Stability of refractory compounds in fused basalt

Summary A study was made of the reaction of titanium, zirconium, niobium, molybdenum, and tungsten carbides, aluminum, zirconium, and silicon nitrides, titanium, zirconium, chromium, and molybdenum borides, molybdenum disilicide, chromic oxide, and refractories based on silicon nitride and carbide with molten basalt at a temperature of 1400°C. It has been established that molybdenum disilicide exhibits the highest stability in molten basalt.Translated from Poroshkovaya Metallurgiya, No. 2 (50), pp. 47–49, February, 1967.     

Reaction of Tungsten with Melts in the Cu - Co System

We have studied the solubility at 1200°C of tungsten in Cu - Co melts and the growth kinetics of a W₆Co₇ layer at the tungsten — melt interface. We have established the composition of the melt in the three-phase equilibrium tungsten — W₆Co₇ — melt: 0.0195 Co, 4.8 · 10⁻⁵ W, the rest is Cu (in atomic fractions). In the studied composition range for the melt, the solubility of tungsten is described well by the expression: lgX_W = −7.117 + 25.7 · X_Co ^1/2 − 41.06 · X_Co, where X_W and X_Co are the atomic fractions of the corresponding elements in the melt. We have determined the correlation between the growth rate for a layer of tungsten-containing phase at the tungsten — melt interface and the thermodynamic characteristics of the melt. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(443), pp. 81–86, May–June, 2005.     

Formation of oxide films on the surface of tungsten disilicide during anodic oxidation

Oxide-layer formation on the surface of tungsten disilicide is studied during its electrochemical polarization in a 3% NaCl solution at 20 °C. It is indicated that the total thickness of the oxide layer that forms over a period of 90 min does not exceed 10 nm. The following multistage mechanism of anodic oxidation is established: tungsten oxides (for example, W₃O and W_O3) in which the greater the polarization, the greater the extent to which the metal is oxidized, the lower tungsten silicide W₅Si₃, and silica are formed. It is demonstrated that a double electrolytic layer on the interphase boundary between the tungsten disilicide and 3% NaCl solution is formed owing to dissociation of the silanolic groups situated a priori on the surface of the silicide. Here, the surface of the electrode acquires a negative charge, while the positively charged diffusion layer contains hydroxone ions. As a silica film forms on the WSi₂, the material becomes more corrosion-resistant. On the whole, the scheme established for electrochemical oxidation of WSi₂ complies with the mechanism of its high-temperature oxidation.     

Stability and Heat Resistance of Silicide Coatings on Refractory Metals. Part 1. Effect of Subsurface Layer on Process of Phase Redistribution in System WSi2 ― W

The effect of a subsurface layer of metal silicide on the phase composition stability of high-temperature MeSi₂-type coatings on refractory metals was investigated. Using the system WSi₂ W as a prototype it was experimentally determined that a subsurface layer of the tungsten disilicide, and the distribution profile of silicon in the diffusion zone upon high-temperature heating have a substantial effect on the formation of a barrier layer of lower silicide which determines the stability of the system as a whole. It is proposed that the search for more stable silicide coatings on refractory metals should be directed toward the creation of diffusion barriers on not only the inner interface MeSi₂ Me, but also the external surface of the coating. The process of coating formation must be accompanied by the formation of a metal silicide on the external surface with the highest possible concentration of silicon.     

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.     

Thermodynamic Analysis of High-Temperature Oxidation of Material in the Cr – TiB2 – TiSi2 System in Air

We have carried out a thermodynamic analysis of the chemical processes in the system Cr – TiB₂ – TiSi₂ at temperatures of 1400 K to 1800 K in air. We have established that titanium diboride plays the role of a protective layer while titanium disilicide is the substrate for a layer protecting from reaction with air.     

Investigation of Liquid-Phase Sintering in W ― Sn ― Si Pseudoalloys

The kinetics of reaction between particles of tungsten and silicon during liquid-phase sintering of W Si Sn pseudoalloys was studied. Specimen growth caused by the formation of tungsten disilicide was observed. The growth rate in the investigated ranges of time and temperature obeyed a linear law. The rate constant was determined, and also the activation energy of the process, which agreed with the standard heat of formation of WSi₂.     

Kinetics of disilicide layer growth at the interface of tungsten with molten copper, silver, and tin saturated with silicon

The kinetics of WS₂ layer growth at the interface of tungsten with molten metals saturated with silicon is studied. Research is performed at 1200°C using melts based on copper, silver, and tin. It was established that WSi₂ layer growth in these melts obeys a “parabolic” rule but the corresponding growth rate constants differ markedly, i.e., from 3.4·10^?11 m²/sec (melt based on copper) to 1.5·10^?13 m²/sec (melts based on silver and tin). The reasons for this difference are discussed.     

硅对高频燃烧红外吸收法测定高硅样品中碳和硫的影响

利用扫描电镜(SEM)和能谱仪(EDS)观测了经高频感应炉燃烧后高硅样品熔体中硅的存在形态和组成,从不同角度考察了样品中硅对熔融效果产生的影响,以及对高频感燃烧红外吸收法测定碳和硫结果的影响。结果显示:仅使用纯钨做助熔剂时,含硅样品熔体中的硅主要以球状的二氧化硅颗粒镶嵌于氧化钨和氧化铁等基体中,硅的存在使得高频燃烧红外吸收法测定碳和硫的准确度变差,且随着样品中硅含量的增加,这种影响的程度逐步增大。通过对影响机理进行分析认为:生成的二氧化硅使得熔体的流动性变差,且接受电磁感应形成感应电流的能力也同时减弱,从而对测定结果的精密度和正确度造成影响。实验在使用钨助熔剂的基础上,进行了添加纯锡或纯铁助熔剂的试验,结果表明:添加一定量纯锡或纯铁助熔剂,可以降低样品熔点,提高熔池温度,大大改善熔融效果,从而提高碳和硫测定的精密度和正确度。     

Thermodynamic Analysis of Chemical Reaction between Components of the Cr – CrB2 – CrSi2 System with Air at Elevated Temperatures

We have carried out a thermodynamic analysis of chemical processes in the Cr – CrB₂ – CrSi₂ system at 1400 K to 1800 K. We have established that chromium diboride fulfills the function of a protective layer, while chromium disilicide protects against reaction with air at temperatures up to 1473 K.     

Hardening of the surface layers of a hollow billet formed by centrifugal casting

One of the methods to increase the mechanical properties of steel is its hardening via the introduction of a refractory fine-grained phase into a melt. A method of fabrication of a tube blank by centrifugal casting accompanied by hardening with a refractory phase is considered. The introduction of fine tungsten and silicon carbides is shown to improve the structure of grade 15 steel and to increase the wear resistance of a tube blank made of this steel.     

Observations on the evolution of potassium bubbles in tungsten ingots during sintering

The present article describes the evolution of potassium bubbles during sintering of tungsten ingots pressed from doped powder. In the manufacture of incandescent lamp filaments, tungsten powder is produced by reduction of blue tungstic oxide which is doped with potassium disilicate and aluminum chloride. The reduced tungsten particles contain submicron pores. Analytical transmission electron microscopy (TEM) identifed two types of pores in reduced tungsten powder. First, there are pores which contain particles which consist of potassium, aluminum, and silicon, and second, there are pores which contain aluminum and silicon alone. On sintering at 2100 °C or 2300 °C, potassium aluminosilicate particles migrate together with grain boundaries to necks which form between tungsten particles. Sintering at 2100 °C or 2300 °C reduces the potassium, aluminum, and silicon concentrations of the particles. Atomic absorption spectroscopy (AAS) also measured reductions in the bulk potassium, aluminum, and silicon concentrations. In the present study, analytical TEM and Auger electron spectroscopy (AES) were used to describe the decomposition of dopant particles and the evolution of elemental potassium bubbles in sintered ingots.     

连铸结晶器保护渣配炭量及配炭方式的研究

本文研究了保护渣熔化速度、熔融特性和熔融结构与渣中配炭量、配炭种类和配炭方式的关系。研究发现,炭质材料影响保护渣熔融行为的程度取决于炭质材料的含碳量,分散度和着火点,复合配炭优于目前广泛采用的单一配炭,采用复合配炭后,保护渣的熔融结构呈含有熔融层,半熔层、烧结层和粉渣层的多层熔融模型,熔化速度随含碳量的改变增减较缓。熔融特性在较宽的温度范围内保持相对稳定,使之能适应于连铸工艺参数在连铸过程中的突变。     

Formation of Nanocrystalline Structure of TaSi2 Films on Silicon

The nanocrystalline structure and mechanical properties of TaSi₂ films deposited by sputtering of TaSi₂ target have been investigated by x-ray diffraction, cross-sectional transmission electron microscopy (TEM), four-point electrical resistance measurement, and cyclic depth-sensitive nanoindentation. The purpose of this work is to study the formation of nanocrystalline structure in TaSi₂ films on a silicon substrate. As revealed, a decrease in the deposition rate leads to an increase in the O and C impurity content in the films. Contamination of the film by O and C atoms during a low-rate deposition causes the formation of an amorphous phase in the deposited films. Upon annealing, the amorphous structures crystallize into mixtures of disilicide and a small amount of polysilicide, i.e. TaSi₂ and Ta₅Si₃, respectively. After annealing at 970 K, the formation of a nanocrystalline structure with a grain size about 10 nm takes place in the film produced at a deposition rate of 0.2 nm/sec. The formation of a nanocrystalline structure changes drastically the mechanical properties of the film. The nanohardness and elastic modulus increase significantly, and the film becomes brittle and overstressed. After deposition in the film produced at the 1 nm/sec deposition rate mainly Ta disilicide and the amorphous phase are observed. After annealing, the amorphous phase near the Si substrate coexists with column-shape grains of Ta disilicide of size 150 × 500 nm. The annealed thin film becomes nonuniform in thickness. The nanohardness and elastic modulus increase.     

直拉法硅表面层熔体流动所遵循的线性规律

本文定量地研究了硅自由表面层熔体的流动规律,用大量实验数据确立了自由表面层熔体径向流动速度与熔体纵横比(D/H)值的线性对应关系。根据这个关系可以成功地解释和处理硅单晶生长过程中的许多相关问题。     

高压空气环境下钨电极失效的研究

在模拟试验舱中进行了高压TIG焊试验,钨极的失效形式主要有烧蚀、钨极尖端粘上金属、钨极熔化3种形式,烧蚀占总失效的90%,钨极尖端粘上金属占到9%,钨极熔化占到1%;给出了钨极失效的几何判据。对钨极凸缘形成机理进行了研究,研究表明,凸缘是电极尖端温度合适的C粗糙区内,钨重结晶和钨的氧化物分解、沉积共同作用的结果。分析了影响高压焊接钨极失效的因素,建立了高压TIG焊钨极失效的故障树,对故障树进行了定性和定量分析,找出它的最小割集,计算出事件的结构重要度系数。在此基础上,提出了防止钨极失效的措施。     

Hereditary influence of the structure of charge materials on the density of aluminum alloys of the Al-Si system

The influence of dispersity of charge silicon and structure of initial charge alloys on the density of silumins of the Al-Si system is investigated using the improved express-method for determining the melt density. It is established that the structure of charge materials substantially and stably affects this characteristic, which should be taken into account in preparation technologies of alloys of the Al-Si system.     

Theory and technology of sintering processes,thermal and thermochemical treatment reaction kinetics for interaction of tungsten with copper-zirconium melts

We investigated the growth kinetics of layers of W₂Zr phase on tungsten interacting with copper-silicon melts at 1150°C. We determined the limiting value of the zirconium content in the copper melt (40.1 at. %). Below this value, we do not observe the indicated layer. We discuss the mechanism of reactive diffusion in the system and the effect of zirconium additives on the liquid phase sintering of the copper-tungsten materials. We have discovered well-defined differences between the growth rates of the layers on polycrystalline and single-crystal specimens of tungsten.Institute of Problems in Materials Science, National Academy of Sciences of Ukraine. Kiev. Translated from Poroshkovaya Metallurgiya. Nos. 3–4(384). pp. 25–29. March–April 1996. Original article submitted October 25, 1994.     

液—固掺杂垂熔钨坯中改性元素的行为

对传统粉末冶金方法(液—固混合制成掺杂钨粉,经过压制、烧结、旋锻、拉拔等工序)制成的钨坯和钨丝,用扫描电子显微镜进行了断面及磨面分析。结果发现:在晶界孔穴内,元素Si和Al的含量丰富;在晶内孔穴内,除元素Si、Al外,还富集了元素K。只用钾泡理论解释改性元素改善高温钨丝的抗下垂性能是不够的,应以分子筛理论和SiO_2-Al_2O_3系统相图予以补充。