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.     

An overview of the oxidation performance of silicide diffusion coatings for vanadium-based alloys for generation IV reactors

This study focuses on the development of new protective coatings for the vanadium-based alloy V-4Cr-4Ti. Halide-activated pack-cementation (HAPC) technique was used to develop V_xSi_y multilayered diffusive silicide coatings. The outer layers (coatings) were formed of VSi₂ doped with 27 at.% Cr or TiSi₂. These compounds exhibited a very low oxidation rate at 650 °C, both in air and at a low oxygen pressure (He, 5 ppm O₂). The coatings formed mainly of MSi₂ were found to be insensitive to pesting and largely unreactive to liquid sodium (<10 ppm O₂) during a 360 h compatibility test at 550 °C.     

Oxidation behavior at 300–1000 °C of a (Mo,W)Si2-based composite containing boride

The oxidation behavior of a (Mo,W)Si₂ composite with boride addition was examined at 300–1000 °C for 24 h in dry O₂. The oxidation kinetics was studied using a thermobalance, and the oxide scales were analyzed using a combination of electron microscopy (SEM/EDX, FIB, BIB) and XRD. Accelerated oxidation was found to occur between 500 °C and 675 °C, with a peak mass gain at 625 °C. The rapid oxidation is attributed to the vaporization of molybdenum oxide that leaves a porous and poorly protective silica layer behind. At higher temperature (700–1000 °C) a protective scale forms, consisting of a dense SiO₂/B₂O₃ glass.     

Critical Al content to form external-alumina scales on Cu–Al alloys

The oxidation of Cu–6.8Al (at.%) alloy has been studied at 800 and 900 °C in 1 × 10⁵ Pa pure O₂. The scales formed at 800 °C are composed of a thin outer CuO layer and an inner protective Al₂O₃ layer. On the contrary, at 900 °C different samples of the alloy present two kinds of different oxidation behavior: one is protective, very similar to that at 800 °C, while the other is intermediate between protective and non-protective, with formation of very thick scale on the partial surface, which is mainly composed of copper oxides. The different behavior presented on a single sample is probably caused by local inhomogeneities of the alloy. It is deduced that at 900 °C the critical Al content to form external-alumina scale on Cu–Al alloy is about 6.8 at.%.     

High-temperature oxidation behavior of multi-phase Mo-containing γ-TiAl-based alloys

Intermetallic titanium aluminides are potential materials for a number of high-temperature components used in aero and automotive engines. In particular, alloys solidifying via the β-phase are of great interest because they possess a significant volume fraction of the disordered body-centered cubic β_o-phase at elevated temperatures ensuring good processing characteristics during hot-working. Nevertheless, the practical use of such alloys at a temperature as high as 800 °C requires improvement of their oxidation resistance. Various attempts have been made including alloying with additional elements such as Nb, Cr, Mo etc. or applying the so-called fluorine effect. However alloying could not provide a sufficient oxidation resistance above 850 °C whereas the fluorine effect protects the base material against environmental degradation up to over 1000 °C. This paper aims to investigate the influence of the phase composition on the oxide scale morphology without and with fluorine effect. The results refer to the oxidation behavior of three β-solidifying γ-TiAl-based alloys in the cast and hot-isostatically pressed condition at 800 °C in air. The behavior of the TNM alloy (Ti–43.5Al–4Nb–1Mo–0.1B, in at.%) was compared with that of two Nb-free TiAl alloys which contain different amounts of Mo (3 and 7 at.%, respectively) and hence a different microstructure (α₂/β_o/γ vs. β_o/γ). During testing in dry synthetic air at 800 °C a mixed oxide scale develops on all three alloys. This behavior was changed via the fluorine effect, as demonstrated for previously investigated TiAl alloys with an Al-content higher than 40 at.% based on α₂/γ and α₂/β_o/γ phases. The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated samples. The reason for this is the change in the oxidation mechanism triggered by the small additions of fluorine in the subsurface zone of the investigated alloys. The results of isothermal oxidation tests at 800 °C in air are presented and discussed in view of chemical composition and microstructure, along with the impact of the phase composition on the efficiency of the fluorine effect. From a microstructural perspective the fluorine effect leads to the formation of an even thinner oxide scale on the β-phase compared to the γ-phase.     

Role of nitrogen on the oxidative stability of Ti5Si3 based alloys at elevated temperature

Ti₅Si₃ has been extensively studied as a candidate material for high temperature application due to its high melting point (2130 °C), low density (∼4.3 g/cm³) and excellent oxidation resistance in oxygen above 1000 °C. However, stoichiometric Ti₅Si₃ alloy experiences accelerated oxidation during exposure in air above 1000 °C. It was proposed that nitrogen was responsible for the increased oxidation in air. In the present study, the isothermal reaction kinetics of Ti₅Si₃ in nitrogen at 1000 °C was investigated. Compared to a slow parabolic oxidation rate in oxygen, a faster linear reaction rate was observed when Ti₅Si₃ is exposed to nitrogen. Further studies on the oxidation behavior for changing nitrogen/oxygen atmospheres showed that Ti₅Si₃ is stable for exposure up to 400 h at 1000 °C when the gas contained 50% N₂. Breakaway oxidation occurs after short exposures when the gas contained at least 75% N₂, and the reaction rate increased as the concentration of N₂ increased. Furthermore, time to breakaway oxidation decreases with the increasing nitrogen partial pressure. Extensive analysis of the oxidation products with SEM and XRD revealed that the formation and fast growth of a nitride-containing subscale interferes with the establishment of the continuous protective silica scale and contributes to the breakaway oxidation.     

Oxidation behavior of a Zr–Cu–Al–Ni amorphous alloy in air at 300–425 °C

The oxidation behavior of Zr–30Cu–10Al–5Ni bulk metallic glass and its crystalline counterpart was studied over the temperature range of 300–425 °C in dry air. In general, the oxidation kinetics of both amorphous and crystalline alloys followed a two- or three-stage parabolic rate law at T⩾350 °C, while at 300 °C the amorphous alloy oxidized following a linear behavior. The oxidation rate constants for the amorphous alloy are slightly higher than those for the crystalline alloy at 350–400 °C. The scale formed on the amorphous alloy consists of mainly tetragonal-ZrO₂ at 300 °C, while a mixture of monoclinic-ZrO₂ (m-ZrO₂) and tetragonal-ZrO₂ (t-ZrO₂) and some CuO were detected at higher temperatures. The scale formed on the crystalline alloy, on the other hand, consists of mainly Al₂O₃, some tetragonal-ZrO₂, and a slight amount of monoclinic-ZrO₂ at 300 °C. At higher temperatures, the crystalline alloy consists of mainly monoclinic-ZrO₂, some CuO and Cu₂O, and limited tetragonal-ZrO₂. It is suggested that the formation of Al₂O₃ (at 300 °C) and CuO/Cu₂O (at 350-400 °C) on the crystalline alloy is responsible for the reduced oxidation rates as compared with those of amorphous alloy.     

The oxidation behavior of a novel Ni-free Zr–Cu-based bulk metallic glass composite in the supercooled liquid and crystallization states

The oxidation behavior of a novel Ni-free (Zr₄₈Cu₃₂Al₈Ag₈,Ta₄)Si_0.75 bulk metallic glass composite (BMG-C) in dry air in the supercooled liquid state (SLS at 430 °C and 480 °C) and the crystallization state (CS at 520 °C and 560 °C) for 100 h were studied herein.Test results showed that the oxidation kinetics of the BMG-C in the SLS and CS followed a multi-stage oxidation rate law. The scales forming in both the SLS and CS consist of t-ZrO₂, and m-ZrO₂, CuO and Ag. In the CS, additional Al₂O₃ was observed. In the substrate area, Cu₁₀Zr₇ crystalline likely formed in the amorphous substrate in the SLS. In the CS, more crystallization phases were found in the substrate, including additional CuZr₂ observed at temperatures ≥520 °C; AlCu₂Zr was observed at 560 °C.The Ta-reinforced phase in the BMG-C was more likely to react with Si in the scales forming Ta₂Si at temperatures ≥480 °C, which resulted in cracks in the Ta. Furthermore, channels between Ta precipitates and the matrix might exist, facilitating oxygen diffusivity. As the oxidation temperature (e.g., CS) and test duration were increased, the effects of the cracks and the channels became more significant and were responsible for the fast-growth oxidation in the final stage of the test.     

High temperature oxidation of Mo–Si–B alloys: Effect of low and very low oxygen partial pressures

The oxidation mechanism of a Mo–Si–B alloy in two different oxygen partial pressure ranges was investigated between 820 and 1200 °C. Oxygen partial pressures between 10^?19 and 10^?12 bar were applied in order to suppress Mo oxide formation. Weight gain kinetics were determined resulting from simultaneous external and internal oxidation. Silica scale formation was found to lead to a droplet shape because of the high evaporation rates of B₂O₃ and limited wetting of the silica. In the oxygen partial pressure range 10^?6–10^?4 bar Mo–Si–B alloys suffer from severe degradation due to continuous formation of volatile MoO₃. Catastrophic oxidation was observed as a consequence of the formation of a highly porous and non-protective silica scale.     

Effects of temperature and the incorporation of W on the oxidation of ZrB2 ceramics

The effects of tungsten additions and temperature on the oxidation behavior of nominally pure ZrB₂ and ZrB₂ containing 4, 6 or 8 mol% of W after oxidation at temperatures ranging from 800 to 1600 °C were investigated. For pure ZrB₂, the protective liquid/glassy layer covering the surface as a result of oxidation was evaporated above 1500 °C. For (Zr,W)B₂ specimens, the liquid/glassy layer was present after exposure up to 1600 °C. The higher stability of the liquid/glassy phase in the W-containing compositions was attributed to the presence of tungsten in the liquid/glassy phase, resulting in improved oxidation resistance for ZrB₂ samples containing W.     

High temperature oxidation of AlCrFe complex metallic alloys

Isothermal oxidation of Al₆₅Cr₂₇Fe₈ and Al₈₀Cr₁₅Fe₅ was studied in the 600–1080 °C range. Formation of transient alumina layers is obtained up to 900 °C. On Al₆₅Cr₂₇Fe₈ transient to α-phase transformations occur when performing oxidation at 1000 °C, together with the possible appearance of (Al_0.9Cr_0.1)₂O₃. At 1080 °C, direct formation of α-alumina is obtained. On Al₈₀Cr₁₅Fe₅, spallation of the oxide layer during the cooling stage is observed following oxidation at 800 and 900 °C, revealing thermal etching of the underneath alloy surface. At 1050 °C the α-Al₂O₃ scale is directly formed but plastic deformation and recrystallization of the underneath alloy into several intermetallic phases is observed.     

Oxidation behavior of magnetron sputtered double layer coatings containing molybdenum,silicon and boron

Protective coating systems were applied to Mo–9Si–8B (at.%) alloys to prevent oxidation at elevated temperatures. The coatings produced by magnetron sputtering and subsequent annealing consisted of an outer oxidation protection layer and an interlayer between this and the substrate. Three amorphous outer layers with different compositions were deposited: Mo–45Si–25B, Mo–55Si–10B and Mo–29Si–15B (all in at.%). The interlayer was selected to give a diffusion barrier with the composition of the Mo₅SiB₂ (T2) phase. All coatings were dense and well-adherent. During vacuum annealing the amorphous as-deposited coatings became crystalline exhibiting mainly the intermetallic Mo₅SiB₂ compound as interlayer and the MoSi₂, Mo₅Si₃ and MoB phases in the top layers. The samples were exposed to dry laboratory air in the pesting regime at 800 °C and above, i.e. at 1000 and 1300 °C for up to 100 h under cyclic conditions. All coatings were protective at 800 and 1000 °C for at least 100 h and showed a marked improvement in mass change compared to the uncoated substrate. For protection at 800 °C higher boron content is preferential, while at higher oxidation temperatures a lower boron content provides improved oxidation protection. At 1300 °C stress induced failures like cracking, spallation and buckling occurred due to the relatively high CTE mismatch between PVD coating and substrate. Even though, the mass change was still markedly reduced as compared to the bare substrate.     

High temperature oxidation of Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C alloy

A newly developed Ti–46Al–6Nb-0.5W-0.5Cr-0.3Si-0.1C alloy was oxidized isothermally and cyclically in air, and its high-temperature oxidation behavior was investigated. When the alloy was isothermally oxidized at 700 °C for 2000 h, the weight gain was only 0.15 mg/cm². The parabolic rate constant, k_p (mg²/cm⁴·h), measured from isothermal oxidation tests was 0.002 at 900 °C and 0.009 at 1000 °C. Such excellent isothermal oxidation resistance resulted from the formation of the dense, continuous Al₂O₃ layer between the outer TiO₂ layer and the inner (TiO₂-rich, Al₂O₃-deficient) layer. The alloy also displayed good cyclic oxidation resistance at 900 °C. Some noticeable scale spallation began to occur after 68 h at 1000 °C during the cyclic oxidation test.     

Magnetron-sputtered oxidation protection coatings for Mo–Si–B alloys

Oxidation protective Mo–70Al, Mo–37Si–15B and Mo–46Si–24B (at.%) coatings with 5–10 μm thickness were deposited on Mo–9Si–8B alloys by magnetron sputtering; and their oxidation behavior was studied at 800, 1000 and 1300 °C in air. On the Mo–70Al layer a dense aluminum borate scale grew at 800 °C; however, this coating rapidly degraded at 1000 °C linked to substrate oxidation at uncoated areas. The Mo–37Si–15B and Mo–46Si–24B layers provided oxidation protection to the Mo–Si–B alloy at 800 and 1000 °C for up to 100 h due to formation of a borosilicate scale. The latter coating was protective for short times even at 1300 °C.     

Characterisation of oxide films formed on Co–29Cr–6Mo alloy used in die-casting moulds for aluminium

Oxide films formed at 700 °C on Co–29Cr–6Mo alloy were characterised extensively to improve the corrosion resistance of the alloy to liquid Al, enabling its use in Al die-casting moulds. Film of duplex layer consisting of an outer CoO-rich layer and an inner Cr₂O₃-rich layer was observed in samples subjected to oxidation for 4 h. With an increase in duration of oxidation, CoO was gradually replaced by Cr₂O₃, resulting in a single-layered oxide film dominantly composed of Cr₂O₃. The oxide film evolved with duration of oxidation treatment indicating the possibility of optimising films for Al die-casting moulds.     

Scale composition and oxidation mechanism of the Ti–46Al–8Nb alloy in air at 700 and 800 °C

It is known that the oxide scale formed on TiAl alloys is generally composed of a mixture of alumina (Al₂O₃) and titania (TiO₂). The presence of niobium changes the activities of Ti and Al and influences the kinetics of oxidation and oxide layer composition. In this work, the Ti–46Al–8Nb alloy was subjected to cyclic oxidation in air at 700 °C (for 2 and 24 h) and 800 °C (for 300 h). Scale composition was analyzed by means of different techniques including X-ray photoelectron spectroscopy, X-ray diffraction and secondary ion mass spectroscopy. The scale consisted of several layers. The outer layer was built of alumina (amorphous or with very fine grains), whereas the inner layer – mainly of titania. After a longer exposure at a higher temperature (800 °C), niobium-rich precipitates and aluminum oxide grains were detected near to the alloy/scale interface and titanium nitride was found in the inner parts of the scale. Oxidation mechanism was studied by two-stage oxidation method using oxygen-18 and oxygen-16 isotopes combined with SIMS analyses. The distribution of oxygen isotopes over the alloy/scale interface indicated mixed inward/outward diffusion at the of reacting species. The experiments using Au markers showed that after longer oxidation time the inward diffusion was a predominant transport process.     

High-temperature oxidation and hot corrosion of Cr2AlC

High-temperature oxidation and hot corrosion behaviors of Cr₂AlC were investigated at 800–1300 °C in air. Thermogravimetric–differential scanning calorimetric test revealed that the starting oxidation temperature for Cr₂AlC is about 800 °C, which is 400 °C higher than other ternary transition metal aluminum carbides. Thermogravimetric analyses demonstrated that Cr₂AlC displayed excellent high-temperature oxidation resistance with parabolic rate constants of 1.08 × 10⁻¹² and 2.96 × 10⁻⁹ kg² m⁻⁴ s⁻¹ at 800 and 1300 °C, respectively. Moreover, Cr₂AlC exhibited exceptionally good hot corrosion resistance against molten Na₂SO₄ salt. The mechanism of the excellent high-temperature corrosion resistance for Cr₂AlC can be attributed to the formation of a protective Al₂O₃-rich scale during both the high-temperature oxidation and hot corrosion processes.     

High-temperature oxidation behavior of minor Hf doped NiAl alloy in dry and humid atmospheres

Oxidation behavior of NiAl and 0.05 at.% Hf doped NiAl alloys were investigated at 1100 °C in dry and humid atmospheres. Hf doping significantly improved the cyclic oxidation resistance. Water vapour promoted the formation of voids at the scale/alloy interface and accelerated scale spallation. Also, water vapour had effect on the phase transformation from θ- to α-Al₂O₃ at the initial oxidation stage. In humid atmosphere, more Hf segregated at the scale/alloy interface to form oxide pegs. Pre-oxidation process in O₂ + Ar could compromise the effect of humid atmosphere on the oxidation kinetics of the NiAl alloys.     

Formation and stability of the clathrate-I structure in the systems Sr–(Ni,Cu,Zn)–Ge based on experimental and DFT studies

The phase relations in the Ge-rich corner of the Sr–Ni–Ge system are investigated at 700 °C by X-ray powder diffraction and electron probe microanalysis. The formation of one ternary compound, SrNiGe₃, crystallizing in the BaNiSn₃-structure type, is observed. No clathrate-I compound was detected in as cast or annealed state.Theoretical calculations based on density functional theory (DFT) were performed to gain information on the heat of formation for three different clathrate-I compounds, Sr₈T₆Ge₄₀ (T = Ni, Cu, Zn) and relevant other Ge-rich ternary compounds, namely SrT₂Ge₂ (T = Cu, Zn) and SrNiGe₃. Based on these data and the phase relations at 700 °C energies of formation of the clathrate-I phases were calculated and predictions of the phase equilibria at 0 K are provided.     

The effect of grain refinement on the adhesion of an alumina scale on an aluminide coating

To understand the effect of grain refinement on the thermally grown alumina scale adhesion to the metal substrate, two δ-Ni₂Al₃ coatings, one coarse-grained (∼70 μm) and the other ultrafine-grained (generally below ∼500 nm), were prepared. The cyclic oxidation in air at 1100 °C shows that the ultrafine-grained (UFG) coating is better oxidation resistant than the coarse-grained (CG) coating due to the formation of a more adherent alumina scale. The latter is intrinsically correlated with the fact that the aluminide grain refinement helps to increase the oxide/metal strength through a route to prevent the formation of large-sized voids at the interface.