about 762 K and 15.86 J/g, respectively. The results obtained provide good candidate materials for fabricating bulk metallic glass composites and related bulk nanocrystalline materials.     

Structure and magnetic properties of bulk amorphous Fe60Co10Ni10Zr7B13 alloy formed by mechanical synthesis and hot pressing

Formation and magnetic properties of the bulk amorphous Fe₆₀Co₁₀Ni₁₀Zr₇B₁₃ alloy are described. The amorphous powder was prepared by high-energy ball milling of the amorphous ribbon and then was pressed under high pressure and temperature to obtain a bulk alloy in the form of a disc 5 mm in diameter and 2.5 mm thick. The compacted discs are fully amorphous as confirmed by X-ray diffraction and Mössbauer spectroscopy measurements. Differential scanning calorimetry curve recorded for the amorphous ribbon shows two exothermal peaks related to two stages of crystallization. Similar thermal behavior is observed for the powder samples. Hysteresis loops obtained for the amorphous ribbon, as-milled and compacted powders reveal their soft magnetic properties.     

Internal friction and structural relaxation of amorphous Fe---Mn---P alloys

Measurements of internal friction and dynamic modulus have been carried out on amorphous Fe_83−xMn_xP₁₇(x=9,12, or 15) alloys in the temperature range 300–800 K, as a function of applied frequency, by a forced oscillation method. The characteristic transition temperatures and activation energies for crystallization were measured by differential scanning calorimetry (DSC). The viscous flow of the sample was measured using a thermomechanical analyzer (TMA) under continuous heating conditions. Internal friction increases rapidly with temperature starting from 450 K for 0.1 Hz, 474 K for 0.5 Hz, 486 K for 1.0 Hz, 511 K for 5 Hz, and 525 K for 10 Hz for the Fe₇₄Mn₉P₁₇ specimen. It was found that the internal friction peak temperature was very closely the same as the steady state viscous flow temperature from the TMA curves for Fe₇₄Mn₉P₁₇ and Fe₆₈Mn₁₅P₁₇ but not for Fe₇₁Mn₁₂P₁₇. It is assumed that the internal friction peak does not correspond to the glass transition temperature or crystallization temperature for these alloy systems. Two maxima in the free-volume fraction and two minima in the viscosity were also found for the samples which exhibit a ‘mid-contraction’ in their TMA displacement curves. The internal friction peak occurred at the steady state viscous flow temperature. This occurrence means that the origin of the internal friction peak is related to the viscous flow behavior and free-volume fluctuation for this alloy system.     

The study of transformation kinetics of the amorphous PdSi alloys

The kinetics of transformation behavior of roller-quenched amorphous Pd₈₃Si₁₇ and Pd₈₀Si₂₀ alloys after linear and isothermal heating is reported. The transformation was examined with electron microscopy, electron diffraction and electrical resistivity measurement. The crystallization of amorphous alloys with 17 at% Si begins with metastable ordered fcc solid solution. The ordered fcc solution is transformed to a ordered metastable phase with, probably, orthorhombic structure. The crystallization of amorphous alloys with 20 at% Si begins by formation of spherulites with lamellar structure. Using electron diffraction we found that spherulites consist of two phases - orthorhombic Pd₃Si silicide and Pd-rich silicide. From resistivity measurements, activation energies of 28.5 kcal/mol for Pd₈₃Si₁₇ and 80 kcal/mol for Pd₈₀Si₂₀, respectively, were calculated.     

Estimation of diffusivity governing primary nanocrystallisation and its relation to thermal stability of amorphous phases

Thermal stability and changes of both the average size of Al nanocrystals and their crystallised volume fraction formed in a series of the amorphous Al–(Ni,Co,Fe)–(Gd,Y,Tb) alloys as well as of α-Fe(Si) in the Fe_73.5Si_13.5B₉Cu₁Nb₃ alloy under isothermal conditions have been experimentally studied by a combination of X-ray diffraction (XRD) analysis, differential scanning calorimetry (DSC) and electrical resistance measurements. The experimental data have been fitted with the analytical models describing the diffusion-limited growth of nanocrystals and nanocrystallisation kinetics accounting for impingement of diffusion fields and the values of the effective diffusivity governing this process have been estimated. The obtained values of the diffusivity in Al-based amorphous alloys follow the Arrhenius-type dependencies with correlation between the pre-exponential factors and the activation energies somewhat different from that found for impurity diffusion coefficients in amorphous alloys. It has been established that at the onset crystallisation temperatures varying from 453 to 778 K, the values of the effective diffusivity in the investigated amorphous alloys are in the narrow range of 1.7–4.7 × 10^? 20 m² s^? 1, which indicates a crucial role of the effective diffusivity for the thermal stability of nanocrystals forming amorphous alloys and the possible reason for this is discussed.     

Mechanical properties and ion irradiation of bulk amorphous Zr55Cu30Al10Ni5 alloy

The bulk amorphous Zr₅₅Cu₃₀Al₁₀Ni₅ alloy is one of the widely studied Zr-based alloys due to very attractive thermal and mechanical properties. Alloy ribbons and bulk samples were synthesized by Cu mould casting and characterized by SEM/EDS, DSC and XRD. Mechanical properties like Vicker’s hardness, nanohardness and elastic modulus etc. were measured. Ion irradiation of the samples was carried out to enhance the surface properties without altering the amorphous nature of the bulk material. Ion irradiation by singly charged Ar⁺ enhanced the hardness as well as elastic modulus considerably.     

The enthalpy relaxation of amorphous Fe---Co alloys with metal-metalloid and metal-metal compositions

The enthalpy relaxation behaviour of metal-metalloid type (Fe_0.5Co_50.5)₈₃P₁₇ and metal-metal type (Fe_0.5Co_0.5) _90-Zr10 amorphous alloys was investigated with a DSC measurement. The annealing temperature (T_a) dependence of the annealing-induced reversible enthalpy relaxation evaluated with an endothermic showed the maximum at thesame T_a for the two alloys, but the magnitude of the relaxation was considerably smaller for the (Fe_0.5Co_0.5)₉₀Zr₁₀ alloy. Moreover, it was found that the amorphous (Fe_0.5Co_0.5)₉₀Zr₁₀ alloy has a larger atomic packing density and a higher activation energy of the enthalpy relaxation than the amorphous (Fe_0.5Co_0.5)₈₃P₁₇ alloy. Rhe small enthalpy relaxation and high activation energy for the amorphous (Fe_0.5Co_0.5)₉₀Zr₁₀ alloy were ascribed to the highly packed structure of the metal-metal type amorphous alloy.     

Thiol-ene/hyperbranched polymer hybrid thin films: Cure behavior and gas barrier properties

In this work, novel thiol-ene (TE)/hyperbranched polymer(HBP) hybrid thin films have been prepared by a simple photopolymerization method and the TE/HBP system was also investigated in terms of thermocuring behavior and gas barrier properties. The effect of added HBP on thermal curing (cross-linking) of TE polymer was examined with dynamic and isothermal differential scanning calorimeter (DSC) and compared with photocuring reaction of identical system. The structure of prepared TE/HBP hybrid thin film was analyzed by X-ray diffraction (XRD). Themomechanical analysis (TMA) provided extensive data on the coefficient of linear thermal expansion (CTE), dimensional change upon heating, and isothermal profile of TE/HBP hybrid thin films. Light transmittance as well as water/oxygen permeability of the resulting TE/HBP hybrid thin film was also measured depending on the HBP content.     

Effect of strain rates on the fracture morphologies of Zr-based bulk metallic glasses

The effect of strain rates from 1 × 10⁻⁴ s⁻¹ to 2 × 10³ s⁻¹ on tensile fracture morphologies of Zr_52.5Al₁₀Ni₁₀Cu₁₅Be_12.5, Zr₆₅Al₁₀Ni₁₀Cu₁₅, and Zr_52.5Al₁₀Ni_14.6Cu_17.9Ti₅ bulk amorphous alloys was investigated by scanning electron microscopy. The results show that the tensile fracture morphologies of three compositions of bulk amorphous alloys are dependent on strain rate. At low strain rates, the tensile fracture surface morphology of Zr-based bulk metallic glasses presents cleavage veins. However, the morphology will become microvoid-coalescence dimples when the strain rate is high enough.     

Ni59Zr20Ti16Sn5 amorphous alloy obtained by melt spinning and mechanical alloying

Amorphous Ni₅₉Zr₂₀Ti₁₆Sn₅ alloys were fabricated by melt spinning and by mechanical alloying (MA) techniques. Additionally the melt spun ribbon was powdered by ball milling. Differential scanning calorimetry (DSC) measurements revealed two or three-steps crystallization of the amorphous alloys. MA powders exhibited the lowest crystallization temperature of the first DSC peak, the melt spun ribbon – the highest one. On the other side, the lowest value of activation energy of crystallization was calculated for ball milled ribbon. Further studies are needed to clarify the differences in thermal stability and crystallization behavior of the alloys.     

Synthesis and mechanical properties of Zr-based bulk amorphous alloys

Design and characterization of bulk amorphous alloys has been an active area of research due to their promising thermal and mechanical properties. An alloy composition Z₆₅Cu₁₇Ni₁₀Al₈ was designed and synthesized by Cu mold casting. In the base alloy 2 at.% Gd was added to study its effect on thermal and mechanical properties. Characterization of the alloys was done by techniques of X-ray diffraction (XRD), differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FESEM). Many thermal parameters were evaluated to investigate the thermal stability and glass-forming ability of the alloys. In addition, the mechanical properties like nanohardness, elastic modulus and elastic recovery were measured. Thermal properties and activation energy reduced while mechanical properties like nanohardness, elastic modulus and percentage elastic recovery (% R) increased with Gd addition.     

Crystallization behaviour of an amorphous Ti50Be40Zr10 alloy

The crystallization behaviour of an amorphous Ti₅₀Be₄₀Zr₁₀ alloy during a continuous heating mode from room temperature to 973 K and isothermal annealing at temperatures above the glass transition temperature is examined by differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS) measurement and large-angle X-ray diffractometry (LAXD). DSC indicated two well-defined exothermic peaks, a slight shoulder at the higher temperature side of the second peak and a small heat evolution at higher temperature. The Kissinger plot for the first and the second peak gives a straight line, from which the apparent activation energy is estimated to be 269 and 413 kJ/mol respectively; the enthalpies for the first and second crystallization process are 1.04 kJ/mol and 4.39 kJ/mol for a heating rate of 20 K/min. The SAXS intensities increase sharply after annealing at about 673 K (corresponding to the first peak in the DSC curves); the scattering is due to the formation of fine-scale crystalline Ti particles by the LAXD. The size of the particles does not change significantly while the number of scattering particles increases, indicating that the reaction is almost nucleation controlled and the growth is very limited. Another crystalline phase would appear in addition to the Ti particles on annealing at temperatures above about 753 K (corresponding to the second peak in the DSC curves), where the SAXS intensities decrease compared with those for only the first-stage of crystallization. The crystalline phase might be a metastable cubic phase with the lattice parameter a₀?0.2994 nm.The sequence in the crystallization of the initial non-crystalline material is amorphous → microcrystalline (MS I) → crystalline (MS II; S III), although the structure of crystalline phase in the final stage (S III) was not identified. It is also likely that cold-rolling does not have a perceptible effect on the crystallization behaviour of the present amorphous alloy.     

Oxidation Kinetics of Amorphous,Polycrystalline, and Nanocrystalline Co33Zr67 Alloys

The oxidation kinetics of amorphous, polycrystalline, and nanocrystalline CoZr₂ alloys have been studied under isothermal conditions in air atmosphere with a microbalance. DSC and X-ray diffraction were used to study the influence of oxidation on crystallization. The three alloys under investigation which have the same composition but differ in the microstructure showed noticeable differences in their oxidation behaviour: the oxidation of the amorphous alloy is characterized by a very fast initial oxygen absorption step, completed in less than 1–2 min, followed by a slow oxide growth obeying a logarithmic time law at 400 °C. The polycrystalline thermodynamically stable tetragonal alloy absorbs small amounts of oxygen and obeys a parabolic rate law in the temperature range 500–800 °C. The activation energy of oxygen diffusion in tetragonal CoZr₂ was determined from the temperature dependence of the parabolic rate constant. The intermediate nano-crystalline phase with a local structure more closely packed and ordered than that of the precursor amorphous phase, shows an oxidation behaviour similar to that of the tetragonal phase, but with a different time exponent.     

A comparison of the crystallization behavior of liquid-quenched and vapor-quenched amorphous Cu60Ti40

The crystallization behavior of two Cu₆₀Ti₄₀ amorphous alloys, one prepared by a vapor-quench (VQ) process and one by a liquid-quench (LQ) process, has been studied using differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. Microstructural studies show that the two alloys have the same transformation sequence under similar annealing conditions. However DSC measurements of the crystallization temperature during isochronal annealing and of the incubation time for crystallization during isothermal annealing show that the LQ alloy is more thermally stable and therefore more structurally relaxed. TEM analysis of the partially annealed microstructures gives some insight into the mode of crystal nucleation and growth as a function of temperature. During isothermal annealing significant differences are observed for the two alloys in the shape and orientation of the crystals and in the crystal structure formed. Whereas the VQ alloy transforms polymorphicaly into the intermetallic Cu₃Ti₂, the LQ alloy forms the Ti-rich phase, Cu₄Ti₃. The two phases are closely related in structure and composition. The differences observed for the two alloys are discussed in terms of greater structural relaxation and short range order in the LQ alloy resulting from the mode of synthesis.