Phase relations and thermodynamics of the system Fe-Cr-0 in the temperature range of 1600 °C to 1825 °C (1873 to 2098 K) under strongly reducing conditions

Equilibrium relations involving alloy and oxide phases in the system Fe-Cr-O were determined in the temperature range from 1600 °C to 1825 °C (1873 to 2087 K). Compositions of coexisting alloy and spinel phases were established as a function of oxygen pressure by equilibrating liquid Fe-Cr alloys with iron chromite (Fe_3-xCr_xO₄) solid solutions at 1600 °C and 1700 °C. Combinations of these experimental data and thermodynamic calculations were used to construct composition-oxygen pressure diagrams for the system at 1600 °C and 1700 °C. Additional runs for selected mixtures were made at still higher temperatures (1700 °C to 1825 °C), and thermodynamic parameters were derived for spinel-containing phase assemblages at temperatures up to 1865 °C. The spinel phases occurring in the present system are typically in the high-chromium range of the solid-solution series Fe₃O₄-Cr₃O₄,i.e., in the range between stoichiometric iron chromite (FeCr₂O₄) and Cr₃O₄. The activities of the various oxide components of the spinel solid solution at 1600 °C were calculated from experimentally determined parameters for coexisting alloy and spinel phases, as well as by statistical-mechanical modeling of the same spinel solid solution based on crystal-chemical considerations. The agreement between the two sets of results was excellent. Temperature variation of parameters characterizing the univariant equilibria spinel + Cr₂O₃ + alloy and spinel + alloy + liquid oxide was established. The univariant curves were found to display temperature maxima of 1715 °C ± 5 °C and approximately 1865 °C, respectively. In analogy with relations in the Cr-O system, the increase in divalent chromium of the liquid oxide phase with decreasing oxygen potential was identified as the main cause of the sharp decrease in liquidus temperatures of chromites in contact with Fe-Cr alloys of high Cr contents. Formerly Graduate Research Assistant, Department of Metallurgy, The Pennsylvania State University L.S. DARKEN and ARNULF MUAN, formerly Professors of Geochemistry and Materials Science, The Pennsylvania State University, University Park, PA 16802, are deceased.     

Measurement and modeling of Alloy-Spinel-Corundum equilibrium in the Ni-Mn-Al-0 system at 1873 K

The oxygen content of liquid Ni-Mn alloy equilibrated with spinel solid solution, (Ni,Mn)O. (1 +x)A1₂O₃, and α-Al₂O₃ has been measured by suction sampling and inert gas fusion analysis. The corresponding oxygen potential of the three-phase system has been determined with a solid state cell incorporating (Y₂O₃)ThO₂ as the solid electrolyte and Cr + Cr₂O₃ as the reference electrode. The equilibrium composition of the spinel phase formed at the interface of the alloy and alumina crucible was obtained using EPMA. The experimental data are compared with a thermodynamic model based on the free energies of formation of end-member spinels, free energy of solution of oxygen in liquid nickel, interaction parameters, and the activities in liquid Ni-Mn alloy and spinel solid solution. Mixing properties of the spinel solid solution are derived from a cation distribution model. The computational results agree with the experimental data on oxygen concentration, potential, and composition of the spinel phase.     

Measurement and modeling of Alloy-Spinel-Corundum equilibrium in the Ni-Mn-Al-0 system at 1873 K

The oxygen content of liquid Ni-Mn alloy equilibrated with spinel solid solution, (Ni,Mn)O. (1 +x)A1₂O₃, and α-Al₂O₃ has been measured by suction sampling and inert gas fusion analysis. The corresponding oxygen potential of the three-phase system has been determined with a solid state cell incorporating (Y₂O₃)ThO₂ as the solid electrolyte and Cr + Cr₂O₃ as the reference electrode. The equilibrium composition of the spinel phase formed at the interface of the alloy and alumina crucible was obtained using EPMA. The experimental data are compared with a thermodynamic model based on the free energies of formation of end-member spinels, free energy of solution of oxygen in liquid nickel, interaction parameters, and the activities in liquid Ni-Mn alloy and spinel solid solution. Mixing properties of the spinel solid solution are derived from a cation distribution model. The computational results agree with the experimental data on oxygen concentration, potential, and composition of the spinel phase.     

The metal saturation line and tie-lines in the nickel-cobalt-sulfur ternary system between 1273 and 1573 K

The metal saturation line and the tie-lines in the Ni-Co-S ternary system have been determined between 1273 and 1573 K. The experiments were conducted by equilibrating the liquid sulfide with the metallic alloy phase. The liquid sulfide phase was sampled and chemically analyzed. The alloy was analyzed by electron microprobe. Combining the present results with the available literature data, the thermodynamic properties of this system were calculated.     

High-Temperature Compression Behavior of Cast and Homogenized IN939 Superalloy

Hot deformation behavior of IN-939 superalloy was investigated in this work. Hot compression experiments were performed at temperatures of 1273 K, 1323 K, 1373 K, and 1423 K (1000 °C, 1050 °C, 1100 °C, and 1150 °C) at strain rates of 0.001, 0.01, 0.1, and 1 s^?1 up to a true strain of 0.8. Then variations in stress-strain curves as well as changes in microstructures of various hot-deformed samples were studied. At 1273 K to 1323 K (1000 °C to 1050 °C), dynamic recovery (DRV), and at 1373 K to 1423 K (1100 °C to 1150 °C), dynamic recrystallization (DRX), were recognized to be the main mechanisms of the alloy softening during hot compression tests. The relationships between flow stress, strain rate, and temperature were mathematically modeled with three well-known equations, and on the basis of those equations, the activation energy of hot deformation was calculated. For improvement of the proposed models, it was necessary to conduct the investigation at two temperature ranges: 1373 K to 1423 K (1100 °C to 1150 °C), in which DRX occurred, and 1273 K to 1323 K (1000 °C to 1050 °C), where DRV as well as γ′ precipitation happened. For each of the temperature ranges, a different value for activation energy was obtained, which in conjunction with the related model, can be used for simulating the deformation behavior of the alloy.     

Flame Pyrolysis Spraying of Alumina-Yttria Ceramic Coating

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Phase relations and gibbs energies in the system Mn-Rh-O

Phase relations in the system Mn-Rh-O are established at 1273 K by equilibrating different compositions either in evacuated quartz ampules or in pure oxygen at a pressure of 1.01 × 10⁵ Pa. The quenched samples are examined by optical microscopy, X-ray diffraction, and energy-dispersive X-ray analysis (EDAX). The alloys and intermetallics in the binary Mn-Rh system are found to be in equilibrium with MnO. There is only one ternary compound, MnRh₂O₄, with normal spinel structure in the system. The compound Mn₃O₄ has a tetragonal structure at 1273 K. A solid solution is formed between MnRh₂O₄ and Mn₃O₄. The solid solution has the cubic structure over a large range of composition and coexists with metallic rhodium. The partial pressure of oxygen corresponding to this two-phase equilibrium is measured as a function of the composition of the spinel solid solution and temperature. A new solid-state cell, with three separate electrode compartments, is designed to measure accurately the chemical potential of oxygen in the two-phase mixture, Rh + Mn_3−2xRh_2xO₄, which has 1 degree of freedom at constant temperature. From the electromotive force (emf), thermodynamic mixing properties of the Mn₃O₄-MnRh₂O₄ solid solution and Gibbs energy of formation of MnRh₂O₄ are deduced. The activities exhibit negative deviations from Raoult’s law for most of the composition range, except near Mn₃O₄, where a two-phase region exists. In the cubic phase, the entropy of mixing of the two Rh³⁺ and Mn³⁺ ions on the octahedral site of the spinel is ideal, and the enthalpy of mixing is positive and symmetric with respect to composition. For the formation of the spinel (sp) from component oxides with rock salt (rs) and orthorhombic (orth) structures according to the reaction, MnO (rs) + Rh₂O₃ (orth) → MnRh₂O₄ (sp),ΔG° = -49,680 + 1.56T (±500)J mol⁻¹ The oxygen potentials corresponding to MnO + Mn₃O₄ and Rh + Rh₂O₃ equilibria are also obtained from potentiometric measurements on galvanic cells incorporating yttria-stabilized zirconia as the solid electrolyte. From these results, an oxygen potential diagram for the ternary system is developed.     

Feasibility study of the new rutile extraction process from natural ilmenite ore based on the oxidation reaction

Phase relations in the Fe₂O₃-FeTiO₃-TiO₂ system were investigated by equilibrating synthetic samples in evacuated sealed quartz tubes at a temperature of 1373 K. The equilibrium partial pressure of oxygen was measured by the electromotive force (EMF) method in the temperature range of 1273 to 1373 K. The phase diagram and oxygen partial pressure diagram in the titanium-iron-oxygen ternary system were then constructed at 1373 K. Rutile extraction from natural ilmenite ore was discussed from the thermodynamic viewpoint. It is found that rutile can be produced from common natural ilmenite ores not only by the reduction as the conventional titanium-rich slag process but also by an oxidation. Then, the oxidation experiment was conducted in air using Australian ilmenite ore to obtain rutile as one of the coexistent phases. Magnetic separation and leaching experiments for synthesized pseudobrookite and reagent rutile were conducted to confirm the possibility of separation of rutile from pseudobrookite. A new rutile extraction process was then proposed.     

The thermodynamics of spinel interphase formation at diffusion-bonded Ni/Al2O3 interfaces

Interface microstructural development during solid state diffusion bonding of Ni to single-crystal α-Al₂O₃ has been studied by electron microscopy. Nickel aluminate spinel (NiAl₂O₄) interphase layers ∼ 1 μm thick formed under high vacuum bonding conditions. Very high vacuum (VHV) annealing caused the spinel to disappear, indicating that its stability depends critically upon the oxygen activity. High vacuum diffusion bonding utilizing initially oxygen-free Ni and oxygen-containing Ni established that spinel formation requires a threshold oxygen activity, and furthermore, that the source of the required oxygen can be oxygen initially dissolved in the Ni. Thermodynamic calculations confirm that the threshold oxygen level necessary to stabilize the spinel increases from 0.006 at.% (60 at.ppm) at 1273 K to 0.025 at.% (250 at. ppm) at 1663 K. Further analysis indicates the spinel exhibits a maximum thickness determined by the difference between the initial and threshold oxygen concentrations and the Ni thickness. Considering the solubility limit of oxygen in solid Ni, the spinel thickness is limited to ∼0.005 times the Ni thickness. The reaction is explored further in the context of diffusion path concepts with a calculated NiAlO phase diagram.     

High-Temperature Oxidation Resistance of a Nanoceria Spray-Coated 316L Stainless Steel Under Short-Term Air Exposure

Nanoceria coatings using a spray method were implemented on a 316L stainless steel (SS). Coated and uncoated coupons were exposed to dry air at 1073 K to 1273 K (800 °C to 1000 °C) for short time periods (up to 24 hours) and in situ measurements of oxidation were carried out using a highly sensitive thermogravimetric balance. From the experimental outcome, activation energies were determined in both, coated and uncoated 316 SS coupons. The estimated exhibited activation energies for oxidation in the coated and uncoated conditions were 174 and 356 kJ/mol, respectively. In addition, the developed scales were significantly different. In the coated steel, the dominant oxide was an oxide spinel (Fe, Mn)₃O₄ and the presence of Fe₂O₃ was sharply reduced, particularly at 1273 K (1000 °C). In contrast, no spinel was found in the uncoated 316L SS, and Fe₂O₃ was always present in the scale at all the investigated oxidation temperatures. The coated steels developed a highly adherent fine-grained scale structure. Apparently, the nanoceria particles enhanced nucleation of the newly formed scale while restricting coarsening. Coarse grain structures were found in the uncoated steels with scale growth occurring at grain ledges. Moreover, the oxidation rates for the coated 316L SS were at least an order of magnitude lower than those exhibited by the steel in the uncoated condition. The reduction in oxidation rates is attributed to a shift in the oxidation mechanism from outward cation diffusion to inward oxygen diffusion.     

Effect of Yttrium Alloying on Intermediate to High-Temperature Oxidation Behavior of Mo-Si-B Alloys

The oxidation behavior of 0.2 Y-alloyed Mo-9Si-8B (at. pct) was investigated in a wide temperature range from 923 K to 1673 K (650 °C to 1400 °C). Formation of a thin yttrium-silicate scale at the outer layer along with the thick silica-rich inner layer containing Y-rich oxide inclusions was detected beyond 1573 K (1300 °C). A substantial improvement in the oxidation resistance of the alloy could be realized at 1073 K to 1273 K (800 °C to 1000 °C) with the addition of yttrium. The formation of a viscous silica-rich protective scale could prevent the permeation of MoO₃ at the initial stages of oxidation at this temperature regime. The growth of the internal oxidation zone followed a parabolic rate at 1273 K to 1673 K (1000 °C to 1400 °C), and the activation energy values calculated for both the outer oxide scale and internal oxidation zone formation indicated the inward diffusion of oxygen as the dominant rate controlling mechanism. The microstructural and kinetic data obtained for internal and external oxidation indicate that yttrium-silicate scale reduces the inward diffusion of oxygen, thereby improving the oxidation resistance of the alloy at high temperatures in any oxidizing environment.     

Oxidation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Scale-Forming MAX Phases in Turbine Environments

High temperature oxidation of alumina-forming MAX phases, Ti₂AlC and Cr₂AlC, were examined under turbine engine environments and coating configurations. Thermogravimetric furnace tests of Ti₂AlC showed a rapid initial transient due to non-protective TiO₂ growth. Subsequent well-behaved cubic kinetics for alumina scale growth were shown from 1273 K to 1673 K (1000 °C to 1400 °C). These possessed an activation energy of 335 kJ/mol, consistent with estimates of grain boundary diffusivity of oxygen (~375 kJ/mol). The durability of Ti₂AlC under combustion conditions was demonstrated by high pressure burner rig testing at 1373 K to 1573 K (1100 °C to 1300 °C). Here good stability and cubic kinetics also applied, but produced lower weight gains due to volatile TiO(OH)₂ formation in water vapor combustion gas. Excellent thermal stability was also shown for yttria-stabilized zirconia thermal barrier coatings deposited on Ti₂AlC substrates in 2500-hour furnace tests at 1373 K to 1573 K (1100 °C to 1300 °C). These sustained a record 35 µm of scale as compared to 7 μm observed at failure for typical superalloy systems. In contrast, scale and TBC spallation became prevalent on Cr₂AlC substrates above 1423 K (1150 °C). Cr₂AlC diffusion couples with superalloys exhibited good long-term mechanical/oxidative stability at 1073 K (800 °C), as would be needed for corrosion-resistant coatings. However, diffusion zones containing a NiAl-Cr₇C₃ matrix with MC and M₃B₂ particulates were commonly formed and became extensive at 1423 K (1150 °C).     

A study of the thermodynamics and phase relationships of the Fe−Co−S−O quaternary system

The oxygen potentials of several three- and two-phase equilibria in the Fe−Co−S−O quaternary system were measured atP _SO2=1, 0.1, and 0.01 atm over wide temperature ranges. The measurements were carried out using a solid oxide electrolyte emf technique. The equilibria measured are sp+ε+δ, s+ε+ξ, sp+ξ+η, and sp+δ. The symbols sp, ε, δ, ξ, and η denote the spinel, monoxide, monosulfide, metal sulfate, and Fe₂O₃ phases, respectively. Compositions for several of the equilibrated phases were measured using electron probe microanalysis. The present results and literature data for the constituent ternary systems were used to obtain thermochemical solution parameters for the sp, ε, δ and ξ solid solution phases. The calculated potential-composition stability diagrams for SO₂ pressures of 1, 0.1, and 0.01 atm at 973, 1023, and 1073 K, respectively, are in good agreement with the experimental results. OMRAN A. MUSBAH, formerly Research Associate at the University of Wisconsin-Madison     

Thermodynamics of oxygen behaviour in cobalt-nickel alloys

In this study the concentration and chemical potential of oxygen in liquid Co-Ni alloys equilibrated with cobalt-nickel aluminate spinel solid solutions and alumina have been determined at 1773, 1823 and 1873K as a function of nickel concentration. The oxygen content of the melt has been measured by suction sampling and inert gas fusion analysis. The corresponding oxygen potential has been determined with the following solid state cell: Mo, Mo+MoO₂ | (MgO)ZrO₂ | (Co, Ni) melt + AI₂O₃ + (Co, Ni)O·(1+x)Al₂O₃, Mo. The effect of nickel on the activity coefficient of oxygen in Co-Ni alloys has been determined. The results for the activity coefficient have been modelled with Wagner's interaction parameters and also the more recent exponential method of St. Pierre et al. at the three temperatures.     

Rare earth application for heat-resisting alloys

High-temperature oxidation resistance of Al2O3-and Cr2O3-forming heat-resisting alloys with rare earths(yttrium-implanted FeCrAl,-added FeCrAl,-added FeCrAlPt alloys,Y2O3-or CeO2-coated NiCrSi,yttrium-or lutetium-added NiCr and NiCrSi) was studied in oxygen at high temperatures,by mass gain measurements,mass change measurements,amount of spalled oxide,observation of surface appearance,X-ray diffraction(XRD),scanning electron microscopy(SEM),electron probe X-ray microanalysis(EPMA) and transmission electron microscopy(TEM).After oxidation at 1573 K for 18 ks in oxygen,oxide scale on FeCrAl alloy spalled from the entire surface,however,yttrium-implanted FeCrAl alloys showed good oxide adherence.After oxidation at 1473 K for 18 ks in oxygen,mass gain of FeCrAlY alloys decreased with increasing yttrium of up to 0.1 wt.% follwed by an increase with the yttrium content,and the mass gain of FeCrAl0.005Pt0.05Y alloy with appropriate additions of platinum and yttrium was lower than that of FeCrAl0.1Y alloy.Yttrium-added FeCrAl alloys showed good oxide adherence.TEM analysis revealed that the alumina/alloy interface of FeCrAl0.005Pt0.05Y alloy showed good coherency.The scale surface of FeCrAl alloy was rough,however,those of FeCrAlY and FeCrAlPtY alloys were smooth.Cyclic oxidation of NiCrSi,Y2O3-or CeO2-coated NiCrSi alloys was studied up to 10 cycles(1 cycle:300 s) at 1523 K in oxygen.Mass change of NiCrSi alloy increased up to 3 cycles and then decreased up to 10 cycles because of oxide spallation during cooling.On the other hand,mass change of Y2O3-or CeO2-coated NiCrSi alloy increased up to 10 cycles,and these alloys showed good oxide adherence.Granular Cr2O3 particles on Y2O3-coated NiCrSi alloy were in size smaller than these on CeO2-coated NiCrSi alloy.This result suggested that oxidation rate of Y2O3-coated NiCrSi alloy was lower than that of CeO2-coated NiCrSi alloy.After oxidation at 1473 and 1573 K for 18 ks in oxygen,mass gain of yttrium-or lutetium-added NiCr and NiCrSi alloys decreased.Oxide scales on NiCrS     

Thermodynamic properties of oxygen in yttrium-oxygen solid solutions

The oxygen potential in yttrium-oxygen (Y-O) solid solutions was measured by equilibration with titanium-oxygen (Ti-O) solid solutions. Yttrium and titanium samples were immersed in calcium-saturated CaCl₂ melts at temperatures between 1108 and 1438 K, and oxygen levels in the two metals were measured. With the Ti-O system acting as a reference, oxygen potentials in Y-O solid solutions were determined. By this technique, it was possible to make reliable measurements of extremely low oxygen potentials (as low as 10^?44 atm at 1273 K), far beyond the range of solid oxide electrolyte sensors.     

Phase equilibria in the Mg-Al-Ca ternary system at 773 and 673 K

The isothermal sections of the Mg-Al-Ca ternary system at 773 and 673 K were determined by phase analysis with electron-probe microanalysis (EPMA) and transmission electron microscopy (TEM). The C36 phase exists between the C14 (Mg₂Ca) and C15 (Al₂Ca) phases, and its stoichiometry is close to Mg₂Al₄Ca₃. The α-Mg phase equilibrates with the C14 and C36 phases at 773 K, but with C14, C15, and β phases at 673 K, due to the decomposition of the C36 phase into C14 and C15 phases. These intermetallic phases have significant solid-solubility in the ternary system.     

Normal spectral emissivities of liquid Ag-Cu alloys in the visible and infrared regions

The wavelength and composition dependencies of normal spectral emissivities for liquid Ag-Cu alloys have been measured over a wavelength range between 450 and 1500 nm at temperatures from 1273 to 1423 K in a cold crucible furnace. The spectral emissivities abruptly increase in the visible region with decreasing wavelength. The wavelength at the abrupt increase in the emissivity shifts to a shorter wavelength as the concentration of silver in the alloy increases. This suggests that the abrupt increases are attributed to the direct interband transition, and the energy gap between the d band and the Fermi level increases with an increase in silver concentration in the alloy. To the contrary, the spectral emissivities in the infrared region were found to exhibit weak negative wavelength dependencies, which are attributed to the intraband transition. The composition dependencies of spectral emissivities observed in the infrared region have been explained by the Drude model.     

Internal oxidation and mechanical properties of TZM-Mo alloy

The internal oxidation behavior of the bcc alloy TZM-Mo (Mo-0.5 wt pct Ti-0.08 wt pct Zr-0.02 wt pet C) was investigated in low-pressure O₂, CO, and H₂O environments at 1098 and 1273 K. The results indicate that a diffusion process controls the kinetics of the oxygen absorption at 1098 K, while bulk diffusion and gas-metal interaction at the specimen surface both affect the rate at 1273 K. The carbon content of TZM in these experiments increased initially and then decreased. Decarburization became significant only after extended exposure at 1273 K. The deformation and fracture behavior of both oxidized and heat-treated TZM specimens were studied at temperatures to 1589 K. TZM specimens showed an increase in strength and a linear decrease in ductility with oxygen content. Oxidized TZM lost its ductility completely at an oxygen level of 300 ppm at room temperature, 1366, and 1589 K, but 500 ppm was required at 1098 K. The ductility of embrittled TZM was increased significantly with heat treatment at high temperatures and was almost completely restored after annealing at 1973 K. The change in mechanical properties is discussed in terms of internal oxidation and precipitation of oxides.