Crystalline Solid Solutions Research Articles

Amorphous and crystalline phase formation during suspension plasma spraying of the alumina–zirconia composite

The focus of this study is the amorphous phase formation in the alumina–yttria stabilized zirconia composite coatings during thermal spray deposition. The investigated processes include conventional and suspension plasma spraying. The focus of this paper is on suspension spraying, while making a comparison of the two processes. Through the study of the in-flight collected particles and coatings produced from the two processes, the comparison of fragmentation, melting and mixing phenomena became possible. Scanning electron microscopy, differential scanning calorimetry and X-ray diffractometric studies helped better understanding of the formation and the nature of amorphous and crystalline phases within the as-sprayed coatings. The results support the importance of melting and mixing phenomena during spraying on the amorphous phase formation, so that longer exposure at high temperature (lower in-flight particle velocity) results in higher amorphous contents due to more complete melting and mixing. The comparison of the atmospheric and suspension plasma spray methods presents several similarities in terms of melting and mixing behaviour and the resulting phases. The two methods are, however, different in fragmentation and the eventual crystallite sizes. The formation of crystalline supersaturated solid solutions of alumina and zirconia in SPS coatings is confirmed.

A mean-field model for amorphization in crystalline solid solutions

Based on our understanding of the atomic mechanisms of melting in binary crystalline solid solutions, we propose a unified two-order-parameter mean-field model to describe solid-state amorphization under polymorphic constraints. In this framework, we treat both thermal melting and amorphization on equal footing as a topological-order-to-disorder transition. The primary order parameter is defined such that it represents the loss of the long-range order. The elastic strain field induced by composition disorder that plays an important role at low temperature is described by a bilinear coupling with the primary order parameter. We show that solid-state amorphization can be considered as an extension of a melting transition at low temperature under polymorphic constraints. This theory can also address endothermic melting as well as exothermic melting at low temperature. We present phase diagrams and the corresponding thermodynamic quantities for a model binary crystalline solution system at and near the transitions.

Prediction of Favored and Optimized Compositions for Cu−Zr−Ni Metallic Glasses by Interatomic Potential

For the Cu-Zr-Ni system, an interatomic potential was constructed under the newly proposed formulism named smoothed and long-range second-moment approximation of tight-binding. Applying the constructed potential, molecular dynamics simulations were carried out to compare the relative stability of crystalline solid solution versus its disordered counterpart. Simulations not only reveal that the origin of metallic glass formation is the crystalline lattice collapsing while the solute concentrations exceed critical values, but also determine a quadrilateral region, within which the metallic glass formation is energetically favored. Moreover, the energy differences between the crystalline solid solutions and the disordered states were considered as the driving force for amorphization and were computed by molecular static calculations. The calculation results located an optimized composition area with the driving force much greater than those outside. In addition, the alloys around the composition of Cu(16)Zr(60)Ni(24) were identified to have maximum driving force, and the atomic configurations were also analyzed by the Voronoi tessellation method.

Fractal patterns in Au–Ta multilayer films upon ion beam mixing

Metallic glass has been formed in Au50Ta50 multilayer films upon 200 keV xenon ion mixing at an irradiation dose of 1 × 1015 Xe+/cm2. Electron diffraction and high-resolution transmission electron microscopy analysis confirm that the metallic glass consists of two amorphous phases, evolved from Au and Ta lattices, respectively, and also reveal the formation of a fractal morphology with a fractal dimension of 1.73 ± 0.05 in the dual-phase glass features. In similar Au65Ta35 multilayer films, a fractal pattern is also observed at a dose of 1 × 1015 Xe+/cm2, while the pattern, with a fractal dimension of 1.74 ± 0.05, is composed in this case of a crystalline Au-based solid solution and a Ta-based amorphous phase. Interestingly, the fractal dimensions of the two irradiation-induced fractal patterns match quite well with that expected on a cluster diffusion-limited aggregation model.

Thermodynamic calculation and interatomic potential to predict the favored composition region for the Cu–Zr–Al metallic glass formation

For the Cu-Zr-Al system, the glass forming compositions were firstly calculated based on the extended Miedema's model, suggesting that the amorphous phase could be thermodynamically favored over a large composition region. An n-body potential was then constructed under the smoothed and long-range second-moment-approximation of tight-binding formulism. Applying the constructed Cu-Zr-Al potential, molecular dynamics simulations were conducted using solid solution models to compare relative stability of crystalline solid solution versus its disordered counterpart. Simulations reveal that the physical origin of metallic glass formation is crystalline lattice collapsing while solute concentration exceeding the critical value, thus predicting a hexagonal composition region, within which the Cu-Zr-Al ternary metallic glass formation is energetically favored. The molecular dynamics simulations predicted composition region is defined as the quantitative glass-forming-ability or glass-forming-region of the Cu-Zr-Al system.

Antireflection coatings for solar cells based on an alloy of a mixture of MgO and SiO2

Alloys of a mixture of MgO and SiO2 oxides are obtained using radiant heating. X-ray phase analysis shows that the mixture alloys are multiphase crystalline structures and solid solutions; they contain a glass phase and traces of the original oxides. For the mole: mole concentration, forsterite is identified in the composition of the composite material. It is found that films of an alloy of MgO and SiO2 deposited on the surface of glass and silicon wafers exhibit high mechanical strength and adhesion; they are transparent in the range of sensitivity of solar cells and can be used as antireflection coatings.

Percolative nanostructure and superconductivity in the mixed crystalline cuprateNd1 − xPrxBa2Cu3O7 − δ

The Pr-substituted cuprate Nd1 − xPrxBa2Cu3O7 − δ (NdPr-123) has been investigated with particular attention on the relationship between thesolid solution structure and the superconducting properties concerning the resistivityρ and diamagneticsusceptibility χ. The compound NdPr-123 is regarded as a mixed crystalline solid solution that consists ofunit-cells of Nd-123 and Pr-123, where a percolative nanostructure is constructed. The propertiesρ andχ are largely characterized by such structural inhomogeneity. Several characteristic points (x1,x2,x3) are observedin χ as afunction of x. Occurrence of these characteristic points is well explained in terms of anonclassical percolation process, where the Nd-123 cell adjacent to the Pr-123cells is converted into a different cell, thereby creating virtual percolationthresholds that appear as the characteristic points. The effective Pr-concentrationx* is newly considered in reference to the cell-conversion. The concept ofx* will provide clues for understanding various issues on the Pr-substitutedcuprates. In addition, as an alternative case, a thoroughly different type ofeffective Pr-concentration has also been discussed in reference to the irregularsubstitution of Pr at Ba sites, which may happen to take place in RPr-123 (R = rare earth including Nd).

Proposed truncated Cu–Hf tight-binding potential to study the crystal-to-amorphous phase transition

Proposed truncated Cu–Hf tight-binding potential was constructed by fitting the physical properties of Cu, Hf, and their stable compounds, i.e., Cu5Hf, Cu8Hf3, Cu10Hf7, and CuHf2. Based on the constructed potentials, molecular dynamics simulations were carried out to compare the relative stability of the crystalline solid solution and the disordered state. Simulation results not only reveal that the physical origin of crystal-to-amorphous transition is the crystalline lattice collapsing when the solute atoms exceeding the critical concentration, but also predict that the glass forming range (GFR) of the Cu–Hf system is 21–77 at. % Cu, which covers the GFRs determined by various metallic glass-producing techniques. Ion beam mixing experiments of the Cu–Hf system were conducted using 200 keV xenon ions and the results show that a uniform amorphous phase can be obtained in the Cu23Hf77 sample, matching well with the GFR determined by the interatomic potential, which, in turn, provides additional evidence to the relevance of the constructed Cu–Hf potential.

Phase Diagrams and New Crystal Structures Predicted for the LaX3/LaY3 Systems (X, Y = F, Cl, Br, I)

AbstractThe low‐temperature phase diagrams for the quasi‐binary lanthanum halogenides LaX3/LaY3 (X,Y = F, Cl, Br, I) were calculated at an ab initio level, without any recourse to experimental information. The starting point of our general approach is the global exploration of the enthalpy landscapes for many different compositions in these systems. Candidates for both ordered stoichiometric modifications and crystalline solid solution phases are identified, and their free enthalpies are computed at an ab initio level. From this the low‐temperature phase diagrams are derived. We found that in all six systems under investigation only crystalline ordered phases should be present. We predict the existence of several thermodynamically stable and metastable phases in these systems.

Amorphization of equimolar alloys with HCP elements during mechanical alloying

This study prepares two equimolar alloys, entirely composed of HCP elements, BeCoMgTi and BeCoMgTiZn, from elemental powders by mechanical alloying. No crystalline solid solutions and compounds formed during milling except an amorphous phase formed gradually until full amorphization was attained. The amorphization processes of these two alloys conform to type II according to the Weeber and Bakker classification based on binary alloys. The inhibition of crystalline solid solutions and compounds before amorphization relates to chemical compatibility, high entropy effect and large atomic size difference effect.

Glass Forming Region of Cu−Ti−Hf Ternary Metal System Derived from the n-Body Potential through Molecular Dynamics Simulation

An n-body potential is constructed for the Cu-Ti-Hf ternary metal system under a recently proposed formalism named long-range empirical potential. Applying the proven relevant Cu-Ti-Hf potential, molecular dynamics simulations are carried out using solid solution model to compare the relative stability of the crystalline solid solution versus its disordered counterpart as a function of solute concentration. The simulation results not only reveal that the physical origin of the crystal-to-amorphous transition is the collapse of the crystalline lattice, while the solute atoms exceed the critical value, but also predict a region in the composition triangle energetically favored for the Cu-Ti-Hf ternary metallic glass formation. Interestingly, the prediction directly from the n-body potential is supported by the experimental observations and is in accordance with the so-called structural difference rule.

Formation of nanostructured and amorphous β-Al 3Mg 2 based alloys by rapid solidification and mechanical milling

Al–Mg alloys close to β-Al 3Mg 2 intermetallic phase were processed by melt-spinning and mechanical milling in order to make it nanocrystalline or amorphous alloy. X-ray diffraction, differential thermal analysis and transmission electron microscopy were carried out to investigate the sequential microstructural evolution and stability during the synthesis process. It was found that in the melt-spun ribbons of all the alloys there was no new phase formation but nanophase was observed with a minimum crystallite size of ∼45 nm. However mechanically milled alloys yielded nanocrystalline phase with lower crystallite size and amorphous phase in some alloys. The minimum crystallite size was found to be in the range of 8–22 nm after 20 h of milling. Thermodynamic modeling using the Miedema semi-empirical approach was also done to predict the glass-forming range of the Al–Mg system. From Miedema model it can be inferred that, for the amorphous phase to be more stable than the crystalline solid solution the crystallite size should be below ∼20 nm in the present alloy systems.

Alteration of crystalline zircon solid solutions: a case study on zircon from an alkaline pegmatite from Zomba–Malosa, Malawi

A natural, altered zircon crystal from an alkaline pegmatite from the Zomba–Malosa Complex of the Chilwa Alkaline Province in Malawi has been studied by a wide range of analytical techniques to understand the alteration process. The investigated zircon shows two texturally and chemically different domains. Whereas the central parts of the grain (zircon I) appear homogeneous in backscattered electron images and are characterised by high concentrations of trace elements, particularly Th, U, and Y, the outer regions (zircon II) contain significantly less trace elements, numerous pores, and inclusions of thorite, ytttrialite, and fergusonite. Zircon II contains very low or undetectable concentrations of non-formula elements such as Ca, Al, and Fe, which are commonly observed in high concentrations in altered radiation-damaged zircon. U–Pb dating of both zircon domains by LA-ICPMS and SHRIMP yielded statistically indistinguishable U–Pb weighted average ages of 119.3 ± 2.1 (2σ) and 118 ± 1.2 (2σ) Ma, respectively, demonstrating that the zircon had not accumulated a significant amount of self-irradiation damage at the time of the alteration event. Electron microprobe dating of thorite inclusions in zircon II yielded a Th–U-total Pb model age of 122 ± 5 (2σ) Ma, supporting the age relationship between both zircon domains. The hydrothermal solution responsible for the alteration of the investigated zircon was alkaline and rich in CO3 2−, as suggested by the occurrence of REE carbonates and CO2-bearing fluid inclusions. The alteration of the crystalline, trace element-rich zircon is explained by an interface-coupled dissolution-reprecipitation mechanism. During such a process, the congruent dissolution of the trace element-rich parent zircon I was spatially and temporally coupled to the precipitation of the trace element-poor zircon II at an inward moving dissolution-precipitation front. The driving force for such a process was merely the difference between the solubility of the trace element-rich and -poor zircon in the hydrothermal solution. The replacement process and the occurrence of mineral inclusions and porosity in the product zircon II is explained by the thermodynamics of solid solution-aqueous solution systems.

Crystal chemistry, Rietveld refinements and Raman spectroscopy studies of the new solid solution series: Ba 3− xSr x(VO 4) 2 (0 ≤ x ≤ 3)

The new solid solution series Ba 3− x Sr x (VO 4) 2 (0 ≤ x ≤3) has been synthesized and studied by a combination of X-ray powder diffraction and Raman vibrational spectroscopy. This continuous solid solution crystallise in the hexagonal system with R 3 ¯ m space group. The structure has been determined at room temperature from X-ray diffraction by the Rietveld method analysis. It is formed by a 3D network of (Ba/Sr) (1)(VO 4) 2 4− layers linked into a crystal network by (Ba/Sr) 2 + (2) cations. The vibrational spectra of this crystalline orthovanadate solid solution series are interpreted by means of factor group analysis in terms of space group R 3 ¯ m ( D 3 d 5 ). Assignments of the V–O vibrational stretching and bending modes, as well as some of the external modes, have been made. While all the modes show a monotonous shift as a function of the composition x, a break in the curves of intensities, full width at half maximum and band areas as a function of x is observed and attributed to the statistical distribution of Ba and Sr ions in the same crystallographic sites.

Miscibility of In x Ga1−x As alloys: a study using atomistic simulations

Atomistic simulations are used in combination with the two potential energy functions, namely, the Valence Force Field (VFF) model and the Tersoff model, to study the solution thermodynamics of In x Ga1−x As alloy. The simulation data, in the form of a T − x diagram, is contrasted with the results obtained by using the Ho and Stringfellow approach. It is observed that for the VFF model, the upper critical solution temperature obtained from simulation data is approximately 850 K, which is higher than the 729 K predicted by the Ho and Stringfellow treatment. The composition range for which the two-phase heterogeneous region exists is wider than that predicted by the Ho and Stringfellow approach. The Tersoff model predicts a complex miscibility diagram, where the 850 K temperature corresponds to the approximate ‘eutectic’ temperature. Further improvement of model predictions may be made possible by investigation of temperature and composition dependent interaction parameter in a modified regular solution theory, and investigation of non-random, non-ideal solution models in the Ho and Stringfellow treatment, development of temperature dependent VFF model parameters and adjustment of Tersoff model parameters to account for longer range interactions which exist at temperatures above 850 K. The miscibility diagram constructed using the Tersoff model simulation data can be used to provide information on the phase stability and equilibrium Indium content at any given temperature for the crystalline solid solution.

Glass-forming region of the Ni–Nb–Ta ternary metal system determined directly from n-body potential through molecular dynamics simulations

An n-body Ni–Nb–Ta potential is constructed to conduct molecular dynamics simulations using 129 solid solution models with various compositions. Comparing the relative stability of solid solutions versus their disordered counterparts, simulations determine two critical solid-solubility lines, which define a region in the composition triangle. If an alloy is located inside the defined region, a disordered state is energetically favored; if it is located outside, a crystalline solid solution is preserved. The region is therefore named as the metallic glass-forming region.

Proposed thermodynamic method to predict the glass formation of the ternary transition metal systems

By considering the energetic competition between the crystalline solid solution and glass phase, a thermodynamic method is proposed to predict/determine the glass forming range of a ternary metal system and in terms of the dynamics, the parameter gamma(ABC)* is further defined to search for a proper alloy with superior glass forming ability in the system. 10 more ternary/binary metal systems, e.g. the Cu-Zr-Ti and Cu-Hf-Ti systems, were studied and the predicted alloys of superior glass forming ability match well with those reported from experimental observations.

Mechanism of amorphisation in Cu–Ru, a binary alloy with a positive heat of mixing

Cu–Ru has a positive heat of formation and does not form equilibrium alloys. Nevertheless, amorphous alloys have been obtained by He (Phys. Rev. B 75, 045431 (2007)) by ion mixing of multilayers. Analysis of the free energies of the competing phases (the glass and the crystalline solid solutions based on Cu and Ru) leads us to propose that formation of glasses occurs as a result of kinetic frustration between the hcp and fcc solid solutions. These two have lower free energies than the glass, but those free energies are very similar, so a strong driving force for the formation of a particular crystalline phase does not exist. In addition, formation and growth of hcp and the fcc phases appears equally difficult from a kinetic point of view. Very small embryos can form but their growth will be frustrated by the presence of embryos of the other phase.

Synthesis optimization, structural evolution and optical properties of Y 0.9Er 0.1Al 3(BO 3) 4 nanopowders obtained by soft chemistry methods

This paper describes the structural evolution of Y 0.9Er 0.1Al 3(BO 3) 4 nanopowders using two soft chemistry routes, the sol–gel and the polymeric precursor methods. Differential scanning calorimetry, differential thermal analyses, thermogravimetric analyses, X-ray diffraction, Fourier-transform infrared, and Raman spectroscopy techniques have been used to study the chemical reactions between 700 and 1200 °C temperature range. From both methods the Y 0.9Er 0.1Al 3(BO 3) 4 (Er:YAB) solid solution was obtained almost pure when the powdered samples were heat treated at 1150 °C. Based on the results, a schematic phase formation diagram of Er:YAB crystalline solid solution was proposed for powders from each method. The Er:YAB solid solution could be optimized by adding a small amount of boron oxide in excess to the Er:YAB nominal composition. The nanoparticles are obtained around 210 nm. Photoluminescence emission spectrum of the Er:YAB nanocrystalline powders was measured on the infrared region and the Stark components of the 4 I 13/2 and 4 I 15/2 levels were determined. Finally, for the first time the Raman spectrum of Y 0.9Er 0.1Al 3(BO 3) 4 crystalline phase is also presented.

A mechanistic study of Ag 50Cu 50 solid solution formation by mechanical alloying

This work investigates the phase-transformation kinetics of Ag 50Cu 50 powder mixtures subjected to mechanical processing by ball-milling. A gradual dissolution of Ag and Cu into each other and the subsequent formation of crystalline metastable solid solutions were observed. A quantitative analysis of X-ray diffraction patterns was carried out to reconstruct kinetic curves. Based on a series of experimental evidences, a phenomenological kinetic model was developed to interpolate experimental data by means of mathematical functions. This allowed the fraction of powder trapped in individual collisions to be estimated and to show that only approximately 5% is involved in reactive events. Local dissolution processes determine the formation of the Ag 89Cu 11 and Ag 12Cu 88 metastable solid solutions already at the first impact.