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.     

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.     

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.     

Effect of Zr Addition on the High-Temperature Oxidation Behaviour of Mo–Si–B Alloys

Mo–Si–B alloys are promising candidates for structural high-temperature applications due to their excellent high-temperature mechanical properties along with high melting temperatures and oxidation resistance. After an initial period with high weight loss rates as a consequence of the volatilization of Mo-oxide, a protective borosilica (glass) layer develops on the alloy surface and steady-state oxidation is achieved. Aiming at improved mechanical properties of Mo–Si–B alloys which exhibit a continuous Mo solid solution matrix as a consequence of a powder metallurgical production route, small amounts of Zr were added. The presence of oxygen in the alloy leads to the formation of thermodynamically very stable Zr-oxide precipitates in the bulk alloy causing an enhancement of its mechanical properties. It was observed that the addition of Zr (distributed in the alloy matrix) also has significant influence on the oxidation behaviour of Mo–Si–B alloys by reducing the period for the formation of the protective and stable silica scale. Furthermore, the weight loss due to vaporization of Mo-oxides is consequently reduced. Besides this beneficial effect, Zr is harmful for the oxidation resistance at temperatures beyond 1,200 °C. This is mainly due to the increased oxygen transport through defects in the silica scale.     

The Effect of Microstructure on the Electrical Properties of Gas-Atomized Copper–Iron Metastable Alloys

With the increase in global demand for highly functionalized materials, there is continued interest in exploiting the material properties of metals either individually or in the form of alloys. Copper–iron alloy is considered unique with its remarkable combination of strength and high electrical conductivity. Due to the low cost of iron, this alloy is expected to replace alloys like Cu–Ag and Cu–Nb. In order to explore the microstructural features, copper–iron alloy with three different compositions (10, 30, and 50 at.% Fe) were prepared by a gas atomization process. A detailed microstructural characterization was performed using scanning electron microscopy, X-ray diffraction, and electron backscattered diffraction. Spark plasma sintering was used to sinter the powders to evaluate their electrical conductivities. The mechanism of the microstructure formation is also discussed in detail. As the Fe content increases, the Fe-rich phase changes its shape from spherical to irregular with a concomitant sharp decrease in the electrical conductivity of the alloy.     

Oxidation Behavior of Ni–Cr–Fe-Based Alloys: Effect of Alloy Microstructure and Silicon Content

The oxidation behavior of Ni–Cr–Fe-based alloys in a low oxygen partial pressure atmosphere (H₂–H₂O) was investigated in terms of the effect of alloy microstructure and their silicon content. It was found that the formation and growth kinetics of the oxide scale are rather sensitive to the alloy microstructure and their corresponding Si contents. Oxide ridges were found to form in areas with eutectic structure, while a thin and homogeneous oxide scale formed on austenite matrix. The thicknesses of the oxide ridges and the oxide layer on the austenite matrix were dependent of their corresponding Si contents. The austenite/carbide phase boundaries in eutectic structure can offer fast diffusion paths for metal outward diffusion, which leads to the formation of ridge-like oxide features. The continuous SiO₂ sub-layer formed at the oxide scale/metal interface on the austenitic matrix acted as an effective diffusion barrier to metal outward diffusion, resulting in rather thin and uniform oxide scales.     

Simultaneous Internal Oxidation and Nitridation of Ni–Cr–Al Alloys

A series of Ni–Cr–Al alloys was subjected to thermal cycling to 1100°C in air for up to 260 1-hr cycles. All alloys exhibited poor corrosion resistance. Repeated scale spallation led to subsurface alloy depletion in aluminum and, to a lesser extent, chromium. This caused transformation of the prior alloy three-phase structures (-Cr+-NiAl+-Ni) to single-phase -nickel solution. Destruction of the external scale allowed gas access to this metal, which was able to dissolve both oxygen and nitrogen. Inward diffusion of the two oxidants led to development of a complex internal-precipitation zone: Al₂O₃ and Cr₂O₃ beneath the surface, followed by Al₂O₃, then AlN, then AlN+Cr₂N, and, finally, AlN alone in the deepest region. This distribution is shown to reflect the relative stabilities of the precipitates and the higher permeability of nitrogen. Diffusion-controlled kinetics were in effect initially, but mechanical damage to the internal-precipitation zone led to more rapid gas access and approximately linear kinetics in the long term.     

Effect of Intergranular Precipitation on the Internal Oxidation Behavior of Cr–Mn–N Austenitic Stainless Steels

The effect of intergranular precipitation on the internal oxidation behavior of Cr–Mn–N austenitic steels at 1000 °C in dry air atmosphere was investigated using scanning electron microscope, transmission electron microscope, and X-ray diffraction analysis. The results show that intergranular M23C6 carbide morphologies play an important role on the internal oxidation behavior of Cr–Mn–N steels. During the period of the oxidation, both discontinuous chain-shaped and continuous film-shaped intergranular M23C6 carbides precipitated along the grain boundaries. Internal oxides of silica preferentially intruded into the matrix along grain boundaries with discontinuous M23C6 carbide particles, while silica was obviously restricted at the interfaces between the external scale and matrix on the occasion of continuous film-shaped M23C6 carbides. It is seemed that reasonable microstructure could improve the oxidation resistance of Cr–Mn–N steels.     

Effect of Zr Addition on Microstructure and Properties of Cu–Ag–Ti Alloys

Effects of small addition of Zr on the microstructures and properties of as-cast Cu₅₀Ag_46?xZr_xTi₄ alloys were investigated by differential scanning calorimeter (DSC), x-ray diffraction, microscope, and property testings. The results show that the melting point of the Cu₅₀Ag₄₆Ti₄ alloys does not change obviously with the addition of Zr at the melting points of about 779 °C. Adding Zr reduces the volume fraction and size of the board strips of Cu₃Ti phase, promotes the uniform distribution of the new phase Cu₄AgZr, and improves the Vickers hardness and shear strength of the based alloys. Moreover, increasing the Zr content can improve the high-temperature oxidation resistance of alloys.     

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.     

Oxidation Behavior of Two-Phase γ + σ Nb-Ti-Al Alloys

The oxidation behavior of a series of single-and two-phase Nb-Ti-Al alloys, selected from the sameextended + tie-line, was investigated at1200°C in air. The single-phase alloy suffered from extensive internal oxidation andoxidized at a much higher rate than the single-phase alloy. In a two-phase + microstructure, the phase was preferentiallyattacked to form internal alumina andTi-rich nitride. This preferential attack of limited the extent to which the phase wasinternally oxidized, but also interrupted the formationof a continuous alumina scale. The single-phase alloyalso did not form a continuous alumina scale. Theinability of the phase to form continuousalumina was attributed to a combination of nitrideformation and internal oxidation. The oxidation behaviorof the two-phase + Nb-Ti-Al alloys isdiscussed in terms of mechanisms developed for theoxidation of binary, two-phase alloys.     

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.     

Microstructure and High-Temperature Oxidation Behavior of Dy-Doped Nb–Si-Based Alloys

Microstructures and oxidation behaviors of four Dy-doped Nb–Si-based alloys at 1250℃ were investigated. The nominal compositions of the four alloys are Nb–15Si–24Ti–4Cr–2Al–2Hf–xDy(at.%), where x = 0, 0.05, 0.10 and 0.15,respectively. Results showed that the four alloys all consisted of Nbss, αNb_5Si_3 and γNb_5Si_3, and the addition of Dy produced no obvious effect on the phase constitution and the microstructures of Nb–Si-based alloys. After oxidation at 1250℃ for 58 h, it was found that the addition of Dy accelerated the oxidation rate of Nb–Si-based alloys and caused a larger weight gain, accompanied by the formation of a more porous and less protective oxide scale. The oxides of Nb_2O_5,Ti_2Nb_(10)O_(29), TiNb_2O_7, Ti_(0.4)Cr_(0.3)Nb_(0.3)O_2 and glassy SiO_2 were formed on Dy-doped Nb–Si-based alloys. The hightemperature oxidation mechanism of Dy-doped Nb–Si-based alloys was discussed.     

Effects of Cooling Rate and Component on the Microstructure and Mechanical Properties of Mg–Zn–Y Alloys

The microstructure and mechanical properties of Mg–6Zn–1Y and Mg–6Zn–3Y(wt%) alloys under different cooling rates were investigated. The results show that the second dendrite arm spacing(SDAS) of Mg–6Zn–1Y and Mg–6Zn–3Y is reduced by 32 and 30% with increasing cooling rates(Rc) from 10.2 to 23 K/s, which can be predicted using a empirical model of SDAS=68 R 0:45:45cand SDAS=73 R 0c, respectively. The compressive strength of both alloys increases with increasing the cooling rate, which is attributed to the increase of volume fraction(Vf) of secondary phases under high cooling rate. The interaction of the cooling rate and component with SDAS has been theoretically analyzed using interdependence theory.     

Effect of the Application of a Mechanical Load on the Oxide-Layer Microstructure and on the Oxidation Mechanism of Ni–20Cr Foils

The effect of a tensile load on the oxidation kinetics and mechanism ofNi–20Cr was studied by comparison of the oxidation behavior ofNi–20Cr thin strips in air under classical conditions, i.e., withoutany applied mechanical load and under tensile creep, at temperatures between500 and 900°C. The study was performed mainly by comparisons of crosssections of oxidized samples observed by SEM. The results obtained clearlyindicate that applying a tensile load induces an increase in the oxidationrate, does not modify the oxide-film morphology, but promotes the formationof internal oxidation at low temperatures, 500–600°C, and notablyincreases the thickness of the intermediate NiCr₂O₄layer at 900°C. This is related to the acceleration of anionic diffusionwhen a tensile load is applied, due to the formation of fast-diffusion byshort-circuit paths.     

The Effect of Doping on the Corrosion Behavior of Quasi-Crystalline Alloys in the Al–Cu–Fe–Cr System

The electrochemical behavior of five alloys of variable compositions in the Al₆₅Cu₂₅Fe_10–хCr _х system in dependence on the number of QC phases in acidic and alkaline media has been investigated by the potentiodynamic method. It has been established that the samples’ corrosion stabilities increase along with the increase of the solution pH. Higher stability was manifested by alloys with a predominant quasi-crystalline (dexagonal and icosahedral) structural component.     

Influence of Initial Microstructure on the Strengthening Effect of Extruded Mg–8Sn–4Zn–2Al Alloys

The Mg–8Sn–4Zn–2Al(TZA842, in wt%) alloys with different initial microstructure(as-cast-AC and homogenization treatment-HT) subjected to hot extrusion. Also, the strengthening responses to AC and HT for the extruded TZA842 alloys were reported. The results revealed that the alloy subjected to HT shows finer grain size, more homogenous microstructure and weaker basal texture than those of counterpart subjected to AC. In addition, compared with TZA842-AC alloy, precipitates were finer and uniformly dispersed in TZA842-HT owing to the utilization of HT. Moreover, the TZA842-HT alloy showed higher yield strength of 200 MPa, ultimate tensile strength of 290 MPa and elongation(EL) of17.9% than those of TZA842-AC, which was mainly attributed to the combined effects of grain boundary strengthening,precipitation strengthening, solid solution strengthening and weak texture. Strengthening mechanism for both alloys was discussed in detail.     

Effect of Strontium and Magnesium Additions on the Microstructure and Mechanical Properties of Al–12Si Alloys

Metals and Materials International - In this study, a binary Al–12Si, eight ternary Al–12Si–Sr, and six quaternary Al–12Si–0.1Sr–(0.2–1)Mg alloys were...     

The Effect of Pre-Oxidation Treatment on the High-Temperature Oxidation of Co–Re–Cr Model Alloys at Laboratory Air

The aim of a pre-oxidation treatment at low oxygen partial pressure is to promote the formation of a Cr₂O₃ (or Al₂O₃) scale in such a way that the oxide layer can reliably prevent the contact of oxygen with the metallic substrate also at high oxygen partial pressure. In this study, the pre-oxidation treatment was applied to the two alloys Co–17Re–23Cr and Co–17Re–30Cr (at.%). Pre-oxidation of the Co–Re–xCr alloys was found to be non-protective for the metallic substrate at high oxygen partial pressure despite the formation of a Cr₂O₃ layer. Different kinds of Cr₂O₃ scale damage were observed depending on the Cr concentration. The Cr₂O₃ scale formed on the alloy Co–17Re–23Cr loses its protective properties as a result of cation transport by lattice and grain boundary diffusion, while the Cr₂O₃ scale formed on the alloy Co–17Re–30Cr degraded as a consequence of scale cracking. In addition, the effect of alloying with Si was investigated and found to be promising.