Si Solid Solutions Research Articles

Ductilization of Mo–Si solid solutions manufactured by powder metallurgy

Mo–1.5 at.% Si alloys with additions of either Y 2O 3 or Zr were manufactured by mechanical alloying. The Y 2O 3 particles reduced the grain size and increased the room temperature strength, but did not alleviate the brittleness of previously investigated Mo–1.5 at.% Si without Y 2O 3. Additions of Zr, on the other hand, resulted not only in a fine grain size and an extremely high bend strength (∼2 GPa), but also in limited bend ductility at room temperature. Zr additions are seen to be beneficial for three reasons. First, Zr reduces the grain size. Second, Zr getters detrimental oxygen by forming ZrO 2 particles (which in turn help to pin the grain boundaries). Third, in situ Auger analysis shows that Zr reduces the concentration of Si segregated at the grain boundaries. This is thought to enhance the grain boundary cohesive strength and thus leads to the observed ductility.

Effect of additives on the structure characteristics, thermal stability, reducibility and catalytic activity of CeO2–ZrO2 solid solution for methane combustion

MyOx-modified CeO2–ZrO2 (M = Al, Ba, Cu, La, Nd, Pr, Si) solid solutions with the atomic ratio of Zr/Ce = 1 were prepared by the reverse microemulsion method, and the effect of different additives on the structure characteristics, thermal stability, reducibility, and catalytic activity of CeO2–ZrO2 solid solution for methane combustion were investigated. According to their different effects, MyOx can be classified into three groups. The first group includes SiO2 and Al2O3 which do not vary the crystalline phase of CeO2–ZrO2 solid solution but distort the crystal lattice obviously. They are the most effective additives for improving the surface area, thermal stability, and reducibility of CeO2–ZrO2, and they can also promote the catalytic activity of Pd/CeO2–ZrO2 for methane combustion. The second group includes La2O3, Pr2O3, and Nd2O3, which can also keep the same crystalline phase, distort the crystal lattice, and improve the surface area and thermal stability of the solid solution, but their effects are much weaker and they decrease the reducibility of the solid solution. The third group includes BaO and CuO, whose effects on the property of CeO2–ZrO2 are much different. BaO and CuO, especially CuO, can decrease the thermal stability, and reduction extent of CeO2–ZrO2. CuO-modified CeO2–ZrO2 calcined at 550 °C shows the comparable high activity for the methane combustion, but after being calcined at 900 °C, CuO-modified CeO2–ZrO2 would separate into three phases as CeO2, ZrO2, and CuO, resulting in the much lower activity for the methane catalytic combustion.

Structural transformation in mechanosynthesized bcc Fe–Al–Si(Ge) solid solutions during heating

X-ray diffractometry and Mössbauer spectroscopy study of Fe 50Al 25Si 25 and Fe 50Al 25Ge 25 alloys obtained by mechanical alloying (MA) of elementary powders was carried out. Phase transformation during heating of synthesized products was studied using differential scanning calorimetry (DSC). After 2.5 h of MA monophase alloys containing bcc Fe(Al, Ge) solid solutions Fe(Al, Si) are formed. Fe(Al, Si) is partially ordered B2 type and Fe(Al, Ge) is completely disordered. DSC curves of synthesized alloys displayed the presence of exothermal peaks caused by phase transformation. The metastable Fe(Al, Si) solid solution transformed into FeAl 1− x Si x (B2) and FeSi 1− x Al x (B20) equilibrium phases. The Fe(Al, Ge) solid solution transformed into equilibrium phases through intermediate stage of Fe 6Ge 3Al 2 metastable phase formation. The Fe 6Ge 3Al 2 phase dissociated into three equilibrium phases: FeAl 1− x Ge x (B2), χ-Fe 6Ge 5 and η-Fe 13(Ge, Al) 8 (B8 2). The structure of Fe 6Ge 3Al 2 was calculated by Rietveld method, the distribution of Al and Ge in the elementary cell and its parameters were calculated. Mössbauer study showed that Fe(Al, Si) and Fe(Al, Ge) solid solutions are paramagnetic. In the equilibrium state the alloy containing Si is also paramagnetic while the alloy with Ge showed ferromagnetic properties.

Microstructure and tribological behavior of Ti–Si eutectic alloys with Al addition

The effect of Al element alloying on the microstructure and tribological behavior of Ti–Si eutectic alloys has been studied. The experimental results show that Al element changes the microstructure from large eutectic cells that consist of layered tablet phases Ti5Si3 and α-Ti (for the Al-free alloy) to near-equiaxed or rod-like Ti5Si3 particle reinforced continuous α-Ti (Al, Si) solid solutions. This microstructural change greatly improves the ductility and reduces brittle fracture of massive superficial materials during wear process. Microplough and local delamination are the main wear mechanisms of Al-added Ti–Si alloys. Therefore, the wear resistance and friction stability are simultaneously improved.

Trace element effects on precipitation in Al–Cu–Mg–(Ag, Si) alloys: a computational analysis

In order to optimize mechanical properties of age-hardenable Al–Cu–Mg–(Ag, Si) alloys, the nucleation of the strengthening precipitate Ω must be enhanced while competing phases such as S and θ (or θ″/θ ′) must be suppressed. One strategy is to promote the formation of the precursors, i.e., certain solute clusters that aid in Ω nucleation, using trace element additions such as Ag, and/or the elimination of deleterious impurities, e.g., Si. This paper describes the use of computational analysis tools to identify those elements that are either helpful or harmful. Using a multi-component quasi-chemical model, short-range-order parameters were calculated based on CALPHAD databases to indicate cluster formation in Al–Cu–Mg–(Ag, Si) solid solutions. To anticipate the orientation of the plate-like clusters that controls the habit-plane selection of the subsequent precipitates, the anisotropy of the associated strain was considered using the second order elastic tensors obtained from first principle total energy calculations.

Trace element effects on precipitation in Al?Cu?Mg?(Ag, Si) alloys: a computational analysis

In order to optimize mechanical properties of age-hardenable Al–Cu–Mg–(Ag, Si) alloys, the nucleation of the strengthening precipitate Ω must be enhanced while competing phases such as S and θ (or θ″/θ′) must be suppressed. One strategy is to promote the formation of the precursors, i.e., certain solute clusters that aid in Ω nucleation, using trace element additions such as Ag, and/or the elimination of deleterious impurities, e.g., Si. This paper describes the use of computational analysis tools to identify those elements that are either helpful or harmful. Using a multi-component quasi-chemical model, short-range-order parameters were calculated based on CALPHAD databases to indicate cluster formation in Al–Cu–Mg–(Ag, Si) solid solutions. To anticipate the orientation of the plate-like clusters that controls the habit-plane selection of the subsequent precipitates, the anisotropy of the associated strain was considered using the second order elastic tensors obtained from first principle total energy calculations.

AES depth profiling of thermally treated Al/Si thin-film structures

Interdiffusion in sputter deposited bi- and multilayer structures onto smooth silicon substrates consisting of crystalline c-Al and amorphous a-Si layers was studied. The bilayer consisted of c-Al(300 nm) and a-Si(400 nm) layers and the c-Al/a-Si multilayer of four pairs of c-Al(45 nm) and a-Si(85 nm) layers. Diffusion processes took place in the isothermally heated c-Al/a-Si structures in an argon atmosphere for 20 min at temperatures between 150°C and 270°C, or, at a linear heating rate of 5°C min −1 between room temperature and different final temperatures (300–500°C). AES depth profiling of as-deposited and heat-treated samples enabled identification of the main migrating element in the early stage of reaction, which is Si, and observation of new reaction products. In a few annealed multilayers, a mixture of Al–Si solid solutions and precipitates was found with up to 30 at% Si, and in the Si-rich layer up to 10 at% Al. For the multilayer structures annealed at temperatures higher than 250°C, it was found that diffusion processes resulted in the exchange of the Al- and Si-rich layer positions.

Comparison of the thermal stabilities of NiSi films in Ni/Si, Ni/Pd/Si andNi/Pt/Si systems

The effects of different interlayer materials (Pd and Pt) deposited between Nifilms and Si substrates on the NiSi thermal stability are discussed. Ni0.943Pd0.057Siand Ni0.945Pt0.055Sisolid solutions were formed when the samples were annealedat high temperatures and the lattice parameters of Ni0.943Pd0.057 Siwere calculated according to Vegard’s law. The NiSi thermal stability wasenhanced by interposing a Pd or Pt interlayer, and the sample with the Ptinterlayer had the highest NiSi thermal stability among all the samples studied.This is attributed to the reduction of the interface energy between NiSi and Sisubstrates and the decrease of the driving force for the nucleation of NiSi2,induced by formation of the NiSi(200) preferred orientation and the solid solutionrespectively.

Transport properties of Al–Si solid solutions: theory

Al–Si solid solutions synthesized under high pressure demonstrate striking physical properties, among which are enhanced superconductivity and peculiarities of low-temperature transport properties. To find the reason behind the latter we have performed a first-principles study of the electronic spectra and Fermi surfaces of Al–Si solid solutions. Two electronic topological transitions (ETT's), taking place in the system with increasing concentration of Si and pressure, have been revealed. Based on these data and the theory of ETT's for substitutional solid solutions we have calculated concentration dependencies of the resistivity, thermoelectric power and Hall constant. We show the results to be in quantitative agreement with experiment and to reproduce nicely the experimentally observed peculiarities.

Characterization of silicide precipitates in Nb–Si and Nb–Ti–Si alloys

Abstract The present paper describes the morphology, chemistry and crystallography of silicide precipitates that were observed in Nb–Si and Nb–Ti–Si solid solutions. Although the stable structure of Nb3Si is tetragonal (tP32), within the bcc Nb solid solution, fine-scale Nb3Si precipitates were observed with a metastable orthorhombic crystal structure. These precipitates were observed with an acicular morphology, and with sizes ranging from 2 nm to 1 μm in the assolidified condition. The orientation relationship between these precipitates and the Nb matrix is (100)//(100) and [010]//[010]. It is suggested that the metastable orthorhombic structure forms owing to ease of nucleation of this structure in the bcc Nb matrix. The crystallography of these precipitates is described, and these findings are compared with previous data on the Nb–Si system.

Graded SiGe crystals as X-ray collimators

Many X-ray applications could benefit from a beam conditioner providing a parallel beam with a small cross-section. The limiting values may vary with an application, however, not a single commonly used X-ray optical device is able to achieve these two requirements simultaneously without a drastic loss of the primary beam intensity. The most promising way to overcome this limitation is using crystals with a gradient of the lattice parameter. Recent technological successes in growing high-perfection crystals of Ge–Si solid solutions [A. Erko, F. Schäfers, W. Gudat, N.V. Abrosimov, S.N. Rossolenko, V. Alex, W. Schröder. Nucl. Instr. Meth. Phys. Res. A 374 (1996) 408] make this goal achievable. The high quality of these crystals is experimentally confirmed [S. Keitel, C. Malgrange, T. Niemueller, J.R. Schneider. Acta Crystallogr. A 55 (1999) 855]. This study shows that the gradient crystals allow making high-quality beam collimators overcoming the flux gain restrictions imposed by the flat or curved single crystals. Several types of collimators both for divergent and convergent beams are suggested.

Transient behavior of photoconductivity in Ge-Si single crystals

The transient behavior of photoconductivity in single crystals of Ge–Si solid solutions was studied. The parameters of sticking and recombination centers were determined. The results suggest that impurity agglomerates and silicon precipitates act as recombination and sticking centers, respectively.

The effects of dilution on the competing exchange state in (Tb, Y)Mn 2X 2 (X = Ge, Si)

Neutron-diffraction and magnetometric measurements have been performed on Tb 1− x Y x Mn 2Ge 2 ( x = 0.1, 0.2, 0.3) and Tb 0.9Y 0.1Mn 2Si 2. Replacement of the Tb atoms by Y decreases the ordering temperatures of Tb moments and gives an intermediate phase in (Tb, Y)Mn 2X 2 (X = Ge, Si) solid solutions.

Structural and dynamical properties of metastable Al:Si solid solutions calculated by the embedded-atom method.

Metastable fcc ${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$ solid solutions in the low Si concentration range are modeled by the embedded-atom method. The potential parameters for Si are fitted to the experimental lattice constant of the solutions, the cohesive energy, and the elastic properties of the metallic simple-cubic phase of Si determined by ab initio computations. The model reproduces and explains the concentration dependence of the vibrational density of states, the decrease of the shear modulus with increasing x, and allows us to simulate the first stages of the alloy decomposition into Al and Si.

Interfacial reactions and silicide formation in Si/Ni/Si and Si/Cr/Si sandwich layers

AbstractThe preparation of well‐characterized silicide thin films for microelectronics needs a control of interfacial reactions and diffusion processes during heat treatment of metal/semiconductor systems. Two sandwich structures of Si(33 nm)/Me(50 nm)/Si(33 nm), where Me = Ni or Cr, with a total thickness of each structure of 116 nm were sputter deposited onto smooth silicon‐(111) substrates. The reactions of both metals with amorphous silicon thin films were activated in a differential scanning calorimeter (DSC), at a heating rate of 40°C/min−1, between room temperature and different higher temperatures. Auguer electron spectroscopy depth profiles showed that the Si/Ni/Si sandwich structure reacted almost completely during heat treatment up to 320°C and formed reaction products with a composition close to Ni3Si2. Selected area diffraction patterns revealed that this is a mixture of Ni2Si and NiSi silicides. A much less pronounced reaction between Si and Cr was observed in the Si/Cr/Si sandwich structure, even with heating to 630°C, resulting in CrSi2 silicide and different Cr–Si solid solutions. The results of AES depth profiling studies of the thermally treated sandwich structures are discussed in terms of diffusion processes, movement of interfaces and formation of silicides. The additional information obtained with differential scanning calorimetry and transmission electron microscopy enables a detailed identification of reaction products formed in the early stage of the thermally treated Si/Me/Si structures.

The kinetics of decay of supersaturated solid solutions Al(Si) produced under high pressure

Calorimetric investigations of the decay of supersaturated Al(Si) solid solutions prepared under high pressure are presented. It is established that the decay of the metastable state of Si in a solution occurs in several stages. The complicated process of the decay may be described in the framework of a simple model considering the mutual positions of Si and Al atoms in nearest coordination spheres.

Formation of AlSi and AlGe solid solutions and equation of state

The pressure effect on the solid solubility of the AlSi and AlGe systems is studied using the electronic theory based on pseudo- potentials and the virtual crystal approximation for the disordered alloy. The atomic concentration x-dependence of the equilibrium volume satisfying energy-minimum condition for Al 1− x Si x and Al 1− x Ge x solid solutions with fcc lattice is in good agreement with an available experimental data. The equation of state of these alloy systems is presented quantitatively and the compression effect on the heat of solution is investigated. The heat of solution ΔE S ( x, P) for Al 1− x Si x and Al 1− x Ge x solid solutions under pressure P is small for the Al-rich region and decreases as the crystal is compressed. Consequently, using the Helmholtz free energy of formation under pressure F' s ( x, P, T), we predict the extended solid solutions under pressure in AlSi and AlGe alloy system is consistent with the experimental tendency.

Aging Process in Supersaturated Al-10 at % Si Alloy Solution Treated at High Pressure

Supersaturated Al-10 at%Si solid solutions were prepared by high-pressure and temperature techniques. The elastic constants, lattice constants, electrical resistivity, density and hardness of these obtained specimens were measured in each process of isochronal and isothermal aging. At isochronal temperatures of about 100°C°C to 300°C, rapid variations of several characteristics appeared, and the lattice parameters overshot that of pure aluminum. The relation between the Al-matrix and the precipitated phase of silicon and the activation energies for precipitation are also discussed.

Dynamical properties of high pressure quenched F.C.C. AlSi solid solutions

Using high pressure quenching, we have prepared a homogeneous Al 0.94Si 0.06 solid solution. By means of inelastic neutron scattering, the phonon density of states has been determined and compared with that of aluminium. The specific heat between 0.12 and 7 K is also presented. Experimental evidence is found for 1. (i) a softening of the transverse acoustic modes of phonons at low frequencies, 2. (ii) an important enhancement of the superconducting transition temperature and 3. (iii) at low temperature a contribution to the specific heat with a linear temperature dependence. It is noticeable that all these three effects are usually simultaneously found in amorphous metals. In this crystalline solid solution, the metallic state of silicon atoms is emphasized to explain them and also the non-equilibrium state.

Aging process in supersaturated Al-10at% Si alloy solution treated under a high pressure.

Supersaturated AI-10 at % Si solid solutions were prepared by using high pressure and high temperature techniques. The elastic constants, lattice parameters, electrical resistivity, density and hardness of these obtained specimens were measured by the process of isochronal and isothermal aging. For isochronal temperatures of about 100°C to 300°C, rapid changes of several characteristics were observed. In the aging process, the variations of elastic constants which had two stages appeared and the lattice parameters overshot those of pure-aluminum. The relation between the Al-matrix and the precipitated phase of silicon and the activation energies for precipitation are also discussed.