SiO(g) formation from SiC in mixed oxidizing-reducing gases

The formation of SiO(g) from SiC by either active oxidation or an oxidation-reduction process is discussed. The Wagner criterion for the transition from active to passive oxidation is generalized for any oxidant. Kinetic modeling of both active oxidation and oxidation-reduction is described.Symbols - - MW _SiC/MW _Si - _CO CO(g) boundary layer thickness - _ox gaseous-oxidant, boundary layer thickness - stoichiometric factor from Eqs. (2)–(4), # of CO produced/# of oxidant (on oxygen atom basis) - gas viscosity - concentration of diffusing gas species in boundary layer - concentration of major gas species in boundary layer - _oxide density of SiO₂ - D diffusion coefficient of diffusing species in gas-boundary layer - D _CO diffusion coefficient of CO(g) - D _ox gas diffusion coefficient of oxidant - J flux, rate of weight loss limited by diffusion in gas-boundary layer - J _CO flux of CO(g) - J _ox flux of gaseous oxidant - K _g linear oxide growth constant, weight/(length² time) - k _g linear oxide growth constant, length/time - k _l linear volatilization constant for SiO₂,k _lo+k _ls, weight/(length² time) - k _lo linear volatilization constant for oxygen from SiO₂, weight/(length² time) - k _ls linear volatilization constant for silicon from SiO₂, weight/(length² time) - k _l linear volatilization constant for SiO₂, length/time - k _p parabolic oxide growth constant, weight²/(length⁴ time) - k _p parabolic oxide growth constant, length²/time - L sample length parallel to gas flow direction - (M/A)₁ specific weight change due to oxygen gain and associated carbon loss in paralinear oxidation - (M/A)_1L limiting value of weight change due to oxygen gain and associated carbon loss in paralinear oxidation - (M/A)₂ specific weight change due to silicon loss and associated carbon loss in paralinear oxidation - MW _C molecular weight of carbon - molecular weight of O₂ - MW _Si molecular weight of silicon - MW _SiC molecular weight of silicon carbide - molecular weight of silica - n number of oxygen atoms per oxidant molecule - P _CO ^eq eqiilibrium CO(g) pressure - P _CO ^g CO(g) pressure outside of boundary layer - P _CO ⁱ CO(g) pressure at SiC-gas interface - P _ox ^g oxidant gas pressure outside of boundary layer - P _ox ⁱ oxidant gas pressure at SiC-gas interface - R gas constant - t time - T absolute temperature - v linear gas velocity - x oxide thickness - x _L limiting oxide thickness achieved in paralinear oxidation - x _t oxide thickness at which transition from linear to parabolic growth occurs     

On the nature of “easy paths” for the diffusion of oxygen in thermal oxide films on aluminum

Thermal oxide films grown on electropolished aluminum specimens have been investigated by transmission electron microscopy of stripped oxide films and ultramicrotomed sections. Particular attention has been focused on the nucleation sites -Al ₂ O ₃ crystals and the relationship of such sites to surface features on the electropolished aluminum surface. It is evident that easy paths for the diffusion of oxygen, or the nucleation sites of -Al ₂ O ₃ crystals, are not distributed randomly over the electropolished aluminum surface, but form preferentially in the amorphous oxide layer grown over preexisting metal ridges. Thus, the diffusion of molecular oxygen through cracks in the amorphous oxide layer represents the most realistic and acceptable basis for explaining the local growth of the -Al ₂ O ₃ crystals in thermal oxide films on aluminum, although the cracks have not yet been observed directly.Present address: Alcan International, Ltd., Banbury Laboratories, Banbury, Oxford, OX16 7SP, United Kingdom.     

On the oxidation ofα-Fe andε-Fe2N1−z : I. Oxidation kinetics and microstructural evolution of the oxide and nitride layers

The oxidation of -Fe and -Fe₂N_1–z at 573 K and 673 K in O₂ at 1 atm was investigated by thermogravimetrical analysis, X-ray diffraction, light-optical microscopy, scanning electron microscopy and electron probe X-ray microanalysis. Upon oxidation at 573 K and 673 K, on -Fe initially -Fe₂O₃ develops, whereas on -Fe₂N_1–z initially Fe₃O₄ develops. In an early stage of oxidation the oxidation rate of -Fe₂N_1–z appears to be much larger than of -Fe. This can be attributed largely to an effective surface area available for oxygen uptake, which is much larger for -Fe₂N_1–z than for -Fe due to the porous structure of -Fe₂N_1–z as prepared by gaseous nitriding of iron. The development of a magnetite layer in-between the hematite layer and the -Fe substrate, at a later stage of oxidation, enhances layer-growth kinetics. After 100 min oxidation at 673 K the (parabolic) oxidation rates for -Fe and -Fe₂N_1–z become about equal, indicating that on both substrates the oxide growth is controlled by the same rate limiting step which is attributed to short-circuit diffusion of iron cations. Oxidizing -Fe₂N_1–z increases the nitrogen concentration in the remaining -iron nitride, because the outward flux of iron cations, necessary for oxide growth, leads to an accumulation of nitrogen atoms left behind.     

General laws governing variation in properties of cast alloys of the Al-Zn-Mg system

Among existing high-strength corrosion-resistant aluminum alloys, those of the Al-Mn-Zn system are most promising. They have a different phase composition, depending on the content of magnesium and Zinc: + (Al₃Mg₂), + + T(Al₂Mg₃Zn₃), + T, + T + (MgZn2) and + The majority of industrial Al-Mg-Zn alloys correspond to the phase regions + T and + T + with respect to composition. A high level of strength and satisfactory overall corrosion resistance are characteristic for these alloys. Al-Mg-Zn alloys may, however, tend to the most dangerous form of corrosion - stress-induced corrosion cracking. Using methods of experiment planning in the study, we investigated Al-Zn-Mg alloys of various compositions for the purpose of selecting alloy compositions with a high level of mechanical properties and stress-induced corrosion cracking.All-Union Scientific-Research Institute of Aviation Materials. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 24–28, December, 1994.     

Diffusional transport during the cyclic oxidation of γ+β, Ni-Cr-Al(Y,Zr) alloys

The cyclic oxidation behavior of several cast +, Ni-Cr-Al(Y, Zr) alloys and one LPPS +, Ni-Co-Cr-Al(Y) alloy was examined (, fcc; , NiAl structure). Cyclic oxidation was performed by cycling between 1200°C and approximately 70°C. Oxide morphologies and microstructural changes during cyclic oxidation were noted. Recession of the high-Al phase was nonparabolic with time. Kirkendall porosity resulting from diffusional transport within the alloy was observed in the near-surface -phase layer of one alloy. Concentration profiles for Ni, Cr, and Al were measured in the -phase layer after various cyclic oxidation exposures. It was observed that cyclic oxidation results in a decreasing Al concentration at the oxide-metal interface due to a high demand for Al (a high rate of Al consumption) associated with oxide scale cracking and spalling. In addition, diffusion paths plotted on the ternary phase diagram shifted to higher Ni concentrations with increasing cyclic oxidation exposures. The alloy with the highest rate of Al consumption, and highest Al content, underwent breakaway oxidation after 500 1-hr cycles at 1200°C. Breakaway oxidation occurred when the Al concentration at the oxide-metal interface approached zero. The relationship between the Al transport in the alloy and breakaway oxidation is discussed.     

Microstructural Study of Oxidized γ-TiAl

The microstructural development of oxidized-TiAl is presented with a focus on oxidation inair. The investigations were carried out usingconventional, analytical, and, especially,energy-filtered transmission electron microscopy (EFTEM). Threeimportant points were studied in detail: (1) thenitrogen effect, (2) thesurface-finish effect, and (3) thesubsurface zone. Nitrogen leads to the formation of TiN andTi₂AlN at the metal-scale interfaceinterrupting alumina and thereby preventing thedevelopment of a continuous alumina layer. TheAl-depletion layer formed during the oxidation process develops from a single-phaselayer, consisting of a cubic phase, to a two-phaselayer, consisting of the cubic phase and₂-Ti₃Al. The cubic phase isnot known in the system Ti-Al-O-N. Oxidation in oxygen depends on the surfacepreparation of the sample with rapid oxidation kineticsfor fine polishing and slow kinetics for a 600-gritSiC-paper finish. The rougher surface finish leads to the development of a recrystallization zonenear the surface and supports the formation of acontinuous alumina layer in the early stages ofoxidation. As for the oxidation in air, the cubic phaseis formed first underneath the oxide scale,followed by ₂-Ti₃Alformation.     

Effect of silicon surface-cleaning procedures on the growth of Cu3Si on Si(100)-oriented wafers

The effect of a SiO ₂ layer, partially removed by wet chemical processes or thermally grown, on the mechanism of nucleation and growth of Cu ₃ Si from the reaction between a Si(100) wafer and gaseous copper chloride was studied. For oxide layer thicknesses less than 2 nm, the number of Cu ₃ Si nuclei and the area of silicon reacting with CuCl per unit area are inversely proportional to thickness, whereas the size of the more numerous nuclei increases from 4–10 m. In this case, the nuclei are octahedrally shaped. The superficial erosion of Cu ₃ Si nuclei and the formation of pits have been explained by the reaction between Cu ₃ Si and CuCl. For thicker layers, 10e40nm, the Cu ₃ Si nuclei are deformed, and the reaction is strongly inhibited. A SiO ₂ layer of 45 nm prevented active-site formation.     

Dependence of the Protective Concentration of a Mixture of Inhibitors from Several Reaction Series on the Polarities of Substituents in Their Molecules

Protective concentrations of mixtures of inhibitors from several reaction series (RSs), which decrease the corrosion rates a given number of times, are exponentially dependent on the sum of partial polarities of substituents in compounds belonging to the chosen RSs, provided that the total concentration of compounds for each RS is constant. The dependence is linearized in semilogarithmic log C _prvs – m coordinates.     

Reversible shape memory effect in alloys of the Mn - Cu system

The reversible shape memory effect is understood as the capacity of specimens deformed in a martensitic state to change shape (dimensions) both in heating in the process of a reverse transformation and in subsequent cooling as a result of a forward transformation. This is clearly manifested in manganese-rich Mn -Cu alloys susceptible to a diffusionless anti ferromagnetic (martensitic) transformation, as a result of which the crystal structure changes from face-centered cubic to face-centered tetragonal (f.c.c. f.c.t.).Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 34–37, September, 1996.     

X-ray topographic study of β-NiAl upon growth of an α-Al2O3 film

The Berg-Barrett X-ray topographic method was employed as a microstructural technique to seek correlations of the metal substructure to the morphological features of -Al₂O₃ films grown on -NiAl. An analysis of diffraction micrographs using {112} and {002} reflections from individual grains in -NiAl revealed its subgrain structure to a depth of 30 . The dimensions of these subgrains were directly related to the density of oxide ridges in the -Al₂O₃ films and to the dimensions and shapes of cavities at the NiAl-Al₂O₃ interface.     

Effect of Plastic Deformation on the Structure and Properties of High-Nitrogen Alloys of the Fe – Cr System

The structure, strength, and ductility of alloys of the Fe – Cr – N system bearing from 15 to 24% chromium and from 0.4 to 1.3% nitrogen are studied after rolling at 20, 450, 800, 900, 1000, 1100, and 1200°C. It is shown that the highest hardening due to cold plastic deformation is provided in alloys with metastable -solid solution that undergoes transformation into strain martensite ( ). In hot plastic deformation the alloys are hardened due to the formation of a fragmented structure. The data of electron microscopic studies are used to determine the role of the phase composition in the hardening of the alloys.     

Structural conversions in VT22 titanium alloy during aging

Conclusion For two-phase high-strength alloys of the VT22 type, as for -alloys of titanium, the presence of an incubation period for the formation of -phase during aging is characteristic. The signs for this process are the following: enchanced etchability of the surface of thin sections; formation of zones which do not have an interface with the matrix but differ from it in etchability (zones of presegregation of -phase); a change in the intensities of the -phase lines on the x-ray diffraction patterns. Decomposition of the -solid solution in titanium alloys of the VT22 type is a multistep process, including formation of segregates (onset of stratification of the -solid solution), regions of the Guinier—Preston zone type, various intermediate states, coherent states of the formed -phase, and finally the appearance of isolated -phase, having an interface with the matrix -solid solution.Pskov Filiate of St. Petersburg State Technical University. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 33–37, August, 1992.     

On the oxidation ofα-Fe andε-Fe2 N1−z II. Residual strains and blisters in the oxide layer

The development of residual strains in the iron-oxide layers growing on -Fe and -Fe₂N_1–z at 673 K in O₂ at 1 atm was investigated by X-ray diffraction at room temperature. After correction for thermal-strain development due to cooling after oxidation, it was found that tensile growth strains occur in magnetite and compressive growth strains occur in hematite. The growth strains in the oxides on -Fe are (in absolute sense) 2–3 times as large as those in the oxides on -Fe₂N_1–z. Buckling of the oxide layer occurs in the case of an -Fe substrate, which is attributed mainly to relaxation of the growth strains in magnetite and hematite. Thermal-strain development during cooling enhances the tendency for buckling. Buckling is not observed for oxide layers on -Fe₂N_1–z, which could be due to the smaller values of absolute strain in the oxide layer on -nitride. The absolute values of the growth strains in the oxide layer on -nitride being smaller is attributed to microstructural changes in the nitride layer during oxidation.     

Effect of prolonged heating on the thermal stability of phases inα +β titanium alloys

Conclusions With prolonged low-temperature heating, alloys VT3-1 and VT9 tend to the stable condition corresponding to the heating temperature. This is accompanied by a change in the quantity of phase and the concentration of -stabilizing elements in phase.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 42–46, May. 1977.     

Influence of original structure and heating rate on the properties of steel after hardening from the intercritical range

Conclusions The original condition and heating rate determine the mechanical properties of the steel after hardening from the intercritical range. The best properties, particularly ductility, _t=1600 MPa, _0.2=1250 MPa, =14%, anda _n=0.9 MJ/m², are obtained after preliminary hardening from 930°C, tempering at 200°C, a second hardening from 800°C (5% ferrite), and tempering at 200°C. Full hardening from 930°C with subsequent tempering at 200°C (without preliminary hardening) makes it possible to obtain _t=1550 MPa, _0.2=1200 MPa, =9%, anda _n=0.9 MJ/m².Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 52–56, May, 1981.     

Structure formation in nitrided layers of titanium alloys

Conclusions Nitriding of -, pseudo-- and +-Ti alloys forms diffusion layers consisting of a nitride layer and a gas-saturated layer. In +-Ti alloys, a third additional transient zone forms. During the nitriding process, redistribution of the alloying elements present in the alloys takes place, which is caused by their differing affinities for nitrogen.Nitriding schedules also have an effect on the alloying element redistribution. Nitriding temperature or time increase causes either thickening of the diffusion layer zones or formation of a qualitatively new structure in them.Lvov Physicomechanical Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 11–14, June, 1986.     

TEM Study of Scale Microstructures Formed on Ni–10Cr and Ni–10Cr–5Al Alloys with and Without Y Addition

A study was conducted to investigate the effect of Y addition on the isothermal-oxidation behaviors of Ni–10Cr, Ni–10Cr–0.5Y, Ni–10Cr–5Al, and Ni–10Cr–5Al–0.5Y alloys. The alloys were oxidized in air for 50 hr at 1000°C. The oxides formed on the alloys were characterized using primarily cross-sectional transmission-electron microscopy techniques along with light microscopy, scanning-electron microscopy, and X-ray diffraction. Although the Al-containing alloys showed comparatively better oxidation behavior, all alloys exhibited nonprotective scaling, as suggested by the thick oxides formed. The major component of the outer oxide was NiO. However, modified Y-containing alloys formed protective layers (i.e., -Cr₂O₃ for NiCrY and -Al₂O₃ for NiCrAlY) at the scale–alloy interface following the nonprotective scaling. The spalling resistance of the modified Y-containing alloys was better than their counterpart unmodified Y-free alloys, while their overall oxidation mechanism remained unchanged after Y addition.     

Oxidation ofγ′-Ni3Al andγ′-Ni3Al(Si) intermetallic compounds at low-oxygen pressures

The oxidation behavior of -Ni₃Al and -Ni₃Al(Si) (Ni₇₅Al₂₀Si₅) intermetallic compounds was studied at 1073 K and oxygen partial pressures of 59×10^–6 atm, 1.2×10^–14 atm, and 1.2×10^–19 atm by means of a manometric apparatus, and Rhines packs of NiO/Ni and FeO/Fe, respectively. Oxidation kinetics were determined either by recording weight gains or by measuring the internal-oxidation-zone depths. The structures and morphologies of oxides were also studied. Relatively low oxidation rates occurred for both compounds when oxidized in the manometric apparatus, while fast internal oxidation was observed for both compounds in the NiO/Ni pack. The fastest oxidation occurred in the -Ni₃Al compound in the FeO/Fe pack. A healing Al₂O₃ layer was formed on the surface of -Ni₃Al(Si) compound in the FeO/Fe pack, indicating a synergistic effect between the solute elements in the compound at the oxygen pressures corresponding to the dissociation of FeO. The oxidation rate was found to depend on the volume expansion associated with solute-atom oxidation.     

Resistance of 1201 aluminum alloy to plastic deformation at various strain rates

Conclusion With an increase in the rate of plastic strain there is a change in the resistance of 1201 alloy to plastic deformation, which is revealed in a nontraditional decrease in _0.2 and increase in , and A_p. The characteristics _k and _t increase nonlinearally.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 56–61, August, 1989.     

On the Similarity Criteria for the Processes of High-Temperature Sulfur Corrosion of Heat-Resistant Alloys

A similarity parameter for the high-temperature sulfur corrosion of heat-resistant alloys is suggested. This parameter allows to put in order published experimental data of different authors. Original data on the weight loss by 826, -70, 26, and several materials for protective coatings also correlate with the similarity parameter. The dependences thus obtained allow to predict the operation life of materials (by their weight loss) and to rank nickel-based alloys by their corrosion resistance, depending on their elemental composition.