Si Al Alloy Research Articles

Control of Si Crystal Growth during Solidification of Si-Al Melt

The growth of Si crystals from a Si-55.3 at%Al melts was investigated with the aim of developing a new Si refining process for SOG-Si production. Si crystals were grown by directional solidification of the Si-Al alloy. The temperature at the bottom of the sample was controlled from 1273 K (the alloy’s liquidus temperature) to 1173 K. Bulk Si crystals were successfully obtained in this study, and the crystal growth was found to be controlled by the diffusion of Si in the melt. By controlling the crystal growth conditions, the Al content of the Si crystals could be decreased to the level of the solid solubility of Al in Si; furthermore, other impurity elements could be efficiently removed by this refining method. [doi:10.2320/matertrans.M2010001]

Electromagnetic wave absorption properties of flaky Fe–Ti–Si–Al nanocrystalline composites

Nanocrystalline Fe–Ti–Si–Al flaky powders with large aspect ratio have been fabricated by high-energy planetary milling melt-spun ribbons. Magnetic composites made of thermally annealed Fe–Ti–Si–Al flaky powders embedded in epoxy resin exhibit excellent electromagnetic wave absorption properties in a wide frequency range from 100 MHz to 2.65 GHz. Ti additions can enhance the electrical resistivity of Fe–Si–Al alloy without changing the crystallographic structure, resulting in the reduction of eddy current loss at high frequencies. Moreover, the substitution of Ti for Fe enhances the cut-off frequency of flaky powder–polymer composites. Meanwhile, compared to the Ti-free composites, reflection loss of the Ti-doped composites (3 mm in thickness) is lower than −10 dB in the frequency range from 593 MHz to 1.83 GHz, indicating that the composites can effectively work in a wider frequency bandwidth.

Improvement of mechanical properties of AL (A356) cast alloy processed by ECAP with different heat treatments

Severe plastic deformation (SPD) by equal channel angular pressing (ECAP) has been used as a reliable method to achieve an ultra-fine and homogenous microstructure in the bulk materials. In this research, study was conducted on the structures and mechanical properties of A356 (7%Si) Al alloy produced by severe plastic deformation through ECAP. Special attention was paid to the effect of heat-treatments such as annealing, solution, quenching and pre/post-aging on the properties of the materials. The heat treated as cast structure was successfully deformed in ECAP process. The yield strength, ultimate strength, and hardness of the ECAP samples were improved after pre/post-artificial aging. Mechanical properties improvement was achieved after ECAP process in the samples with the pre-aging treatment. The results show near 114% improvement of the ultimate tensile strength (UTS) and 166% elongation from the as-cast structure. From experimental work, it was also found that the acceleration of precipitation kinetics after ECAP process is not important factor to achieve improvement of mechanical properties, but distribution of Si particles with precipitates formed during T6 is important in high UTS achievement along with high elongation.

In situ observation of solidification phenomena in Al–Cu and Fe–Si–Al alloys

Synchrotron radiation enables the observation of solidification in metallic alloys. In situ observations of solidification for Al–Cu alloys (5, 10 and 15 wt-%Cu) are reported. Nucleation and fragmentation of dendrite arms were often observed in the 15 and 10%Cu alloys when unidirectional solidification was performed from the planar interface. In contrast, nucleation and fragmentation were rarely observed in the 5%Cu alloys. The nucleation ahead of the solidifying front and the fragmentation in the mushy region strongly depended on alloy composition. This paper also presents in situ observation of solidification of Fe–10Si–0·5Al (at-%) alloys. The dendritic growth of δ-Fe was clearly observed using this technique. The development of X-ray imaging techniques enables the solidification of various conventional cast alloys such as Al, Ni and Fe alloys to be observed and will be increasingly used to investigate solidification phenomena.

Microstructure and oxidation resistance of Mo–Si and Mo–Si–Al alloy coatings prepared by electro-thermal explosion ultrahigh speed spraying

Mo–Si and Mo–Si–Al alloy coatings were synthesized on the surface of nickel-based superalloy substrate using electro-thermal explosion ultrahigh speed spraying technology. The coatings have dense microstructure with submicrometer size grains. The microhardness of the coatings is in the range of 1100–1350 HV. The weight loss of Mo–Si alloy coatings is found to be 2 mg/cm 2 on exposure at 500 °C for 840 h and 20 mg/cm 2 on holding at 900 °C for 360 h, respectively. However, Al alloyed in Mo–Si coatings obviously slows down the oxidation at high temperature. The weight loss of Mo–Si–Al alloy coatings is only 0.5 mg/cm 2 at 500 °C for 840 h and 7 mg/cm 2 at 900 °C for 360 h, respectively. The weight loss ratio of Mo–Si–Al alloy coatings decreases about 70% comparing to that of Mo–Si alloy coatings. The pest disintegration of the MoSi 2 can be suppressed by electro-thermal explosion ultrahigh speed spraying technology and alloying Al.

Interactions between solute atoms in Fe–Si–Al–C alloys as studied by mechanical spectroscopy

In Fe–Si–Al–C alloys, point-defect relaxation includes both the interstitial carbon Snoek-type relaxation, split into a “pure iron” (Fe–C–Fe) Snoek peak and an “interstitial–substitutional” (Fe–C–Me; Me = Al, Si) peak, and the substitutional Zener relaxation. The influence of Al and Si, with varying Al/Si ratio, on these effects is used to study the qualitative characteristics of substitutional–interstitial (Si–C, Al–C) and substitutional–substitutional (Al–Al, Si–Si, Al–Si) interactions in these alloys. Concerning the latter, there is a mutual compensation of the elastic distortion fields, produced in the Fe matrix by the relatively bigger Al and smaller Si atoms, respectively, which largely suppresses the Zener relaxation in the ternary Fe–Si–Al alloys, probably without affecting the elastic dipole strength of interstitial carbon. From the kinetic behavior of the two components of the Snoek-type relaxation, it is concluded that the substitutional–interstitial interaction is generally attractive and sufficient for modifying the distribution of the C atoms, but not for trapping to become dominant.

Production and Properties of a 90%Si-Al Alloy for Electronic Packaging Applications

The high silicon content Si-Al alloy is a typical heat dissipation material that used in the electrical packaging field. A spray forming process wais used to produce a 90%Si-Al alloy specimen as a heat dissipation material in the present study. Then the spray formed 90%Si-Al specimens weare hot pressed at 1000°C with different pressure ranged from 2MPa to 10MPa to increase their density. The physical properties of the experimental alloy specimen weare measured. And the microstructure wass are observed by using optical microscopy and scanning electronic microscopy. The results showed that the Spray forming wais suitable forto producinge a 90%Si-Al alloy. With hot pressure of 10MPa, the relative density value of 90%Si-Al reachedcan obtain 94%. The typical physical properties such as the thermal conductivity, coefficient of thermal expansion and electrical conductivity of 90%Si-Al alloy are acceptable as a heat dissipation material for many electronic packaging applications.

Screen-Printed Al Back Contacts on Si Solar Cells: Issues and Some Solutions

AbstractThis paper identifies some mechanisms that lead to problems in back Al contact formation. Major issues are related to a basic problem that the Al melt has a large surface tension and tries to ball up during the firing step. Other issues arise from dissolution of the Si-Al interface and entrapment of glass within the Si-Al alloy. Si diffusion into Al can be applied to control the melt, while cooling rate can help improve the structure of various regions of the back contact for a favorable series resistance. We also discuss a modified time-temperature profile that can lead to a deep and uniform back-surface field.

Hybridized kinetic energy functional for orbital-free density functional method

We propose a hybridized kinetic energy functional, a T TF + b T vW , where T TF is the Thomas–Fermi functional and T vW the von Weizsäcker functional while a and b are adjustable parameters. The new functional is implemented in orbital-free plane-wave density functional method, in which a conjugate-gradient line-search scheme of electronic minimization is incorporated. Calculations with the fitted a and b show that this kinetic energy functional can describe the structures of small Si, Al and Si–Al alloy clusters with reasonable accuracy.

Refining of silicon during its solidification from a Si–Al melt

To develop a low-cost process for refining silicon for use in solar cell materials, we have studied a low-temperature refining technique referred to as “solidification refining of silicon with a Si–Al melt at low temperature”. To discuss the purification in the solidification process, the segregation ratios of impurity elements between solid silicon and the Si–Al melt were determined experimentally and evaluated by thermodynamic analysis. Since the determined values were much smaller than the segregation coefficients between solid and liquid silicon, the solidification of silicon from the Si–Al melt at low temperature is found to be more effective for purification than general silicon solidification. A new technique for agglomerating solidified silicon crystals during solidification of Si–Al alloy was established using induction heating. A refining test, by means of this agglomeration method, revealed the effective purification during silicon solidification from the Si–Al melt.

The effects of thermomechanical training treatment on the deformation characteristics of Fe–Mn–Si–Al alloys

We have studied the effects of the thermomechanical training treatment on the deformation mode for Fe–30Mn–(6 − x)Si– xAl ( x = 0, 1, 2 and 3 mass%) alloys, which exhibit complicated deformation processes due to coexistence of the stress-induced fcc to hcp martensitic transformation and the mechanical twinning of the fcc matrix in addition to slip deformation. The stress–strain curves at each training cycle were analyzed for the alloys, and the deformation microstructures were observed with atomic force microscopy on a Fe–30Mn–5Si–1Al alloy before and after the training treatment. It was found that the deformation mode shifts from the mechanical twinning to the stress-induced martensitic transformation, as the training proceeds.

Electrochemical characterization of Ti–Si and Ti–Si–Al alloy anodes for Li-ion batteries produced by mechanical ball milling

Alloys of Ti–Si and Ti–Si–Al were produced by mechanical milling. The alloys were prepared by ball milling the pieces of prealloyed melt-spun ribbons or mechanical alloying from elemental powder mixture. The influence of the milling time on the electrochemical performance of the alloys as anode in lithium-ion batteries was investigated. It appears that a critical milling time is required to attain good cycle performance. However, prolonged milling results in the reduction of reversible capacity. For Ti 28Si 72 alloy showing good cycling stability with small capacity, the partial substitution of Ti with Al enhances the reversible capacity with comparable cycle performance. The factors attributable to the better cyclability are discussed.

Assessment of Si and Al diffusion for the production of high Si and high Si–Al electrical steel

A trend in the production of electrical steel is to increase the concentration of Si and Al (over 3 wt%) in order to reduce magnetic losses through increased electrical resistivity. This can hardly be done in early stages of the production process due to the appearance of brittle phases; however, a concentration increase of (Si, Al) can be reached by surface deposition of Si and/or Al, followed by diffusion into the bulk. Diffusion of Si and/or Al modifies substantially the mechanical and magnetic properties of the substrate and a study of the diffusion process leading to Fe–Si–Al alloys with homogeneous or inhomogeneous distribution of (Si, Al) is needed. The industrial production of these alloys requires models predicting the concentration profile of (Si, Al) resulting from the diffusion process and the changes of the mechanical and magnetic properties associated with the resulting concentration profiles of (Si, Al). This paper assesses the diffusion process using commercial software for the modelling of diffusion-controlled transformations. Experimental validation was made on some base alloys and results for the 3 wt% Si material are reported. Substrates were coated by dipping in a molten Al–25 wt% Si alloy, followed by annealing at different temperatures and times. The results show that Si concentration profiles can be accurately predicted. However, when considering both Si and Al diffusion their interaction causes important disagreements between experimental results and model calculations.

Production and Properties of a Spray Formed 70%Si-Al Alloy for Electronic Packaging Applications

The high silicon content Si-Al alloy is a typical heat dissipation material that used in the electrical packaging field. A spray forming process is used to produce a 70%Si-Al alloy specimen as a heat dissipation material with a diameter of 76.2 mm (3 inch) and a thickness of 6 mm. Then the spray formed Si-Al alloy specimens are hot pressed at 570°C with different pressure ranged from 200 MPa to 700 MPa to increase their density. The physical properties of the experimental alloy specimen are measured. And the microstructures are observed by using optical microscopy and scanning electronic microscopy. The results show that Spray forming is suitable to produce a 70%Si-Al alloy. The size of primary Si phase in the spray formed 70%Si-Al alloy is refined only 20∼30 μm. The relative density of 70%Si-Al alloy after spray forming is about 90%. With a following hot pressure of 700 MPa, the relative density value can obtain 98%. The typical physical properties such as the thermal conductivity, coefficient of thermal expansion and electrical conductivity of spray formed 70%Si-Al alloy are acceptable as a heat dissipation material for many electronic packaging applications.

Determination of stacking fault probability in fcc Fe–Mn–Si–Al alloy by electron diffraction

The stacking disorder in the fcc structure of Fe–23Mn–2Si–2Al alloy after tensile testing at −75°C was investigated by electron diffraction, using a relationship between the stacking fault probability and the shift of diffraction spots deduced in the present work (and based on Kakinoki’s theory [Acta Crystallogr. 23, 875 (1967)] and Kajiwara’s previous work [Jpn. J. Appl. Phys. 9, 385 (1970); J. Phys. Soc. Jpn. 22, 795 (1967)]). Shifts of diffraction spots along the [111] direction in the fcc structure were observed, and the stacking fault probabilities in two selected areas with different densities of stacking faults were determined as α=0.15 and α=0.35. The stacking fault probabilities measured by electron diffraction are much larger than average values determined by x-ray diffraction, indicating that the distribution of stacking faults is localized. A mechanism for the γfcc→εhcp transformation is suggested, whereby during deformation the following sequences take place: dissociation of perfect dislocation→localization of stacking faults→evolution from stacking disorder to stacking order→the formation of the perfect hcp martensite (α=1) or the hcp martensite with stacking faults (α→1). This proposed mechanism for strain-induced hcp martensite formation should be also applicable to the thermally induced hcp martensite.

Property measurements on spray formed Si-Al alloys

A novel Si-Al alloy was prepared by spray forming process for electronic packaging. Property measurements on spray-formed Si-Al alloys after hot pressing were carried out. The results indicate that the alloys (Si-(30%–40%)Al) have advantageous physical and mechanical characteristics, including low coefficient of thermal expansion (6.9×10 −6–8.7×10 −6/K), high thermal conductivity (118–127 W/(m-K)), low density (2.421×10 3–2.465×10 3 kg/m 3), high ultimate flexural strength (180–220 MPa) and Brinell hardness (162–261). The alloys are easy to machine to tight tolerances using standard machine tools and they can be electroplated with gold finishes and soldered with Sn-Pb alloy without any difficulty.

Effects of aspect ratio and particle size on the microwave properties of Fe–Cr–Si–Al alloy flakes

The structural and electromagnetic properties of gas atomized Fe–Cr–Si–Al alloy (SMSS) powders flattened by ball milling were studied. It is found that the aspect ratio of the powders with the same particle sizes of 48–55 μm increases with the milling time. With the increase of the aspect ratio, the complex permittivity, permeability and intensity of reflectivity peak increase, respectively, and the matching frequency shifts to low frequency due to the improved wave impedance matching and the increase of real part of complex permittivity and permeability. A similar trend of the microwave properties with the decrease of the particle size can be also observed.

Crystallization kinetics of ultrathin amorphous Si film induced by Al metal layer under thermal annealing and pulsed laser irradiation

The crystallization kinetics of ultrathin a-Si induced by Al under thermal annealing and pulsed laser irradiation has been studied. Under thermal annealing, the crystallization temperature and activation energy for crystallization of a-Si with a thin Al metal layer was reduced to around 340°C and 3.3eV, respectively. The reaction exponent was determined to vary from 1.5 to 1.8, corresponding to a crystallization process in which grain growth occurs with nucleation, and the nucleation rate decreases with the progress of grain growth. Under high power pulsed laser irradiation, the crystallization and reamorphization of a-Si were found to take place sequentially in a-Si∕Al. The reamorphization of a-Si in contact with a thin Al metal layer can be attributed to the melting of a-Si∕Al initiated at the interface, due to the low melting temperature of Si–Al alloy and the rapid solidification that followed. Considering only the crystallization process, the activation energy for crystallization of a-Si induced by Al, estimated to be about 0.22eV, was nearly an order of magnitude lower than that under thermal annealing. This may be explained by the explosive crystallization of a-Si by mechanical impact with a high power pulsed laser. In the meantime, the reaction exponent, determined to range from 1.9 to 2.2, was slightly higher than that under thermal annealing, indicating that the decrease of nucleation rate with the progress of grain growth during crystallization was slower, and the crystallization process became more nucleation dominant.

The microscopic mechanism of silicon precipitation in Al/Si system

A Homo-Heterogeneous Mechanism (HHM) for the precipitation of silicon atoms in aluminum is proposed. The main point of HHM is that the formation of critical nucleus consists of two steps. At the first step the pre-critical nucleus is formed. It plays a role of a preferable site for the further hetero-nucleation and then for the growth of Si precipitates on these nuclei. The reported results are based on ab initio calculations of total energy for equilibrium configurations of silicon clusters embedded into aluminum lattice. HHM explains the results of experimental studies of silicon precipitation in quenched Si–Al alloy obtained in this work and by other authors.

Ni3Si(Al)/a-SiOx core–shell nanoparticles: characterization, shell formation, and stability

We have used an electrochemical selective phase dissolution method to extract nanoprecipitates ofthe Ni3Si-type intermetallic phase from two-phase Ni–Si and Ni–Si–Al alloys bydissolving the matrix phase. The extracted nanoparticles are characterizedby transmission electron microscopy, energy-dispersive x-ray spectrometry,x-ray powder diffraction, and electron powder diffraction. It is found that theNi3Si-type nanoparticles have a core–shell structure. The core maintains the size,the shape, and the crystal structure of the precipitates that existed in thebulk alloys, while the shell is an amorphous phase, containing only Si and O(SiOx). Theshell forms around the precipitates during the extraction process. After annealing the nanoparticles innitrogen at 700 °C, the tridymite phase recrystallizes within the shell, whichremains partially amorphous. In contrast, on annealing in air at1000 °C, no changes in the composition or the structure of the nanoparticles occur. It issuggested that the shell forms after dealloying of the matrix phase, where Si atoms,the main constituents of the shell, migrate to the surface of the precipitates.