Investigation into the Structure Formation and Properties of Materials in the Copper–Titanium Disilicide System

The structure formation and properties during infiltration, free sintering, and spark-plasma sintering in Cu–(12.5–37.5 vol %) powder materials Ti₃SiC₂ are investigated by electron microscopy, X-ray phase analysis, and energy-dispersion analysis. The independence of the phase composition of composite materials (CMs) on the sintering method and temperature in a range of 900–1200°C is established. The peculiarities of formation of the CM structure during sintering are the intercalation of silicon from titanium carbosilicide and the formation of a carbon solid solution based on Ti₅Si₃(C) titanium disilicide, small amounts of titanium carbide, silicon carbide, and TiSi₂ silicide. An increase in Ti₃SiC₂ in the CM certainly lowers electrical conductivity, but considerably increases the hardness, strength, and electroerosion wear resistance of CM electrodes for electroerosion broaching.     

Reaction of a molten mixture of titanium,aluminum, and silicon oxides with hot-pressed silicon nitride

The formation temperature of a liquid phase and the solidification temperature of a molten mixture of Al₂O₃-TiO₂-SiO₂ oxides on a silicon nitride substrate are determined. Data are obtained for the change in kinetics. It is established that the intensity of interaction of molten Al₂O₃-TiO₂-SiO₂ with silicon nitride depends on the oxide mixture composition. With heating there are two possibilities: improvement and worsening of Si₃N₄ crystallite wetting with a liquid phase as well as solidification of the melt. The temperature range where a liquid phase exists for actual materials is about 15°C, which markedly worsens the process of structure formation with Si₃N₄ during sintering.Translated from Poroshkovaya M etallurgiya, No. 5, pp. 39–44, May, 1993.     

Silicon volatilisation in the form of SiO during slagging by limestone in BOF

In this paper, the silicon volatilisation phenomenon during slagging by limestone in basic oxygen furnace (BOF) and its influencing factors were studied by industrial experiment and thermodynamic calculation. In our estimation, the volatilisation ratio of silicon in this industrial experiment is about 13.01–47.82%. Thermodynamic analysis showed that the silicon volatilisation phenomenon happens after charging limestone directly into BOF because CO₂ from limestone decomposition could massively oxidise the silicon in the hot metal into gaseous SiO in the hot spot zone. The mass of produced SiO increases, then decreases with the increase of the limestone addition and the carbon content of hot metal, and SiO mass is proportional to silicon content of hot metal. Compared with lime slagging, the strong stirring effect of CO₂ from limestone decomposition, massive foaming slag formation, great increment of furnace gas are all favourable to silicon volatilisation.     

The effect of strontium on the Mg2Si precipitation process in 6201 aluminum alloy

A transmission electron microscopy (TEM) study of a 6201 aluminum alloy to which controlled strontium additions were made has revealed important differences compared to the same alloy free of strontium. In the as-cast state, strontium favors the formation of α-AlFeSi (Al₈Fe₂Si) rather than β-AlFeSi (Al₅FeSi) phase, resulting in a greater quantity of excess silicon present in the strontium-treated alloy. During heat treatment, the excess silicon allows a greater density of finer β″-Mg₂Si precipitates to form, leading to increased tensile strength values and increased electrical resistivity. Strontium also retards the growth of the precipitates formed during heat treatment and inhibits formation of the equilibrium β-Mg₂Si phase. As a result, the strontium-treated alloy exhibits a resistance to overaging.     

Sintering of alloys of zirconium diboride with molybdenum disilicide

Summary The sintering of zirconium diboride with molybdenum disilicide is accompanied by the formation of a solid solution based on zirconium diboride, formation of a liquid phase at temperatures above 1800°C, and partial vaporization of silicon in the ZrB₂+15% MoSi₂ alloy. At temperatures up to 1800°C, solidphase sintering takes place; at low temperatures, this is accompanied by specimen growth due to heterodiffusion processes resulting from the difference in the partial diffusion coefficients of the components and to the vaporization of excess silicon in the case of the ZrB₂+15% MoSi₂ alloy.At temperatures above 1800°C, shrinkage is caused by the formation of a liquid phase, which disappears during sintering. Under these conditions, grain recrystallization and growth in the solid solution of Mo and Si in zirconium diboride in the case of 15% MoSi₂ alloys are not completed even after 4-h holding at temperatures of 1800, 1900, and 2000°C.Translated from Poroshkovaya Metallurgiya, No. 9(45), pp. 11–16, September, 1966.     

Thermodynamic and Kinetic Behavior of B and Na Through the Contact of B-Doped Silicon with Na2O-SiO2 Slags

Boron (B) is the most problematic impurity to be removed in the processes applied for the production of solar grade silicon. Boron removal from liquid silicon by sodium-silicate slags is experimentally studied and it is indicated that B can be rapidly removed within short reaction times. The B removal rate is higher at higher temperatures and higher Na₂O concentrations in the slag. Based on the experimental results and thermodynamic calculations, it is proposed that B removal from silicon phase takes place through its oxidation at the slag/Si interfacial area by Na₂O and that the oxidized B is further gasified from the slag through the formation of sodium metaborate (Na₂B₂O₄) at the slag/gas interfacial area. The overall rate of B removal is mainly controlled by these two chemical reactions. However, it is further proposed that the B removal rate from silicon depends on the mass transport of Na in the system. Sodium is transferred from slag to the molten silicon through the silicothermic reduction of Na₂O at the slag/Si interface and it simultaneously evaporates at the Si/gas interfacial area. This causes a Na concentration rise in silicon and its further decline after reaching a maximum. A major part of the Na loss from the slag is due to its carbothermic reduction and formation of Na gas.     

Formation of silicon carbide in the reaction of reduction of silica by carbon

Conclusions In the initial stages of heat treatment of H₂SiO₃ and sucrose a mixture of highly dispersed defective SiO₂ particles and carbon material is formed. Then as the result of contact interparticle interaction of a radical character disintegration (activation) of the carbon particles and envelopment of them by a layer of SiO₂ accompanied by deformation of the Si-O-Si bonds occur. Filling of the pores of the carbon material with silicon oxide creates in subsequent higher temperature treatment favorable conditions for formation of SiC. The particles formed as the result of the relatively low-temperature solid-state reaction are non-uniform in composition. Their core consists of uninteracted carbon and after it follow a layer of silicon carbide and an outer layer of SiO₂. A switch to the area of high synthesis temperature makes it possible to approach a stoichiometric composition of the silicon carbide.Translated from Poroshkovaya Metallurgiya, No. 9(321), pp. 57–62, September, 1989.     

Thermodynamic Assessment of the Possibility of the Deposition of Silicon Borides from Their Halogenides

The results of the predictive calculation of thermodynamic properties (enthalpy, entropy, and heat capacity) of boron silicide required for a thermodynamic analysis of the Si–B–Cl–H system performed with the help of the TERRA software complex are presented. Cases of the formation of condensed phases SiB₄ and SiB₆ in the reaction mixture are considered. To evaluate process parameters (temperature, pressure, and ratio of initial reagents) of deposition from the gas phase of silicon borides, thermodynamic calculations of the Si B–Cl–H system formed by SiCl₄, BCl₃, and H₂ for a temperature range of 1000–2200 K and pressure range of 0.00001–0.1 MPa are performed. It is shown that the thermodynamic stability of higher chlorides in the Si–B–Cl system drops with a decrease in pressure and the fraction of lower chlorides increases; i.e., initial silicon and boron chlorides destruct. However, no condensed phases SiB₄ and SiB₆ is formed, because their formation requires the introduction of hydrogen. It is determined that, varying the parameters of chemical deposition from the gas phase, it is possible to fabricate both single-phase and multiphase coatings. The results found in this study are of scientific and practical interest for developers of various production processes (gas-phase, liquid-phase, etc.) of silicon borides.     

Selective removal of dissolved silicon and aluminium ions from gas liquor by hydrometallurgical methods

Sasol raw gas liquor emanating from the gasification plant is contaminated by trace amounts of silicon, iron and aluminium which can crystallize or precipitate from gas liquor to form synthetic clay during either: (1) gas liquor transportation from the gasification plant to the phenosolvan plant or (2) during the liquid–liquid extraction process. Phenol is a major high value component of gas liquor which can be selectively recovered by diisopropyl ether (DIPE) from gas liquor during a liquid–liquid extraction process. A colloidal alumina/silica precipitate readily occurs on the heat exchanger plates during the liquid–liquid extraction process. This gelatinous precipitate formation is an operational problem which results in a severe blockage of the heat exchanger plates and eventually needs to be unblocked with hydrofluoric acid, a toxic and corrosive chemical. The precipitate formation can be attributed to the presence of the aluminium, silicon and iron species in the gas liquor.Characterisation studies identified metal ions such as Al³⁺, Ca²⁺, Fe³⁺, Mg²⁺, Si⁴⁺ and K⁺ in all liquid samples analysed in this study. Major anions detected in the liquid samples are sulphate, fluoride, chloride, bicarbonate, carbonate and nitrate. Thermodynamic modelling of inorganic compounds in the gas liquor sample indicates that Al(OH)₃, CaCO₃ and Fe(OH)₃ are dominant precipitates between pH 8 and 10 and a temperature of 40 °C.In this study, various purification processes such as: (1) desilication, (2) flocculation, and (3) selective precipitation were evaluated to selectively remove silicon, aluminium and iron species from the phenol-containing gas liquor. In addition a complexing of Al (III) and Fe (III) ions with citric acid was investigated to retain both aluminium and silicon species in the phenol-containing gas liquor during the extraction of phenol. The concentration of phenol present in the gas liquor must not be affected during the removal of aluminium, silicon and iron ions from the gas liquor. The preliminary results obtained indicate that in terms of removing the aluminium and silicon species from the gas liquor, the addition of seed crystals such as alumina, gibbsite and silica to gas liquor coupled with flocculation proved superior to all hydrometallurgical processes evaluated in this study. The availability of this technology to eliminate or substantially reduce silica and alumina fouling would reduce expensive maintenance and down time requirements and also enhance the plant efficiency.     

Effect of Mg on Solidification Microstructures of Homogenous and Functionally Graded A390 Aluminum Alloys

Hypereutectic Al?CSi alloys are used in components that require high resistance wear and corrosion, good mechanical properties, low thermal expansion and less density. The size and morphology of hard primary silicon particles present in Al?CSi alloys greatly influences the mechanical properties. Addition of Mg leads to formation of intermetallic Mg₂Si phases, which contributes towards the properties of high silicon alloy as well as alters the nature and quantity of primary silicon formed. The high silicon alloy subjected to centrifugal casting leads to the formation of functionally gradient material, which provides variation in spatial and continuous distribution of primary phases in a definite direction exhibiting selective properties and functions within a component. The present study is to evaluate the effect of Mg on solidification microstructures of homogenous and functionally graded A390 aluminium alloys. The addition of Mg from 3 to 5?% in A390 alloy using Al?C20Mg master alloy has shown a transformation from primary silicon rich matrix to Mg₂Si rich matrix. Centrifugal casting shows the gradient distribution of primary silicon and Mg₂Si phases towards the inner periphery of the casting.     

Thermodynamic Assessment of the Reduction of WO<Subscript>3</Subscript> by Carbon and Silicon

An interesting process in terms of resource conservation is the arc surfacing of worn components by means of powder wire in which the filler contains tungsten oxide WO₃ and a reducing agent (carbon and silicon). Thermodynamic assessment of the probability of 21 reactions in standard conditions is based on tabular data for the reagents in the range 1500–3500 K. This range includes the temperatures at the periphery of the arc and in the upper layers of the surfacing bath. The reactions assessed include direct reduction of WO₃ by carbon and silicon, indirect reduction of WO₃ by carbon, and reaction of tungsten compounds with carbon and silicon to form tungsten carbides and silicides. The possible reaction products considered are W, WC, W₂C, WSi₂, W₅Si₃, CO, CO₂, SiO, and SiO₂. The reduction of the oxide is written for 1 mole of O₂, while the reactions of tungsten compounds with carbon and silicon compounds are written for 2/3 mole of tungsten W. The probability of the reactions is estimated in terms of the standard Gibbs energy. In the range 1500–3500 K, the standard states of the reagents are assumed to be as follows: W(so); WO₃(so, li), with phase transition at 1745 K; WC(so); W₂C(so); C(so); CO(g); CO₂(g); WSi₂(so, li), with phase transition at 2433 K; W₅Si₃(so, li), with phase transition at 2623 K; Si(so,li), with phase transition at 1690 K; SiO(g) and SiO₂(so, li), with phase transition at 1996 K. To assess the influence of the possible evaporation of tungsten oxide WO₃ in the arc (T_b = 1943 K) on the thermodynamic properties, the thermodynamic characteristics of two reactions are considered; the standard state in this temperature range is assumed to be WO₃(g). Thermodynamic analysis of the reduction of tungsten oxide WO₃ shows that the temperature of the melt and the composition of the powder wire may affect the composition and properties of the layer applied. At high melt temperatures (>2500 K), the formation of tungsten and also tungsten carbides and silicides is likely. These reactions significantly change the composition of the gas phase, but not that of the slag phase in the surfacing bath. Below 1500 K, the most likely processes are the formation of tungsten silicides and tungsten on account of the reduction of WO₃ by silicon. In that case, the slag phase becomes more acidic on account of the silicon dioxide SiO₂ formed. However, this temperature range is below the melting point of WO₃ (1745 K). In the range 1500–2500, numerous competing reduction processes result in the formation of tungsten and also tungsten carbides and silicides in the melt. The reaction of tungsten compounds with carbon and silicon to form carbides and silicides is less likely than reduction processes. Evaporation of tungsten oxide WO₃ in the arc increases the thermodynamic probability of reduction; this effect is greatest at low temperatures.     

Oxidation Resistance of Niobium Coated with Titanium Disilicide

Coatings based on TiSi₂ have been used to protect niobium alloys from corrosion at temperatures up to 1300°C. Kinetic oxidation curves are given for these coatings on niobium. The phase compositions of the coatings have been determined and also of the layers formed during oxidation. EPMA has been applied to the element distributions in the coating, from which it is concluded that the silicon is mobile. The high resistance to oxygen of coatings based on TiSi₂ is due to the formation of films of TiO₂ and SiO₂ on them.

     

Development of a diffusion barrier layer for silicon and carbon in molybdenum—a physical vapor deposition approach

During the last 2 decades, research on high-temperature, oxidation-resistant coating systems for refractory metals has focused on a variety of silicides (e.g., Mo and Ta silicides), due to their excellent resistance to oxidation. However, commercialization efforts have been thwarted in large measure due to the diffusion of silicon from the coating to the substrate, resulting not only in the depletion of silicon from the coating, but also the formation of less oxidation-resistant subsilicides. Consequently, the development of a high-temperature, diffusion barrier layer for silicon has assumed considerable importance. Furthermore, introduction of carbon in the system, e.g., during the synthesis of MoSi₂-SiC composite thin films on molybdenum substrates, results in the diffusion of both silicon and carbon into the substrate, necessitating the development of a barrier layer for both elements. This article examines the possibility of using a novel approach—that of reactive radio frequency (RF) sputtering—for synthesizing a diffusion barrier (based on the Mo-Si-C-N quaternary system) for both silicon and carbon. It is shown that reactive rf magnetron sputtering of a composite target (MoSi₂ + 1.96 moles SiC) in an argon-nitrogen atmosphere results in the formation of an amorphous layer, of an as-yet undetermined stoichiometry, preventing the diffusion of both silicon and carbon into the molybdenum substrate. This layer is thermally and chemically stable up to at least 1260 °C. Submission of a patent application has been made.     

Relative Influences of Aluminium and Silicon on the Kinetics of Bainite Formation from Intercritical Austenite

Bainite formation from intercritical austenite is of great practical importance for the production of TRIP‐assisted steels. Silicon and aluminium play important roles during this transformation by delaying carbide precipitation, thus favouring the carbon enrichment of untransformed austenite, which makes its stabilisation down to room temperature possible. Previous studies have shown a strong dependence of bainite formation kinetics on both chemical composition and transformation temperature. In the present work, the effect of silicon and aluminium contents on bainite formation kinetics is investigated experimentally using dilatometry combined with microscopical observations. The experimental results are analysed by comparison with thermodynamic parameters, such as the activation energy G¹ for nucleation of bainite and the carbon content C_to corresponding to the T_o‐curve. It is shown that the faster transformation kinetics induced by the substitution of silicon by aluminium can be ascribed (i) to a higher driving force for nucleation, (ii) to a higher carbon content C_to at the To‐curve and (iii) to the precipitation of carbide in austenite in steels with a low Al content.     

Oxidation Resistance of Niobium Coated with Titanium Disilicide

Coatings based on TiSi₂ have been used to protect niobium alloys from corrosion at temperatures up to 1300°C. Kinetic oxidation curves are given for these coatings on niobium. The phase compositions of the coatings have been determined and also of the layers formed during oxidation. EPMA has been applied to the element distributions in the coating, from which it is concluded that the silicon is mobile. The high resistance to oxygen of coatings based on TiSi₂ is due to the formation of films of TiO₂ and SiO₂ on them.     

Effect of Lithium Carbonate on the Structure Formation of Ceramics Based on Si3N4

Structure formation in the system Li₂CO₃ Si₃N₄ both during heating in the powder state (500-1450°C) and also during specimen sintering (1450-1750°C) is studied. The most active formation of binary Li Si nitrides (LiSi₂N₃, Li₂SiN₄, Li₈SiN₄) is observed at 1450-1550°C. With a controlled sintering temperature and the amount of added Li₂CO₃ it is possible to prepare materials based on silicon nitride with a prescribed phase composition and corresponding properties.     

Observation of gold-silicon alloy formation in thin films by high resolution electron microscopy

Thin films (<30 nm) containing mixtures of gold and silicon were co-deposited on (100) surfaces of NaCl from the pure elements co-evaporated at equivalent molar rates. The resulting samples, believed to be homogeneous distributions of the elements, were used to observe alloy formation during electron beam heating in an electron microscope. The formation of the alloy was shown to accompany the displacement and subsequent crystallization of excess silicon. The crystallographic data derived from the microscopic study were found to agree within experimental error with those reported from studies of bulk samples prepared by the rapid quenching of gold/silicon melts and assigned the composition Au₂Si. Samples heated under intense electron beam irradiation resulted in the decompositon of the alloy and the formation of crystalline gold and silicon. Thicker samples (∼100 nm) were prepared by evaporation on (100) NaCl and subsequently heated at temperatures up to 250°C. These specimens examined by the “thin-film” X-ray diffraction technique confirmed the formation of Au₂Si as well as its subsequent decomposition at temperatures above 100°C.     

Effect of impurities on the properties of silicon nitride materials

Conclusions The presence of oxides in silicon nitride leads to the formation of an intergranular vitreous phase which has an adverse effect on the high-temperature physicomechanical characteristics of constructional silicon nitride ceramics. This phenomenon is particularly pronounced with silicon nitride contaminated with calcium, silicon, and alkaline metal oxides. Future development work on oxidation-resistant constructional materials based on silicon nitride should go in the directions of removal of impurities and use of additions forming solid solutions or refractory compounds with Si₃N₄ and SiO₂. Additions which can be employed for this purpose include oxides of AI, Mg, Y, and some rare-earth elements.Translated from Poroshkovaya Metallurgiya, No. 1(193), pp. 75–80, January, 1979.