Application of the model of locally nonequilibrium solidification to the process of structure formation in alloys rapidly quenched by spinning

Solidification of a binary alloy rapidly quenched by spinning [1] has been studied by mathematical simulation.     

Effects of Nb on the formation of icosahedral quasicrystalline phase in Ti-rich Ti-Zr-Ni-Cu-Be glassy forming alloys

As-cast (Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈)_100−xNb_x (0 ? x ? 5) (Ф3) glassy forming alloys were investigated in order to clarify the role of Nb on the formation of icosahedral quasicrystalline phase (I-phase) in Ti-rich Ti-Zr-Ni-Cu-Be glassy system. It is found that an I-phase is formed in Ti-Zr-Ni-Cu-Be glassy alloy by addition of Nb element; however, the nucleation rate of I-phase increases, whereas the grain growth rate decreases with increasing Nb content. Moreover, with increasing Nb content, the thermal stability against crystallization increases, while the temperature range of stability of the I-phase decreases.     

Formation of a new F.C.C. τ2 phase in rapidly quenched Al55Cu35V10 alloy

X‐ray diffraction and transmission electron microscopy experiments were carried out to study the structure of rapidly solidified as‐cast and annealed Al₅₅Cu₃₅V₁₀ alloy. The as‐cast Al₅₅Cu₃₅V₁₀ alloy shows the presence of a new f.c.c. τ2 phase (a=0.58nm) along with a b.c.c. (a = 0.89 nm) phase which after subsequent annealing transforms into single f.c.c. phase (a = 0.58 nm). In this paper, it is also reported that these phases are crystalline approximants to an icosahedral phase on the basis of e/a (valence electron per atom) constant line. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Heterogeneous nucleation mechanisms and formation of metastable phase assemblages induced by different crystalline seeds in a rapidly cooled andesitic melt

Pt-capsules loaded with a Pt-coil and two crystalline seeds immersed into an andesitic melt were rapidly cooled from 1300 to 1100 °C with a rate of 3 °C/min at atmospheric pressure and air oxygen fugacity. Results show that the Pt-coil does not induce any heterogeneous crystallization process as well as iron diffusion process from the melt into the platinum substrate. In contrast, the presence of crystalline seeds in a solidifying andesitic melt significantly alters the phase assemblage, composition and texture of the new-forming crystals in response to a heterogeneous nucleation mechanism and the formation of metastable phases.     

Crystallization of the amorphous phase and martensitic transformations in multicomponent (Ti,Hf,Zr)(Ni,Cu)-based alloys

Shape memory materials with specific microstructure can be obtained from amorphous precursors. Rapidly solidified amorphous and crystalline–amorphous ribbons have been produced by planar flow casting for a number of multicomponent TiNi-based alloys of pseudobinary 50:50 and 55:45 compositions with substitutions of Zr, Hf, Nb for Ti and Cu, Co, Pd, Ag, Al for Ni. The glass transition and crystallization behavior of the amorphous phase as well as the martensitic transformations in the crystallized materials have been studied by differential scanning calorimetry. To control the crystal structure at different stages of crystallization, X-ray diffractometry and transmission electron microscopy were used. The effects of crystallization conditions and the resulting microstructural state on the martensitic transformation characteristics have been investigated. The transformation temperature intervals are shown to depend on the mean size of the parent phase crystals.     

An analytical model to represent crystallization kinetics in materials with metastable phase formation

The aim of this article is to propose a simple analytical model that can describe the isothermal crystallization process in materials when the formation of a stable crystalline phase is preceded by the formation of a metastable phase. This model explains deviations from the well-known Johnson-Mehl-Avrami-Kolmogorov kinetics theory and predicts the three slopes in Avrami’s plot. The model predictions were compared with experimental results obtained from X-ray measurements in the chalcogenide glasses with composition of Ge₂Sb₂Te₅ (thin films) and in aqueous solutions of methylhydrazine monohydrate during isothermal phase transformations. In order to validate the proposed model to represent experimental results, a computer program was developed. This program uses experimental data from measurements of the total volume fraction at different times during isothermal transformations and fits the model parameters that best represent the kinetic behavior of the system.     

Advanced nanostructure Ti0.7In0.3O2 support enhances electron transfer to Pt: Used as high performance catalyst for oxygen reduction reaction

ABSTRACT

The slow rate of the oxygen reduction reaction (ORR) and the instability of Pt based catalysts are two of the most important issues which must be solved in order to make proton exchange membrane fuel cells (PEMFCs) a reality. Here, we present a new approach by exploring robust non-carbon Ti_0.7In_0.3O₂ used as a novel functionalised co-catalytic support for Pt. This approach is based on the novel nanostructure Ti_0.7In_0.3O₂ support with “electronic transfer mechanism” from Ti_0.7In_0.3O₂ to Pt that can modify surface electronic structure of Pt, owing to a shift in the d-band centre of the surface Pt atoms. The 20 wt% Pt/Ti_0.7In_0.3O₂ catalyst shows high activity than that of that of the commercial 20 wt% Pt/C (E-TEK). Our data suggest this enhancement is a result of both the electronic structure change of Pt upon its synergistic interaction with Ti_0.7In_0.3O₂ and the inherent structural and chemical stability and the corrosion-resistance of the Ti_0.7In_0.3O₂ in acidic and oxidative environments.     

Controlling the group II composition in CdZnTe alloys grown by organometallic vapor phase epitaxy: a kinetic model

The organometallic vapor phase epitaxy (OMVPE) of CdTe and ZnTe has been examined in a hot-wall, laminar-flow reactor. It was found that cadmium and zinc are produced in excess on the film surface during OMVPE. The excess group II elements sublime off the surface and are deposited downstream on the cold reactor walls. Based on these and other results, a kinetic model has been derived for CdZnTe OMVPE. The elementary reactions included in this model are the adsorption of the organometallic precursors, the desorption of the alkyl ligands, film growth, and the desorption of Zn and Cd metal. The predictions of the model have been compared to the Zn segregation data reported in the literature. This analysis reveals that the distribution of the group II elements between phases is relatively insensitive to the process conditions, i.e. temperature and VI/II ratio. However, it is strongly influenced by the intrinsic kinetic parameters, i.e. the difference in the Zn and Cd sublimation energies and the relative sticking probabilities of the organometallic precursors.     

Influence of the temperature and the oxygen partial pressure on the phase formation in the system Cu – Fe – O

Copper iron oxides, Cu_1‐xFe_2+xO₄ (0 ≤ x ≤ 0.5), have been synthesized by thermal oxidation of copper ‐ iron mixtures. In this process, the phase formation and the phase stability were investigated as function of the temperature (800°C – 1200°C) and the oxygen partial pressure (1.013 x 10¹ – 1.013 x 10⁵ Pa). The phase formation starts with the reaction of the metallic components to simple oxides (Fe₃O₄, Fe₂O₃, CuO). From these simple oxides, the formation of complex oxides requires a minimum temperature of 800°C. The synthesis of single phase spinel compounds Cu²⁺_1‐2x Cu¹⁺_xFe_2+xO_4±δ is realized for 0.1 ≤ x ≤ 0.5, using specific temperature – p(O₂) – conditions for a given value of x. Remarkably, to achieve our goal, we found that the increase of x implies that of the reaction temperature and/or a decrease of the p(O₂) in the reaction gas stream. Besides, a single phase spinel CuFe₂O₄ does not exist in the temperature / p(O₂)‐field investigated. Using the results of XRD ‐ phase analysis, T ‐ p(O₂) – x – diagrams were constructed. These diagrams allow the prediction of phase compositions expected for different synthesis conditions. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Some aspects on thermodynamic properties, phase diagram and alloy formation in the ternary system BAs–GaAs—Part II: BGaAs alloy formation

We present a detailed analysis of the thermodynamics of the BGaAs alloy formation taking into account the question of boron solubility and Gibbs free energy change. We also predict a Gibbs free energy change for the gas phase formation of BAs to be compared to for GaAs. We show that the experimental boron solubility is actually an open question both theoretically and experimentally, while the maximum concentration obtained up to date is reaching 6–7%. It thus follows that despite the low boron solubility, the Gibbs free energy change for the alloy formation is dominated by the chemical term over the mixing energy change. We could deduce order of magnitude for the ratio of the boron partial pressure and that of gallium. P_B could be estimated as being always much lower than P_Ga; it is shown that the alloy content is probably controlled by the Ga equilibrium partial pressure.     

Morphological instability due to double diffusive convection in directional solidification: the pit formation

Deep interface depression or “pit formation”, as a result of solute accumulation, due to double-diffusive convection in the directional solidification of succinonitrile (SCN) containing ethanol in an ampoule is investigated by a fully nonlinear numerical simulation. The calculated results are consistent with previous observations (Schaefer and Coriell, Metal. Trans. 15A (1984) 2109), and the instability margin falls between the convective and morphological boundaries at a low growth rate. For a high growth rate, the global interface depression becomes deep due to significant release of the heat of fusion; in this case, the critical concentration can be lower than the convective value. Near the instability margin, the pit forms at the center of the interface and is soon followed by constitutional supercooling. Also, the pit shape is affected significantly by the convective solute transport and thus the flow structures. Such pit formation, results from the nonlinear coupling of double-diffusive convection and the interface deformation, and although differs from the traditional mechanisms, it could be an important route to interface breakdown.     

Diffusion in bulk-metallic glass-forming Pd–Cu–Ni–P alloys: From the glass to the equilibrium melt

Since the discovery of bulk-metallic glasses there has been considerable research effort on these systems, in particular with respect to mass transport. Now the undercooled melt between the melting temperature and the caloric glass transition temperature, which has not been accessible before due to the rapid onset of crystallization, can be investigated and theories can be tested. Here we report on radiotracer diffusion measurements in metallic bulk-glass-forming Pd–Cu–Ni–P alloys. Serial sectioning was performed by grinding and ion-beam sputtering. The time, temperature as well as the mass dependence, expressed in terms of the isotope effect E, of Co-diffusion were investigated. In the glassy state as well as in the deeply supercooled state below the critical temperature T_c, where the mode coupling theory predicts a freezing-in of liquid-like motion, the experimentally determined very small isotope effects indicate a highly collective hopping mechanism involving some ten atoms. Below T_c the temperature dependence shows Arrhenius-type behavior with an effective activation enthalpy of 3.2 eV. Above T_c the onset of liquid-like motion is evidenced by a gradual drop of the effective activation energy and by the validity of the Stokes–Einstein equation, which is found to break down below T_c. This strongly supports the mode coupling scenario. The Stokes–Einstein equation is presently tested for other constituents of the alloy. The Co isotope effect measurements, which have never been carried out near T_c in any material, show atomic transport up to the equilibrium melt to be far away from the hydrodynamic regime of uncorrelated binary collisions.     

Investigation of the formation of quasicrystalline Al<Subscript>70</Subscript>-Pd<Subscript>20</Subscript>-Re<Subscript>10</Subscript> phase in situ during annealing

The change in the phase composition of thin-film layered AlPdRe nanostructures during annealing, which led to the formation of a quasicrystalline layer, has been studied in situ. It is shown that the Al₃Pd phase is formed at a temperature above 260°C, which transforms into the AlPd phase at 580°C, and the icosahedral quasicrystalline Al-Pd-Re phase is formed at 680°C.     

Some aspects on thermodynamic properties, phase diagram and alloy formation in the ternary system BAs–GaAs—Part I: Analysis of BAs thermodynamic properties

Owing to the lack of available thermodynamic data based on experimental measurements of heat capacity, decomposition reaction or vapour pressure measurements, the problem of BAs stability is considered. We propose a new set of thermodynamic data for enthalpy of formation, entropy and Gibbs free energy of Bas compound. By using thermodynamic database, our approach is based on the semi-empirical trends and analogy in the variation of those quantities for several binary series in different III–V systems like arsenides, nitrides and phosphides. Thus, the values for BAs were derived by extrapolation from Al to boron-based compounds (BAs, BP and, BN). For pure BAs(s), we predict a low enthalpy of formation in the standard state of at 300 K and a Gibbs free energy of indicating a lower stability of this compound than GaAs. Those values are contradictory discussed with trends in the cohesive energy of several III–V systems. A cohesive energy of 900 kJ/mol (9.4 eV) is proposed in agreement with Philips's rule.     

Fractional composition of large crystallite grains: A unique microstructural parameter to explain conduction behavior in single phase undoped microcrystalline silicon

We have studied the dark conductivity of a broad microstructural range of plasma deposited single phase undoped microcrystalline silicon (μc-Si:H) films in a wide temperature range (15–450 K) to identify the possible transport mechanisms and the interrelationship between film microstructure and electrical transport behavior. Different conduction behaviors seen in films with different microstructures are explained in the context of underlying transport mechanisms and microstructural features, for above and below room temperature measurements. Our microstructural studies have shown that different ranges of the percentage volume fraction of the constituent large crystallite grains (F_cl) of the μc-Si:H films correspond to characteristically different and specific microstructures, irrespective of deposition conditions and thicknesses. Our electrical transport studies demonstrate that each type of μc-Si:H material having a different range of F_cl shows different electrical transport behaviors.     

A statistical mechanical model for chemical disorder in interacting systems: Application to phase separation in alkali silicates,alkaline earth silicates and alkaline earth aluminosilicates

The present model provides a formalism for studying the effects of chemical disorder in a system of interacting atoms in which the number of neighbors of each kind of atom need not be the same. When they are the same, this model reduces to the quasi-chemical approximation.It is applied to a model for phase separation in alkali silicates and alkaline earth silicates which involves a mixture of fourfold and sixfold coordinated species. The sixfold coordinated species represents a pseudo-atom comprised of two silicon atoms each bonded to non-bridging oxygen atoms which are strongly associated.A miscibility dome similar to that observed in sodium silicates and barium silicates is predicted. The suppression of phase separation by alumina is predicted.     

A study of absorption coefficient spectra in a-Si:H films near the transition from amorphous to crystalline phase measured by resonant photothermal bending spectroscopy

Optical absorption spectra of a widegap hydrogenated amorphous silicon film have been estimated by resonant photothermal bending spectroscopy. It is found that excess absorption exists over the photon energy region of 1.2–1.6 eV. This excess absorption decreases by light illumination and does not recover through thermal annealing. The decrease in the excess absorption may be due to oxidization of the film by light illumination.     

From amorphous phase separations to nanostructured materials in sol–gel derived ZrO2:Eu3+/SiO2 and ZnO/SiO2 composites

ZrO₂:Eu³⁺–SiO₂ and ZnO–SiO₂ composites have been synthesized by a sol–gel method by using a specific gelation and drying procedure. In the two cases we were able to produce large and transparent monolithic samples. Microstructural properties of these materials were investigated by thermo-differential and thermo-gravimetric analysis, X-ray diffraction, transmission electron microscopy and small angle X-ray scattering. The existence of a miscibility gap in both systems results in the formation of nanocomposites where crystallized zirconia or amorphous zinc oxide nanoparticles are dispersed in a silica glass matrix. These two kinds of nanocomposites are potential high efficiency luminescent materials because the nanoparticles size is easily controlled by the annealing conditions.