Internal Oxidation of Fe–Mn–Cr Steels,Simulations and Experiments

A multi-element and multi-phase internal oxidation model that couples thermodynamics with kinetics is developed to predict the internal oxidation behaviour of Fe–Mn–Cr steels as a function of annealing time and oxygen partial pressure. To validate the simulation results, selected Fe–Mn–Cr steels were annealed at 950 °C for 1–16 h in a gas mixture of Ar with 5 vol% H₂ and dew points of ? 30, ? 10 and 10 °C. The measured kinetics of internal oxidation as well as the concentration depth profiles of internal oxides in the annealed Fe–Mn–Cr steels are in agreement with the predictions. Internal MnO and MnCr₂O₄ are formed during annealing, and both two oxides have a relatively low solubility product. Local thermodynamic equilibrium is established in the internal oxidation zone of Fe–Mn–Cr steels during annealing and the internal oxidation kinetics are solely controlled by diffusion of oxygen. The internal oxidation of Fe–Mn–Cr steels follows the parabolic rate law. The parabolic rate constant increases with annealing dew point, but decreases with the concentration of the alloying elements.     

Zum Mechanismus der Hochtemperatur-Oxidation von Kupfer-Aluminiumlegierungen

Mechanism of high temperature oxidation of copper aluminium alloys The oxidation (950 °C, oxygen pressure 10^?2 to 760 Torr ) follows a linear kinetic law after the first 3 to 5 min. Low oxygen concentrations produce preferential internal oxidation with Al₂O₃ being precipitated in statistical distribution in the copper matrix or — with higher a/Al contents — forming a barrier layer near the surface. This layer would slow down diffusion of copper ions; this effect would be even more pronounced at high oxygen pressures, where couprous oxide is present. The rate constant of the oxidation is drastically reduced by higher Al contents (> ～ 1%).     

Corrosion characteristics of T91 steel in lead–bismuth eutectic with different oxygen concentrations at 500°C

Corrosion tests of T91 were carried out in the stagnant lead–bismuth eutectic (LBE) containing 10^?6, 10^?7, and 10^?8 wt% oxygen at 500°C for up to 2000 h, respectively. The dependence of corrosion characteristics on oxygen concentrations in LBE was obtained. The results show that the oxide-scale structure changes from a three-layer magnetite/spinel/internal oxidation zone (IOZ) scale under the oxygen concentration of 10^?6 wt%, to the formation of a two-layer spinel/IOZ scale under the oxygen concentrations of 10^?7 and 10^?8 wt%. In addition, with the decrease of the oxygen concentration, the Cr/Fe ratio in the Fe–Cr spinel slightly increases in a linear rule while the thicknesses of the oxide layers gradually decrease.     

Calculation of Selective Oxidation in Grain and Grain Boundary

The aim of this paper is to identify selective oxidation of high strength steels under different conditions. FISHER model is introduced to simulate diffusion and precipitation of chemical elements in metallic matrices and grain boundary, a scheme of its analytical solution is given. Effects of grain size, dew point and annealing temperature related to concentration of oxygen during annealing treatment are presented. The results are consistent well with literatures. The calculation shows that the selective oxidation of CMnSi TRIP steel at ferritic annealing in a low dew point N₂-10%H₂ atmosphere of ?30 °C result in the formation of internal MnO in the matrix and SiO₂ particles in the subsurface. The calculations agree well with the experimental results.     

Volatilization and Transport Mechanisms During Cr Oxidation at 300 °C Studied In Situ by ToF-SIMS

The oxidation of chromium at 300 °C was investigated in situ by ToF-SIMS for three different oxygen pressures (\(P_{{{\text{O}}_{2} }} = 2.0 \times 10^{ - 7}\), 6.0 × 10^?7 and 2.0 × 10^?6 mbar). Sequential exposure to the ¹⁸O isotopic tracer was performed to reveal the governing transport mechanism in the oxide film. The evolution of the oxide thickness was monitored. Volatilization of Cr₂O₃ was evidenced. A model was used to describe the kinetics resulting from the measurements. Both the parabolic and volatilization constants showed a dependence on oxygen partial pressure like \(P_{{{\text{O}}_{2} }}^{ - 1/n}\), with n = 1.9 ± 0.1, indicating a defect structure mainly consisting of oxygen vacancies. The re-oxidation in ¹⁸O₂ shows a growth of the oxide layer at the metal/oxide interface, demonstrating an oxidation process governed by anionic transport via oxygen vacancies. The diffusion coefficient of oxygen in the oxide was determined by fitting the ToF-SIMS depth profiles. It is 2.0 × 10^?18 cm² s^?1.     

Oxidation of cobalt at high temperature

Precise values of parabolic rate constants of cobalt oxidation have been determined over a wide range of temperature (950–1300°C) and oxygen pressure (6.58× 10^?4?0.658 atm). The dependence of the calculated values of parabolic rate constants k″_p on oxygen pressure and temperature can be described by the following empirical equation: $$k''_p = const. \cdot {\text{p}}_{O_2 }^{{\text{1/n}}} \cdot exp ( - {\text{E}}_{\text{k}} /RT)$$ The exponent 1/n decreases with an increase in temperature from 1/3.40 at 950°C to 1/3.96 at 1300°C, whereas the activation energy E_k decreases with an increase in the oxygen pressure from 41.7 to 38.1 kcal/mole.     

Accelerated Oxidation of V2O5-Deposited Copper

The accelerated-oxidation kinetics of V₂O₅-deposited copper were studied in the temperature range 560?C800 °C in air. The accelerated oxidation followed the parabolic-rate law, indicating that the process was diffusion-controlled. Oxygen diffusion along liquid channels in the oxide scale was inferred to be the rate-limiting step in the overall mechanism. The parabolic-rate constant increased from 3.4 × 10^?6 to 1.6 × 10^?5 kg² m^?4 s^?1 with increasing the deposit mass from 0.075 to 0.225 kg m^?2 at 700 °C.     

High Temperature Oxidation of Micro-alloyed Steel and Its Scale Adhesion

The present work investigated the high temperature oxidation behaviour of the micro-alloyed steel and the adhesion of thermal oxide scale to its steel substrate. Oxidation testing was conducted at 815 °C in oxygen without and with 17.9% v/v water vapour. The oxidation kinetics in the two atmospheres were parabolic with similar rate constants, i.e. 1.13 × 10^?9 and 1.17 × 10^?9 g² cm^?4 s^?1 for the sample oxidised in oxygen without and with water vapour, respectively. The XRD peaks for wustite, magnetite, Ti-doped magnetite and titanium carbide were detected for the sample oxidised in oxygen. For the sample oxidised in the humidified atmosphere, Ti-doped magnetite was dominantly observed, additionally with titanium carbide. A tensile testing machine equipped with an optical lens was used to monitor scale failure during straining. For the sample oxidised for 1 min, the strain initiating the first spallation of the steel oxidised in the humidified oxygen was 1.74 ± 0.14%. This strain was higher than the strain initiating the first spallation of the steel oxidised in oxygen which was 1.00 ± 0.04%, indicating the improved adhesion of scale formed in the atmosphere containing water vapour. Mechanisms of water vapour effect on scale adhesion are discussed.     

Cyclic Oxidation Behavior of IN 718 Superalloy in Air at High Temperatures

Ni-base superalloy IN 718 was cyclically oxidized in laboratory air at temperatures ranging from 750 to 950 °C for up to 12 cycles (14 h/cycle). The kinetic behaviour as well as the surface morphology, and the oxide phases of the scales were characterized by means of weight gain measurements, cyclic oxidation kinetics, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analysis techniques. The results showed that as the oxidation temperature increased, the oxidation rate, the external scale thickness, and internal oxidation zone increased. It was suggested that the oxidation rate was controlled by the diffusion of substrate elements in the alloy and the inward diffusion of oxygen through the oxide scale. The oxidation kinetics followed a sub-parabolic rate law and, the activation energy of oxidation was 249 ± 20 kJ mol^?1. The scaling process was controlled mainly by the diffusion of chromium, titanium, manganese, and oxygen ions through the chromia scale. IN 718 showed low weight gain and very slow reaction rates of substrate elements at 750 °C. At 850 °C, a continuous and very thin oxide scale was formed. At 950 °C, XRD and EDS-elemental mapping analysis revealed that a complex oxide scale had formed. It consisted of an outermost layer of TiO₂?CMnCr₂O₄ spinels, inner layer of Cr₂O₃, and the inner most layer composed of Ni₃Nb enriched with Nb, Ti and Al oxides underneath the chromia layer. The oxide scale at this temperature seemed to be thicker layer, significant spallation and volatilization had apparently occurred, and greater internal corrosion was identified. The doping effect of titanium was observed, where it was found to be diffused through the chromia scale to form TiO₂ at the oxide-gas interface as well as internally and at the oxide alloy interface. The amount of rutile (TiO₂) at the oxide surface increased with temperature. In view of Mn contents in the alloy, the manganese?Cchromium spinel oxide was inferred to have played an important role in cyclic oxidation behaviour of IN 718, where the change in oxidation kinetic was noted. The Al contents would cause internal Al-rich oxide formation at grain boundaries.     

Microstructure and Oxidation Behavior of Al and Al/NiCrAlY Coatings on Pure Titanium Alloy

To improve the oxidation resistance of Ti alloys, a NiCrAlY coating was deposited as diffusion barrier between aluminum overlay coating and pure Ti substrate by air plasma spraying method. The microstructure and oxidation behavior of Al coatings with and without NiCrAlY diffusion barrier were investigated in isothermal oxidation tests at 800 °C for 100 h. The results indicate that the weight gain of the Al/NiCrAlY coating was 4.16 × 10^?5 mg² cm^?4 s^?1, whereas that of the single Al coating was 9.52 × 10^?5 mg² cm^?4 s^?1 after 100 h oxidation. As compared with single Al coating, the Al/NiCrAlY coating revealed lower oxidation rate and excellent oxidation resistance by forming thin Al₂O₃ + NiO scales at overlaying coating/diffusion barrier and diffusion barrier/substrate interfaces. Meanwhile, the inward diffusion of Al and the outward diffusion of Ti were inhibited effectively by the NiCrAlY diffusion barrier.     

Optimization of thermal processing parameters of Ti555211 alloy using processing maps based on Murty criterion

Isothermal compression testing of Ti555211 titanium alloys was carried out at deformation temperatures from 750 to 950 °C in 50 °C intervals with a strain rate of0.001–1.000 s~(-1). The high-temperature deformation behavior of the Ti555211 alloy was characterized by analysis of stress–strain behavior, kinetics and processing maps. A constitutive equation was formulated to describe the flow stress as a function of deformation temperature and strain rate, and the calculated apparent activation energies are found to be 454.50 and 207.52 k J mol~(-1)in the a b-phase and b-phase regions, respectively. A processing map based on the Murty instability criterion was developed at a strain of 0.7. The maps exhibit two domains of peak efficiency from 750 to 950 °C. A *60 % peak efficiency occurs at 800–850 °C/0.001–0.010 s~(-1). The other peak efficiency of *60 % occurs at C950 °C/0.001–0.010 s~(-1), which can be considered to be the optimum condition for high-temperature working of this alloy.However, at strain rates of higher than 1.000 s~(-1)and deformation temperatures of 750 and 950 °C, clear process flow lines and bands of flow localization occur in the hightemperature deformation process, which should be avoided in Ti555211 alloy hot processing. The mechanism in stability domain and instability domain was also discussed.     

Conversion electron Mössbauer spectroscopy on the oxidation of a (110) Fe-57 single crystal 100 Å thick

Conversion electron Mösshauer spectroscopy (CEMS) has been used to measure the initial stages of oxidation on an 100 Å thick (110) Fe-57 single crystal face at 275°C in the pressure range from 2 × 10^?4 to 8 × 10^?3 torr. Only magnetite is formed at 2 × 10^?4 and 8 × 10^?3 torr, a magnetite/haematite conversion occurs at 2 × 10^?3 torr. The various oxide phases have been investigated according to their growth behaviour and kinetics. Complicated changes in the kinetic behaviour are found with the magnetite/haematite conversion. Depth selective CEMS has been used to measure qualitatively the depth distribution of the oxide phases and the iron ion sites.     

Oxidation Behavior of ZrB<Subscript>2</Subscript>–SiC Composites at 1650 °C

The oxidation behavior of ZrB₂–SiC composites in air was studied at 1650 °C. Diffusion-controlled oxidation kinetics were found for the composites studied. A parabolic rate constant of 1.2 × 10^?8 g² cm^?4 s^?1 was measured for ZrB₂–10 % SiC composite. A transition in the oxidation kinetics was observed for ZrB₂–30 % SiC composite with the initial parabolic rate constant being 1.3 × 10^?8 g² cm^?4 s^?1. After exposure for 60 min, the parabolic rate constant was found to be 0.3 × 10^?8 g² cm^?4 s^?1. A single ZrO₂-rich oxide layer was found in the oxide scale structure of ZrB₂–10 % SiC composite. On the other hand, three-layer oxide structures, namely SiO₂-rich top layer, followed by ZrO₂-rich oxide scale and SiC-depleted layer, were found for ZrB₂–30 % SiC composite. The outer layer in the oxide scale structure of ZrB₂–SiC composite was tapered with enhanced oxidation at the corners of the sample. Vortex formation during the viscous flow of B₂O₃–SiO₂–ZrO₂ liquid near the corners on the surface was proposed as the root cause for enhanced oxidation at the corners of the sample.     

Isothermal Oxidation Comparison of Three Ni-Based Superalloys

Ni-based superalloys are used for high-temperature components of gas turbines in both industrial and aerospace applications due to their ability to maintain dimensional stability under conditions of high stress and strain. The oxidation resistance of these alloys often dictates their service lifetime. This study focuses on the isothermal oxidation behavior of three Ni-based superalloys, namely, polycrystalline cast IN738LC, single-crystal N5, and a ternary Ni-Fe-Cr (TAS) powder metallurgy alloy. The isothermal oxidation tests were conducted at 900 °C in the static air up to 1000 h, and the specific aspects studied were the oxidation behavior of these chromia-forming and alumina-forming alloys that are used extensively in industry. In particular, the behavior of oxide scale growth and subsurface changes were analyzed in detail using various techniques such as SEM, EDS, and AFM. From the isothermal oxidation kinetics, the oxidation rate constant, k _p, was calculated for each alloy and found to be; k _p = 2.79 × 10^?6 mg² cm^?4 s^?1 for IN738LC, k _p = 1.42 × 10^?7 mg² cm^?4 s^?1 for N5 and k _p = 1.62 × 10^?7 mg² cm^?4 s^?1 for TAS. Based on a microstructural analysis, IN738LC exhibited a continuous dense outer scale of Cr₂O₃ and discontinuous inner scale of Al₂O₃, whereas N5 and TAS showed a dense outer scale of Al₂O₃ alone. The results suggested that the N5 and PM-TAS alloys are more oxidation resistant than the IN738LC under these conditions.     

The Ablation and Oxidation Behaviors of SiC Coatings on Graphite Prepared by Slurry Sintering and Pack Cementation

SiC coatings were generated on graphite using slurry sintering (SS) and pack cementation (PC). The samples’ ablation features were assessed by an oxyacetylene torch. The rates of mass ablation of the PC–SiC and SS–SiC coatings were approximated 2.17?×?10^?3 and 9.52?×?10^?3 g s^?1, respectively, decreased by 84.1 and 29.6% compared to the uncoated samples. It was mainly attributed to the formation of a SiO₂ layer on the surface. The continuous SiO₂ molten film formed via the PC–SiC oxidation generates a sealing mechanism which can be an obstacle against the oxygen diffusion and hinder more ablation. This is while discontinuous SiO₂ film formed from the thin SS–SiC cannot protect the graphite effectively. The non-isothermal oxidation test shows that without the SiC coating, the sample weight is lost largely from 25 to 1500 °C, and its weight loss was 2.2% after the TGA. However, after coating, the samples possessed excellent oxidation protection and weight losses of SS–SiC and PC–SiC coatings are down to 1.3 and 0.6%, respectively. The more oxidation of the graphite substrate occurred due to the formation of macrocracks in the coating during the TGA and also the formation of holes on SiO₂ glass layer owing to release of CO or CO₂.     

Hot deformation behavior and microstructure evolution of TC4 titanium alloy

The hot deformation behavior of Ti-6Al-4V(TC4) titanium alloy was investigated in the temperature range from 650 °C to 950 °C with the strain rate ranging from 7.7×10-4 s-1 to 7.7×10-2 s-1.The hot tension test results indicate that the flow stress decreases with increasing the deformation temperature and increases with increasing the strain rate.XRD analysis result reveals that only deformation temperature affects the phase constitution.The microstructure evolution under different deformation conditions was characterized by TEM observation.For the deformation of TC4 alloy,the work-hardening is dominant at low temperature,while the dynamic recovery and dynamic re-crystallization assisted softening is dominant at high temperature.     

Hot Deformation Characteristics of GH625 and Development of a Processing Map

The hot deformation behavior of GH625 is investigated by a compression test in the temperature range of 950-1150 °C and the strain rate of 10^?3-5 s^?1. It is found that the flow stress behavior is described by the hyperbolic sine constitutive equation with average activation energy of 421 kJ/mol. Through the flow stresses’ curves, the processing maps are constructed and analyzed according to the dynamic materials model. In the processing map, the variation of the efficiency of the power dissipation is plotted as a function of temperature and strain rate, and the maps exhibit a significant feature with a domain of dynamic recrystallization occurring at the temperature range of 950-1150 °C and in the strain rate range of 0.005-0.13 s^?1, which are the optimum parameters for hot working of the alloy. Meanwhile, the instability zones of flow behavior can also be recognized by the maps.     

Cobalt Ferrite in YSZ for Use as Reactive Material in Solar Thermochemical Water and Carbon Dioxide Splitting, Part II: Kinetic Modeling

The kinetics of 10 wt.% cobalt ferrite (CoFe₂O₄) in 8 mol.% yttria-stabilized zirconia, synthesized via the co-precipitation method and formed into a porous structure, are investigated in support of simulating the performance of a solar thermochemical reactor. Kinetic parameters for the thermal reduction (T-R) of CoFe₂O₄ at temperatures of 1325–1500°C were investigated by thermogravimetry. A nonlinear best fit of a uniform conversion model was used to determine kinetic parameters from experimental data. In the temperature range of 1375–1450°C, the activation energy and preexponential term were found to be 386 ± 13 kJ mol^?1 and 8.8 × 10⁹ ± 2.0 × 10⁸ min^?1, respectively, while increasing at higher temperatures. Simultaneous thermogravimetric analysis and differential scanning calorimetry studies showed an increase in the reaction rate of T-R upon the onset of melting (1440°C). Oxidation studies of the material using CO₂ yield an activation energy and preexponential term of 52.1 ± 6.8 kJ mol^?1 and 2.86 ± 0.2 min^?1, respectively, which is in good agreement with past work. The reaction order for CO₂ was determined to be 0.750 ± 0.08. The reaction kinetics for oxidation using CO₂ were best described by a 3-D diffusion Jander model.     

Thermal properties of diamond/Al composites by pressure infiltration: comparison between methods of coating Ti onto diamond surfaces and adding Si into Al matrix

This study was pertained to the effects of Ti coating on diamond surfaces and Si addition into Al matrix on the thermal conductivity(TC) and the coefficient of thermal expansion(CTE) of diamond/Al composites by pressure infiltration.The fracture surfaces,interface microstructures by metal electro-etching and interfacial thermal conductance of the composites prepared by two methods were compared.The results reveal that Ti coating on diamond surfaces and only12.2 wt% Si addition into Al matrix could both improve the interfacial bonding and increase the TCs of the composites.But the Ti coating layer introduces more interfacial thermal barrier at the diamond/Al interface compared to adding 12.2 wt% Si into Al matrix.The diamond/Al composite with 12.2 wt% Si addition exhibits maximum TC of 534 W·m~(-1)·K~(-1)and a very low CTE of 8.9×10~(-6)K~(-1),while the coating Ti-diamond/Al composite has a TC of 514 W·m~(-1)·K~(-1)and a CTE of 11.0×10~(-6)K~(-1).