Theoretical Analysis of Anomalies in High-Temperature Oxidation of Fe–Cr, Fe–Ni, and Fe–Ni–Cr Alloys

The following anomalies are theoretically analyzed: weakening of the protective ability of dense Cr₂O₃ film during its long-term thermal exposure (because of iron oxidation under the film); lowering of the heat resistance of Fe–Cr and Fe–Ni–Cr alloys during the oxidation (800°C) with an increase in the chromium content over 40 at. %; improving of the protective ability of the films formed at Fe–Ni alloys because of nickel oxidation under the dense FeO film; and the internal oxidation of the Fe 30Ni alloys under the FeO films with the internal formation of FeO oxides and spinel of NiFe₂O₄ type. It is shown that these anomalies can be explained, and the composition of the most heat-resistant alloys calculated, if one takes into account that associates with significantly stronger interatomic bonds than those in ideal solutions can form in solid solutions and cause unlimited solubility of the metallic components in each other.     

The influence of Cr alloying on microstructures of Fe–Al–Mn–Cr alloys

The effects of increasing chromium content on the phase transformations in Fe–Al–Mn–Cr alloys have been investigated by means of transmission electron microscopy and energy-dispersive X-ray spectrometry. The experimental results revealed that increasing the chromium addition would expand both the A12α-Mn and DO₃ phase-field regions.     

Characterization of irradiated single crystals of Fe and Fe–15Cr

Pure Fe and Fe–15Cr single crystals with three different orientations, [1 0 0], [1 1 0] and [1 1 1], were irradiated in the BR2 reactor of SCK-CEN, at a temperature of 300 °C to a dose of 0.2 displacements per atom. Irradiation-induced microstructure changes were studied by transmission electron microscopy. The size distribution and defect densities were measured and the Burgers vectors and the nature of the loops were determined in detail. In the pure Fe specimens, mainly a〈1 0 0〉 edge-type interstitial dislocations loops could be identified. Their average density and size were (4.1 ± 0.4) × 10²¹ m^?3 and (8 ± 2) nm, respectively. In Fe–15Cr, on the other hand, no defects could be observed.     

Modeling of Oxidation Kinetics of Y-Doped Fe–Cr–Al Alloys

Studies using advanced analytical techniques indicated that the reactiveelements (RE) segregate along the oxide grain boundaries and at theoxide–alloy interface during oxidation of -Al₂O₃forming alloys. The segregation results in inward oxygen diffusion along theoxide grain boundaries as the predominant transport process in the oxidegrowth. The present work establishes a mathematical model based on themechanisms of inward oxygen diffusion along the grain boundaries and oxidegrain coarsening. This model has been used to describe the oxidationkinetics of Y-doped Fe–Cr–Al alloys. The results showed a muchbetter agreement with the experimental data than the parabolic rate law. Byusing this model, the exponential number for the grain coarsening of aluminascales during oxidation was calculated to be 3. The activation energyfor oxygen diffusing along the grain boundaries was 450 kJ/mol. They arealso in good agreement with values reported in the literatures.     

Peculiarities of Mechanical Alloying of High-Concentration Fe–Cr Alloys

The kinetics of the mechanical alloying of Fe and Cr powder mixtures with Cr contents of 20–48 at % has been studied by Mössbauer spectroscopy, X-ray diffraction analysis, and temperature measurements of dynamic magnetic susceptibility. It has been found for the first time that the processes of mechanical alloying differ for initial mixtures with chromium contents of 30 at % or more and contents of less than 30 at %. For the first case, the unidirectional dissolution of Cr in Fe is observed through the whole process of mechanical alloying of the mixture in a planetary ball mill. For the second case, the dissolution of Cr in Fe is observed at the beginning stage of milling; however, after milling for 2 h (t_mil), the reverse process, namely, the dissolution of Fe in Cr is likely to dominate. The mechanically alloyed samples are characterized by inhomogeneous Cr and Fe concentration distribution in powder particles, in particular at a Cr concentration in the initial mixture of more than 30 at %.     

Effect of Alloy Composition and Exposure Conditions on the Selective Oxidation Behavior of Ferritic Fe–Cr and Fe–Cr–X Alloys

Selective oxidation behavior of ferritic martensitic Fe–Cr base alloys, exposed in various atmospheres containing combinations of O₂, CO₂, and H₂O, were studied at various temperatures relevant to oxy-fuel combustion. This paper begins with a discussion of the required Cr content to form a continuous external chromia scale on a simple binary Fe–Cr alloy exposed in oxygen or air based on experiments and calculations using the classic Wagner model. Then, the effects of the exposure environment and Cr content on the selective oxidation of Fe–Cr alloys are evaluated. Finally, the effects produced by alloying additions of Si, commonly present in various groups of commercially available ferritic steels, are described. The discussion compares the oxide scale formation on simple binary and ternary Fe–Cr base model alloys with that on several commercially available ferritic steels.     

Oxidation Characteristics of Fe–18Cr–18Mn-Stainless Steel Alloys

Air oxidation studies of Fe–18Cr–18Mn stainless steels were conducted at 525, 625, and 725 °C. Alloys were evaluated with respect to changes in oxidation properties as a result of interstitial additions of nitrogen and carbon and of minor solute additions of silicon, molybdenum, and nickel. Interstitial concentrations possibly had a small, positive effect on oxidation resistance. Minor solute additions significantly improved oxidation resistance but could also reduce interstitial solubility resulting in formation of chromium carbides. Loss of solute chromium resulted in a slight reduction in oxidation protection. Oxidation lasting over 500 h produced a manganese rich, duplex oxide structure: an outer sesquioxide and an inner spinel oxide.     

Prediction of chromium depleted-zone evolution during aging of Ni–Cr–Fe alloys

The phenomenological and analytical study of depleted chromium zone in Ni–Cr–Fe alloys has been carried out in order to predict the evolution of chromium profiles resulting from carbide precipitation during aging. A diffusional model has been developed to take into account the two stages of chromium concentration evolution: dechromization and rechromization. The model has been verified using the available experimental data found in the literature for a nickel alloy type: Inconel 690. The changes of the chromium concentration at the carbide–matrix interface xⁱ_Cr(t) and near the grain boundary x_Cr(x,t), as a function of annealing conditions, can be assessed with a reasonable accuracy by the proposed model and compared to the previous thermodynamic and kinetic models.     

Influence of Fe on Microstructure and Mechanical Properties in Pdoped Ni–Cr–Fe Alloys

The influence of Fe on the microstructure and mechanical properties of P-doped Ni–Cr–Fe alloys has been investigated.Results showed that increasing Fe content refined the dendrite microstructure and enhanced the solubility of P in as-cast alloys. The change of microhardness in different dendrite regions was attributed to the segregation of P atoms in solid solution state, which had strengthening effects. Increasing Fe contents from 15.2 to 60.7 wt% reduced the yield strength and tensile strength but had little influence on the elongation of alloys. The stress rupture life of alloys after heat treatment decreased with the increment of Fe contents, and the failure fracture modes transferred from transgranular to intergranular fracture mode. The change of fracture modes was due to the weakness of grain boundaries caused by the increment of Fe.In addition, the precipitation of M_(23)C_6 was believed to be related to the segregation of P toward grain boundaries, which led to the fluctuation of carbon and chromium atoms near the grain boundaries in alloys with low Fe contents. Consequently, the increment of Fe decreased the strength of matrix and changed the existence of P atoms and the precipitates at grain boundaries.     

Wear characteristics of Fe–25Cr–C–B eutectic cast alloys

Abstract

To clarify the characteristics of Fe–25Cr–C–B cast alloys, a pin on disc friction and wear test was conducted on Fe–25Cr–0C–2.2B, Fe–25Cr–2.2C–1.0B and Fe–25Cr–3.5C–0B eutectic alloys, at various sliding velocities ranging from 0.125 to 1.99 m s^-1. The effects of sliding velocity on the wear resistance of these alloys were studied by the pin on disc friction and wear test, SEM and an X-ray diffraction method. The results show that the effects of sliding velocity on the increase in wear loss were different due to the differences in structure among the alloys. The X-ray diffraction method shows the presence of Fe₂ O₃ and Fe₃ O₄ in the alloys after conducting wear tests for almost all of the wear conditions. From the sliding velocity dependence of wear loss, worn surface observation after the wear tests and X-ray diffraction results, the relationships between the type of oxide and wear loss for Fe–25Cr– 0C–2.2B and Fe–25Cr–2.2C–0B alloys are not clear. However, the wear loss of Fe–25Cr–3.5C–0B alloy decreases at a sliding velocity of 0.5 m s^-1 or lower, due to the presence of red Fe₂ O₃ oxide on the worn surface. The wear loss peaks at a sliding velocity of 0.95 m s^-1, and decreases again at a sliding velocity of 1.99 m ^-1 due to the presence of black Fe₃ O₄ oxide.     

Compressive behavior of an extruded nanocrystalline Al–Fe–Cr–Ti alloy

The investigation of the compressive behavior of an extruded nanocrystalline Al–Fe–Cr–Ti alloy reveals that the ductility of the alloy is determined by the status of the oxide film at the prior powder particle boundaries. When properly extruded, the alloy exhibits more than −0.45 compressive strain at 25 °C coupled with superior specific strengths up to 400 °C. The enhanced strength is attributed to grain refinement, intermetallic precipitation, and solid solution.     

Effect of Sulfur Partial Pressures on Oxidation Behavior of Fe–Ni–Cr Alloys

Fe–Ni–Cr alloys containing different contents of Si with and without pre-formed oxide scale at the surface were tested in oxidation environments at 1,050?°C with varied sulfur partial pressures. The oxide-scale growth on Fe–Ni–Cr alloys was accelerated by increasing sulfur partial pressures in the oxidizing-carburizing environments. This accelerated oxidation was characterized by the formation of plate-shaped MnCr₂O₄ spinel crystallites and the nodular clusters at the site of scale spallation. Pre-oxidized Fe–Ni–Cr alloys generally did not suffer from sulfur attack because of excellent protection of pre-formed oxide scale. Scale spallation and sulfur attack were found only on high-Si alloy subjected to the maximum sulfur potential, which was attributed to accelerated oxidation and selective oxidation and sulfidation at the sites where oxide scale spallation had occurred. For bare alloys in absence of pre-formed oxide layers, scale spallation was found to occur at lower level of sulfur potential on low-Si alloy than on high-Si alloy. A higher content of Si is necessary for the formation of protective silica sub-layer, which is believed to be the main cause of the difference in scale spallation observed.     

Microstructure and corrosion behavior of Zr–1.0Cr–0.4Fe–xMo alloys

Possibility of using Mo as an alloying element in Zr-based alloys was attractive in terms of microstructure refinement and mechanical properties strengthening. In this research, Zr–1.0Cr–0.4Fe–xMo(0, 0.2, 0.4, and 0.6) alloys with different Mo contents were prepared by vacuum arcmelting method, the microstructure and the corrosion resistance of these alloys were investigated. Addition of Mo has a refinement effect on the microstructure; with the increase of Mo content, the a-laths in the as-cast samples and the grain size in the annealed samples decrease. Zr–1.0Cr–0.4Fe–xMo alloys have large numbers of fine second-phase particles(SPPs) in the matrix, the area fraction of the SPPs is more than 10 %. With the increase of Mo content, the population density of the SPPs increases significantly, while the average diameter of the SPPs decreases. Mo addition also affects the texture; the intensity of basal pole texture aligning normal direction decreases with the increase of Mo content in the alloys.Compared with Zr-4 and Zr–1Nb, Zr–1.0Cr–0.4Fe–xMo alloys have excellent corrosion resistance in 500 °C/10.3 MPa steam. The corrosion rates of Mo-containing alloys are higher than that of Mo-free alloy, which is mainly attributed to the solute Mo atoms in the Zr matrix.Change of the SPPs features due to the increase of Mo content alleviates the degradation of corrosion resistance in some degree, but it is not the dominant factor.     

Effect of Ce content on mechanical properties of Ce/Cr coated open-cell Ni–Cr–Fe alloy foams

The Ce/Cr coating was homogenously deposited onto the reticulated open-cell Ni–Cr–Fe alloy foam by the pack cementation process. The mechanical properties of the Ce/Cr coated alloy foams were investigated by the quasi-static compression test. Simultaneously, the deformation and failure mechanisms of Ce/Cr coated alloy foams were discussed. The results show that the adding amount of CeO₂ powders influences the mechanical properties of the Ce/Cr coated alloy foams. Despite an increase in density as compared to the uncoated foams, the Ce/Cr coated foams exhibit improvement in both yield strength and energy-absorption performance. Especially, the energy-absorption performance of 2% Ce/Cr (mass fraction) coated alloy foam is averagely 1.9 times as high as that of the bare Ni–Cr–Fe alloy foam. In addition, the mechanical properties of the Ce/Cr coated alloy foams increase with the increase of strain rate. The distortion and cracking are mainly the deformation behavior of the Ce/Cr coated alloy foam, confirmed by SEM images.     

Formation of Vacancy-Type Dislocation Loops in Hydrogen-Ion-Implanted Fe–Cr Alloy

Fe-10 at.%Cr alloy was implanted with hydrogen ions at room temperature, followed by annealing at high temperatures.The annealing process made the defects develop into large dislocation loops. The nature of the dislocation loops formed after annealing was studied by the evolution of loops under in situ electron irradiation in high-voltage electron microscope.It indicated that only interstitial-type loops were observed when annealed at 550 °C and below, but vacancy-type loops started to form at the temperature higher than 600 °C. According to the previous study of our group, the presence of chromium element made the formation temperature of vacancy-type loops higher than that in pure iron. The effect of alloying elements on the formation temperature of the vacancy-type loops was discussed.     

Structure of strengthening particles of niobium carbide in Fe–Cr–Ni cast refractory alloys

Methods of optical and electron microscopies were used to study the structure of particles of niobium carbide in a cast refractory Fe–Cr–Ni–C alloy modified by Nb and Ti. Particles of niobium carbide in the structure of the cast alloy are predominantly multiphase polycrystalline clusters that are inhomogeneous in the chemical composition and crystal structure. The misorientation angle between individual crystals that compose the carbide particles is 30°–60°. The polycrystalline character of carbides is probably associated with significant thermal stresses that arise at the interphase boundaries in the structure of the alloy upon the primary cooling of the ingot. To explain the polymorphism of the cluster of niobium carbide, a further analysis of the structural and geometrical crystallography is required.     

The Effect of Water Vapor on Selective Oxidation of Fe–Cr Alloys

Binary Fe–Cr alloys containing 10 and 20 mass% Cr were studied with respect to isothermal oxidation behavior at 900 and 1,050 °C in Ar–20%O₂, Ar–7%H₂O and Ar–4%H₂−7%H₂O. Thermogravimetric analyses in combination with analytical studies using SEM/EDX and Raman Spectroscopy revealed, that in atmospheres in which water vapor is the source of oxygen, Cr exhibits a higher tendency to become internally oxidized than in the Ar–O₂ gas. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the scale, the present results thus reveal that the presence of water vapor also affects the transport processes in the alloy. This mechanism is an “easy” explanation of the frequently observed effect that Fe–Cr alloys with intermediate Cr contents (e.g. 10–20%, depending on temperature) exhibit protective chromia-rich scale formation in dry gases but breakaway type Fe-rich oxides in wet gases, provided the oxygen partial pressure is sufficiently high for Fe to become oxidized.     

Phase constitution and microstructure of the Fe–Si–Cr ternary ferritic alloys

Phase constitution and microstructure of Fe-corner of the ternary Fe–Si–Cr system are investigated particularly on the multiphase fields involving bcc disordered phase, A2 phase, and bcc ordered phases, B2 and D0₃ phases. The (A2 + D0₃) two-phase region is found to exist in compositions between 12 and 15 at.% Si at 10% Cr at 873 K.     

Influence of Mo in the structure of rapidly solidified Fe–Mo–Cr–Mn–Si–C alloy

Abstract

An Fe–Mo–Cr–Mn–Si–C alloy was prepared in an induction furnace and was cast into cylindrical rod in a copper mould in castmatic equipment (low pressure casting). A single phase non-equilibrium featureless (no visible microstructures after deep etching) phase was observed over a certain range of thickness of the rod. In this present work, the extent of the featureless phase was studied with different concentrations of Mo (5–25 wt-%) for 5·5 mm diameter of cylindrical rod at a cooling rate of 1100 K s^–1. Light optical microscopy, scanning electron Microscopy and Vickers hardness tests were used to analyse the samples. The amount of the featureless area varies as the Mo content changes and the maximum featureless area was obtained for 7 wt-% of Mo. This single phase featureless structure exhibits very high hardness (>1350 HV) which can be used in many interesting applications with or without suitable heat treatments.