Stability and Heat Resistance of Silicide Coatings on Refractory Metals. Part 2. Stability and Heat Resistance of Silicide Coatings on Tungsten and Molybdenum at 1500-2000°C

The kinetics of diffusion redistribution of phases within the system WSi₂ W on heating tungsten silicide in air in the temperature range 1500-2000°C is studied. The stability and heat resistance of silicide coatings on tungsten is mainly governed by the diffusion of silicon towards the interphase boundaries W W₅Si₃, W₅Si₃ WSi₂, and WSi₂ SiO₂, formation at them of diffusion barriers of lower silicide W₅Si₃, and also a protective SiO₂ film at the outer boundary of the silicide coating. It is established that the transition rate for the higher to the lower tungsten silicide WSi₂ W₅Si₃ is on average four times slower than the transition rate for MoSi₂ Mo₅Si₃. It is shown that an increase in silicon concentration in the WSi₂ surface layer stimulates formation of diffusion barrier compounds at interphase boundaries. This leads to an increase in the stability of the phase composition and heat resistance of a silicide coating on metals. In particular at 1700°C the transition rate for molybdenum silicide on tungsten MoSi₂ (Mo, W)₅Si₃ is about twenty times slower than the transition rate for MoSi₂ Mo₅Si₃, and less by a factor of about eleven than the transition rate for WSi₂ W₅Si₃. Here there is also an increase in the heat resistance of silicide coatings on tungsten and molybdenum. It is shown that the SiO₂ film on tungsten silicide does not lose its protective properties up to 2000°C.     

Phase relations of a silicide/silica reaction couple at 2273 K

The phase relations of a zirconium silicide/silica reaction couple have been investigated at 2273 K in air. After annealing times from 4 to 49 hours, the reaction couples, created by encapsulating zirconium disilicide in a quartz ampule, developed an interdiffusion zone and an inner core consisting of solidified zirconium silicide. The interdiffusion zone consisted of a silica layer, zirconia precipitates, and prior liquid silicide globules dispersed in prior liquid silica. Zirconia precipitates formed from the oxidation of the silicide melt between the protective silica layer and the two liquid regions of silicide and silica, as a result of oxygen diffusion. At 2273 K, the inner core consisted of liquid zirconium silicide (ZrSi)_L, although the microstructure analysis indicated formation of proeutectic ZrSi₂ and a eutectic microstructure of ZrSi₂ and Si upon solidification. A constant oxygen potential developed within the interdiffusion zone and protected the silicide from oxidation even after 49 hours at temperature.     

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.     

Growth kinetics of SiO2 nanofilm on MoSi2 in anodic polarization

The anodic oxidation of MoSi₂ ceramics in 3% NaCl solution is shown to be a multistage process. Auger electron spectroscopy established that only silica forms on the MoSi₂ surface between 1.5 and 2.0 V, while molybdenum passes completely into solution. The growth kinetics of silica is studied using chronoamperometry under controlled potential conditions. The resulting kinetic curves show two stages. At the first stage, the reaction rate (current density) falls by one order for the first few minutes when SiO₂ nanofilm begins to form. Then the diffusion-limited process, which fits parabolic kinetics, is established. On the whole, the model describing the electrochemical formation of oxide nanofilm on molybdenum disilicide agrees with the Mott-Cabrera theory, which was earlier proposed for high-temperature oxidation processes. __________ Translated from Poroshkovaya Metallurgiya, Vol. 47, No. 1–2 (459), pp. 161–167, 2008.     

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.     

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.     

Effects of Duty Cycle,Current Frequency,and Current Density on Corrosion Behavior of the Plasma Electrolytic Oxidation Coatings on 6061 Al Alloy in Artificial Seawater

In this study, the effects of duty cycle, current frequency, and current density on corrosion behavior of the plasma electrolytic oxidation (PEO) coatings on 6061 Al alloy in artificial seawater (3.5 wt pct NaCl solution) were investigated. To serve this purpose, the electrical parameters of a unipolar pulsed current were applied during the PEO process on 6061 Al alloy in alkaline silicate electrolyte with and without Al₂O₃ nanoparticles. The coating morphology and microstructure were characterized by the scanning electron microscopy. The corrosion behavior and electrochemical response of the specimens treated by plasma electrolytic oxidation were analyzed by the electrochemical impedance spectroscopy and the potentiodynamic polarization in artificial seawater. It was found that PEO coatings formed in the presence of Al₂O₃ nanoparticle had lower porosity and exhibited better corrosion behavior compared with the coatings formed in the absence of Al₂O₃ nanoparticle in the structure. This can be attributed to the nanoparticles’ incorporation and penetration through the PEO coatings. On the other hand, the decrease in the current density and increases in the duty cycle and frequency lead to further reduction of the nanoparticles’ incorporation and distribution on the coating surface.

     

Control of surface carburization and improvement of dynamic fracture behavior in tungsten heavy alloys

A carburization technique using a Cr powder layer has been developed to control the diffusion depth of carbon in W-Ni-Fe heavy alloys. The aged heavy alloy samples were covered with a Cr powder layer of about 1-mm thickness and then packed with carbon black powder. The packed samples were heat-treated at 1150 °C for 10 minutes in H₂ and then for 50 minutes in N₂. The carburization treatment resulted in the formation of Cr₇C₃ and Fe₃W₃C around the tungsten grains from the sample surface with a thickness of 40 to 50 μm. This carburized layer was much thinner than that formed without a Cr powder layer under the same experimental conditions. With the surface carburization, the surface hardness increased by ∼75 pct, from 508 to 888 VHN, and the impact energy decreased by ∼31 pct, from 123 to 85 J. After the carburization treatment, the main fracture behavior in a dynamic torsional test changed from smearing of the matrix to cleavage of the tungsten grains. A high-speed impact test showed that the surface carburization of penetrators induced the formation of many cracks around the penetrator surface, enhanced the self-sharpening, and improved the penetration performance. It appears that the developed technique provides an easy method of carburization without serious deterioration of the toughness of the material.     

Electrochemical oxidation of the carbide wastes of hard alloys using alternating current

The electrochemical oxidation of the carbide wastes of a W-Co alloy has been studied by gas, electron-probe microanalysis, and X-ray diffraction analyses. The experiments are carried out using halfwave sinusoidal alternating current. It is established that a CO₂ + CO mixture forms under such conditions in a gaseous phase in volumetric ratio of 2: 1 and that a tungsten deposit forms in an anode sludge mainly in the form of hydrated tungsten oxide WO₂(OH)₂. Marketable products are obtained in the form of pure CO₃O₄ and WO₃.     

Oxidation of Alloy 600 and Alloy 690: Experimentally Accelerated Study in Hydrogenated Supercritical Water

The objective of this study is to determine whether the oxidation of Alloys 600 and 690 in supercritical water occurs by the same mechanism in subcritical water. Coupons of Alloys 690 and 600 were exposed to hydrogenated subcritical and supercritical water from 633 K to 673 K (360 °C to 400 °C) and the oxidation behavior was observed. By all measures of oxide character and behavior, the oxidation process is the same above and below the supercritical line. Similar oxide morphologies, structures, and chemistries were observed for each alloy across the critical point, indicating that the oxidation mechanism is the same in both subcritical and supercritical water. Oxidation results in a multi-layer oxide structure composed of particles of NiO and NiFe₂O₄ formed by precipitation on the outer surface and a chromium-rich inner oxide layer formed by diffusion of oxygen to the metal-oxide interface. The inner oxide on Alloy 600 is less chromium rich than that observed on Alloy 690 and is accompanied by preferential oxidation of grain boundaries. The inner oxide on Alloy 690 initially forms by internal oxidation before a protective layer of chromium-rich MO is formed with Cr₂O₃ at the metal-oxide interface. Grain boundaries in Alloy 690 act as fast diffusion paths for chromium that forms a protective Cr₂O₃ layer at the surface, preventing grain boundary oxidation from occurring.     

Catalytic effect of cobalt on the carburization kinetics of tungsten

The kinetics of the gas-phase carburization of tungsten by methane were studied and found to be controlled by two surface reactions and a later diffusion limiting reaction. The two surface reactions were found to produce the compounds W₂C and W₂C-WC respectively. The diffusion limiting reaction produced only WC. Extensive carburization caused cracking of the tungsten material. Cobalt was found to form a thin film on each tungsten surface which was easily converted to eta-phase (Co₃W₃C) when carburization began. A model was proposed and kinetic equations were developed which predict the rate of reaction for both cylinders and powders. The reaction rate curves for the uncatalyzed reaction using the same equations was also developed. CHARLES F. DAVIDSON, formerly with Fansteel Research Center, Salt Lake City, Utah     

Microstructure, Chemical and Phase Composition of Chromium Silicide Diffusion Coatings on Carbon Steels

The microstructure, chemical and phase composition of chromium silicide diffusion coatings on steels 20, 45, U8, and U10 are studied. It is established that the phases Cr²³C⁶ and Cr⁷C³ with a silicon content in them of up to 0.25 at.% form at the surface of chromium silicide coatings. The maximum silicon content at 7% in steels 20 and 45 is observed beneath the carbide layer at a depth of 40–50 µm, but for steel U10 it is 1.9% at the carbide layer-matrix boundary. It is shown that the corrosion resistance of chromium silicide diffusion coatings increases markedly compared with uncoated specimens: by a factor of 15–30 depending on the acid used (H₂SO₄, HCl, H₃PO₄, HNO₃, CH₃COOH). There is also an increase in the cavitation resistance of articles with chromium silicide coatings in water and 3% NaCl solution.__________Translated from Poroshkovaya Metallurgiya, Nos. 1–2(441), pp. 23–30, January–February, 2005.     

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.     

Synthesis, heat resistance, and optical properties of boron silicide and materials based on it

Conditions of synthesis from elements of pure superfine powder of boron silicide B₄Si are investigated. It is shown that the silicide phase forms on boron particles and B₄Si particles inherit the shape and size of initial boron particles, which makes it possible to control the particle size of the powder obtained. The oxidation of pure boron silicide and materials based on it in air and their optical properties are studied. It is shown that chromium diboride added to boron silicide increases the oxidation resistance of B₄Si and hardly decreases its absorbance. The B₄Si-CrB₂ material can be successfully introduced into the compositions of heat-protective coatings operating in air at 1300 °C. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 3–4 (454), pp. 45–52, 2007.     

微弧氧化对7050铝合金腐蚀行为的影响

采用微弧氧化(MAO)技术在7050铝合金表面制备了陶瓷膜层,运用扫描电子显微镜(SEM)和能谱分析仪(EDS)表征陶瓷膜微观结构,采用动电位极化曲线、电化学阻抗谱(EIS)和慢应变速率拉伸试验(SSRT)研究了微弧氧化膜对7050铝合金在3.5%(质量分数) NaCl水溶液中腐蚀和应力腐蚀开裂(SCC)行为的影响.结果表明:微弧氧化膜层由表面疏松层与内部致密层组成,表面疏松层主要由Al₂O₃组成,内部致密层由氧化铝与铝烧结而成.微弧氧化膜层可以有效抑制7050铝合金表面的腐蚀萌生及明显降低腐蚀速率,且使7050铝合金的应力腐蚀敏感性出现显著下降.      

OXIDATION PROTECTION MECHANISMS DURING THE SINTERING OF Mo/FeSi2 MIXTURES

Abstract

A powder mixture of molybdenum and iron disilicide can be caused to react exothermically without excessive oxidation of the molybdenum owing to the formation of an unreactive and self-healing coating. The method depends upon a chemical means, the formation of a surface barrier of a molybdate that provides an environment in which molybdenum losses are suppressed by the exclusion of oxygen. A sodium compound added to the mixture of metal powders to be alloyed reacts with the molybdenum oxide to form a sodium molybdate. During the sintering process, the sodium molybdate migrates to the surface where it forms a protective glaze which prevents oxygen in the environment from reacting with the metallic molybdenum during alloying. Once the alloy is formed, molybdenum losses through oxidation and sublimation cannot occur since the molybdenum is in the form of MoSi₂. A discussion of the chain of chemical reactions believed to occur in the sintering process is based upon identification of evolved reaction products, as well as X-ray identification of solid reaction products.     

Corrosion behaviour of Al1.3CrFeNi chemically complex alloy

In this study, corrosion behaviour of double-phase Al_1.3CrFeNi chemically complex alloy was investigated, including hot corrosion and electrochemical corrosion. Hot corrosion behaviour of Al_1.3CrFeNi alloy was explored in molten 75 wt-% Na₂SO₄?+?25 wt-% NaCl salt. The result revealed that the corrosion kinetic curve of Al_1.3CrFeNi alloy followed the exponential rate rule through mass loss measurement. In addition, it prevented that the formed corrosion layer had obvious stratification including external granular Al₂O₃ and inner porous Cr₂O₃ when corrosion time was up to 100?h. Besides, it should be noted that the Al_1.3CrFeNi alloy was sensitive to the molten salt containing chlorine, which makes the alloy surface leave voids and bring about acceleration of corrosion. Meanwhile, electrochemical corrosion resistance of Al_1.3CrFeNi alloy in NaCl solution with different concentrations (0.6, 1.0 and 2.0?mol?L^?1) was investigated at room temperature. The results revealed that Al_1.3CrFeNi alloy showed superior corrosion resistance in NaCl solution due to the existence of Al and Cr which aid the formation of protective oxide layer.     

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.