α‐Fe layer formation during metal dusting of iron in CO‐H2‐H2O gas mixtures

The formation of an α‐Fe layer between cementite and graphite was observed and investigated during metal dusting of iron in CO‐H₂‐H₂O gas mixtures at both 600°C and 700°C. The condition to form this phenomenon is determined by the gas composition which depends on temperature. The iron layer formation was observed for CO content less than 1 % at 600°C and less than 5 % at 700°C. With increasing CO contents, no α‐Fe layer was detected at the cementite/graphite interface by optical microscopy. In this case cementite directly contacts with the coke layer. The morphologies of the coke formed in the gas mixtures with low CO contents were also analysed. Three morphologies of graphite have been identified with 1 % CO at 600°C: filamentous carbon, bulk dense graphite with columnar structure, and graphite particle clusters with many fine iron containing particles embedded inside. At 700°C with 5 % CO the coke mainly consists of graphite particle clusters with some filamentous carbon at the early stage of reaction. Coke analysis by X‐ray diffraction shows that both α‐Fe and Fe₃C are present in the coke. The mechanism of α‐Fe accumulation between cementite and graphite is discussed in this paper.     

Effect of nitriding on the electrochemical behaviour of Ti‐Al‐Mn alloy

The effect of technological conditions of nitriding such as process time duration and chemical composition of saturating medium, on the corrosion behaviour of nitrided coatings in 14 M solution of sulphuric acid was analyzed. The investigations were done on the alloy Ti‐5,0 Al‐2,0 Mn. The nitriding was carried out in nitrogen both at atmospheric pressure and rarefied nitrogen pressure (1 Pa) at the temperature 850°C and time processing in the range from 5 to 20 h in nitrogen‐containing gas only, and in powder electrode graphite and nitrogen‐containing gas. It was shown that technological conditions of nitriding determine the protective properties of nitrided coatings. It was indicated that the optimal structure of the nitride layer for best corrosion protection is the thin nitride TiN_x with high surface quality and a gas‐saturated layer. Nitriding in graphite powder effects positively the protective properties of nitride coatings due to reducing the nitride‐forming process.     

Improvement of oxidation resistance in graphite for MgO–C refractory through surface modification

Graphite, used as a carbon source in a conventional magnesia–carbon (MgO–C) refractory, was modified with an acid reagent, resulting in a negative charge on the surface of graphite, to enhance the coating efficiency of aluminum (Al) phase, which was compared to the pristine graphite through its dispersibity and oxidation behavior. The graphite particles with and without surface modification were added, respecticely, in an Al(NO₃)₃ suspension used as a coating reagent, and then filtered at room temperature. The modified graphite shows better disperbility than the pristine graphite, indicating that the coating efficiency of Al precursor is enhanced in the modified graphite. With respect to oxidation behavior, the modified graphite without the coating layer is totally reacted with oxygen at heat treatment of 900 °C in air. However, the Al-coated graphite starts to react with oxygen at heat treatment of 900 °C and fully reacted with oxygen at heat treatment of 1000 °C, showing the gray and white colors, respectively. It is verified that the Al layer is individually and uniformly formed on the surface of graphite and the oxidation resistance of graphite is enhanced owing to the increased coating efficiency of Al precursor.     

Synthesis of TiC composite by salt route and grain refinement of aluminium alloy

Abstract

The synthesis of Al–3Ti–0˙75C master alloy grain refiner was carried out by adding K₂TiF₆ and graphite powder together in aluminium melt from 800 to 1200°C. The reaction between the halide salts and graphite in aluminium melt was also carried out at different time intervals at 1200°C. The in situ formation of TiC particles in the Al/K₂TiF₆/graphite system involved the formation of Al₃Ti followed by formation of TiC at varying temperature and time. It is observed that TiC and Al₃Ti phases together showed the best grain refining efficiency in comparison to only by TiC or Al₃Ti alone.     

Formation of oxynitride layers on titanium alloys by gas diffusion treatment

Titanium alloys were oxynitrided in controlled nitrogen-oxygen gas atmospheres between 650 °C and 950 °C for 5 h to 10 h with two different techniques of gas diffusion treatment. One technique was performed in an oxygen-containing (oxygen amount ≥ 0.4 %) nitrogen environment. The other technique was performed in a deoxygenated (oxygen amount < 0.01 % to 0.0005 %) nitrogen environment with subsequent cooling in an oxygen-containing nitrogen environment (with an oxygen pressure of 1 Pa). The surface microhardness of oxynitrided samples increased due to the strengthening effect of titanium oxynitrides (TiN_xO_y). The maximum microhardness of the titanium oxynitrides was obtained with a near-equiatomic composition of nitrogen and oxygen in TiN_xO_y under optimal oxygen partial pressure and temperature-time conditions.     

Uranium metal by carbon reduction of oxide

Uranium metal was prepared by reacting an oxide (UO₂ or U₃O₈) with graphite powder under vacuum conditions. U₃O₈ prepared from production grade UO₃ gave the highest uranium content per unit volume of charge compared to other uranium oxide sources.The process consists of two separate vacuum induction heating steps. The first step involves heating a stoichiometric mixture of oxide and carbon in a graphite crucible to temperatures in the range 1550–1950°C to prepare an electrically conducting intermediate product. The final, or refining, step to obtain the metal consists of heating this intermediate product by self-induction at temperatures in the neighborhood of 2200°C in an urania crucible.When a mixture of U₃O₈ and carbon is being heated, considerable reaction takes place at temperatures well below 1000°C and some CO₂ is evolved. About 7.2 moles of carbon per mole of U₃O₈ are required for complete reduction to metal. The carbon reduction of UO₂ yields practically no CO₂ and therefore requires a carbon to oxygen ratio of unity. The quality of the final metal is essentially the same regardless of whether UO₂ or U₃O₈ is used as the starting oxide. The total carbon, oxygen plus nitrogen content in the metal is in the range of 400 ppm with better than an 80% overall yield on a 200 g of metal scale.     

Role of nitrogen on the oxidative stability of Ti5Si3 based alloys at elevated temperature

Ti₅Si₃ has been extensively studied as a candidate material for high temperature application due to its high melting point (2130 °C), low density (∼4.3 g/cm³) and excellent oxidation resistance in oxygen above 1000 °C. However, stoichiometric Ti₅Si₃ alloy experiences accelerated oxidation during exposure in air above 1000 °C. It was proposed that nitrogen was responsible for the increased oxidation in air. In the present study, the isothermal reaction kinetics of Ti₅Si₃ in nitrogen at 1000 °C was investigated. Compared to a slow parabolic oxidation rate in oxygen, a faster linear reaction rate was observed when Ti₅Si₃ is exposed to nitrogen. Further studies on the oxidation behavior for changing nitrogen/oxygen atmospheres showed that Ti₅Si₃ is stable for exposure up to 400 h at 1000 °C when the gas contained 50% N₂. Breakaway oxidation occurs after short exposures when the gas contained at least 75% N₂, and the reaction rate increased as the concentration of N₂ increased. Furthermore, time to breakaway oxidation decreases with the increasing nitrogen partial pressure. Extensive analysis of the oxidation products with SEM and XRD revealed that the formation and fast growth of a nitride-containing subscale interferes with the establishment of the continuous protective silica scale and contributes to the breakaway oxidation.     

Coke formation during metal dusting of iron in CO‐H2‐H2O gas with high CO content

Iron carburisation and coke formation during metal dusting of iron have been investigated in the gas mixture of 75%CO‐24.8%H₂‐0.2%H₂O at 600°C and 700°C. In all cases, cementite is formed at the surface, together with a coke layer on the top. In the coke layer, two morphologies of graphite are identified: compact bulk graphite with a uniform thickness and a columnar structure, and filamentous carbon with iron‐containing phases at the tip or along its length. The examination of coke formation in different stages of reaction at 700°C reveals that the coke contains two layers. The inner layer is composed of filaments, while the outer layer consists of the compact columnar graphite. After 2 h reaction the top compact graphite layer has suffered a serious deformation and has formed fractures because of the growth of catalytic filamentous carbon underneath. These filaments grow outside from these fractures and finally cover the whole surface after 4 h reaction. At 600°C, however, the coke contains a thick bulk graphite layer and non‐uniformly distributed filaments on the top. The bulk graphite layer is composed of many graphite columns which are loosely piled and are vertical to the surface. Each graphite column consists of many fine graphite fibres in parallel with the columnar axis. Filaments grow outside preferably from the gaps among these graphite columns and along the grinding scratches. TEM analysis of the coke detects very convoluted filaments with iron‐containing particles at the tip or along their length. XRD and TEM analyses show that these particles are Fe₃C rather than metallic iron.     

Metal dusting of nickel-base alloys

Metal dusting, the disintegration of metallic materials into fine metal particles and graphite was studied on nickel, Fe Ni alloys and commercial Ni-base alloys in CO H₂ H₂O mixtures at temperatures between 450–750°C. At carbon activities a_c > 1 all metals can be destroyed into which carbon ingress is possible, high nickel alloys directly by graphite growth into and in the material, steels via the intermediate formation of instable carbide M₃C. Protection is possible only by preventing carbon ingress. Chromium oxide formation is the best way of protection which is favoured by a high chromium concentration of the alloy and by a surface treatment which generates fast diffusion paths for the supply of chromium to the surface. The metal dusting behaviour of Alloy 600 is described in detail. A ranking of the metal dusting resistance of different commercial nickel-base alloys was obtained by exposures at 650°C and 750°C.     

Kinetics and Mechanism of the Oxidation of Molybdenum

The rates of formation of the different oxides on molybdenum in pure oxygen at 1 atm pressure have been determined in the temperature range 500° to 770°C. The rate of vaporization of MoOs is linear with time, and the energy of activation for its vaporization is 53,000 cal per mol below 650°C and 89,600 cal per mol at temperatures above 650°C. The ratio Mo0_{3(vaporizing)}/Mo0_3(surface) increases in a complicated manner with time and temperature. There is a maximum in the total rate of oxidation at 600°C. At temperatures below 600°C, an activation energy of 48,900 cal per mol for the formation of total Mo0₈ on molybdenum has been evaluated. The suboxide Mo0₂ does not increase beyond a very small critical thickness. At temperatures above 725°C, catastrophic oxidation of an autocatalytic nature was encountered. Pronounced pitting of the metal was found to occur in the temperature range 550° to 650°C. Marker movement experiments indicate that the oxides on molybdenum grow almost entirely by diffusion of oxygen anions.     

Evaluation of Ti<Subscript>3</Subscript>Si Phase Stability from Heat-Treated,Rapidly Solidified Ti-Si Alloys

Ti-base alloys containing significant amounts of silicon have been considered for high temperature structural applications. Thus, information concerning phase stability on the Ti-Si system is fundamental and there are not many investigations covering the phase stability of the Ti₃Si phase, specially its dependence on oxygen/nitrogen contamination. In this work the stability of this phase has been evaluated through heat-treatment of rapidly solidified Ti-rich Ti-Si alloys at 700 °C and 1000 °C. The rapidly solidified splats presented nanometric scale microstructures which facilitated the attainment of equilibrium conditions. The destabilization of Ti₃Si due to oxygen/nitrogen contamination has been noted.     

The Ablation and Oxidation Behaviors of SiC Coatings on Graphite Prepared by Slurry Sintering and Pack Cementation

SiC coatings were generated on graphite using slurry sintering (SS) and pack cementation (PC). The samples’ ablation features were assessed by an oxyacetylene torch. The rates of mass ablation of the PC–SiC and SS–SiC coatings were approximated 2.17?×?10^?3 and 9.52?×?10^?3 g s^?1, respectively, decreased by 84.1 and 29.6% compared to the uncoated samples. It was mainly attributed to the formation of a SiO₂ layer on the surface. The continuous SiO₂ molten film formed via the PC–SiC oxidation generates a sealing mechanism which can be an obstacle against the oxygen diffusion and hinder more ablation. This is while discontinuous SiO₂ film formed from the thin SS–SiC cannot protect the graphite effectively. The non-isothermal oxidation test shows that without the SiC coating, the sample weight is lost largely from 25 to 1500 °C, and its weight loss was 2.2% after the TGA. However, after coating, the samples possessed excellent oxidation protection and weight losses of SS–SiC and PC–SiC coatings are down to 1.3 and 0.6%, respectively. The more oxidation of the graphite substrate occurred due to the formation of macrocracks in the coating during the TGA and also the formation of holes on SiO₂ glass layer owing to release of CO or CO₂.     

Effects of the injection temperature of N2 gas on pressureless infiltration for fabricating Al−Mg/Al2O3 composites

Pressureless infiltration of molten Al−Mg alloys into particulate Al₂O₃ preforms has been known to occur only in a nitrogen atmosphere. In order to understand the pressureless infiltration mechanism of Al−Mg alloys into particulate Al₂O₃ preforms, nitrogen was injected at 25°C, 300°C, and 600°C. The higher the injection temperature of the nitrogen gas was, the lower the infiltration temperature of the molten Al−Mg alloys into the particulate Al₂O₃ preform was. Pressureless infiltration of the Al−6Mg alloy occurred at 700°C when the nitrogen gas was injected at 600°C. The formation of an Mg−N compound (Mg₃N₂) on Al₂O₃ particles, which improves wettability by decreasing the interfacial energy between the Al−Mg alloys and the Al₂O₃ particles, enabled the formation of the Al−Mg alloy/Al₂O₃ composite via pressureless infiltration. Increasing the injection temperature close to the melting point of the Al−Mg alloys appeared to enhance the formation of Mg₃N₂ on the surface of the Al₂O₃ particles.     

Constitution of iron-boron alloys in the low boron range

The solid solubility of boron in iron has been determined by saturating iron with respect to Fe₂B at several temperatures from 870° to 1135°C. In alpha iron the maximum solubility was found to be 0.002 pct B and in gamma iron the solubility varied from 0.001 pct at 915°C to 0.020 pct B at 1165°C. The eutectic and peritectoid temperatures were found to be 1165° and 911°C respectively and the gamma iron solidus was located approximately at 0.020 pct B.     

Oxidation of orthorhombic titanium aluminide Tl-22AL-25NB in air between 650 and 1000 °C

The oxidation behavior of orthorhombic titanium aluminide alloy Ti-22Al-25Nb was studied in air between 650 and 1000 °C by isothermal thermogravimetry and postoxidation scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction. Microhardness measurements were performed after exposure to gage hardening due to nitrogen and oxygen ingress. The parabolic rate constant of Ti-22Al-25Nb was of the same order as conventional titanium alloys and Ti₃Al-based titanium aluminides at and below 750 °C. Between 800 and 1000 °C, the oxidation resistance of Ti-22Al-25Nb was as good as that of γ-TiAl based aluminides; however, the growth rate changed from parabolic to linear after several tens of hours at 900 and 1000 °C. The mixed oxide scale consisted of TiO₂, AlNbO₄, and Al₂O₃, with TiO₂ being the dominant oxide phase. Underneath the oxide scale, a nitride-containing layer formed in the temperature range investigated, and at 1000 °C, internal oxidation was observed below this layer. In all cases, oxygen diffused deeply into the subsurface zone and caused severe embrittlement. Microhardness measurements revealed that Ti-22Al-25Nb was hardened in a zone as far as 300 μm below the oxide scale when exposed to air at 900 °C for 500 h. The peak hardness depended on exposure time and reached five times the average hardness of the bulk material under the above conditions.     

Filamentous carbon formation caused by catalytic metal particles from iron oxide

The thermodynamic equilibrium between metallic iron, iron oxides, iron carbides and an hydrocarbon/hydrogen mixture was calculated at 600°C. On the basis of the metastable Fe‐C‐O phase diagram, both metallic iron and iron oxides can be directly converted into carbides in reducing and carburizing atmosphere. Thermogravimetric (ATG) measurements have been performed in iC₄H₁₀‐H₂‐Ar atmosphere at 600°C on reduced and pre‐oxidised iron samples. The kinetic of coke formation was studied on both surface states by sequential exposure experiments. The initial stages of the transformation were characterised by scanning electron microscopy (SEM) observations and X‐ray diffraction (XRD) analysis. On a reduced surface, the results are consistent with the mechanism currently proposed to explain catalytic coke formation. Cementite (Fe₃C) is formed on the iron surface after carbon supersaturation (a_c > 1). The graphite deposition on its surface (a_c = 1) induces its decomposition. Iron atoms from cementite diffuse through the graphite and agglomerate to small particles that act as catalysts for further carbon deposition. A new mechanism of catalytic particle formation is proposed when an oxide scale initially covers the iron surface. In the carburizing and reducing atmosphere, magnetite (Fe₃O₄) can be directly converted into cementite (Fe₃C). XPS analysis confirm that, in this process, metallic iron is not an intermediary specie of the oxide/carbide reaction. At the same time, graphite deposition occurs at the metal/oxide interface through the cracks present in the oxide scale. Iron carbide in contact with graphite is partially decomposed and acts as catalyst for graphitic filaments growth.     

The effect of Fe2Al5 as reducing agent in intermediate steps of 'Al2O3/TiC-Fe composite production process

Al₂O₃/TiC-Fe cutting tools can be produced in-situ by using ilmenite, aluminum and graphite. The formation mechanism of the system was investigated in some researches. Most of them have diagnosed the aluminum as reducing agent of TiO₂, but in this study, it is proven that Fe₂Al₅ acts as the reducing agent for TiO₂ in the intermediate steps of the reactions. To achieve this goal, firstly, the reactions of synthesized ilmenite, aluminum and graphite system were investigated in detail. Then, pure Fe₂Al₅ was synthesized, and mixed with definite amount of TiO₂. The sample mixture was heat treated at the same temperature at the real reaction temperature, namely, 930 °C. The final products were analyzed with XRD. It was found that the product heat treatment of F₂Al₅ and TiO₂ mixture was the same as the sample of ilmenite, aluminum and graphite mixture with a molar ratio of 1:2:1 heat treated at 930 °C.     

Ferroelectric properties of dysprosium-doped Bi4Ti3O12 thin films crystallized in various atmospheres

Dysprosium-doped Bi4Ti3O12 （Bi3.4Dy0.6Ti3O12, BDT） ferroelectric thin films were deposited on Pt（111）/Ti/SiO2/Si（111） substrates by chemical solution deposition （CSD） and crystallized in nitrogen, air and oxygen atmospheres, respectively. X-ray diffraction （XRD） and scanning electron microscopy （SEM） were used to identify the crystal structure, the surface and cross-section morphology of the deposited ferroelectric films. The results show that the crystallization atmosphere has significant effect on determining the crystallization and ferroelectric properties of the BDT films. The film crystallized in nitrogen at a relatively low temperature of 650 ℃, exhibits excellent crystallinity and ferroelectricity with a remanent polarization of 2Pr = 24.9 ℃/cm^2 and a coercive field of 144.5 kV/cm. While the films annealed in air and oxygen at 650 ℃ do not show good crystallinity and ferroelectricity until they are annealed at 700 ℃. The structure evolution and ferroelectric properties of BDT thin films annealed under different temperatures （600-750 ℃） were also investigated. The crystallinity of the BDT films is improved and the average grain size increases when the annealing temperature increases from 600 ℃ to 750 ℃ at an interval of 50 ℃. However, the polarization of the films is not monotonous function of the annealing temperature.     

Phase reaction and sintering behavior of a Al2O3–20wt%AlN–5wt%Y2O3 system

Two major problems exist in the processing of AlN. The first is the difficulty in achieving full densification even at relatively high sintering temperatures. The second is the formation of the spinel phase, AlON. Pure AlN sintered at temperatures up to 2000°C have produced no more than 90–93% densification in the former case, while AlN rich ternary systems (AlN–Al₂O₃-sintering agent) have resulted in the detrimental formation of AlON well before full densification can occur. This paper reports on the phase reaction and sintering behavior of a ternary Al₂O₃–AlN–Y₂O₃ system near the critical temperature range of 1600–1700°C, in a carbo-thermal reduction furnace in a fully nitrogen environment. Full densification (>98%) for AlN without the formation of AlON was achieved by sintering an initial Al₂O₃ rich ternary system (Al₂O₃–20wt%AlN–5wt% Y₂O₃) at a relatively low temperature of 1680°C. Formation of the AlON was delayed until 1700°C, at which a stoichiometric γ-AlON (Al₃O₃N) with spinel type structure was obtained. Thermal conductivity values for a sintered substrate obtained with low oxygen content within the AlN matrix reached 125 W m⁻¹ K⁻¹.     

Slag-metal-graphite reactions and the activity of silica in lime-alumina-silica slags

Reduction of silicon from blast-furnace-type slags by carbon-saturated iron is a very slow reaction even under conditions of rapid stirring. Equilibrium under atmospheric pressure of carbon monoxide was approached from both sides, and the silica-silicon relation was established at temperatures of 1425° to 1700°C for slags containing up to 20 pct Al₂O₃.Silicon carbide is formed by reaction of graphite with high silica slags and the conditions for its formation were determined.Activities of SiO₂ and of CaO in their binary solutions at 1600°C were determined. The effects on the activity of SiO₂ were established for additions of 10 to 20 pct Al₂O₃ and of 10 pct MgO.