Study on crystallization kinetics of Al65Cu20Ti15 amorphous alloy

The crystallization behavior and thermal stability of amorphous phases of Al₆₅Cu₂₀Ti₁₅ alloy obtained by mechanical alloying were investigated by using in-situ X-ray diffraction and differential scanning calorimetry (DSC) under non isothermal and isothermal conditions. The result of a Kissinger analysis shows that the activation energy for crystallization is 1131 kJ/mol. The higher stability against crystallization of Al₆₅Cu₂₀Ti₁₅ amorphous alloy is attributed to the stronger interaction of atoms in the Al-Cu-Ti system and formed of complicated compound like Al₅CuTi₂ and Al₄Cu₉ as primary phases. The isothermal crystallization was modeled by using the Johnson-Mehl-Avrami (JMA) equation. The Avarami exponents suggest that the isothermal crystallization is governed by a three-dimensional diffusion-controlled growth.     

Microstructural evolution of decagonal quasicrystal in the undercooled Al72Ni12Co16 alloy melts

The microstructure evolution of decagonal quasicrystals in Al₇₂Ni₁₂Co₁₆ alloy was investigated by the electromagnetic melting and cyclic superheating method. Single-phase decagonal quasicrystals have been obtained when the undercoolings were larger than 60 K. The decagonal quasicrystals formed at various undercoolings show different microstructural morphologies. Furthermore, grain refinement was found near the undercooling of 120 K. Based on current thermodynamic and dendrite growth theories, a dimensionless superheating parameter was adopted to explain the effect of processing conditions on the microstructure of Al₇₂Ni₁₂Co₁₆ alloy. The result indicate that the fine equiaxied microstructure of decagonal quasicrystal (D-phase) formed near on undercooling of 120 K originates from the break-up of dendrites.     

Structure and morphology of [icosahedral Al62Cu25.5Fe12.5]100−x[decagonal Al70Co15Ni15]x alloys, for x=10(03 and 510)

Alloys made from mixtures of Al₆₂Cu_25.5Fe_12.5 icosahedral quasicrystal (IQC) and Al₇₀Co₁₅Ni₁₅ decagonal quasicrystal (DQC) were studied by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The transition from IQC to DQC was thus discussed by studying the evolution of their constituent phases in each alloy. Three approximant phases were found as common phases in most of the pseudo-binary alloys: λ-Al₁₃Fe₄, β-AlFe and τ3-Al₃Ni₂. It is found that, with the increment of the DQC content in the alloy, the λ phase changes from Al₁₃Fe₄ to Al₁₃Co₄ and the τ3 phase changes from Al₃Cu₂ to Al₃Ni₂. The formation of these phases were found also to follow the evolution of their corresponding e/a-constant lines in the Al–(Cu,Ni)–(Fe,Co) pseudo-ternary phase diagram. Under this framework, the roles played by the related approximants in the transition process are briefly discussed.     

Magnetic properties of quasicrystalline Al65Cu22Fe13 powders synthesized by solid-phase diffusion

The magnetic properties of quasicrystalline Al₆₅Cu₂₂Fe₁₃ powders synthesized by solid-phase diffusion as a result of thermal treatment in vacuum and in a hydrogen atmosphere have been studied. The magnetic properties of the samples synthesized in hydrogen were found to be much better than those of the samples synthesized in vacuum. It is shown that an increase in the treatment time in vacuum decreases the number of magnetic nanoclusters and elimninates the time instability of their magnetic properties (magnetic susceptibility and magnetization).     

Structural study of amorphous Al0.20As0.50Te0.30, Al0.10As0.40Te0.50 and Al0.10As0.20Te0.70 by X-ray diffraction (II)

A spherical-shaped model of Al_0.20As_0.50Te_0.30, Al_0.10As_0.40Te_0.50 and Al_0.10As_0.20Te_0.70 amorphous alloys has been performed by the random Monte Carlo method. These models describe quite well the experimental radial distribution functions and abide by the expected coordination numbers apart from the threefold coordinated Te, of which an excess has appeared. The structures are formed, basically, of distorted tetrahedra around the Al atoms whose corners are occupied by As or Te atoms. Also, a separated phase model for Al_0.10As_0.20Te_0.70 alloy has been built taking into account the results of thermodynamical study on this amorphous alloy system. The fitting of this model was better than that of the model generated under the hypothesis of a continuous phase.     

Structural study of amorphous Al0.20As0.50Te0.30, Al0.10As0.40Te0.50 and Al0.10As0.20Te0.70 by x-ray diffraction (I)

An analysis of the atomic radial distribution function of Al_0.20As_0.30Te_0.30, Al_0.10As_0.40Te_0.50 and Al_0.10As_0.20Te_0.70 amorphous alloys obtained from quenching of the molten mixture of the elements was performed. A structure in which all the Al atoms are tetrahedrally bonded to the other types of atoms in the material, would satisfy the requirements of the experimental curve. Tetrahedral groups might be linked to each other by As and Te atoms, or directly through a Te or As atom belonging to more than one tetrahedra.     

Diffusion and viscosity in molten Pd40Ni40P20 and Pd40Cu30Ni10P20 alloys

The self diffusion of ⁶²Ni and the shear viscosity in liquid Pd₄₀Ni₄₀P₂₀ and Pd₄₀Cu₃₀Ni₁₀P₂₀ have been measured. The used methods were the long-capillary technique for diffusion measurement and the gas-film-levitation for viscosity measurement. The temperature dependence of diffusion in the equilibrium melt follows the prediction of the mode-coupling theory. The Stokes-Einstein relation describes well the momentum and mass transport in both melts. The ⁶²Ni diffusion is equal in both alloys whereas the normalized bulk viscosity is higher in Pd₄₀Cu₃₀Ni₁₀P₂₀. Thermodynamic and structural considerations are invoked to propose a qualitative explanation for this behavior.     

Elasticity, thermal expansion and compressive behavior of Mg65Cu25Tb10 bulk metallic glass

The existence of special covalently bonded short-range ordering structures in a Mg₆₅Cu₂₅Tb₁₀ bulk metallic glass (BMG) is confirmed by thermal expansion and compression behavior. Under ambient conditions the linear thermal expansion coefficient obtained is almost constant in the glassy state with a value of 4.0 × 10⁻⁵ K⁻¹. By fitting the static equation of state at room temperature under ambient conditions we find the value for bulk modulus B of 48.7 GPa, which is in excellent agreement with the experimental study by pulse-echo techniques of 44.7 GPa. Unlike many bulk metallic glasses, such as Zr- and Pd-based, which bulk modulus is much larger than 100 GPa, the value B of Mg₆₅Cu₂₅Tb₁₀ BMG falls into the range of SiO₂ and fluorozirconate glass ZBLAN. Moreover, the elastic constant of the Mg₆₅Cu₂₅Tb₁₀ BMG is almost the same as those of ZBLAN. No evidence for the high-pressure phase transitions of the Mg₆₅Cu₂₅Tb₁₀ BMG has been found up to 31.19 GPa at room temperature.     

Atomic structure of Al55(Cr1−xMnx)15Si30 metallic glasses observed by neutron diffraction

An atomic structure of Al₅₅(Cr_1−xMn_x)₁₅Si₃₀ (x = 0, 0.49,1) metallic glasses was studied by neutron diffraction. An advantage of the neutron diffraction technique was fully exploited by utilizing the negative scattering length of Mn to form a neutron zero scattering ‘alloy’ for the component Cr_0.51Mn_0.49 in this quaternary Al---(Cr, Mn)---Si alloy. This allows the atomic distribution of the resulting quasibinary Al---Si metallic glass to be derived directly. Moreover, the (Al, Si)---TM (TM = Mn, Cr) and TM---TM pair correlations were also extracted by taking appropriate linear combinations of the atomic structures for the Al₅₅(Cr_1−xMn_x)₁₅Si₃₀ (x = 0, 0.49, 1) metallic glasses. A sharp first peak in the (Al,Si) ---TM pair correlations thus obtained led to the conclusion that a strong attractive interaction exists between (Al, Si) and TM atoms and, hence, that the presence of the TM atoms is responsible for the formation of an amorphous phase.     

Complex crystallization mode of amorphous/nanocrystalline composite Al86Ni2Co5.8Gd5.7Si0.5

Kinetics of the first crystallization stage of Al₈₆Ni₂Co_5.8Gd_5.7Si_0.5 amorphous alloy and structure of the partially crystallized specimens were investigated by measurements of electrical resistance, X-ray diffraction and transmission electron microscopy. The presence of Al nanocrystals and eutectic colonies consisted of mutually oriented crystals of Al and metastable phase was found. The transient nucleation and slowing-down diffusion-limited growth and the interface-controlled growth of the quenched-in nuclei were identified as mechanisms of formation of the Al nanocrystals and eutectic colonies, respectively.     

A single-crystal X-ray diffraction study of an Al70.5Pd21Mn8.5 icosahedral quasicrystal

X-ray diffraction data were measured on a single grain of a stable Al_70.5Pd₂₁Mn_8.5 perfect quasicrystal, in order to complement the neutron diffraction data obtained previously. The F-type lattice structure of the six-dimensional periodic arrangement was confirmed along with the very strong chemical order, due basically to an uneven distribution of the Pd atoms among the lattice sites. This distribution might be the origin of the high stability of the icosahedral structure in this system.     

Potentiodynamic polarization studies on amorphous Zr46.75Ti8.25Cu7.5Ni10Be27.5, Zr65Cu17.5Ni10Al7.5, Zr67Ni33 and Ti60Ni40 in aqueous HNO3 solutions

Potentiodynamic polarization studies were carried out on virgin specimens of Zr-based bulk amorphous alloys Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 and Zr₆₅Cu_17.5Ni₁₀Al_7.5, and conventional-type binary amorphous alloys Zr₆₇Ni₃₃ and Ti₆₀Ni₄₀ in solutions of 0.2 M, 0.5 M and 1.0 M HNO₃ at room temperature. The values of the corrosion current density (I_corr) for the bulk amorphous alloy Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 were found to be comparable with those of Zr₆₅Cu_17.5Ni₁₀Al_7.5 in 0.2 M and 0.5 M HNO₃, but the value of I_corr for the former was almost three times more than that of the latter in 1.0 M HNO₃. In the case of conventional binary amorphous alloys, Ti₆₀Ni₄₀ showed lower value of I_corr as compared to Zr₆₇Ni₃₃ in 0.5 M and 1.0 M HNO₃ and a comparable value of I_corr in 0.2 M HNO₃. In general, the binary Ti₆₀Ni₄₀ displayed the best corrosion resistance among all the alloys in all the cases and the corrosion current density (I_corr) for all the alloys was found to increase with the increasing concentration of nitric acid. It is noticed that the bulk amorphous alloys do not possess superior corrosion resistance as compared to conventional binary amorphous alloys in aqueous HNO₃ solutions. The observed differences in their corrosion behavior are attributed to different alloy constituents and composition of the alloys investigated.     

Insulator-like electrical transport in structurally ordered Al65Cu20+xRu15−x (x = 1.5, 1.0, 0.5, 0.0 and −0.5) icosahedral quasicrystals

Low-temperature resistivities, in zero-field and 8 T field, and magnetoresistance have been measured down to 1.4-300 K for stable icosahedral quasicrystals Al₆₅Cu_20+xRu_15−x (x = 1.5, 1.0, 0.5, 0.0 and −0.5). The analysis of the magnetoresistance data shows an overwhelming presence of anti weak-localization effect (τ_so ∼ 10⁻¹² s). But the sample with x = −0.5 shows anomalous magnetoresistance and the anti weak-localization effect breaks down (τ_so to be 10⁻¹⁵ s). The in-field σ-T between 5 K and 20 K, for x = 1.0, 0.5, 0.0 and −0.5 samples, and between 1.4 K and 40 K for x = 1.5 sample, follow a power-law behavior with an exponent of 0.5 and above ∼30 K the exponent ranges from 1.17 to 1.58. The observed power-laws basically characterize the presence of critical regime of the metal-insulator (MI) transition, dominated by electron-electron and electron-phonon inelastic scattering events respectively. In samples with x = 1.0, 0.5, 0.0 and −0.5 the in field σ-T has been found to follow ln σ-vs-T^1/4 below 5 K, which indicates the presence of variable range hopping. The observed transport features indicate the occurrence of proximity of metal-insulator transition in these Al-Cu-Ru quasicrystal samples.     

Effect of Al2O3 on phase separation of SiO2-Nd2O3 glasses

The immiscibility boundary and the critical point of SiO₂-Nd₂O₃ system glass were determined as a function of Al₂O₃ addition. The critical temperature of the immiscibility boundary was observed to decrease with the addition of Al₂O₃. Using the regular solution model, the observed decrease of the immiscibility boundary was directly related to the decrease of the concentration fluctuation of Nd₂O₃ in SiO₂. It is concluded that the Al₂O₃ addition to Nd₂O₃ containing silica glass is beneficial in decreasing the concentration quenching effect, deterioration of the optical efficiency due to clustering of rare earth element, because Al₂O₃ addition diminishes the concentration fluctuation of Nd₂O₃ in silica glass.     

The glass transition of Pd40Ni10Cu30P20 studied by temperature-modulated calorimetry

The frequency dependence of the heat capacity in the glass-transition region of Pd₄₀Ni₁₀Cu₃₀P₂₀ was studied by temperature-modulated differential scanning calorimetry (TMDSC) during slow heating and cooling. Such data for low frequencies between 0.1 and 0.01 Hz are not available, especially for metallic glasses. A crossover between mixed static/dynamic and purely dynamic response signals was observed for the lowest frequencies between 1/80 and 1/100 s⁻¹, which allows a direct determination of the average relaxation time at a given cooling rate during the static glass transition. Further, these results were used to evaluate the experimental parameters necessary to truly separate the static and dynamic response in low-frequency modulation calorimetry experiments to obtain the moduli of the dynamic specific heat.     

Direct observations of pseudo-binary NdBa2Cu3Ox–Ba3Cu10O13 phase diagram and NdBa2Cu3Ox crystallization under reduced oxygen atmosphere

The pseudo-binary NdBa₂Cu₃O_x–Ba₃Cu₁₀O₁₃ phase diagrams and the crystallization of NdBa₂Cu₃O_x have been in situ observed using high-temperature optical microscope under three different oxygen atmospheres namely 1%, 0.1% and 0.0097% oxygen in argon. It has been observed that the liquidus line becomes narrower both in composition and temperature with decreasing oxygen pressure. This result suggested that under reduced oxygen atmosphere, the NdBa₂Cu₃O_x crystals could only be grown from a peritectic melt consisting of Nd₄Ba₂Cu₂O₁₀ and liquid. The crystallization temperature of NdBa₂Cu₃O_x was found to decrease logarithmically with decreasing oxygen content in the atmosphere.     

Linear optical properties of BaNaB9O15, BaLiB9O15 and SrLiB9O15 glasses

Transparent BaNaB₉O₁₅ (BNBO), BaLiB₉O₁₅ (BLBO) and SrLiB₉O₁₅ (SLBO) glasses were fabricated via the conventional melt-quenching technique. X-ray diffraction (XRD) and Differential thermal analysis (DTA) studies carried out on the as-quenched glasses confirmed their amorphous and glassy nature, respectively. The optical properties for these as-quenched glasses were investigated. The refractive index, optical band gap, Urbach energy and Fermi energy were determined. The average electronic polarizability calculated from the refractive index expression.     

Crystal growth and characterization of two-leg spin ladder compounds: Sr14Cu24O41 and Sr2Ca12Cu24O41

Single crystals of Sr_14−xCa_xCu₂₄O₄₁ (x=0 and 12) are grown by the travelling solvent floating zone technique using an image furnace. The grown crystals are characterized for their single crystallinity by the X-ray and Neutron Laue method. The magnetic susceptibility measurements in Sr₁₄Cu₂₄O₄₁ show considerable anisotropy along the main crystallographic axes. Low-temperature specific heat measurement and DC susceptibility measurement in Ca-doped crystal showed antiferromagnetic ordering at 2.8 K at ambient pressure. High-pressure AC susceptibility measurement on Ca-doped crystal showed a sharp superconducting transition at 2 K under 40 kbars. T_c onset reached a maximum value of 9.9 K at 54 kbars. The bulk superconductivity of the sample is confirmed by the high-pressure AC calorimetry with T_c max=9.4 K and T_N=5 K at 56 kbars.     

Length and density changes of amorphous Pd40Cu30Ni10P20 alloys due to structural relaxation

The density changes of bulk amorphous Pd₄₀Cu₃₀Ni₁₀P₂₀ due to structural relaxation under isothermal heat treatment were measured using the buoyancy method. The corresponding length changes of amorphous Pd₄₀Cu₃₀Ni₁₀P₂₀ ribbons applying non-isothermal heat treatment are determined applying dilatometry. The measured density changes Δρ(T=const.,t)/ρ_o as a function of isothermal heat treatment increase close to the glass transition up to 0.18%. The measured density changes as a function of temperature and time are furthermore reversible. The experimental data validate the model predictions of the free volume model for length and density changes of amorphous alloys due to structural relaxation applying isothermal and non-isothermal heat treatment.     

A multi-peak transformation kinetics model and its application to the isothermal crystallization of Mg65Cu25Y10 amorphous alloy

Transformation kinetic analytical model plays an important role in the prediction of the microstructural evolution. In this paper, a simple formula has been developed for isothermal mixed nucleation transformation as the kinetic parameters of its JMAK-form formula vary upon time. The explored multi-peak transformation kinetics shows that each peak can be treated as a JMAK case, which is consistent with the classical JMAK model in only one peak case. Thereafter, a method has been developed to deal with the isothermal DSC data of multi-peak overlapping transformation. The isothermal crystallization process of Mg₆₅Cu₂₅Y₁₀ amorphous alloy has been explored and fitted well with the multi-peak kinetics model, which indicates a continuous nucleation, three dimensional interface-controlled growth mechanism with three crystallization peaks overlapping each other.