Effect of water vapour partial pressure on the chromia (Cr2O3)-based scale stability

The kinetics of the Cr₂O₃-based scale oxidation and volatilisation were studied in the presence of water vapour (H₂O). A commercial Cr₂O₃-based scale forming Type 310S stainless steel was examined at the ambient pressure (0.1?MPa) and 550°C in relatively low and high H₂O-containing environments of air-10% H₂O and air-70% H₂O, respectively. The increase in the partial pressure of H₂O (p_H2O) from 10 to 70% resulted in the transition of the oxidation and volatilisation kinetics from the parabolic rate law in air-10% H₂O to the paralinear rate law in air-70% H₂O. The kinetics transition was attributed to the increase in the Cr loss rate from the base scale after coupons exposure in air-70% H₂O. The significant role of Mn alloying element in the base scale protectiveness was also discussed in the context of the Cr₂O₃-based scale stability.     

The thermodynamic activity of MnO in melts containing SiO2, B2O3, and TiO2

The activity of MnO in the systems MnO-Ti₂ and MnO-B₂O₃, and in pseudobinary sections in the systems MnO-TiO₂-SiO₂, MnO-TiO₂-B₂O₃ and MnO-SiO₂-B₂O₃ have been measured at temperatures in the range of 1400 to 1550 °C by means of the technique in which equilibrium is established between the melt, a gaseous atmosphere of fixed oxygen pressure and a Pt-Mn foil immersed in the melt. The excess free energies of melts in the system MnO-TiO₂ at 1500 and 1550 °C are closely represented by the regular solution formalism withG _xs = −55280N _MnO ·N _{TiO 2} J, and the form of the variation ofa _MnO, within the narrow composition range amenable to study at 1400 °C in the system MnO-B₂O₃, is characteristic of a system of polyanions. The negative deviations ofa _MnO from a proposed model behavior, observed in melts in the systems MnO-SiO₂-TiO₂ and MnO-B₂O₃-TiO₂, are explained in terms of the influence of preferred ionic interactions between Ti₄₊ and O₂ and between Mn₂₊ and the O₋ groups on the polysilicate or polyborate anions. The activity of MnO is independent of composition in the system 2MnO · SiO₂−3MnO · B₂O₃ and the thermodynamics of mixing are calculated from application of polymer theory. The influence of B₂O₃ and TiO₂ on the activity of MnO in manganese silicates is discussed with reference to the proposed use of B₂O₃ and/or TiO₂ as substitutes for CaF₂ as the flux for steelmaking slags.     

Effects of Nitrogen Deficiency on the Kinetics and Mechanism of Electrolytic ZrN x Oxidation

Use has been made of potentiodynamic polarization curves, XRD, and scanning electron microscopy (SEM) in the electrolytic oxidation in 3% NaCl solution for specimens of nitrogen-deficient zirconium nitride (ZrN_0.67, ZrN_0.77, ZrN_0.87, and ZrN_0.97), as well as pure zirconium. In all cases, the anodic polarization curves have several stages which characterize during oxidation both active dissolution of ZrN _x and Zr in the electrolyte as well as the formation of surface layers of ZrOCl₂, ZrN _x O _y , and α‐ZrO₂ of monoclinic form. The corrosion resistance of single-phase ZrN _x specimens in 3% NaCl solution decreases in the sequence ZrN_0.97 → ZrN_0.87 → ZrN_0.77, and the initial stages of interaction between the specimen surface and the electrolyte largely determine the subsequent behavior of specimens. It is found that ZrN _x containing a large number of nitrogen atom vacancies, in particular ZrN_0.77, is closer in corrosion behavior to metallic zirconium than it is to stoichiometric ZrN (the reduction in the corrosion resistance is undoubtedly due to the reduction in the ionic-covalent components of the bonds in ZrN _x ).     

Manganese nitrides

Conclusions An investigation was carried out into the kinetics of the process of formation of manganese nitrides in nitrogen and ammonia streams. It was established that the rate of formation of manganese nitrides is higher in ammonia than in nitrogen. The kinetic parameters of manganese nitride formation were calculated.It is shown that in nitrogen the only manganese nitride that can be synthesized in the pure form is Mn_N. In ammonia, depending on the temperature and duration of the nitriding process, it is possible to obtain Mn_N, Mn₂N, and Mn₃N₂ in the pure form, free from any other phases.A study was made of the chemical stability of manganese nitrides of compositions Mn₄N and Mn₃N₂ in water and in concentrated and dilute mineral acids. It was established that these two nitrides are stable in water. In mineral acids they dissolve, the rate of dissolution being higher in concentrated than in dilute acids. In nitric acid Mn₄N and Mn₃N₂ decompose with the liberation of part of the nitrogen in molecular form.Translated from Poroshkovaya Metallurgiya, No. 3(171), pp. 65–70, March, 1977.     

The solubility ofRH2−x in Gd,Er, Tm,Lu and Y from ambient to 850°C

The solubility of hydrogen in Gd, Er, Tm, Lu and Y was determined from 25 to 850°C when the metal was in equilibrium withRH_2?x (x varies between 0.1 and 0.2 depending on the rare earth metal). The room temperature solubilities determined by the lattice parametric method were found to be <0.1, 3.6, 7.7, 20.6 and 19.0 at, pct H in Gd, Er, Tm, Lu and Y, respectively. The change in unit cell volume for each atomic percent hydrogen added was nearly the same for all metals. The solubility of hydrogen increases more rapidly with temperature in those metals with low solubility at room temperature. Thus the solubility of hydrogen at 850°C is nearly the same in all five of the metals studied, that is, 35.0, 36.2, 36.0, 36.0 and 37.3 at. pct H in Gd, Er, Tm, Lu and Y, respectively. The equilibrium pressure of H₂ in these studies was the equilibrium pressure of hydrogen in contact withRH_2?x at the temperature concerned. A change in slope was observed in the solubility curves of the Gd-, Er-, Tm- and Lu-H systems. The logC) (at. pct H inR) was plottedvs 1/T for each system. Straight lines were obtained at temperatures above and below the changes in slope of the solubility curves. A calculation of the approximate ΔH of solution ofRH_2?x in the metal sfrom the slope of the lines gave 4.35, 1.88, 1.28, 0.61 and 0.55 kcal/mole for Gd, Er, Tm, Lu and Y, respecitively in the low temperature portion. The change in slope which occurs at some point between 350°C and 650°C, depending on the metal, indicates a lower heat of solution ofRH_2?x in these metals at the higher temperatures. In Lu there appears to be yet another change in slope in the neighborhood of 250°C.     

In Situ ceramic particle-reinforced aluminum matrix composites fabricated by reaction pressing in the TiO2 (Ti)-Al-B (B2O3) systems

Particulate TiB₂ reinforced aluminum-based metal matrix composites (MMCs) were successfully fabricated by means of the reaction processing method. TiB₂ particulates were formed in situ through the reaction of Ti and B in Ti-Al-B, TiO₂ and B in TiO₂-Al-B, and TiO₂ and B₂O₃ in TiO₂-Al-B₂O₃ systems. The results showed that in situ TiB₂ particulates formed in the Ti-Al-B system had a size of 5 μm and they exhibited block and rodlike structures. Moreover, coarse Al₃Ti blocks several tens of micrometers in size were also formed simultaneously. On the other hand, equiaxed Al₂O₃ and TiB₂ particulates with a size of less than 2 μm were formed in situ in the TiO₂-Al-B and TiO₂-Al-B₂O₃ systems. The Al₃Ti phase was completely eliminated in the TiO₂-Al-B system with increasing B content. Tensile tests revealed that the Al₂O₃ · TiB₂/Al composite fabricated from the TiO₂-Al-B system exhibits excellent mechanical properties. The yield strength of the Al₂O₃ · TiB₂/Al composite appeared to increase with increasing TiB₂ content. The yield strength of the Al₂O₃ · TiB₂/Al composite could be further increased by introducing CuO into the TiO₂-Al-B system. Such an increment in mechanical strength arose from the strengthening effect caused by the Al₂Cu precipitates. The incorporation of CuO had no effect on the in situ reaction process of the TiO₂-Al-B system. Finally, the effect of SiC addition on the microstructure and mechanical properties of the composites fabricated from the TiO₂-Al-B and TiO₂-Al-B-CuO systems was also investigated.     

Antimony activities in copper mattes

A mass spectrometric technique combined with a double Knudsen cell was used to determine the antimony and copper activities in the Cu-Sb binary system at 1373 K and in the two-melt composition range of the Cu−S−Sb ternary system at 1423 K. The antimony and copper activities were calculated based on the intensity ration of the gaseous Sb and Cu species, over the unknown and known activity samples, respectively. γ _Sb ^o were found to be 1.1×10⁻² in molten copper at 1373 K, and 1.8×10⁻² and 0.44 in a copper-rich phase and in a matter phase, of the Cu−S−Sb ternary system at 1423 K, respectively. These values indicate, that antimony can be removed during the matte smelting and slagging stage of the copper smelting process. Interaction parameters of antimony in molten copper slagging stage of the copper smelting process. Interaction parameters of antimony in molten copper at 1423 K were calculated and found to be 10.7, −5.4, and 6.3 for ε _Sb ^Sb · ρ_Sb ^Sb, and ε _Sb ^S , respectively. M. HINO, formerly Visiting Scientist at the University of Toronto     

Crack defect formation during manufacture of fused cast alumina refractories

A sequentially coupled mathematical thermal-stress model, based on the commercial finite-element code ABAQUS, has been developed to rationalize crack defect formation in fused cast αβ-alumina refractories used in the glass industry. The thermal model was validated against thermocouple and pyrometer measurements obtained in an industrial setting. The temperature predictions obtained from the thermal model were employed as input to the elastic strain-rate-independent plastic stress model. The constitutive behavior of αβ-alumina has been determined over a range of temperatures for input to the stress model. The distribution of β-alumina that forms in the center of the casting due to rejection of Na₂O during solidifcation was introduced in the stress model through a user-defined subroutine in order to account for the effect of differences in the thermal contraction behavior and elastic modulus of the αβ- and β-alumina phases. The stress analysis indicates that temperature gradients as well as the different dilatational behavior of the αβ- and β-alumina phases are the main drivers of stress and strain evolution during solidification and subsequent cooling. The β-alumina core, in particular, plays an important role in the generation of tensile stresses and likely gives rise to the generation of the internal cracks observed in industrial castings.     

Cryoscopic studies in fluoride-oxide-silica systems: Part I. systems containing Li+, Na+ and K+

The depressions of the freezing temperatures of NaF and KF by additions of 3M₂O · 2SiO₂, M₂O -SiO₂, M₂O · 2SiO₂ and SiO₂ (where M = Li, Na and K) have been measured and the variations of the activities of NaF and KF with liquidus composition have been calculated. These activities are shown to be in good agreement with a proposed theoretical model of the anionic constitutions of these melts. In melts of M₂O/SiO₂ > 1 linear chain silicate ions and free F⁻ ions are postulated and in melts of M₂O/SiO₂ < 1 reaction between F⁻ and silicate ions to produce polyfluorosilicate anions is postulated. The effect of alkali cation type on the deviation of the liquidus activities from the proposed ideal behavior in melts of M₂O/SiO₂ > 1 and the degrees of polymerization of the fluorosilicate ions in melts of M₂O/SiO₂ < 1 are explained in terms of coulombic interactions and the effects of cation size on the polarization behavior. In the reciprocal systems MF-M₂ ^′O · SiO₂ and MF-M₂ ^′O · 2SiO₂ deviations from proposed ideality are correlated with the signs and magnitudes of the standard free energy changes for the exchange reactions. The conclusions as to the dependency of solution mechanism of fluorides in their alkali silicates on M₂O/ SiO₂ ratio are in agreement with conclusions drawn from studies of infrared absorption in these systems.     

The diffusivity of sulfur in wüstite and its solubility in wüstite and γ iron in equilibrium with each other and a liquid oxysulfide

The solubility of sulfur in wüstite in equilibrium with γ iron and liquid oxysulfide was found to be 0.011 wt pct at 1050°C. The sulfur solubility in γ iron in equilibrium with wüstite and liquid oxysulfide was also determined at 1050°, 1150°, and 1250°C and found to be 135, 165, and 160 ppm respectively. These values are considerably lower than the sulfur-solubility in y iron in the binary Fe-S system saturated with pyrrhotite. The diffusivity of sulfur in wüstite was determined by oxidizing Fe-S alloys in mixtures of CO and CO₂, and analyzing the entire sample for sulfur afterwards. From the amount of sulfur diffused through the growing wüstite layer into the gas phase, the diffusivity of sulfur in wüstite was evaluated, and found to be 4.1 × 10⁻⁸ and 9.6 × 10⁻⁷ cm²/s at 1050° and 1250°C respectively. These values are of the same order as the self-diffusivity of iron in wüstite in equilibrium with iron at the same temperatures.     

Effects of changes in strain path on work hardening in cubic metals

Effects of abrupt changes in strain path on work hardening in stretching 1200 aluminum, OFHC copper, 70-30 brass, a low-carbon ferritic steel, and 310 and 304 austenitic steels have been investigated. Tests were made with first-stage extension in uniaxial and equibiaxial tension. Second-stage stretching was in uniaxial tension. It is shown that all of the above materials have some susceptibility to transient reductions in work-hardening rate,dσ/dε, after changes in strain path. Some of the material and process variables which can influence the form and magnitude of the transients are identified. Within the group of materials tested, stacking fault energy (SFE) appears to be the most generally influential material variable. The magnitudes of prestrain required to give significant reductions indσ/dε·1/σ after strain path changes are higher in the low SFE alloys. Changes in strain path are accompanied by a reduction in the effective extension at the first onset of strain localization, when the reduction indσ/dε is sufficient to causedσ/dε·1/σ to fall below unity. Within the ranges of prestrain explored, the maximum reductions indσ/dε·1/σ were in the range of 0.5 to 1.0. Thus, none of the changes in strain path investigated caused a reduction in the limit of effective uniform elongation, until the prestrain was sufficient to reducedσ/dε·1/σ in monotonic deformation to less than a value in the range of 1.5 to 2.0. For these reasons, the possible reductions in effective uniform elongation were much more severe in the high SFE materials than those in the low SFE alloys. S. PLATIAS, formerly Postgraduate Student, School of Metallurgy and Materials, University of Birmingham P.S. BATE, formerly Research Fellow, School of Metallurgy and Materials, University of Birmingham     

Effects of oxygen atmosphere,FeOx and basicity on mineralogical phases of CaO–SiO2–MgO–Al2O3–FetO–P2O5 steelmaking slag

ABSTRACT

The mineralogical phase of slag after crystallisation is essential to utilisation of steelmaking slag. The mineralogical phases of cooled multicomponent CaO–SiO₂–MgO–Al₂O₃–Fe_tO–P₂O₅ slag with different iron oxide contents and basicities (defined as the ratio of mass percentage of CaO to mass percentage of SiO₂ (w(CaO)/w(SiO₂))) in different atmospheres were investigated in the present work by scanning electronic microscopy and energy dispersed spectroscopy analysis and X-ray diffraction. The mineralogical phases in steelmaking slag cooled in argon are mainly nCa₂SiO₄-Ca₃(PO₄)₂ (thereafter nC₂S-C₃P) solid solution, (Fe, Mn, Mg)O (RO) phase. Some CaMgSiO₄ phases could be found in slag with lower basicity. The mineralogical phases in steelmaking slag cooled in air are mainly nC₂S-C₃P solid solution, spinel phase. The overall crystallisation of slag cooled in both argon and air was enhanced with increasing basicity. However, the crystal sizes become smaller in sample with high basicity. The Fe-enriched phases were transformed from non-faceted RO phase in sample cooled in argon to faceted spinel phases in sample cooled in air. The crystallisation of slag cooled in both argon and air was promoted with increasing FeO_x content. The phosphorus content in solid solution was elevated with decreasing basicity and increasing FeO_x content. It was implied by the present work that appropriate basicity and air oxidation would be beneficial to magnetic separation and phosphorus utilisation.     

Microstructure and properties of In situ Al/TiB2 composite fabricated by in-melt reaction method

A novel in situ reaction process-in-melt reaction method was developed. TiB₂ particles form in situ through the reaction of TiO₂, H₃BO₃, and Na₃AlF₆ in an aluminum alloy melt. The results showed that the in situ TiB₂ particles formed were spherical in shape and had an average diameter of about 0.93 μm. Moreover, the distribution of TiB₂ particles in the matrix was uniform. The interface between the TiB₂ particles and the matrix showed good cohesion. The tensile strength and the yield strength of the composite increase with increasing TiB₂ content. When TiB₂ particle content in the matrix was 10 vol pct, the tensile strength, yield strength, and elongation of Al-4.5Cu/TiB₂ composite were 417 MPa, 317 MPa, and 3.3 pct, respectively.     

Microstructural development and creep deformation in equiaxed γ, γ+α 2, and γ+α 2+B2 titanium aluminides

The development of microstructure and its influence on creep properties have been studied for structures including equiaxed γ, duplex, and other structures of varying α ₂ morphology in two Ti-48Al-2Cr-2Nb alloys. Heat treatments at 1125 °C have been utilized to produce equiaxed γ microstructures in alloys with or without Mo additions. The γ → α transformation produces α ₂ plates with several orientation variants within γ grains during subsequent annealing of the equiaxed γ microstructures below the α transus. Formation of this α ₂ morphology results from rapid up-quenching (UQ), and this structure persists through annealing, cooling, and creep testing. Differences in minimum creep rates for several microstructures containing varying amounts of multi- or single variant γ/α ₂ grains are shown to be minimal. The presence of Mo has also resulted in improved creep resistance in equiaxed γ and γ+α ₂+B2 structures, as compared to similar microstructures in the Ti-48Al-2Cr-2Nb alloy. Deformation during creep at 760 °C at stresses between 200 and 400 MPa occurs by a combination of twinning and dislocation glide without recrystallization, resulting in power-law stress exponents in the range of 6 to 9. Only minimal strain path dependence of the minimum creep rate is detected in a comparison of creep rates in stress jump, stress drop, and single stress tests. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Microstructural development and creep deformation in equiaxed γ,γ + α2 and γ + α2 + B2 titanium aluminides

The development of microstructure and its influence on creep properties have been studied for structures including equiaxed γ, duplex, and other structures of varying α₂ morphology in two Ti-48Al-2Cr-2Nb alloys. Heat treatment at 1125°C have been utilized to produce equiaxed γ microstructures in alloys with or without Mo additions. The γ→α transformation produces α₂ plates with several orientation variants with γ grains during subsequent annealing of the equiaxed γ microstructures below the α transus. Formation of this α₂ morphology results from rapid up-quenching (UQ), and this structure persists through annealing, cooling, and creep testing. Differences in minimum creep rates for several microstructures, containing varying amounts of multi-or single variant γ/α₂ grains are shown to be minimal. The presence of Mo has also resulted in improved creep resistance in equiaxed γ and γ + α₂ + B2 structures, as compared to similar microstructures in the Ti-48Al-2Cr-2Nb alloy. Deformation during creep at 760 °C at stresses between 200 and 400 MPa occurs by a combination of twinning and dislocation glide without recrystallization, resulting in power-law stress exponents in the range of 6 to 9. Only minimal strain path dependence of the minimum creep rate is detected in a comparison of creep rates in stress jump, stress drop, and single stress tests. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlandom, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Kinetics of dissolution of cobalt oxides in acidic media

The kinetics of dissolution of cobalt oxides Co₂O₃ and Co₃O₄ in aqueous solutions of acids (H₂SO₄, EDTA) is experimentally studied. Dissolution rate W increases with the temperature or the EDTA concentration. The reaction orders of dissolution for hydrogen ions in sulfuric acid and EDTA (dlogW/dpH = 0.5 ± 0.1) and for anions (dlogW/dlog[An ⁻] = 0.5 ± 0.1) are determined. A specific feature of the dissolution kinetics in EDTA is a maximum in the dissolution rate of the cobalt oxides at pH −1. The activation energy of the process E _a is 70 kJ/mol in H₂SO₄ and 60 kJ/mol in EDTA. The modeling of the process shows that the CoOH⁺ ion is a surface particle controlling the dissolution rate in mineral acids and the CoHY⁻ ion, in the complexone.     

Environmental effect on room- temperature ductility of isothermally forged tial- base alloys

Isothermally forged TiAl-base alloys (Al-rich, Mn-containing, and Cr-containing TiAl) were heat-treated in various conditions, and equiaxed grain structures consisting of γ and α₂ or Β phases were obtained. The heat-treated alloys were tensile tested in vacuum and air at room temperature, and the environmental effect on tensile elongation was studied. The ductility of the alloys consisting of equiaxed γ grains and a large amount of α₂ grains was not largely affected by laboratory air, and a decrease in the amount of α₂ grains resulted in a large reduction of ductility in air. The Β phase in the Cr-containing alloy improved the ductility in vacuum, but it resulted in a large reduction of ductility in air. Formerly with Kougakuin University, Shinjyuku-ku, Tokyo, Formerly with National Research Institute for Metals, Meguro-ku, Tokyo,     

Characterization and tribological behavior of cast In-Situ Al (Mg,Mo)-Al2O3 (MoO3) composite

Aluminum alloy—based cast in-situ composite has been synthesized by dispersion of externally added molybdenum trioxide particles (MoO₃) in molten aluminum at the processing temperature of 850 °C. During processing, the displacement reaction between molten aluminum and MoO₃ particles results in formation of alumina particles in situ and also releases molybdenum into molten aluminum. A part of this molybdenum forms solid solution with aluminum and the remaining part reacts with aluminum to form intermetallic phase Mo(Al_1−x Fe _x )₁₂ of different morphologies. Magnesium (Mg) is added to the melt in order to help wetting of alumina particles generated in situ, by oxidation of molten aluminum by molybdenum trioxide, and helps to retain these particles inside the melt. The mechanical properties of the cast in-situ composite, as indicated by ultimate tensile stress, yield stress, percentage elongation, and hardness, are relatively higher than those observed either in cast commercial aluminum or in cast Al-Mo alloy. The wear and friction of the resulting cast in-situ Al(Mg,Mo)-Al₂O₃(MoO₃) composites have been investigated using a pin-on-disc wear testing machine under dry sliding conditions at different normal loads of 9.8N, 14.7N, 19.6N, 24.5N, 29.4N, 34.3N, and 39.2 N and a constant sliding speed of 1.05 m/s. The results of the current investigation indicate that the cumulative volume loss and wear rate of cast in-situ composites are significantly lower than those observed either in cast commercial aluminum or in cast Al-Mo alloy, under similar load and sliding conditions. Beyond about 30 to 35 N loads, there appears to be a higher rate of increase in the wear rate in the cast in-situ composite as well as in cast commercial aluminum and cast Al-Mo alloy. For a given normal load, the coefficient of friction of cast in-situ composite is significantly lower than those observed either in cast commercial aluminum or in cast Al-Mo alloy. The coefficient of friction of cast in-situ composite increases gradually with increasing normal load while those observed in cast commercial aluminum or in cast Al-Mo alloy remain more or less the same. Beyond a critical normal load of about 30 to 35 N, the coefficient of friction decreases with increasing normal load in all the three materials.     

Thermodynamic Basis for Glass Formation in Cu-Zr Rich Ternary Systems and Their Synthesis by Mechanical Alloying

Topological factors such as mismatch entropy and configurational entropy, along with thermodynamic entity such as enthalpy of chemical mixing, are found to control glass formation in metallic systems. Taking both these factors into consideration, a parameter called P _HS was proposed to correlate glass forming ability successfully in the Cu-Zr-Ti system. The parameter P _HS (=?H _chem × ?S _σ /k _B ) is a product of enthalpy of chemical mixing and mismatch entropy. Our study indicates that the more negative is the P_HS value within the configurational entropy (?S _config/R) range of 0.9 to 1.0, the higher is the stability of glassy phase resulting in a larger diameter of bulk metallic glass rods. Observed theoretical predictions are supported by experimental results in which the compositions with high negative P _HS resulted in easy amorphous phase formation in comparison with less negative P _HS compositions by mechanical alloying. This criterion was extended to Cu-Zr-Al and Cu-Zr-Ag systems as well, thus establishing a strong correlation between P _HS and the glass forming ability of alloys. The role of size effect, probability of atomic arrangements, and heat of formation among constituent elements in obtaining a larger dimension bulk metallic glasses was addressed in this study.