Oxidation of porous nanocrystalline titanium nitride. III. Oxidation mechanism

The air oxidation mechanism of nanocrystalline TiN at 500 to 900 °C is examined. It is shown that at t ≤ 800 °C the oxidation of titanium nitride is controlled by the diffusion of oxygen and at t > 800 °C the interdiffusion of titanium ions is observed. The oxidation properties of porous TiN are determined by the chemical interaction of oxygen and the reaction surface, which includes the external surface of samples and the internal surface of the pores into which oxygen penetrates. The time and temperature dependence of the weight increment complies with the porous material oxidation model. Active initial oxidation is due to the interaction of oxygen and large internal surface. Short-term self-heating of porous samples is also possible. At t ≤ 800 °C, the pores are obliterated with oxides with time, the internal reaction surface reduces, an external oxide film is formed, the oxygen diffusion and weight increment slow down, and the process stabilizes. With temperature increase, these processes are activated and lead to a smaller weight increment at the final stage (2 to 4 h) at 800 °C as compared with 600 °C. At t > 800 °C the pore obliteration rate increases, but due to the interaction of oxygen and titanium ions that diffuse into the external scale surface, weight increment continuously increases with both time and oxidation temperature. The phase composition of the scale also affects the oxidation mechanism of porous TiN. Oxynitride of terminal composition plays a protective role; the transformation of anatase into rutile is accompanied by a decrease in the oxygen diffusion rate; Ti₂O₃ formed in pores accelerates their obliteration. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 3–4 (454), pp. 95–104, 2007.     

Electrochemical characterization of copper deposited on plasma and thermally modified titanium surfaces

Thin oxide films were grown at temperatures from 373 to 1073 K in plasma and in air on commercially pure titanium substrates. It was determined that the color, thickness, composition, phase, and polarization behavior in a copper electrolyte varied with operating conditions: temperature, oxygen partial pressure, and plasma composition. High-temperature and high oxygen partial pressure plasma produced a thick oxide film. The surface film structure transformed from TiO₂ (anatase) to TiO₂ (rutile) at a temperature of 600 °C. A lower oxide of the form Ti _n O_2n−1, such as Ti₂O₃ (which may be porous) or possibly Ti₃O₅, was formed on a thermally treated sample (400 °C, 80 mtorr O₂, 3 hours). This sample exhibited the lowest potential for copper nucleation and gave a very uniform, smooth, and hole-free copper foil.     

Magnetizing roasting of leucoxene concentrate

The phase transformations occurring during magnetizing roasting of leucoxene concentrate in the temperature range 600–1300°C are studied. It is demonstrated, that in the temperature range 600–800°C, only iron oxides are reduced to a metallic state; at temperatures above 800°C, combined reduction of iron and titanium oxides takes place. At 1050°C, reduced specimens are represented by the Ti₅O₉ and Ti₆O₁₃ Magnéli phases. The formation of iron metatitanate (FeTiO₃), under reduction conditions and the existence of ferrous iron ions in the Magnéli phases slightly degrade the magnetic properties of the products of magnetizing roasting. In high temperature region (1200–1300°C), a similar effect is exerted by the formation of iron dititanate or anosovite in the system. The possibilities of eliminating the undesired factors decreasing the magnetic properties of the products of magnetizing roasting are determined.     

Oxidation of TiAl intermetallic

The oxidation kinetics of TiAl intermetallic at 500–900 °C in air is studied using a gravimetric method, and the phase composition of the scale is studied using an x-ray phase analysis. At t > 600 °C, the kinetics of oxidation is described by a parabolic equation. The oxides TiO₂ (rutile), γ-Al₂O₃, α-Al₂O₃, Ti₂O₃ are found in the scale. It is shown that at the first stage the γ-Al₂O₃ and low-titanium oxides form on the sample surface at t < 70 °C. At t ≥ 850 °C, the Ti₂O₃ forms on the external surface of the scale, TiAl₃ is found in the sublayer at the alloy/scale interface. It is shown that at t ≤ 800 °C the process is controlled by oxygen diffusion. At t > 800 °C, the oxidation mechanism changes: counterdiffusion of titanium ions through interstitial sites in TiO₂ lattice occurs.     

Phase transformations during heating of llmenite concentrates

llmenite concentrates were heated in argon and oxygen in the temperature range 700 °C to 1000 °C to study the behavior of the pseudorutile phase and other changes which occur. Pseudorutile does not persist in argon or oxygen in the temperature range studied. In argon at 700 °C, pseudorutile decomposes into hematite and rutile, while at 1000 °C, it combines with ilmenite to form ferrous-ferritic pseudobrookite solid solution. A new phase “Fe₂O₃-2TiO₂” was identified as an intermediate product during the heating of ilmenite or pseudorutile in oxygen. This compound decomposes into hematite and rutile below 800 °C and to pseudobrookite and rutile above 800 °C. The sequence of reactions during the heating of ilmenite and pseudorutile in oxygen is proposed. Formerly with the Department of Metallurgy, Imperial College of Science, Technology and Medicine, London. Formerly with the Department of Metallurgy, Imperial College of Science, Technology and Medicine, London.     

Carbothermic reduction of llmenite (FeTiO3) and rutile (TiO2)

The mechanically activated carbothermic reduction mechanism of ilmenite has been examined by a combination of steady-state and dynamic thermal techniques coupled with X-ray diffraction. The reaction was found to proceed via an initial, rapid reduction to elemental iron and rutile, which was followed by a slow reduction of rutile to a series of oxides of the general formula Ti _n O_2n−1 until Ti₃O₅ was formed, which was found to be relatively stable. Iron was probably incorporated into the Ti _n O_2n−1 lattice only for n>3, forming mixed oxides of uncertain composition. The formation of TiC was evident at temperatures as low as 1100 °C, but the rate of reaction was extremely slow, presumably due to a solid-state diffusion limitation. Increasing the temperature gave increasing conversion of TiO₂ to TiC until it was the only confirmed product. The effect of iron on the later stages of reduction was removed by examining the reduction of pure rutile. It was found that the reduction of Ti₃O₅ was enhanced by the presence of iron. The separation of iron from the titanium product proved to be high, with > 90 pct of iron removed after the initial reduction. The iron removal increased slowly to almost 100 pct when elemental iron and titanium carbide were the products.     

High-temperature phase relations and thermodynamics in the iron-titanium-oxygen system

Phase equilibria and thermodynamics in the FeO-TiO₂-Ti₂O₃ ternary system were studied at 1500 °C and 1600 °C. In particular, the liquid slag-phase region and its saturation boundary with respect to metallic iron, titania, and lower titanium oxides was investigated. The liquid slag-phase region extends substantially toward an anosovite (Ti₃O₅) composition, and considerable concentrations of divalent iron coexist with trivalent titanium in the liquid-slag phase. This seems to be a consequence of the complete solid solution between ferrous pseudobrookite (FeTi₂O₅) and anosovite (Ti₃O₅), which exists at subsolidus temperatures. The liquid-slag field is significantly enlarged toward the anosovite composition upon increasing the temperature from 1500 °C to 1600 °C. Activities of the components “FeO” and TiO₂ in the liquid-slag region were determined by Gibbs-Duhem integration of the measured oxygen partial pressures at 1500 °C. The FeO shows moderate negative deviation, while titania shows a slight negative deviation in FeO-rich slags and a positive deviation in high-titania slags. The experimentally measured activity values were modeled using regular and biregular solution models, and good agreement was obtained with the biregular solution model.     

Oxidation processes of alloys of the Ni—Ta system. II. Oxidation of the intermetallic compound NiTa

The oxidation kinetics of the intermetallic compound NiTa was studied by the continuous thermogravimetry in air at temperatures ranging from 600 to 1000°C. The scale formed was subjected to X-ray and metallographic sectioning phase analysis. Oxidation of NiTa was shown to occur because of the preferential diffusion of oxygen toward the scale-alloy interface. The kinetics is described as a parabolic function of time. The isotherms indicate that the parabolic oxidation rate constant K _p periodically decreases for t≤800°C but increases and periodically decreases for t>800°C. The temperature dependence of K_p is exponential. At t∼850°C the oxidation rate decreases, indicating a change in the oxidation mechanism. The scale formed on NiTa was found to contain the oxides NiO, NiO·Ta₂O₅(NiTa₂O₆), and Ta₂O₅, as well as Ni. The solid solutions Ni(Ta) and Ni₃Ta were detected in the sublayer of scale adjacent to the alloy. At high temperatures those phases are distributed among the layers: NiO+NiTa₂O₆+Ta₂O₅ in the first, NiTa₂O₆+Ta₂O₅ in the second, Ni+Ta₂O₅ in the third, and Ta₂O₅+Ni(Ta)+Ni₃Ta in the fourth. By analogy with the oxidation of unalloyed tantalum the explanation for the experimental results is that at a p→n phase transition. accompanied by the formation of oxygen vacancies and tantalum interstitials, occurs in the lattice at t∼850°C. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(407), pp. 75–82, May–June, 1999.     

Oxidation of alloys of the system Ni−Ta. I. Oxidation of the intermetallic Ni3Ta

Oxidation kinetics for the intermetallic Ni₃Ta in air at 600–1000°C are studied by a thermogravimetric method. The alloy has an ordered crystal structure (D¹³ _2h-Pmmn) with rhombic lattice parameters a=0.512 nm, b=0.423 nm, and c=0.452 nm. The kinetic isotherms of Ni₃Ta oxidation are described by a parabolic equation. With t≤800°C there is a periodic increase in the rate constant of parabolic oxidation, but with t>800°C there is a periodic decrease of it. In the range 850–875°C the alloy oxidation rate decreases as a result of scale sintering. Oxygen diffusion slows down in the compact scale. X-ray and metallographic analysis of the scale that forms on Ni₃Ta indicates that it contains NiO, NiO·Ta₂O₅, Ta₂O₅ and also Ni and the solid solution Ni(Ta). These phase components are distributed in layers of the scale: NiO (first), NiO+NiO·Ta₂O₅ (second), NiO·Ta₂O₅+Ni+Ta₂O₅ (third), Ta₂O₅+Ni(Ta) (fourth). With a low temperature and short periods of heating there is no Nio·Ta₂O₅ or Ni(Ta) in the scale. Oxidation of Ni₃Ta is controlled by oxygen diffusion in the scale over the direction towards the alloy. With t>850°C this mechanism changes. By analogy with oxidation of tantalum it is assumed that structural changes in the Ta₂O₅ lattice may be responsible for this. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(406), pp. 80–87, March–April, 1999.     

Phase Formation of Zinc Titanate Precursor Powders Prepared by a Hydrothermal Process in Various pH Environments

For the precursor, powders were calcined at 773 K (500 °C) for 1 hour. The anatase TiO₂ and ZnO appear as the predominant and minor phases, respectively, when the precursor powders are obtained at pH 5. The phases of Zn₂Ti₃O₈, anatase TiO₂, and ZnO coexist for the precursor powders obtained at pH 7. In addition, the Zn₂Ti₃O₈ and ZnO are the predominant and minor phases, respectively, for the precursor powders obtained at pH 9.     

Structure and properties of multicomponent eutectic alloys based on chromium and titanium carbide

We have investigated alloys in the Cr-Mo-Ti-C, Cr-Re-Ti-C, and Cr-Mo-Re-Ti-C systems in the eutectic <Cr>+<TiC> crystallization region. We found a four component quasibinary eutectic <Cr, Mo>+<TiC> with 4–8 at. % molybdenum content with melting point 1630°C. Additions of 3–11 at. % Mo or 5–20 at. % Re to the base eutectic alloy Cr₇₉Ti₁₂C₉ doubles the Vickers hardness at 1000°C (to approximately 2000 MPa), and simultaneous introduction of molybdenum and rhenium (the alloy Cr₅₁Mo₈Re₂₀Ti₁₂C₉) raises the hardness to 3000–3500 MPa. Ukrainian Materials Science Institute, National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 15–23, January–February, 1997.     

Isothermal oxidation of TiAl alloy

Isothermal oxidation behavior of Ti-48.6 at. pct Al alloy was studied in pure dry oxygen over the temperature range 850 °C to 1000 °C. The oxidation was essentially parabolic at all temperatures with significant increase in the rate at 1000 °C. Effective activation energy of 404 kJ/mol was deduced. The oxidation products were a mixture of TiO₂ (rutile) and α-Al₂O₃ at all temperatures. An external protective layer of alumina was not observed on this alloy at any of the temperatures studied. A layered structure of oxides was formed on the alloy at 1000 °C.     

Deformation characteristics of age hardened Ti-6Al-4V

A commercial Ti−6Al−4V alloy with an equiaxed grain shape was investigated after solution annealing at 810°C and after aging at 550 and 350°C. Age hardening at both temperatures produced significant increases in Young's modulus and yield strength. Finely dispersed α₂(Ti₃Al) precipitates formed within the α phase upon aging at 550°C, but not when aging at 350°C. However, there is evidence of order, probably of oxygen, in the α grains of specimens which were aged at 350°C. The formation of the ordered Ti₃Al precipitates at 550°C and the occurrence of oxygen ordering at 350°C can account for the increases in Young's modulus and yield strength. since January 1977 with General Electric Co., Lighting Research Division, Nela Park, Cleveland, OH. KANAY GAZIOGLU, formerly with DFVLR, is deceased.     

A sintering study on the β-spodumene-based glass ceramics prepared from gel-derived precursor powders with LiF additive

Beta-spodumene (Li₂O·Al₂O₃·4SiO₂, LAS) powders were prepared by a sol-gel process using Si(OC₂H₅)₄, Al(OC₄H₉)₃, and LiNO₃ as precursors and LiF as a sintering aid agent. Dilatometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and electron diffraction (ED) were utilized to study the sintering, phase transformation, microstructure, and properties of the β-spodumene glass-ceramics prepared from the gel-derived precursor powders with and without LiF additives. For the LAS precursor powders containing no LiF, the only crystalline phase obtained was β-spodumene. For the pellets containing less than 4 wt pct LiF and sintered at 1050 °C for 5 hours the crystalline phases were β-spodumene and β-eucryptite (Li₂O·Al₂O₃·2SiO₂). When the LiF content was 5 wt pct and the sintering process was carried out at 1050 °C for 5 hours, the crystalline phases were β-spodumene, β-eucryptite (triclinic), and eucryptite (rhombohedral (hex.)) phases. With the LiF additive increased from 0.5 to 4 wt pct and sintering at 1050 °C for 5 hours, the open porosity of the sintered bodies decrease from 30 to 2.1 pct. The grains size is about to 4 to 5 μm when pellect LAS compact contains LiF 3 wt pct as sintered at 1050 °C for 5 hours. The grains size grew to 8 to 25 μm with a remarkable discontinuous grain growth for pellet LAS compact contain LiF 5 wt pct sintered at 1050 °C for 5 hours. Relative densities greater than 90 pct could be obtained for the LAS precursor powders with LiF > 2 wt pct when sintered at 1050 °C for 5 hours. The coefficient of thermal expansion of the sintered bodies decreased from 8.3 × 10⁻⁷ to 5.2 × 10⁻⁷/°C (25 °C to 900 °C) as the LiF addition increased from 0 to 5 wt pct.     

Reaction of the hydrogen-absorbing intermetallic compound TiFe with oxygen. III. Phase composition of the scale formed on TiFe

We have studied the phase composition of the overlayer of scale which forms on TiFe at 700°C and 900°C (τ=5 h) by x-ray and metallographic analysis. The upper layers of the scale were shown to consist of TiO₂ and Fe₂O₃ after heat treatment at 700°C, but the lower layers contain mainly TiO₂, FeO, and Fe. The underlayer on the boundary with the scale contains Ti₄Fe₂O (η-phase). The inert marker is covered with rutile at 900°C, and FeTiO₃ + TiO₂ is below the marker. But the next (relatively thick) internal layer consists of FeTiO₃, TiO₂, and Fe. Large pores associated with intensive growth of the FeO phase are detected in the scale formed at 700°C. As a result, the scale cracks. At 900°C, the scale is denser because the pores and cracks are covered by rutile. We have shown that FeO (p-type semiconductor) is nonstoichiometric in the upper layers of the scale, and TiO₂ (n-type semiconductor) is nonstoichiometric close to the boundary with the alloy. The results obtained correlate with the results for the oxidation kinetics previously studied for FeTi, and support a change in the oxidation mechanism when the temperature increases from 700°C to 900°C. Such a change occurs because of the influence of diffusion of the metal ion on oxygen diffusion through the boundary between the scale and the alloy: diffusion of iron through the vacancies in the FeO lattice at 700°C, and interstitial diffusion of titanium ions in the TiO₂ lattice at 900°C.     

Phase transitions during reducing roasting of a leucoxene concentrate with carbon

The phase transitions during the reducing roasting of a leucoxene concentrate with carbon are studied to obtain an anosovite product. Thermodynamic modeling of the reducing roasting is performed, and the influence of the temperature and the amount of a reducing agent on the reduction of rutile to Ti₃O₅-based anosovite is studied. Almost complete reduction of rutile to anosovite occurs in a temperature range of 1350–1400°C in the presence of 2.5–5.0% carbon. The Magnéli phases of various compositions are predominantly formed at lower temperatures and smaller amounts of the reducing agent. At temperatures higher than 1400°C and a reducing agent amount >2.5%, rutile reduction results in the formation of anosovite along with an undesirable titanium carbide phase.     

Oxidation of porous nanocrystalline titanium nitride. I. Kinetics

We used the continuous weighing method to study the oxidation kinetics in air for TiN specimens pressed and sintered from nanocrystalline powders with particle size ≤55 nm. Oxidation was carried out at 500–1000 °C for 240 min. By comparing with the oxidizability of compact titanium, we estimated the total reaction surface S of the porous specimens as a function of their oxidation conditions. The mass of absorbed oxygen Δm was calculated from the mass gain ΔP, taking into account the volatile component N₂. We have shown that the maximum mass gain Δm at 600 °C is due to reaction of oxygen with the largest reaction surface. Within 120 min, external pores close up, S decreases, and then a continuous oxide layer forms in which diffusion of oxygen is slowed down. At 700–800 °C, the process of closing up of the pores is activated, and S decreases by an order of magnitude compared to 600 °C. After the first 40–50 min, a continuous oxide film forms and virtually no further mass gain occurs. As the temperature increases, the oxidation rate increases. At 900 °C, the reaction surface becomes equal to the external surface of the specimen, but the thickness of the scale increases linearly. We hypothesize that for T > 850 °C, counterdiffusion of titanium ions is superimposed on diffusion of oxygen. __________ Translated from Poroshkovaya Metallurgiya, Nos. 1–2(447), pp. 98–103, January–February, 2006.     

Defects and Plastic-Deformation Modes of Bulk-Metallic Glasses

Results of the investigation of the point-defect manifestation in the recovery kinetics of Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 and Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ bulk-metallic glasses (BMGs) irradiated with 2.5 MeV electrons at 80 K (–193.15 °C) are presented. An observation of the pronounced annealing stages at 155 K and 130 K (–118.15 °C and –143.15 °C), and 225 K (–48.15 °C), shows that irradiation generates stable point defects in BMGs. The ultrasonic vibrations (USVs) of different amplitudes were used to investigate their effects on the cluster boundaries. The Kaiser effect is chosen as a tool for examining the boundary-slip initiation and impact of vibrations on the intercluster-boundary structure. Both the acoustic-emission activity and strength decrease due to the specimen pretreatment by USV. This effect is interpreted as a result of boundary softening under the USV. The inherent tensile strength of a Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 BMG (in atomic percent) in the as-cast state was determined by means of high-field mechanical loading using the field-ion microscopy. It was revealed that the strength is characterized by a strong size effect in a nanometer-scale range as a result of the manifestation of the structural nanoheterogeneities and, in part, the existence of the cluster boundaries.     

Equilibrium phase relations and thermodynamics of the Cr-0 system in the temperature range of 1500 °C to 1825 °C

Phase relations and thermodynamic properties of the Cr-O system were studied at temperatures from 1500 °C to 1825 °C. In addition to Cr and Cr₂O₂, a third crystalline phase was found to be stable in the temperature range from 1650 °C to 1705 °C. The atomic ratio of oxygen to chromium of this phase, which decomposes upon cooling to form Cr and Cr₂O₃, was determined as 1.33 + 0.02, in good agreement with the formula Cr₃O₄. Temperatures and phase assem blages for invariant equilibria of the Cr-O system were determined as follows: Cr₂O₃ + Cr + Cr₃O₄, 1650 °C ± 2 °C; Cr₃O₄ + Cr + liquid oxide, 1665 °C ± 2 °C; and Cr₃O₄ + Cr₂O₃ + liquid oxide, 1705 °C ± 3 °C. The composition of the liquid oxide phase at the eutectic temperature of 1665 °C was found to be close to CrO. Relations between oxygen pressure and temperature for the univariant equilibria of the Cr-O system were established by equilibrating Cr and/or Cr₂O₃ starting materials in H₂-CO₂ mixtures of known oxygen potentials at temper atures from 1500 ΔC to 1825 °C. From this information, the standard free-energy changes (ΔGΔ) for various reactions were calculated as follows: 2Cr (s) + 3/2O₂ = Cr₂O₃ (s): ΔG ° = -1,092,442 + 237.94T Joules, 1773 to 1923 K; 3Cr (s) + 2O₂ = Cr₂O₄ (s): ΔG ° =-1,355,198 + 264.64T Joules, 1923 to 1938 K; and Cr (s) + l/2O₂ = CrO (1): ΔG ° =-334,218 + 63.81T Joules, 1938 to 2023 K. Formerly Graduate Research Assistant, The Pennsylvania State University Formerly Professor     

Thermodynamic and nonstoichiometric behavior of promising Hi-Tc cuprate systems via electromotive force measurements: A short review

Electromotive force (EMF) measurements of oxygen fugacities as a function of stoichiometry have been made on the YBa₂Cu₃O _x , GdBa₂Cu₃O _x , NdBa₂Cu₃O _x , and bismuth cuprate systems in the temperature range ∼400 °C to 750 °C by means of an oxygen titration technique with an yttriastabilized zirconia electrolyte. The shapes of the 400 °C isotherms as a function of oxygen stoichiometry for the Gd and Nd cuprate systems suggest the presence of miscibility gaps at values of x that are higher than those in the YBa₂Cu₃O _x system. For a given oxygen stoichiometry, oxygen partial pressures above GdBa₂Cu₃O_x and NdBa₂Cu₃O _x the higher (above x=6.5) than that for the promising YBa₂-Cu₃O_x system. This article is based on a presentation made at “The Milton Blander Symposium on Thermodynamic Predictions and Applications” at the TMS Annual Meeting in San Diego, California, on March 1–2, 1999, under the auspices of the TMS Extraction and Processing Division and the ASM Thermodynamics and Phase Equilibrium Committee.