Kinetics of the irregular binary alloys crystallization

With the help of Monte-Carlo computer simulation method we have investigated the growth of binary crystals having the structure anomally depended on temperature and the liquid phase composition. The dependences of the growth rate and component concentrations in the crystal volume on temperature for different liquid phase compositions were obtained. Kinetic phase diagrams are found here too. A comparison with the results obtained from analytical calculations was also made in this work.     

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.     

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.     

Kinetics of crystallization in binary solutions

A two-dimensional nucleation is required to grow the crystals of low energy planes. Expressions for the size and free energy of the two-dimensional nucleus corresponding to a saddle point in binary system are derived. The concentration of the critical nuclei in the system is related to the critical free energy of the two-dimensional nucleus. A circular island of atoms on the growing surface is considered and the flux of atoms on the surface going to the growing walls of the island from either direction is calculated. The rate of advance of the curved step is determined and hence the velocity of the straight step in the A-B crystal-binary solution interface is found.     

Kinetics of the crystallization in amorphous NiTi thin films

The crystallization kinetics in Ni_50.54Ti_49.46 film was studied by differential scanning calorimetry through continuous heating and isothermal annealing. The activation energy for crystallization was determined to be 411 and 315 kJ/mol by Kissinger and Augis & Bennett method, respectively. In the isothermal annealing study, The Avrami exponents were in the range of 2.63–3.12 between 793 and 823 K, suggesting that the isothermal annealing was governed by diffusion-controlled three-dimensional growth for Ni_50.54Ti_49.46 thin films.     

Magnetothermal investigation of crystallization of Ni–P amorphous alloys

A new magnetic phase analysis method is presented and its application to a certain class of amorphous alloys is thoroughly discussed. Special attention is paid to the size of clusters with strong magnetic properties that form the separating phase. This magnetothermal (MT) method reveals the formation or disappearance of a phase with strong magnetic properties, and in certain cases it can give valuable information, even in the temperature range where the sample is paramagnetic. The accuracy and applicability of the MT method are discussed. Combining this method with differential scanning calorimetry (DSC), the transformations in melt-quenched (MQ) and electrolessly deposited (ED) amorphous Ni–P alloys with hypoeutectic and close to eutectic composition are investigated under the condition of a constant heating rate. Up to four stages of Ni precipitation in the alloys are revealed, depending on the phosphorous content and the method of sample preparation.     

Influence of the B and P content on the thermal stability and crystallization of cast iron based bulk amorphous alloys

The possibility of synthesizing bulk amorphous alloys with additions of B and P to commercial cast iron (Ci) of the chemical composition Fe_81.5Si_3.8C₁₄Tm_0.7 (at.%) was investigated. The effect of the B and P concentration on thermal stability, bulk glass forming ability (BGFA) and microstructure was studied by calorimetric (DSC and DTA) measurements as well as by X-ray diffraction. With the addition of 8.42 and 12.17 B to the commercial Ci bulk amorphous alloys with very wide supercooled liquid regions (77 and 81 K, respectively) could be produced. The partial replacement of Ci by B and P atoms increases the thermal stability and the BGFA. The kinetics of crystallization was investigated both by linear heating and by isothermal DSC measurements. The changes in the microstructure of the crystalline phases formed during linear heating were studied during further isothermal annealing at 833 K.     

Bandgap characters in GaAs‐based ternary alloys

The existence and origins of the bowing character in the bandgap variation of GaAs‐based ternary alloys are theoretically investigated based on two different computational methods. Within the framework of the virtual crystal approximation (VCA), both the empirical sp³s * tight‐binding (TB) method with, and without, the inclusion of the spin‐orbit coupling effects, and the first‐principle full‐potential linear augmented plane wave (FP‐LAPW) technique are applied on both the common‐cation GaSb_xAs_1‐x and the common‐anion Ga_1‐xIn_xAs alloys. These methods are used to calculate the bandgap energy, the partial and total densities of states and the constituent charge ionicity versus the composition x. The results show that the bowing behavior exists in the case of common‐cation alloys (GaSb_xAs_1‐x) as a manifestation of a competition between the anion atoms (As and Sb) in trapping the made‐available‐cationic charges. The bowing parameter is found to be proportional to the electronegativity characters of the competing anions (χanion). Consistent with this in the case of common‐anion alloys (Ga_1‐xIn_xAs), as due to the lack of anion competition, the bowing is just absent and the variation of bandgap energy is found to be rather linear. The excellent agreement between our theoretical results and recent photoluminescence data has corroborated our claim. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase‐controlled crystallization of amorphous calcium carbonate in ethanol‐water binary solvents

The evolution of amorphous calcium carbonate (ACC) into crystals in ethanol/water binary solvents under ambient temperature was investigated, and it was found to depend on the volume ratio of ethanol to water (R). Calcite remained dominant when the amount of water was high (R = 1/3). A slight change in the amount of ethanol (R = 3/1) could lead to a dramatic change in the polymorph from calcite to aragonite. However, when poly (allylamine hydrochloride) (PAH) was added at R = 3/1, almost pure vaterite could be obtained, which has a specific morphological variation (from hollow microspheres to cloud‐like). This study provides an alternative polymorphic route for the CaCO₃ mineral by using the evolution of ACC in different solvent environments, which provides some useful clues for understanding the importance of kinetic control of the morphologies and polymorphs of a wide range of inorganic materials. In addition, this simple mild phase‐controlled synthetic method could be scaled up as a green chemistry route for the industrial production of different polymorphs of CaCO₃.     

The crystallization of amorphous germanium films

Germanium thin films were prepared in an amorphous form by vacuum deposition onto room temperature fused silica substrates. The amorphous—crystalline transition was studied as a function of time and temperature by measuring the optical transmission near 0.65 μm, where the absorption constant is most sensitive to the phase transformation. At a fixed temperature, the time for half the volume of the sample to become crystallized was found to be consistent with the relation t_c = τ exp(E₀/kT), with an activation energy E₀ = 2.96 eV.     

Phase separation and crystallization in amorphous Pd-Si-Sb

It is found that replacing some of the Si in a Pd-Si-Sb glass with Sb can make it more resistant to crystallization than the pure binary material. This is shown by the calorimetric results, in which the crystallization temperature increases and the glass temperature decreases with increasing Sb substitution, and also by the finding that certain alloys, such as Pd_80.5 Si_16.5Sb₃ may be quenched into amorphous rods (by the gradient-quench method) up to 0.5 mm thick.

Calorimetry and small-angle X-ray scattering data show that certain alloys in this system phase separate in the amorphous state. The two phases thus produced independently crystallize. Isothermal measurements indicate that the rate of crystallization of one of the phases is interface-limited, with most nuclei initially present. The other crystallization process does not fit an Avrami law, possibly due to a transformation of the crystalline phase.

A method called “gradient-quenching” (GQ) was used to prepare samples from the melt in such a way that the quench-rate was non-uniform, so all stages of crystallization in slowly-quenched alloys are displayed. Nucleation was frequently found to take place on voids and inclusions.

The use of the calorimetric and GQ methods in conjunction to gather information about crystallization processes in metallic glasses is discussed.     

Electrical conductivity and crystallization in amorphous germanium

A simple model is presented which interprets conductivity variations due to crystallization in amorphous layers and allows the determination of the growth rate of crystallization. The validity of this model is demonstrated in the case of germanium.     

In-situ crystallization of amorphous Fe73−xNbxAl4Si3B20 alloys through synchrotron radiation

In the present work Fe_73−xNb_xAl₄Si₃B₂₀ (x = 5, 10) alloys have been processed by melt-spinning with the aim of studying the crystallization sequence through annealing in suitable temperatures. Melt-spun ribbons were characterized by differential scanning calorimetry (DSC), X-ray diffractometry (XRD) through Cu-Kα (λ = 1.54 Å) and synchrotron radiation (λ = 1.77 Å) and transmission electron microscopy (TEM). Soft magnetic properties were measured through the hysteresis loop tracer. In-situ XRD through synchrotron radiation was very accurate in phase identification. Fe_73−xNb_xAl₄Si₃B₂₀ (x = 5, 10) alloys showed the possibility of forming ferromagnetic amorphous alloys composed of commercial Fe-based master alloys with fine nanocrystalline structure and good soft magnetic properties.     

Crystallization of NANOPERM-type amorphous alloys

In the present work, we have investigated progress of crystallization in the Fe_91?xMo₈Cu₁B_x (x = 12, 15, 17, 20) alloy as a function of annealing temperature. This material belongs to the family of the Fe–M–B–(Cu) alloys (transition metal M = Zr, Nb, Hf, Mo, …) called also NANOPERM. The alloy was found to contain small amounts (<5%) of bcc Fe(Mo) and Mo₂FeB₂ nanocrystallites (<2 nm) located on the surface of the ribbon-shaped samples already in the as-quenched state. Depending upon composition, the nanocrystallites are formed on the air and/or wheel side of the ribbon. They are characterized by atomic force microscopy, conversion electron Mössbauer spectroscopy, and by X-ray diffraction of synchrotron radiation. Fast detection (every 10 s) of the latter during continuous heating of as-quenched specimens enabled an in situ observation of the evolution of the crystallization. For x = 12, the crystallization starts earlier at the wheel side of the ribbon but its progress is more rapid at the opposite, i.e. air side.     

Structural relaxation and crystallization of amorphous Ge films

The irreversible changes in conductivity occurring in evaporated amorphous Ge films just after deposition and during annealing are investigated in detail. Their analysis leads to a model for the structural relaxation of real amorphous networks. Electron diffraction and electron microscope experiments provide additional information on annealing effects and crystallization processes.     

Superconducting state parameters of ternary amorphous superconductors

The theoretical investigations of the superconducting state parameters (SSP) viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ^?, transition temperature T_C, isotope effect exponent α and effective interaction strength N_OV of (Ni₃₃Zr₆₇)_1 − xM_x (x = 0, 0.05, 0.1, and 0.15, M = Cu) ternary amorphous superconductors have been reported using Ashcroft's empty core (EMC) model potential. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used in the present investigation to study the screening influence on the aforesaid properties. The T_C obtained from Sarkar et al. (S) local field correction function is found in excellent agreement with available theoretical data. Quadratic T_C equation has been proposed, which provides successfully the T_C values of ternary amorphous alloys under consideration. Also, the present results are found in qualitative agreement with other such earlier reported data, which confirm the superconducting phase in the superconductors.     

Kinetics of FCC- and diamond type binary crystal growth (II). Peculiarities of Si-Ge and Cu-Ni alloy crystallization

Kinetic equations have been used to study how crystal growth rate and component concentration in the solid phase depend on temperature for different crystallographic orientations of the growing faces of binary crystals silicon-germanium and copper-nickel. The shape of kinetic phase diagram has been calculated. Compared are the main characteristics of the process obtained in a model “with overhangs” of the atomic relief of the interphase with those obtained in a model “without overhangs”. The kinetic equation solutions are compared with the results of Monte Carlo computer simulations. Their quantitative agreement has been shown.     

Kinetic theory of crystallization of amorphous materials

Careful analysis of the Avrami equation [x = 1 ?exp(?Atⁿ)] shows that an activation energy for crystallization (E_c) for amorphous materials can be defined over a selected range of temperatures. This activation energy can be determined experimentally using non-isothermal differential scanning calorimetry (DSC) by determining the crystallization temperature (T_c) as a function of heating rate (φ). A plot of (ln(φ/T_c) has a slope equal to E_c/n. The activation energy for the crystallization of amorphous arsenic obtained by this non-isothermal method is found to be in fair agreement with that obtained from an isothermal DSC experiment.     

Comparison of amorphous and liquid palladium-silicon alloys

The Pd_82,5Si_17,5 alloy has been investigated by X-ray diffraction in an amorphous state obtained by rapid quenching from the liquid state and also in the liquid state at a temperature just above T_f. The differences seen in the interference functions obtained relative to both states are not only due to a temperature effect. From the two atomic radial distribution functions it was possible to evaluate the Pd–Pd distances and the Pd–Si distances as well as the number of nearest neighbours. These distances have the same values, but the displacements of the atoms are more important in the liquid. The contribution of Pd–Pd bonds, for the metal-metal distances equal to 1.65 times the particle diameter, decreases in the liquid.     

Decomposition of La- and Dy-rich amorphous alloys

An Al₂₅La₅₀Ni₂₅ and newly discovered Al₃₈Dy₅₀Ni₁₂, Al₃₈Dy₅₀Co₁₂ and Al₄₁Dy₄₇Ni₆Co₆ glasses prepared by melt-spinning were studied by means of electrical resistance (ER) and calorimetric measurements at constant and at linearly increasing temperatures as well as by X-ray diffraction (XRD) and microhardness studies after different heat treatments. Changes in short-range order during relaxation lead to prepeaks in XRD spectra, ER and hardness increases. The glass transitions are accompanied by endothermal heat effects and ER drops. In Al₂₅La₅₀Ni₂₅, crystallisation sets in above 240 °C via the formation of metastable LaNi and AlLa₃ phases which may transform into La(Ni, Al). The Al₃₈Dy₅₀Ni₁₂ glass crystallises only above 400 °C; fine precipitates of Dy₃Ni and probably DyAl appear before a Dy(Ni, Al) phase forms. The Co-containing materials possess still increased stability but are very brittle.