Si-based Alloys Research Articles

STM Observation of Grain Boundaries in Al-Mg<sub>2</sub>Si Base Alloy Deformed at Room Temperature

STM Observation of Grain Boundaries in Al-Mg<sub>2</sub>Si Base Alloy Deformed at Room Temperature

Atomic bonding in amorphous carbon alloys: A thermodynamic approach

The free energy model previously developed for the prediction of the bonding in amorphous Si-based alloys is extended here to amorphous carbon alloys, a-CxH1−x, containing carbon atoms with sp3 and sp2 hybridization. Predictions have been made for the bonds present in the alloys, with the case of ‘‘chemical’’ ordering at T=0 K corresponding to phase separation into separate C (sp3) and C(sp2) regions. For T≳0 K phase separation is eliminated and there is no evidence for the clustering of graphitic carbon, indicating the importance of the configurational entropy in influencing the bonding in the alloys. Hydrogen atoms are predicted to bond preferentially to C (sp3) atoms for all T. The sp3/sp2 ratio is predicted to increase with increasing H content, as observed experimentally, and also with increasing T due to entropy effects. Predictions have been made for the distribution of bonds in tetrahedral C(sp3)- and planar C(sp2)=C(sp2)-centered units. It is found that essentially no aromatic or graphitic structures are present in typical alloys. The a-CxH1−x alloys have been proposed to consist of five amorphous components: diamondlike, graphitic, polymeric, olefinic, and mixed diamond–graphitic (d–g) components. It is predicted that the polymeric and mixed d–g components dominate in typical plasma-deposited alloy films while the mixed d–g component dominates in hydrogen-free a-C films.

Microscopic observation of γ-phase and ε- and α′-martensite in FeMnSi-based shape memory alloys

The martensitic transformation occurring in FeMnSi-based shape memory alloy has been studied by microstructural characterization and by a color etching method developed to reveal the martensite structure. With this technique, γ-, ε-martensite and α'-martensite can be easily distinguished and quantified. Two types of transformations, γ→ε and γ→ε→α′, occur in the studied alloys. The γ→ε transformation is observed in the specimen after annealing at high temperatures followed by air cooling to room temperature. Increasing annealing temperature enhanced the γ→ε transformation during cooling. Both γ→ε and γ→ε→α′ transformations are found after applying a certain amount of deformation strain. Increasing the amount of applied strain induced the γ→ε→α′ transformation more abundantly. The growth and intersection of α′-martensite is enhanced when the applied strain is large. During reverse heating, α′-martensite directly transformed back to γ without reverting through ε-martensite.

The oxidation of Ni 3Si-base alloys

The isothermal oxidation behavior in air of Ni-19at.%Si and Ni-19at.%Si-8at.%Cr alloys, which are based on the Ll 2 structure of Ni 3Si, was studied in the oxidation temperature range from 500 °C to 900 °C for exposures up to 20 h. The maximum oxidation rate for Ni-19at.%Si was observed at 700 °C. The oxides which formed in this alloy at temperatures up to 700 °C consisted of external NiO layers and internal oxides that preferentially formed in the two-phase (Ll) 2 + f.c.c.) regions of the microstructure adjacent to the sample surface. The increase in oxidation rate with temperature below 700 °C is attributed to these oxides. For oxidation temperatures above 700 °C a sharp decrease in oxidation rate is observed and is attributed to the formation of a continuous SiO 2 protective oxide film at the base of the NiO layers. The addition of 8 at.% Cr significantly modified the oxidation behavior of the alloy with a monotonic increase in weight gain observed as a function of oxidation temperature. The formation of Cr 2O 3 appears to dominate the oxidation behavior of the Ni-19at.%Si-8at.%Cr alloys. At temperatures below 800 °C, the oxidation rates for the Cr-containing alloy were found to be significantly lower than those for the Ni-19at.%Si base alloy. However, at temperatures of 800 °C and above the oxidation rates were larger than in the base alloy, suggesting that the Cr 2O 3 scale is less protective than the SiO 2 scale which forms in the base alloy in this temperature range. The formation of a continuous Ni 3Si layer by Cr depletion that is separated from the external Cr 2O 3 by an Ni-rich oxide region is observed.

Laser cladding on aluminium-base alloys: microstructural features

There is great interest in using high power lasers for surface alloying or cladding on aluminium alloys, in order to enhance hardness and to improve wear resistance. This study reports microstructural investigations and the correlation with the hardening phenomenon. In the first stage, a mixture of AlSi and Ni powders was used for cladding on an AlSi-based cast alloy. In the second stage, hard particles were also added to the powder, in order to increase the elastic modulus of the cladding; either oxides or carbides were used, but the best solution appeared to be the injection of TiC, which has a very high Young's modulus. After the optimization of the laser-processing parameters, sound claddings were obtained. Their main features are as follows: thickness, up to 1 mm, in a single pass; soundness, no pores or cracks; microstructure, dendritic energy-dispersive spectroscopy and X-ray experiments reveal the Al 3Ni and Al 3Ni 2 phases in the dendrites, and mainly Al and Si in the interdendritic areas; when TiC particles are also injected, they are found to be unaffected in the surface layer and randomly distributed); the Vickers hardness, up to 800 HV, when carbides particles are present; thermal stability, no evolution during thermal cycling because of the existence of stable phases and only limited coalescence of the formed dendrites.

Influence of fibre-reinforcement on the microstructure of an AlSi-based alloy

The aim of the present paper is to discuss some aspects, i.e., hardness and tensile strength considering the changes in the microstructure of an Al-Si-based alloy due to fibre-reinforcement. The investigations have been undertaken with an AlSi12CuMgNi alloy and two metal matrix composites reinforced with 20 vol% of either Saffil, a nearly pure alumina fibre with a silica content of about 3-4 %, or Fiberfrax, consisting of 50% SiO[sub 2] and 50% Al[sub 2]O[sub 3], respectively. The tensile tests have been performed on flat specimens that were cut from rectangular plates by spark-erosion. These specimens were heat treated for 100 h at 623 K to a highly overaged conditions (HOA). The aging behavior of the three materials has been investigated for an aging temperature of 473 K by Vickers hardness measurements. These experiments were performed using a Wolpert Dia Testor 2 with a force of 200 N. Tensile experiments were carried out on an Instron-testing machine with a constant strain rate of 3.3[center dot]10[sup [minus]4] s[sup [minus]1]. These investigations have been undertaken at temperature of 295, 473, 623 and 723 K, respectively.

Defect densities and hydrogen diffusion in hydrogenated amorphous Si-based alloys

The density of Si dangling bond defects in plasma-deposited Si-rich a-Si1−xCx:H and a-Si1−xNx:H is argued to depend primarily on deposition temperature via the hydrogen diffusion rate. The optimum temperature equals about 0.62 of the bulk-bonded hydrogen evolution temperature, or about 500 °C for a-Si3N4:H. A model of hydrogen diffusion in alloys is proposed.

Small-angle structure and atomic order in Ge- and Si-based liquid alloys

The chemical and topological order of AgSi, AgGe and AlGe liquid alloys are investigated from the thermodynamic equations. On mixing, the component elements (Si, Ge, Al and Ag) undergo noticeable structural change. This suggests that AgSi is a segregating system, AlGe is chemically more ordered and AgGe undergoes inversion from order to segregation as a function of concentration.

The study of optical nonlinearity in amorphous Si-based alloys

Amorphous all-dielectric Fabry-Perot interference filters were adopted as a prototype for studying optical bistability. Room-temperature optical bistability has been demonstrated in hydrogenated amorphous silicon-carbon alloys at wavenlength about 532 nm. The switching intensity which should be required <0.5 W/μm 2

The dissociation of dislocations in SixGe1-x alloys

The knowledge of the structure of dislocations in semiconductors is of importance for an understanding of their mechanical and electrical properties. In electronic devices dislocations are known to act as recombination centers for charge carriers and as traps for impurity atoms. In addition dislocations play an important role in the mechanical relaxation of elastic stresses in lattice-mismatched heterostructures. In Si-based epitactical heterostructures alloys of Si and Ge are used to adjust the lattice parameters by variation of the alloy composition thereby modifying the elastic stresses in a controlled manner. Such strained layer structures are of potential application for electronic components with improved performance.We have investigated dislocations in SixGe1-x crystals grown from the melt by the Czochralski and Bridgman method, respectively. The dissociation of dislocations into Shockley partials in SixGe1-x. alloys with low (x ≈ 0.05) and high (x ≈ 0.5) Si content was characterized by analytical electron microscopy at 300 kV.

CALCULATION FOR g-VALUES OF THE AMORPHOUS Si AND AMORPHOUS Si-BASED ALLOY SEMICONDUCTORS

In this paper, the g-values of the ESR signals in amorphous Si and Si-based alloys have been calculated by using the improved CNDO/2 molecular orbital method. The effects of the structural relaxation on the g-valucs have been also discussed. The calculated results are in accord with the experiments.

Urbach edges and angular distortions in a-Si:H and related alloys

We propose a general discussion of the experimental correlation between the valence band tail slope parameter k B T v and the rms deviation ΔΘ b of the SiSi bond angle distribution which has been observed by various authors in doped and undoped a-Si:H, as well as in a-SiGe:H and a-SiN:H alloys. In the simplified case of a Gaussian distribution of bond angle values with a single harmonic bending potential, an exponential distribution is expected for the elastic energy fluctuations, with a decay parameter scaling with ΔΘ b 2. For Si-centered tetrahedra with at least two Si nearest neighbors, the Keating formalism yields realistic estimates of the elastic potential site-to-site fluctuations along the SiSi skeleton of Si-based alloys. The quantitative agreement between calculated and experimental k B T v values obtained for a set of well characterized PECVD a-SiN:H alloys lead us to attribute the valence band tails states to distorted sites where the SiSi bond angle is at least 2 ΔΘ b away from the ideal tetrahedron value.

Hydrogen in some PdSi-based amorphous alloys and in the crystallized states

The solubility of hydrogen in amorphous and crystallized forms of PdSi-based alloys has been investigated by electrode potential and electrical resistance measurements following electrochemical introduction of hydrogen. The solubility data for the amorphous alloys can be explained by the model proposed by Kirchheim et al., but the data for the crystallized state did not necessarily agree with the model. The variation in electrical resistance with increasing hydrogen concentration in the amorphous alloys was found to be of the form R − R 0)/ R 0 & ([H]/[M]) α , where α & 1, in the concentration range above [H]/[M] & 10 −4 , but the resistance in the crystallized states decreased initially with hydrogen concentration.

Amorphous Si and Si-based alloys from glow-discharge plasma

Abstract

プラズマＣＶＤによる薄膜形成 アモルファスシリコンとシリコン系合金薄膜作製における最近の話題

プラズマＣＶＤによる薄膜形成 アモルファスシリコンとシリコン系合金薄膜作製における最近の話題

Guiding principle for preparing highly photosensitive Si-based amorphous alloys

A guiding principle was proposed for the preparation of highly-photosensitive hydrogenated amorphous SiGe (a-SiGe:H) as well as SiC (a-SiC:H) alloy materials via the plasma chemical-vapor-deposition (CVD), which was derived from the experimental analyses of the behavior of adsorbed precursors on the growing surface by means of a radical separation technique. According to this principle, Triode method and H 2-dilution method of starting gas materials were used for the deposition of a-SiGe:H and a-SiC:H, and highly-photoconductive alloy materials were prepared in the optical-gap range between 1.3 and 2.3eV.

Ｍｎ系分散物を含むＡｌ‐１％Ｍｇ２Ｓｉ‐０．４％Ｓｉ合金の延性に及ぼす結晶粒度の影響

The effects of grain size and Mn on ductility of Al-1%Mg2Si-0.4%Si base alloys were examined by tensile test.When grain size was large as 600μm, elongation was slightly increased with increase of Mn at peak aged (200°C) condition of the alloys containing upto 0.5%Mn. While, if grain size was so small as about 50μm, elongation was almost independent of Mn in the alloys containing 0.20.8%Mn.Although grain size varied from 50 to 600μm, intergranular fracture surface was always observed at peak aged (150 and 200°C) condition of 0.5%Mn addition alloys, which had the Mn bearing dispersoids of a mean diameter of 0.2μm.The slip band spacing was decreased with decrease of grain size in 0.5%Mn addition alloys.

Exafs Study of a-Si1−xGex:H, a-Si1−x Cx:H and a-Si1−xNx:H at the Si K-Edge

AbstractThe structure of three series of hydrogenated amorphous Si-based alloys was studied by EXAFS at the Si K-edge. Average bond-lengths and first shell compositions were determined. It is shown that the two atomic species are randomly distributed in a-Si1−xGex:H whereas a chemical ordered composition is favored in a-Si1−xNx:H

Reactive Deposition of a-Silicon and Si-Based Alloys

ABSTRACTHigh photoconductive a-Si:H(F) and St-based alloys were prepared from “precursors” made by oxidation of silane with F2 or reduction of SiFn (n=1,2,) with atomic hydrogen. Both crystalline and amorphous silicon were prepared at will, in the latter case, by controlling the flow of hydrogen. A marked reduction in the localized states in the vicinity of valence band was eatablished in the a-Si:H(F) prepared under optimal condition. A novel photoconductive film with high photoconductive gain for near-tr light was successfully made by multiplying a-Si:H/a-SiGe:H(F) periodically by a new preparation technique without a rise in the dark conductivity.

Problems in a-Si Photoelectric Devices; Photoreceptor and Vidicon

AbstractThe history in developing a-Si image devices, photoreceptor and vidicon is briefly reviewed. Some problems to be solved are discussed for the device structures and preparation techniques of Si-base alloys and Si itself from technical point of view.