Chemical compatibility of a TiAl-Nb melt with oxygen-free crucible ceramics made of aluminum nitride

The problem of uncontrolled oxygen contamination of intermetallic TiAl ingots is considered for the application of crucibles and molds based on traditional oxide ceramics. A synthesized Ti-45.9Al-8Nb (at %) alloy is solidified in alternative oxygen-free crucibles made of high-purity aluminum nitride (99.99% AlN) upon holding at 1670°C for 5, 12, and 25 min and subsequent quenching in a high-purity argon atmosphere. The initial material and the solidified ingots are studied by scanning electron microscopy, optical microscopy, X-ray diffraction, electron-probe microanalysis, and gas-content chemical analysis. The key features of the interaction of the TiAl-Nb melt with AlN ceramics are revealed. Partial thermal dissociation of the crucible material according to the reaction AlN → Al + N and the reaction of atomic nitrogen with the melt lead to the formation of a solid 6.4-μm-thick TiN coating on the ingot surface and provide perfect wettability of the crucible by the melt and easy removal of solidified casting items from the mold. The TiN coating serves as a diffusion barrier that hinders the diffusion of nitrogen and residual oxygen from the pores in the crucible toward the melt. As a result, no oxide particles are detected in the ingots. However, few single microprecipitates of two nitride phases ((Ti,Al) _x N _y , NbN) are detected in the near-bottom region, 300 μm thick, in the alloy after holding at 1670°C for 25 min. The total oxygen contamination in a two-phase α₂ + γ ingot does not exceed 1100 wt ppm, which is 1.5–2 times lower than that obtained in the experiments performed with modern advanced oxide crucibles made of yttrium ceramics Y₂O₃. AlN is shown to be a promising crucible material that can be considered as an alternative to oxide ceramics in the metallurgy of TiAl intermetallics.     

Reoxidation of Ni‐ and Ni‐Fe‐Alloys by Al2O3‐SiO2 Refractory Materials

Reactions at the refractory/melt interface during ingot casting of Ni‐ and Ni‐Fe‐alloys were studied. The casts were performed using different alumino‐silicate bricks as refractory materials. Samples taken from the casting channel before and after casting were investigated using light and scanning electron microscopy with XPS. Thermodynamic calculations were performed with FactSage and the results were compared with the results from industrial tests. After the melt has infiltrated the surface layer of the bricks, refractory corrosion starts with an attack of Mn and Mg of the melt on SiO₂ and Fe₂O₃ of the refractory bonding matrix. Despite the presence of elements with higher oxygen affinity in the melt, low‐melting alumino‐silicate phases are predominantly built by the reaction with Mn and Mg. In a second step this liquid phase either traps non‐metallic inclusions from the melt or, at higher contents of Zr, Ti, Mg, Y etc. in the melt, causes massive reoxidation and inclusion formation. The refractory materials investigated show an increasing trend for reoxidation with an increasing amount of SiO₂ in glassy phases of the refractory bonding matrix. By the use of a refractory material with higher mullite content in the bonding matrix or by use of alumina bricks a strong reoxidation of the melt and intense inclusion formation can be avoided. These observations are also valid for other alloys with higher contents of elements with high affinity to oxygen.     

Tests on the oxidation-inhibiting effect of CaB6 in refractory MgO-C materials

The oxidation-inhibiting action of B₄C as an antioxidant is to be attributed to two possible mechanisms. The first describes the formation of reactive gases (Mg vapour and B₂O₂) that react with oxygen reducing its partial pressure in pores of the material. The second mechanism results in the formation of a borate layer which includes carbon between the oxide grains and thus protects it from further oxidation. The present study made an examination as to whether these mechanisms also play a part in the use of CaB₆. The results confirm the pattern of chemical behaviour in MgO-C materials with boron-containing antioxidants recorded in previous tests. However, the temperature at which the protective effect begins lies in this case at about 200°C lower when compared to B₄C. It is obviously due to the formation of a eutectic melt in system MgO-CaO-B₂O₃ at approx. 1080°C.     

Formation and Evaluation of Protective Layer Over Magnesium Melt Under SF6/Air Atmospheres

Molten magnesium oxidizes rapidly when exposed to air causing melt loss and handling difficulties. The use of certain additive gases such as SF₆, SO₂, and CO₂ to form a protective MgO layer over a magnesium melt has been proposed. The oxidation behavior of molten magnesium in air containing various concentrations of SF₆ was investigated. Measurements of the kinetics of the oxide layer growth at various SF₆ concentrations in air and temperatures were made. Experiments were performed using a thermogravimetric analysis unit in the temperature range of 943 K to 1043 K (670 °C to 770 °C). Results showed that a thin, coherent, and protective MgF₂ layer was formed under SF₆/Air mixtures, with a thickness ranging from 300 nm to 3 μm depending on SF₆ concentration, temperature, and exposure time. Rate parameters were calculated and a model for the process was developed. The morphology and composition of the surface films were studied using scanning electron microscope and energy-dispersive spectroscope.     

Protective Coatings La–Mn–Cu–O for Stainless-Steel Interconnector 08Х17Т for SOFC,Obtained by the Electrocrystallization Method from Non-Aqueous Solutions

A novel method for the formation of the protective layer on stainless steel interconnectors for solid oxide fuel cells was developed. The method was based on the electrocrystallization of metals from non-aqueous solutions on the stainless-steel interconnector with consecutive thermal treatments. Suggested method was applied for the stainless-steel 08X17T. Chemical composition of the electrolyte for the electrocrystallization was made in order to obtain the oxide protective layer of the stainless-steel interconnector of the following composition: LaMn_0.9Cu_0.1O₃. As a result, a uniform oxide layer was formed on the stainless-steel interconnector surface, protected the stainless-steel from the high-temperature oxidation leading to degradation of the functional properties of the interconnector. Forming coatings were characterized by means of grazing incidence X-rays diffraction, X-rays photoelectron spectroscopy and scanning electron microscopy. Elemental analysis and phase composition have shown that the main components of the protective coatings are found to be compounds with perovskite and spinel structures. The protective coating in the contact with cathode material based on lanthanum-strontium manganite shows significant decrease of chromium propagation from the stainless steel as a result of the diffusive firing in comparison with the sample of the stainless steel without the protective coating. Electrical resistance of the interconnector with the protective coating does not show noticeable degradation during at least 500 h at the temperature 850°C in ambient air.     

Production of niobium powder by direct electrochemical reduction of solid Nb2O5 in a eutectic CaCl2-NaCl melt

The method of electro-deoxidation was used to reduce solid Nb₂O₅ to niobium metal in a CaCl₂-NaCl eutectic melt. The direct electrochemical reduction of Nb₂O₅ was achieved by electrolysis in the eutectic melt at 1123 and 1173 K, respectively, at a controlled potential of 3.1 V, below the decomposition potential of the salts. Analysis of the anodic reaction gases carried by a flow of dry, high-purity argon confirmed that the preferred cathodic reaction is the oxygen ionization of the cathode to form oxygen ions that are successively dissolved in the chloride melt and then discharged at the graphite-rod anode. Chlorine ions are unlikely to be discharged at the anode. The niobium metal powders prepared contained as low as 2311 mass ppm oxygen, clearly demonstrating that the electrodeoxidation method is applicable to the reduction of solid Nb₂O₅ in chloride melts. The kinetics and mechanisms of reduction of the porous pellets of Nb₂O₅ are discussed in detail based on the measured current-time behavior, together with the microstructural analysis of the sintered Nb₂O₅ pellets and the phases present in the partially reduced samples obtained at various times of reduction.     

The interaction between high-alumina concrete and low-temperature melt on the base of potassium cryolite

Using the gravimetrical method, the kinetics of the interaction of high-alumina concrete with the melt of fluoride KF-AlF₃ in the temperature range from 700 to 800°C is considered. Such analysis allows one to observe the variation in the weight of the examined material placed into the molten salt mixture during the experiment and to calculate the rate of the weight increment. As a result of the investigations, the way the concrete binding material interacts with the melt is revealed. The character and kinetics of this interaction are affected by the concentration of dissolved Al₂O₃, melt temperature, and preliminary thermal treatment of the concrete. After the experiments with the melt saturated by alumina (5 wt % Al₂O₃, T = 700°C), the samples totally kept their form and sizes. When the alumina content in the melt is lower than 2.5 wt %, the samples are subjected to corrosion. It is shown that it is possible to use high-alumina concrete as a construction material for electrolysis baths in the future, if potassium cryolite is used.     

Structural and phase transformations with friction in a bronze-tungsten disulfide composite

The nature of secondary structures which form protective layers on the friction track of a (bronze - tungsten disulfide composite) — steel pair under vacuum friction conditions using a screen cooled with liquid nitrogen and the action of an additional current is studied. It is established that in all of the cases in question there is no reciprocal mass transfer of the materials in contact. A homogeneous protective layer covering the working friction surface under the action of an additional current is the interaction product of dispersed initial components of the bronze - tungsten disulfide composite and a layer of new chemical compounds of the Me₂C type with similar parameters to a high-temperature modification of the compound Cu₂S. It is assumed that formation of this compound is the result of intense pulse action of microvolumes of the material at points of contact of the body with the counterbody.Institute of Metal Physics, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, No. 4(364), pp. 30–36, April, 1993.     

Penetration and chemical reactions in carbon cathodes during aluminum electrolysis: part i. laboratory experiments

Penetration of metallic sodium and salt melt into cathodic carbon materials was studied in a laboratory aluminum electrolysis cell. Variables were cryolite ratio, current density, N₂ or Ar atmosphere, degree of graphitization, and strain on the carbon samples. The carbon sample was analyzed by optical and electron microscopy, ash analysis, sodium detection by phenolphtalein, density, and X-ray diffraction (XRD). The developed XRD technique proved especially useful and allowed quantitative determination of concentration profiles for the crystallized compounds. Sodium was found to be the primary penetration agent and its velocity and saturation concentration increased with increased cryolite ratio of the melt and decreased graphitization of the carbon material. Sodium was found to be important as a wetting agent facilitating melt penetration. Al₄C₃ was not present within the pores of the carbon material. Melt penetration was enhanced by polarization and formation of nitrogen compounds. NaCN was formed directly from the elements and was destabilized by the advancing melt. In nitrogen atmosphere, A1N was the major nitrogen component and was formed within the melt phase. Formerly with the Institute of Inorganic Chemistry, The Norwegian Institute of Technology     

NiAl powder alloys: I. Production of NiAl powders

The influence of five methods of production of Ni₅₀Al₅₀ powder alloys on the processes occurring during reactive alloy formation of nickel monoaluminide during heating is considered. It is shown that, when powder mixtures obtained by agitation in ball mills and cladded composite powders with a low level of internal stresses are used, it is possible to produce a material with a nearly equilibrium phase composition in the course of reactive sintering due to an exothermic effect with the participation of a liquid phase (aluminum melt) in the reaction. The sintered material is porous and has an island structure. Mechanical alloying in a high-energy ball mill (attritor) results in the formation of layered Ni/Al granules with a developed interface and a high level of internal stresses and defects, which makes it possible to decrease the temperatures of initiation of reactive interaction by ∼300°C. This interaction develops in the solid phase according to a slow diffusive mechanism leading to the formation of intermediate nickel aluminides and hindering the achievement of equilibrium phase composition. The microingot granules (∼80 wt % particles 100–400 μm in size) produced by melt spraying by gases (N, Ar) has the composition of the melt, but grain boundaries are depleted of aluminum in comparison with the volume. The NiAl powders (∼90 wt % particles <40 μm in size) produced by combined hydride-calcium reduction are characterized by a highly homogeneous nickel and aluminum distribution, and their composition is close to equilibrium. These two types of powders are selected as the initial material for investigating the compacting and production of NiAl-based alloys.     

Conceptual Protection Model for Especially Heat-Proof Materials in Hypersonic Oxidizing Gas Flows

This article is a continuation of the publication cycle of the authors on the subject matter “Multifunctional Protective Coatings for Especially Heat-Loaded Constructional Elements of Hypersonic Systems.” A conceptual physicochemical operation model of the protective coating in a high-speed high-enthalpy oxidizing gas flow taking into account and leveling main surface fracture sources by the gas flow is proposed. The model is successfully implemented when developing a whole series of alloys of the Si–TiSi₂–MoSi₂–B–Y system intended to form thin-layer coatings from them by any method of the stratified deposition providing the reproduction of the structure, phase composition, and morphological features of the deposited material in the coating. During the deposition, the formation of a microcomposition layer is provided. This layer is a refractory silicide framework with the cells filled by a low-melting (relative to the melting point of framework-forming phases) eutectic structural component. This layer transforms into a multilayer system with a series of functional layers (anticatalytic, reradiative, antierosion, heat-proof, and barrier-compensation layers) of micron and submicron thicknesses during high-temperature interaction with oxygen-containing media (the synergetic effect). The protection ability is provided by the formation of self-restoring oxide vitreous film based on alloyed silica. The self-restoring effect consists of rapid filling of random defects with a viscoplastic eutectic component and protective film formation accelerated when compared with known coatings. The high resistance to the erosion carryover is provided by the presence of a branched dendritic-cellular refractory framework. Coatings MAI D5 and MAI D5U, designed in the scope of the proposed concept, are successfully approved in high-speed high-enthalpy oxygen-containing gas flows affecting the samples and constructional elements made of especially heat-proof material of various classes (niobium alloys, carbon–carbon and carbon–ceramic composite materials, and graphitized carbon materials). The protective ability of coatings of 80–100 μm in thickness in flows with the Mach number of 5–7 and enthalpy of 30–40 MJ/kg is no shorter than 600 s at T_w = 1800°C, 200 s at 1900°C, and 60 s at 2000°C, including the constructive elements with sharp edges.     

Differences in oxides on large-and small-grained 304 stainless steel

The oxides formed on large-grained (∼40 °m) and small-grained (∼4 μm) 304 stainless steel oxidized in air at 800 °C have been examined and compared by Auger electron spectroscopy to learn more about the role of grain boundaries in the oxidation of the materials. For vacuum preannealed specimens, relatively thick iron oxides formed over the grains and thin, chromium-rich oxides formed over grain boudaries of large-grained material. The oxide formed over the entire surface of the small-grained material was a thin chromium-rich layer similar to that formed over grain boundaries of the large-grained samples. The oxidation of both small- and large-grained samples was consistent with selective formation of Cr₂O₃ at grain boundaries followed by a lateral diffusion of Cr and spreading of Cr₂O₃. A protective Cr₂O₃ layer formed readily on small-grained material but not on large-grained material. In contrast to the differences in oxide morphology for small- and large-grained preannealed specimens, oxide morphologies were similar for small- and large-grained material when the outer surface layer was removed by polishing after annealing and before oxidation tests. Surface differences, not adequately defined by Auger and SEM studies, caused marked changes in oxide morphologies for large-grained material. The difference in oxidation behavior before and after polishing was attributed to enhanced oxidation at grain boundaries during the vacuum preannealing treatment and to differences in defect concentrations in the surface region.     

Oxygen adsorption/desorption behavior of YBaCo4O7+δ and its application to oxygen removal from nitrogen

A new medium-temperature (200-400℃) adsorbent material for oxygen removal and air separation, YBaCo4O7+β, was prepared by the solid-state reaction method. This new adsorbent could adsorb a large quantity of oxygen in the temperature range of 200-370 ℃. Ad-sorbed oxygen could be released by raising temperature over 400 ℃ or by switching the atmosphere from oxygen to nitrogen. This oxygen adsorption and desorption process had good reproducibility. Taking advantage of this unique oxygen intake/release behavior, a nitrogen puri-fication process was investigated. The results showed that YBaCo4O7+δ material was a promising candidate for the oxygen sorption process and could be used to produce high-purity nitrogen or to remove trace oxygen from other gases.     

Effect of Holding Time on Surface Films Formed on Molten AZ91D Alloy Protected by Graphite Powder

Graphite powder was adopted to prevent the AZ91D magnesium alloy from oxidizing during the melting and casting process. The microstructure of the resultant surface films formed at 973 K (700 °C) holding for 0, 15, 30, 45, and 60 minutes was investigated by scanning electron microscopy, energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) after mechanical polishing and chemical etching. The results indicated that the surface films were composed of a protective layer and the underneath particles with different morphology. The protective layer was continuous with a thickness of 200 to 1000 nm mainly consisting of MgO, MgF₂, and C, while the underneath particles mainly consisted of MgF₂ and MgAl₂O₄. The surface films were the result of the interaction between the graphite powder, the melt, and the ambient atmosphere. The number and the size of the underneath particles, determining the thickness uniformity of the surface films, and the unevenness of the microsurface morphology increased with holding time. The mechanism of holding time on the resultant surface films was also discussed.     

The kinetics of oxidation of iron-silicon and iron-aluminum alloy melts by pure oxygen or oxygen-bearing gases

The kinetics of oxidation of Fe−Si and Fe−Al melts by pure oxygen, and that of pure Fe by He−O₂, N₂−O₂, or Ar−O₂ mixtures have been investigated by a modified Sieverts' method at 1600°C. Considerable decrease in the oxidation rate has been observed for the alloy melts containing a few percent of Si or Al since formation of a silica- or alumina-rich oxide layer on the melts prevents further progress of the exothermic chemical reaction. The oxidation rate for melts high in Al has been considered to be limited by the diffusion of ions through the oxide layer. Addition of diluents to O₂ markedly and continuously decreases the oxidation rate of a pure Fe melt. The latter rate has been show to be controlled by the diffusion of O₂ across the gaseous boundaries at gas/melt interfaces.     

Dissolution rate of lead(II) oxide in an equimolar KCl-PbCl2 melt

Dissolution rates (W) of lead(II) oxide in a KCl-PbCl₂ equimolar melt are determined experimentally using the gravimetric method at T = 773, 823, and 873 K. It is shown that, as the temperature increases from 773 to 873 K, the initial magnitude of W increases from 23.9 to 45.6 mg/(cm² min) with a conventional roughness coefficient of 10. Then the values of W for all temperatures are leveled, being close to zero after 25 min, which indicates the diffusion mode of the process in natural convection conditions. The activation energy of the interaction between PbO and the KCl-PbCl₂ melt was 37.370 ± 0.118 kJ/mol. The limiting concentration of PbO in the equimolar KCl-PbCl₂ melt was determined, being 9.1, 10.6, and 13.5 wt % at T = 773, 823, and 873 K, respectively.     

Modeling of porosity during spray forming: Part II. Effects of atomization gas chemistry and alloy compositions

In this article, the effects of gas chemistry and alloy composition on the level of porosity in deposited materials are investigated by using a porosity model established in Part I of this article. The calculated results reveal that atomization gas chemistry has a significant influence on the level of porosity during spray forming, which can be rationalized on the basis of the influence of gas properties such as gas density, viscosity, and gas constant on the melt flow rate. The alloy properties that predominantly affect the variation of porosity with melt flow rate include melt viscosity, density, surface tension, solvent melting point, liquidus temperature, and equilibrium partition coefficient. A material property factor, μ _mγ_m/ρ ² _m , plays an important role in determining the processing conditions required to attain a minimum amount of porosity in deposited materials.     

Solidification and Re-melting Phenomena During Slurry Preparation Using the RheoMetal™ Process

The melting sequence of the enthalpy exchange material (EEM) and formation of a slurry in the RheoMetal? process was investigated. The EEM was extracted and quenched, together with a portion of the slurry at different processing times before complete melting. The EEM initially increased in size/diameter due to melt freezing onto its surface, forming a freeze-on layer. The initial growth of this layer was followed by a period of a constant diameter of the EEM with subsequent melting and decrease of diameter. Microstructural characterization of the size and morphology of different phases in the EEM and in the freeze-on layer was made. Dendritic equiaxed grains and eutectic regions containing Si particles and Cu-bearing particles and Fe-rich particles were observed in the as-cast EEM. The freeze-on layer consisted of dendritic aluminum tilted by about 30 deg in the upstream direction, caused by the rotation of the EEM. Energy dispersion spectroscopy analysis showed that the freeze-on layer had a composition corresponding to an alloy with higher melting point than the EEM and thus shielding the EEM from the surrounding melt. Microstructural changes in the EEM showed that temperature rapidly increased to 768 K (495 °C), indicated by incipient melting of the lowest temperature melting eutectic in triple junction grain boundary regions with Al₂Cu and Al₅Mg₈Si₆Cu₂ phases present. As the EEM temperature increased further the binary Al-Si eutectic started to melt to form a region of a fully developed coherent mushy state. Experimental results and a thermal model indicated that as the dendrites spheroidized near to the interface at the EEM/freeze-on layer reached a mushy state with 25 pct solid fraction, coherency was lost and disintegration of the freeze-on layer took place. Subsequently, in the absence of the shielding effect from the freeze-on Layer, the EEM continued to disintegrate with a coherency limit of a solid fraction estimated to be 50 pct.     

Structure of a bimetallic strip produced by plasma spraying of a TiAl powder on a niobium sheet

Ti-48 at % Al alloy granules produced by centrifugal spraying are milled into a powder with a particle size of 40–100 μm, and are applied onto a niobium foil using plasma spraying in an argon atmosphere. The fabricated TiAl/Nb bimetallic strip consists of a 100-μm-thick niobium layer and a porous 300-to 400-μm-thick TiAl layer formed by flattened particles. Directly after the preparation of the bimetallic strip, the surface of the TiAl porous layer is rough. Vacuum annealing at 1000, 1100, and 1200°C for 0.5–1.5 h leads to intense pore healing. After deposition and annealing, the interlayer adhesion is strong. The preparation of TiAl granules and spraying of the powder is accompanied by aluminum depletion of the Ti-48 at % Al alloy to 42–45 at % and an increase in the fraction of the α₂-Ti₃Al phase in the deposited layer. The prepared material has a duplex structure. An intermediate diffuse layer characterized by a variable composition and thickness is formed at the interface. This layer consists of two solid solutions; one of them, which is formed at the TiAl layer, is an α₂-Ti₃Al-based solid solution of niobium and the other, which is formed at the niobium foil, is a niobium-based solid solution of titanium and aluminum.     

Interfacial Reactions in Al-Mg (5083)/SiCp Composites during Fabrication and Remelting

The interfaces of aluminum alloy composites (5083) reinforced by SiC particles (as-received, oxidized 3.04 wt pct and 14.06 wt pct) were studied. The composites were fabricated by compocasting and certain samples were also remelted at 800 °C for 30 minutes. The reaction mechanisms between SiC _p and liquid Al and between the SiO₂ layer and Al(Mg) are discussed. The crystal boundaries of the MgO (or MgAl₂O₄) reaction products are believed to be the diffusion paths (or channels) during the interfacial reactions. A SiO₂ layer, formed by oxidation of the SiC particles prior to their incorporation into the melt, plays an important role in preventing the SiC _p from being attacked by the matrix. The interfacial reaction products are affected by both the alloy composition and the thickness of the initial SiO₂ layer.