Interaction between MoO3 and metals Te, Cd, Sb

Eighteen compositions of MoO₃-Te at 800 °C and seven of each of MoO₃-Cd (at 500 °C) and MoO₃-Sb (at 600 °C) were heat treated in vacuum-sealed quartz ampules. The phases of the heat-treated compositions were analyzed using x-ray diffraction (XRD) patterns. The interactions in the three systems are summarized. Three phases in equilibrium are (1) in the MnO₃-Te system at 800 °C, Te, Mo₄O₁₁, TeMo₄O₁₃—(0<x_{MoO 3}<0 889) and MoO₃, Mo₄O₁₁, TeMo₄O₁₃—(0.889<x_{MoO 3}<1); (2) in the MoO₃_Cd system at 500 °C, Cd, MoO₂, CdMoO₄—(0<x_{MoO 3}<0.6667) and MoO₃, MoO₂, CdMoO₄—(0.6667<x_{MoO 3}<1); and (3) in the MoO₃-Sb system at 600 °C, Sb, MoO₂, Sb₄Mo₁₀O₃₁—(0<x_{MoO 3}<0.734) and MoO₃+MoO₂+Sb₄Mo₁₀O₃₁ (0.734<x_{MoO 3}<1). The results lead to construction of ternary phase diagrams: Te-MoO₃-TeMo₄O₁₃, Cd-MoO₃-CdMoO₄, and Sb-MoO₃-Sb₄Mo₁₀O₃₁.     

Effect of ammonium dimolybdate (ADM) on the reduction of molybdenum trioxide

Ammonium dimolybdate ((NH₄)₂Mo₂O₇) or molybdenum trioxide (MoO₃) is used as starting raw materials for manufacturing Mo powders. In the initial step, usually carried out in rotary calciners, (NH₄)₂Mo₂O₇ or MoO₃ is reduced to MoO₂. Agglomeration of powder due to melting of eutectic formed between MoO₃ and Mo₄O₁₁ and due to melting of MoO₃ occurs during this reduction step resulting in several manufacturing issues. The reduction from (NH₄)₂Mo₂O₇ involves an endothermic reaction however, reduction of MoO₃ occurs only by exothermic reaction. It is hypothesized that addition of (NH₄)₂Mo₂O₇ to MoO₃ will decrease agglomeration of powders due to the endothermic reaction involved in the reduction process. The current paper details experiments carried out to verify the hypothesis. MoO₃ containing varying amounts (NH₄)₂Mo₂O₇ were reduced at 550 °C, 650 °C and 750 °C in hydrogen atmosphere. The results show lower agglomeration of powder with addition of (NH₄)₂Mo₂O₇. The thermal analysis results confirm reduction of MoO₃ at lower temperatures with the addition of (NH₄)₂Mo₂O₇.     

Phase Relations in the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-MoO<Subscript>3</Subscript> System in the Solid State. The Crystal Structure of AlVO<Subscript>4</Subscript>

Phase relations in the ternary oxide system Al₂O₃-V₂O₅-MoO₃ in the solid state in air have been investigated by using the x-ray diffraction (XRD) and differential thermal analysis/thermogravimetric (DTA/TG) methods. It was confirmed that in the subsolidus area of the Al₂O₃-V₂O₅-MoO₃ system, there exist seven phases, that is Al₂O₃, V₂O_5(s.s.), MoO₃, AlVO₄, Al₂(MoO₄)₃, AlVMoO₇, and V₉Mo₆O₄₀. Seven fields, in which particular phases coexist at equilibrium, were isolated. The crystal structure of AlVO₄ has been refined from x-ray powder diffraction data. Its space group is triclinic, , Z = 6, with a = 0.65323(1) nm, b = 0.77498(2) nm, c = 0.91233(3) nm, α = 96.175(2)°, β = 107.234(3)°, γ = 101.404(3)°, V = 0.42555 nm³. The crystal structure of the compound is isotypic with FeVO₄. Infrared (IR) spectra of AlVO₄ and FeVO₄ are compared.     

Agglomeration during reduction of MoO3

Molybdenum powder is manufactured in a two step process starting from MoO₃. The first step reduction of MoO₃ to MoO₂ is carried out in rotary calciners. Agglomeration of powder occurs during this reduction stage resulting in several manufacturing issues. The evolution of agglomeration during the reduction of MoO₃ was investigated in the current study. As-received MoO₃ and MoO₃ milled for 0.5 h were used as the starting powders. The powders were reduced at 550 °C, 650 °C and 750 °C in a hydrogen atmosphere. The starting and reduced powders at various temperatures were analyzed using BET surface area, XRD, and SEM techniques. The surface area of the reduced powders was monitored for quantifying the degree of agglomeration. The surface area was found to be minimum for the samples reduced at 650 °C. SEM observations confirmed the agglomeration of powders during reduction process. XRD analysis showed complete reduction of MoO₃ to MoO₂ at 650 °C and 750 °C. The agglomeration of the powders was either due to melting of eutectic formed between MoO₃ and Mo₄O₁₁ or due to partial melting of MoO₃. The reduction of MoO₃ is recommended to be completed at a low temperature to prevent agglomeration of the oxide powders.     

Oxidation behaviour of Mo–Si–B–(Al,Ce) ultrafine-eutectic dendrite composites in the temperature range of 500–700 °C

A comparative study has been carried out to investigate the effects of Al and/or Ce additions on microstructure of Mo–Si–B alloys and their isothermal oxidation behaviour at 500 and 700 °C in laboratory air for 24 h. Microstructure of arc melted Mo–Si–B–(Al, Ce) alloys consists of bcc α-Mo dendrites embedded in ultrafine lamellar Mo₃Si and Mo₅SiB₂ eutectic matrix. Isothermal oxidation kinetics of ultrafine structured Mo–Si–B alloy at 500 °C has been found to show hardly any mass change during 24 h exposure. Addition of Al to Mo–Si–B alloy refines the microstructure, decreases the net mass loss at 700 °C by ～43%, whereas Ce does not bring about any significant change. The enhanced oxidation resistance of Mo–Si–B–Al alloy is due to the formation of Al–O rich inter-layer at the alloy/oxide interface along with the formation of a protective and dense Al₂(MoO₄)₃ outer layer, which reduces the sublimation of MoO₃ at 700 °C. Various transient/complex oxides formed on the alloys during their high temperature exposure have been examined to determine the oxidation mechanisms.     

Oxidation behaviour of the Mo–Si–B and Mo–Si–B–Al alloys in the temperature range of 700–1300 °C

The isothermal oxidation kinetics of molybdenum silicide based alloys with composition (in at.%) as 76Mo–14Si–10B (MSB), 77Mo–12Si–8B–3Al (MSB3AL), and 73.4Mo–11.2Si–8.1B–7.3Al (MSB7.3AL) processed by reaction hot pressing of elemental powders, have been investigated in the temperature range of 700–1300 °C in dry air for 24 h. The microstructures of all the alloys have shown the presence of α-Mo, Mo₃Si, Mo₅SiB₂ and SiO₂ or α-Al₂O₃ phases. The oxidation kinetics and the resulting scale characteristics depend on the alloy composition and temperature of exposure. While all the three alloys show unabated loss of mass causing pest disintegration at 700 °C, the MSB3AL and MSB7.3AL alloys undergo large mass loss in the range of 800–900 °C as well. The loss in mass has been attributed primarily to volatilization of MoO₃ as well as spallation. The oxide scales formed in the range of 700–800 °C show SiO₂ and MoO₃, while those formed at 900 °C or above contain Mo, MoO₂ and SiO₂. In addition, α-Al₂O₃ or mullite has been found in the oxide scales of MSB3AL and MSB7.3AL alloys. The oxidation resistance of the Mo–Si–B alloys can be enhanced in the range of 700–800 °C by pre-oxidation treatment at 1150 °C to form a protective scale of B₂O₃–SiO₂.     

Magnesium-assisted formation of metal carbides and nitrides from metal oxides

A series of carbides (TiC, V₂C, Mo₂C) were synthesized by the corresponding metal oxides (TiO₂, V₂O₅, MoO₃), CaC₂ and magnesium as starting materials in a stainless steel autoclave at 600 °C. Through similar processes, transition metal nitrides (TiN, VN and CrN) could also be produced by employing the corresponding metal oxides (TiO₂, V₂O₅, Cr₂O₃), NaNH₂, and magnesium as starting materials at 550 °C. The FE-SEM image showed that the TiC sample was mainly consisted of flower-like microstructures. TEM image showed the other carbides (V2C, Mo2C) and the obtained nitride (TiN, VN, CrN) were consisted of nanoparticles. The possible synthesis mechanism of TiC had been described.     

Mechanism and kinetic study of hydrogen reduction of ultra-fine spherical MoO3 to MoO2

The mechanism and kinetics of hydrogen reduction of ultra-fine spherical MoO₃ to MoO₂ have been investigated in the present work. The results show that the reduction of MoO₃ to MoO₂ obeys the two-step reduction mechanism with the generation of intermediate product Mo₄O₁₁. The final product MoO₂ always keeps the same morphology as Mo₄O₁₁. The experimental data can be well described by using the dual interface reaction model. It was found that the rate controlling steps for the first (from MoO₃ to Mo₄O₁₁) and second reactions (from Mo₄O₁₁ to MoO₂) were interface chemical reaction and diffusion, respectively, with the activation energies extracted to be 122 kJ/mol and 114 kJ/mol. When the reaction extent (defined as the ratio of weight loss of MoO₃ at time t to the theoretical maximum weight loss from MoO₃ to MoO₂ due to the removal of oxygen during the reduction process) is in the range of 0 to 0.7, both the reduction of MoO₃ to Mo₄O₁₁ and Mo₄O₁₁ to MoO₂ occurred simultaneously; when the reaction extent is in the range of 0.7 to 1, only the reduction of Mo₄O₁₁ to MoO₂ occurred in the reduction process.     

Oxidation mechanisms in Mo-reinforced Mo5SiB2(T2)–Mo3Si alloys

Results of a systematic investigation of the oxidation in a Mo–Si–B alloy containing the three phases, Mo, Mo₃Si, and Mo₅SiB₂ (T2) are presented. The relative kinetics of B-containing silica-scale formation, permeation of MoO₃ through the viscous scale, the viscosity of the B-containing SiO₂ scale, volatilization of MoO₃ and B₂O₃ from the silica scale, are identified as key parameters that determine the kinetics of the oxidation of the alloy in the temperature range of 500–1300 °C. The oxidation is worst in the intermediate temperature range, 650–750 °C, where MoO₃ begins to volatilize but B₂O₃ does not, resulting in gaseous MoO₃ bubbling through a low viscosity borosilicate scale. In this temperature range, the scale provides insufficient protection suggesting that attempts to improve the oxidation resistance of this system must focus on this temperature range.     

Constitution of the systems {V,Nb,Ta}-Sb and physical properties of di-antimonides {V,Nb,Ta}Sb2

The binary phase diagrams {V,Nb,Ta}-Sb below 1450 °C were studied by means of XRPD, EPMA, and DTA measurements. In the V-Sb system, five stable binary phases were observed in this investigation: V_3+xSb_1−x, ℓT-V₃Sb₂, hT-V_2−xSb, V_7.46Sb₉, V_1−xSb₂. The V-Sb phase diagram is characterized by two degenerate eutectic reactions: L↔V_3+xSb_1−x+(V) (T > 1450  °C at 18.1 at.% Sb) and L↔V_1−xSb₂+(Sb) (T=(621 ± 5)°C at ∼99 at.% Sb), three peritectic reactions: L + V_3+xSb_1−x↔hT-V_2−xSb (T=(1230 ± 10)°C at ∼42 at.% Sb), L + hT-V_2−xSb↔V_7.46Sb₉ (T=(920 ± 10)°C at ∼87 at.% Sb), and L + V_7.46Sb₉↔V_1−xSb₂ (T=(869 ± 5)°C at ∼88 at.% Sb), a peritectoid reaction: V_3+xSb_1−x + hT-V_2−xSb↔ℓT-V₃Sb₂ at (875 ± 25)°C, a eutectoid reaction: hT-V_2−xSb↔ℓT-V₃Sb₂+V_7.46Sb₉ at (815 ± 15)°C and congruent melting of V_3+xSb_1−x (T > 1450 °C). An X-ray single crystal study of V₅Sb₄C_1−x proved the existence of interstitial elements in the octahedral voids of a partially filled Ti₅Te₄-type structure (x∼0.5; R_F2 = 0.0101), therefore this phase (earlier labeled “V₅Sb₄”) was excluded from the binary equilibrium phase diagram. V₅Sb₄C_1−x is the first representative of a filled Ti₅Te₄-type structure.A re-investigation of the Nb-Sb system removed the contradiction between the hitherto reported phase diagrams and confirmed the version derived by Melnyk et al. (see ref. [1]).Three binary phases exist in the Ta-Sb system: Ta_3+xSb_1−x, Ta₅Sb₄, TaSb₂. Due to instrumental limits (≤1450 °C), only the peritectic reaction of TaSb₂: L + Ta₅Sb₄ ↔ TaSb₂ ((1080 ± 10)°C at ∼92 at.% Sb) and a degenerate Sb-rich eutectic reaction (L↔TaSb₂+(Sb); (622 ± 5)°C; ∼99 at.% Sb) have been determined.Physical properties (mechanical and transport properties) of binary di-antimonides were investigated with respect to a potential use of these metals either as diffusion barriers or electrodes for thermoelectric devices based on skutterudites. All group-V metal di-antimonides have low metallic-type resistivity and relatively high thermal conductivity. Magnetic field has little influence on the resistivity of V_1−xSb₂ at low temperature, while on {Nb,Ta}Sb₂ it increases the resistivity, especially on NbSb₂. The coefficient of thermal expansion (CTE) decreases from V_1−xSb₂ to TaSb₂, particularly the CTE value of NbSb₂ is in the range of average n-type filled skutterudites. In contrast to the CTE value, elastic moduli increase from V_1−xSb₂ to TaSb₂. The value for V_1−xSb₂ is in the range of Sb-based skutterudites, whereas the values for {Nb,Ta}Sb₂ are significantly higher.     

Subsolidus phase relations in the system ZnO–P2O5–MoO3

The subsolidus phase relations of the ternary system ZnO–P₂O₅–MoO₃ were investigated by means of X-ray diffraction (XRD). Seven binary compounds and eight 3-phase regions were determined, and no ternary compound was found in this system. The phase diagram of pseudo-binary system Zn₃(PO₄)₂–Zn₃Mo₂O₉ was also constructed through XRD and differential thermal analysis (DTA) methods, and the result reveals this system is eutectic system. The eutectic temperature is 904 °C and the corresponding component is 30% Zn₃Mo₂O₉ and 70% Zn₃(PO₄)₂.     

Microstructure and mechanical properties of diffusion bonded joints between titanium and stainless steel with copper interlayer

Abstract

The solid state joining of titanium to stainless steel with copper interlayer was carried out in the temperature range of 850–950°C for 7·2 ks in vacuum. The interface microstructures and reaction products of the transition joints were investigated with an optical microscope and a scanning electron microscope. The elemental concentration of reaction products at the diffusion interfaces was evaluated by electron probe microanalysis. The occurrence of difference in intermetallics at both interfaces (SS/Cu and Cu/Ti) such as CuTi₂, CuTi, Cu₄Ti₃, χ, FeTi, Fe₂Ti, Cr₂Ti, α-Fe, α-Ti, β-Ti, T₂(Ti₄₀Cu_60?xFe_x; 5<x<17), T₄(Ti₃₇Cu_63?xFe_x; 5<x<7) and T₅(Ti₄₅Cu_55?xFe_x; 4<x<5) has been predicted from the ternary phase diagrams of Fe–Cu–Ti and Fe–Cr–Ti. These reaction products were detected by X-ray diffraction technique. The maximum tensile strength of ～91% of Ti strength and shear strength of ～74% of Ti strength along with ～ 7·2% ductility were obtained for the joint bonded at 900°C due to better coalescence of mating surfaces. At a lower joining temperature of 850° C, bond strength is poor due to incomplete coalescence of the mating surfaces. With an increase in the joining temperature to 950°C, a decrease in bond strength occurred due to an increase in the volume fraction of brittle Fe–Ti base intermetallics.     

A study on mechanochemical behavior of MoO3–Mg–C to synthesize molybdenum carbide

The influence of Mg value in the MoO₃–Mg–C mixture on the molybdenum carbide formation and the mechanism of reactions during mechanochemical process were investigated. In keeping with this aim, magnesium and carbon contents of the mixture were changed according to the following reaction: 2MoO₃ + (6 − x) Mg + (1 + x) C = (6 − x) MgO + Mo₂C + x CO. The value of x varied from 0 to 6. Differential thermal analysis (DTA) results for sample with stoichiometric ratio (x = 0) revealed that in the early stage, carbon reduced the MoO₃ to MoO₂ and subsequently highly exothermic magnesiothermic MoO₂ reduction occurred after magnesium melting. Also, it was indicated that the exothermic reaction temperature shifted to before magnesium melting in the 11 h-milled sample (x = 0) and all the exothermic reactions happened, simultaneously. According to the experimental findings, molybdenum carbide (Mo₂C) was synthesized in the mixture powder with stoichiometric ratio (x = 0) after 12 h milling process and the type of reactions was mechanically induced self-sustaining reaction (MSR). However, at lower Mg content in the MoO₃–Mg–C mixture (0 < x ≤ 2), the magnesiothermic reduction occurred in MSR mode and activated the carbothermal reaction. Further decrease in Mg value (2 < x ≤ 3) resulted in MSR mode magnesiothermic reaction and gradual carbothermal reduction. In samples with lower magnesium contents, partial molybdenum oxide reduction proceeded through a gradual mode magnesiothermic reaction.     

Thermophysical properties of Sm2（Zr（1-x）Cex）2O7 ceramics

Sm₂(Zr_1−x Ce _x )₂O₇ (x = 0.1, 0.2, and 0.3) ceramics were prepared by solid reaction method at 1600°C for 10 h using Sm₂O₃, ZrO₂, and CeO₂ as starting reactants. The phase compositions, microstructures, thermal expansion coefficients, and partial thermal conductivities of these materials were investigated. X-ray diffraction (XRD) results reveal that Sm₂(Zr_0.9Ce_0.1)₂O₇ with pyrochlore structure and Sm₂(Zr_1−x Ce _x )₂O₇ (x = 0.2 and 0.3) with fluorite structure were synthesized, and scanning electrical microscopy (SEM) images show that the microstructures of these products are very dense. The linear thermal expansion coefficients increase with increasing temperature in the temperature range from ambient to 1200°C, and the thermal expansion coefficients increase with increasing content of doped CeO₂. The thermal conductivities of Sm₂(Zr_0.8Ce_0.2)₂O₇ and Sm₂(Zr_0.7Ce_0.3)₂O₇ decrease gradually with an increase in temperature. These results show that the synthesized ceramic materials can be explored as novel prospective candidate materials for use in new thermal barrier coating systems in the future.     

A study of the corrosion properties of plasma nitrocarburized and oxidized AISI 1020 steels

Plasma nitrocarburized AISI 1020 steels were oxidized for 15, 30 and 60 min to evaluate their corrosion and microstructural properties. After plasma nitrocarburizing for 3 h at 570°C in a gas mixture comprising 85 vol.% N₂, 12vol.% H₂ and 3 vol.% CH₄, the compound layer composed of ɛ-Fe_2–3(N,C) and γ’-Fe₄(N,C) phases and the diffusion layer above the matrix were observed. The top oxide layer, consisting mainly of magnetite (Fe₂O₄) and hematite (Fe₂O₃) phases, forms after post-oxidation treatment at 500°C. However, the oxide layer was severely degraded by spallation as a result of increases in post-oxidizing time. The difference in corrosion resistance should be attributed to the thickness of the top oxide layer, which was governed by post-oxidizing time.     

The oxidation behaviour of uniaxial hot pressed MoSi2 in air from 400 to 1400 °C

MoSi₂ samples were prepared by hot uniaxial pressing from a 2 μm grain-size powder of commercially available MoSi₂. The oxidation behaviour of MoSi₂ was systematically studied from 400 °C to 1400 °C, which includes the pest-oxidation temperature range. It was observed that the rate and mechanism for oxidation of MoSi₂ change significantly with increasing temperature. Five temperature regimes have to be considered regarding both kinetic results and cross-sections: i) 400 < T < 550 °C; ii) 550 ≤ T < 750 °C; iii) 750 ≤ T < 1000 °C; iv) 1000 ≤ T < 1400 °C; v) T ≥ 1400 °C. In the first range, pesting did not occur in samples that were free of cracks and residual stresses and the oxidation kinetics were governed by surface or phase boundary reactions. Above 550 °C, there was a change in the physical properties of the oxidation products due to the evaporation of MoO₃. The formation of Mo₅Si₃ was observed above 800 °C showing that the thermodynamic previsions were satisfied above this temperature. At higher temperatures (>1000 °C), the oxide scale became very protective and transport in the silica scale (amorphous and β cristobalite) governed the oxidation kinetics. The Mo₅Si₃ phase did not appear anymore at 1400 °C, indicating that another oxidation mechanism has to be proposed.     

采用TRPO从过氧化氢溶液中萃取钼和钨的数学模型和反应机理（英文）

为了解利用三烷基氧化膦(TRPO)从过氧化氢溶液中萃取钼和钨的化学行为,采用斜率法、拉曼和红外光谱研究钼和钨的萃取反应机理。通过建立数学模型,获得钼或钨的萃取分配比(D_Mo或D_W)关于平衡pH值、TRPO浓度和温度等变量的经验公式,并进一步在H⁺-W-Mo-H₂O₂溶液中验证经验公式的可靠性。结果表明:经验公式计算的D_Mo或D_W和实验值吻合度良好。实验条件下,20℃时钼和钨的萃取平衡常数分别为K_Mo^app=8.51×10³(0.74≤pH_e≤1.70)、K_Mo^app=99.89×10³(1.7e≤4.62)和K_W^app=2.65×10³(0.92e&l...     

Improvement of oxidation resistance of arc-melted Mo76Si14B10 by microstructure control upon minor Fe addition

Addition of Fe refines the microstructure of Mo_76-xSi₁₄B₁₀Fe_x (x = 0, 0.5, 1 at.%) composites containing α-Mo, Mo₃Si and Mo₅SiB₂ phases, increases the hardness from 950 Hv (x = 0) to 1031 Hv (x = 1), and improves the oxidation resistance at temperature in the range of 800–1300 °C. The hardness of the base alloy substrate decreases only by <7% than that of as-solidified ingots, indicating good microstructural stability of the composite for high temperature application.     

Measurement of 900 °C Isothermal Section in the Mo-Ni-Zr System

The isothermal section of the Mo-Ni-Zr system at 900 °C was investigated by characterization of eighteen equilibrium alloys. Electron probe microanalysis (EPMA) and x-ray diffraction (XRD) were used to identify the phases and obtain their compositions. The existence of two ternary compounds, Zr₆₅Mo_18?x Ni_16.5+x (τ₁, cF96-Ti₂Ni) and Zr₆₅Mo_27.3Ni_7.7 (τ₂, hP28-Hf₉Mo₄B), was confirmed in the Zr-rich corner, and the compositions of the two phases were determined. The isothermal section of the Mo-Ni-Zr system at 900 °C consists of 15 three-phase regions and 29 two-phase regions. The following three-phase equilibria were well established: (1) (Ni) + Ni₇Zr₂ + Ni₅Zr, (2) MoNi + MoNi₃ + Ni₇Zr₂, (3) Ni₇Zr₂ + MoNi + (Mo), (4) (Mo) + Ni₇Zr₂ + Ni₃Zr, (5) (Mo) + Ni₃Zr + Ni₂₁Zr₈, (6) (Mo) + Ni₂₁Zr₈ + Ni₁₀Zr₇, (7) (Mo) + Ni₁₀Zr₇ + NiZr, (8) (Mo) + Mo₂Zr + NiZr, (9) NiZr₂ + Mo₂Zr + τ₁, (10) τ₁ + Mo₂Zr + τ₂, (11) τ₂ + Mo₂Zr + (Zr)ht, (12) NiZr₂ + τ₁ + (Zr)ht and (13) τ₁ + τ₂ + (Zr)ht. Several binary phases, such as MoNi₃, Ni₇Zr₂ and Mo₂Zr, dissolve appreciable amount of the third component.     

The Effect of Water Vapor on the Initial Stages of Oxidation of the FeCrAl Alloy Kanthal AF at 900 °C

The effect of water vapor on the initial stages of oxidation of the FeCrAl alloy Kanthal AF is reported. Polished samples were exposed isothermally at 900 °C for 1, 24, 72 and 168 h in a well-controlled environment consisting of dry O₂ or O₂ + 40% H₂O. The samples were investigated using a combination of gravimetry and several surface-analytical techniques, including XRD, SEM, EDX, FIB, AES and TEM. The presence of water vapor significantly accelerates oxidation during the first 72 h. A two-layered oxide forms in both the dry and wet environments. The bottom layer consists of inward-growing α-Al₂O₃ while the outer layer initially consists of outward-growing γ-Al₂O₃. A straight and narrow Cr-enriched band is present at the top of the lower (α-Al₂O₃) oxide, corresponding to the original sample surface. In dry O₂, the top (γ-Al₂O₃) layer is converted into a mixture of γ-Al_2−x (Mg,Fe) _x O_3−(x/2), MgAl₂O₄ and α-Al₂O₃. This transformation does not occur in O₂ + H₂O. The initial acceleration of oxidation by H₂O is attributed to the stabilization of the outward-growing γ-alumina layer by the hydroxylation of the γ-Al₂O₃ surface. A schematic mechanism of the early stages of oxidation of FeCrAl alloys is presented, emphasizing the influence of water vapor.