The Effects of Molybdenum on the High-Temperature Oxidation Resistance of Thin Foils of Fe–20Cr–5Al at Very High Temperatures

Thin foils of Fe–20Cr–5Al alloys are susceptible to breakawayoxidation once the aluminum content of the substrate has fallen below somecritical value. The combined addition of 0.1 wt.% lanthanum and 0, 1, or 2wt% molybdenum has a beneficial effect on the high-temperature oxidation ofsuch foils. Lanthanum has the well-known reactive-element effect on adhesionof the protective alumina scale, thereby increasing the time to onset ofbreakaway oxidation, while, for alloys containing molybdenum, breakawayoxide spreads relatively slowly over the specimen in comparison to alloysthat contain no molybdenum. In particular, molybdenum-containing alloys areable to develop a protective Cr₂O₃ layer at the breakawayoxide–substrate interface. Conversely, molybdenum-free alloys form aninternal-oxide zone in the substrate adjacent to this interface, rather thana Cr₂O₃ layer, so breakaway oxide spreads rapidly. A martensitic phase isobserved in the substrate adjacent to the breakaway oxide formed on Fe–20Cr–5Al–La specimens, which means that the-phase has transferred to the -phase at the temperature ofthe oxidation test (1150°C). Conversely, -phase is retained inthe molybdenum-containing alloy, even after breakaway takes place, sincemolybdenum, which is a strong ferrite former, is enriched in the alloyadjacent to areas of breakaway oxide. The diffusion rate of chromium isslower in the than in the -phase so a continuouschromium-rich oxide layer, which is effective in inhibiting breakawayoxide from spreading, cannot be established at the breakawayoxide–substrate interface for the molybdenum-free alloys.     

Oxidation Kinetics and Surface Chemistry of an Fe–Cr–Al–Y Alloy Medium Made of 12-μm Diameter Fibers at Elevated Temperatures in Dry Air

Fiber media composed of Fe–Cr–Al–Y alloy are being used increasingly as materials for high-temperature applications for their excellent oxidation resistance. The oxidation kinetics of Fe–Cr–Al–Y alloy fiber medium as a heat-resistant material for high-temperature applications was studied in dry air at 1073, 1188, 1255, and 1318 K. The oxidation process followed the parabolic kinetic law. The alumina-scale growth was found to be influenced by short-circuit diffusion and the presence of stresses related to oxide-scale growth. The surface of the oxide scale formed on the fiber medium was analyzed using X-ray photoelectron spectroscopy, which revealed that the outer surface of the oxide scale formed on the fiber medium composed of 12-m diameter Fe–Cr–Al–Y alloy fibers, consisted of -Al₂O₃, -Al₂O₃, and Cr-oxide. The metastable -Al₂O₃ subsequently partially transformed into the more stable -phase following a time-temperature-transformation relationship. The surface morphology and the cross section of the oxide scale formed on the fiber medium in the temperature range 1073–1318 K in dry air, have been studied by scanning-electron spectroscopy (SEM) and focused-ion beam, respectively.     

Influence of Silicon and Aluminum Additions on the Oxidation Resistance of a Lean-Chromium Stainless Steel

The effect of Si and Al additions on the oxidation of austenitic stainless steels with a baseline composition of Fe–16Cr–16Ni–2Mn–1Mo (wt.%) has been studied. The combined Si and Al content of the alloys did not exceed 5 wt.%. Cyclic-oxidation tests were carried out in air at 700 and 800°C for a duration of 1000 hr. For comparison, conventional 18Cr–8Ni type-304 stainless steel specimens were also tested. The results showed that at 700°C, alloys containing Al and Si, and alloys with only Si additions showed weight gains about one half that of the conventional type-304 alloy. At 800°C, alloys that contained both Al and Si additions showed weight gains approximately two times greater than the type-304 alloy. However, alloys containing only Si additions showed weight gains four times less than the 304 stainless. Further, alloys with only Si additions preoxidized at 800°C, showed zero weight gain in subsequent testing for 1000 hr at 700°C. Clearly, the oxide-scale formation and rate-controlling mechanisms in the alloys with combined Si and Al additions at 800°C were different than the alloys with Si only. ESCA, SEM, and a bromide-etching technique were used to analyze the chemistry of the oxide films and the oxide–base-metal interface, in order to study the different oxide film-formation mechanisms in these alloys.     

Phase stability,oxidation, and interdiffusion of a novel Ni−Cr−Al−Ti−Si bond-coating alloy between 900 and 1100°C

A novel, low-expansion experimental Ni–Cr–Al–Ti–Si bond-coating alloy was investigated in the as-cast state concerning its phase stability, oxidation resistance in air, and interdiffusion with single-crystal IN-100 at 900, 1000, and 1100°C. Isothermal oxidative thermogravimetry was employed up to 500 hr. Interdiffusion was compared to a commercial Ni–Co–Cr–Al–Y alloy on IN-100. Oxidized Ni–Cr–Al–Ti–Si specimens and diffusion couples were characterized by metallography, SEM, EDX, XRD, and XRF. The Ni–Cr–Al–Ti–Si alloy provides good oxidation resistance in air at least up to 1000°C. The alloy is an alumina former. Due to its coarse microstructure, other oxides (e.g., rutile) may form and considerably dominate the oxidation behavior. The kinetics of oxidation were correlated with temperature, formation of phases, and morphology of oxides. Interdiffusion fluxes between Ni–Cr–Al–Ti–Si and IN-100 were mainly directed to the superalloy. They were faster than in Ni–Co–Cr–Al–Y/IN-100 diffusion couples.     

Metal Dusting of Fe–Cr and Fe–Ni–Cr Alloys Under Cyclic Conditions

Metal dusting is the disintegration of alloys into carbon and metal particles during high-temperature exposure to carbon-bearing gases. Model Fe–Cr and Fe–Ni–Cr alloys were studied to test the hypothesis that M₃C formation is necessary for metal dusting to occur. The alloys were exposed to a 68% CO–26% H₂–6% H₂O gas mixture at 680°C (a_c=2.9) under thermal cycling conditions. Equilibrium calculations predicted the formation of M₃C at the surface of Fe–25Cr, but not Fe–60Cr. All compositions were expressed in w/o, weight percent. Alloys of Fe–25Cr with 2.5, 5, 10, and 25 w/o nickel additions were also exposed to the same conditions to study the role of nickel in destabilizing the precipitation of M₃C and, hence, altering the resistance to metal dusting. Metal dusting was observed on all the alloys except Fe–60Cr. For Fe–25Cr, Fe–25Cr–2.5Ni, and Fe–25Cr–5Ni, the carbonization and dusting process was localized, and its incidence decreased in Fe–25Cr–2.5Ni, consistent with the increased destabilization of M₃C precipitation. However, Fe–25Cr–10Ni and Fe–25Cr–25Ni both underwent extensive dusting in the absence of protective Cr₂O₃ formation. The carbon deposits formed consisted of carbon filaments, which contained particles at their tips. These were shown by electron diffraction to be exclusively Fe₃C in Fe–25Cr, Fe–25Cr–2.5Ni, and Fe–25Cr–5Ni, and a mixture of austenite and (Fe,Ni)₃C in Fe–25Cr–10Ni and Fe–25Cr–25Ni.     

Influence of Yttrium-Alloying Addition on the Oxidation of Alumina Formers at 1173 K

The oxidation behavior of three commercial Fe–Cr–Al alloys, Kanthal APM, Kanthal A1, and Kanthal AF (containing alloying additions of yttrium), has been investigated during isothermal exposures in air at 1173 K. After an initial transient stage, a diffusional process appears to predominantly control the oxidation kinetics of both alloys. During the transient stage, relatively important mass gains have been registered and the presence of yttrium does not seem to have a significant effect on the oxidation rate. On the contrary, the reactive element markedly influences the parabolic oxidation rate and the composition of the oxide scale. In situ X-ray diffraction (XRD) shows that yttrium promotes the transformation of transition alumina into -Al₂O₃, leading to the formation of a more protective oxide scale.     

Thermodynamic Analysis of the Silicon Effect on Chemical and Electrochemical Stability of Iron–Chromium Alloys

Phase diagrams Fe–Si–O and Cr–Si–O and potential–pH diagrams for the systems Fe–Si–H₂O, Cr–Si–H₂O, and alloy Fe + 25% Cr + 3% Si (-phase)–H₂O at 25°C are constructed. Thermodynamics of the silicon effect on the chemical and electrochemical stability of iron–chromium alloys is discussed.     

A study on the oxidation characteristics of cast irons containing aluminum

Isothermal-oxidation characteristics of cast irons containing aluminum (5–15% Al) from 700 to 1000°C in air have been studied. In addition to massgain measurements, the morphology and composition of the oxide scales have been examined by SEM-EDX system and XRD analysis. A normal Fe–5Al–C alloy does not develop protective, adherent scales. Even the addition of misch metal and calcium silicide to such an alloy does not improve its oxidation resistance. But aluminum cast iron develops considerable oxidation resistance only when a sufficient quantity of silicon is also present in the alloy. Treatment of the alloy with misch, metal and calcium silicide together assists in protective scale formation. Among the alloys investigated Fe–15Al–Si–C treated with misch metal and calcium silicide shows minimum oxidation at 1000°C.     

The third-element effect in the oxidation of Ni–xCr–7Al (x = 0, 5, 10, 15 at.%) alloys in 1 atm O2 at 900–1000 °C

The oxidation in 1 atm of pure oxygen of Ni–Cr–Al alloys with a constant aluminum content of 7 at.% and containing 5, 10 and 15 at.% Cr was studied at 900 and 1000 °C and compared to the behavior of the corresponding binary Ni–Al alloy (Ni–7Al). A dense external scale of NiO overlying a zone of internal oxide precipitates formed on Ni–7Al and Ni–5Cr–7Al at both temperatures. Conversely, an external Al₂O₃ layer formed on Ni–10Cr–7Al at both temperatures and on Ni–15Cr–7Al at 900 °C, while the scales grown initially on Ni–15Cr–7Al at 1000 °C were more complex, but eventually developed an innermost protective alumina layer. Thus, the addition of sufficient chromium levels to Ni–7Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum. This effect is interpreted on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most reactive component in binary alloys.     

TEM Study of Scale Microstructures Formed on Ni–10Cr and Ni–10Cr–5Al Alloys with and Without Y Addition

A study was conducted to investigate the effect of Y addition on the isothermal-oxidation behaviors of Ni–10Cr, Ni–10Cr–0.5Y, Ni–10Cr–5Al, and Ni–10Cr–5Al–0.5Y alloys. The alloys were oxidized in air for 50 hr at 1000°C. The oxides formed on the alloys were characterized using primarily cross-sectional transmission-electron microscopy techniques along with light microscopy, scanning-electron microscopy, and X-ray diffraction. Although the Al-containing alloys showed comparatively better oxidation behavior, all alloys exhibited nonprotective scaling, as suggested by the thick oxides formed. The major component of the outer oxide was NiO. However, modified Y-containing alloys formed protective layers (i.e., -Cr₂O₃ for NiCrY and -Al₂O₃ for NiCrAlY) at the scale–alloy interface following the nonprotective scaling. The spalling resistance of the modified Y-containing alloys was better than their counterpart unmodified Y-free alloys, while their overall oxidation mechanism remained unchanged after Y addition.     

Interdiffusion Behavior of Ni–Cr–Al–Y Coatings Deposited by Arc-Ion Plating

Ni–Cr–Al–Y coatings were deposited on the Ni3Al–base superalloys IC-6 and K17 by arc-ion plating. The results indicated that a small amount of substrate atoms, such as Co, Ti, Mo, etc., existed in the Ni–Cr–Al–Y coatings near the substrates, probably due to sputter and antisputter. For the alloy K17, the impeding effect of Al was not obvious, because the temperatures of the substrate and the coating were high (420–480°C) during the deposition process and interdiffusion was accelerated. However, for alloy IC-6 which contains Al, as well as a high concentration of Mo, the diffusion of Cr was impeded. Vacuum heat treatment at 1050°C drastically increased diffusivities and the presence of Al and Mo was not enough to prevent some Cr diffusion. Thus, the coating became more uniform and close to the desired composition.     

High-Temperature Oxidation Behavior of Iron–Chromium–Aluminum Alloys

The detailed oxidation behavior of Fe–10Cr alloys, containing aluminumin the range of 2–8% by weight, was studied in pure oxygen at 1 atmpressure. The investigations were performed over the temperature range950–1050°C under cyclic conditions (3-hr cycles) in each case. Thecyclic-oxidation resistance, as measured by the specific weight-gain values,was observed to progressively improve with increasing aluminum content inthe alloy. For a particular aluminum content, however, the oxidationresistance decreased with increasing temperature. Following the initialtransient-oxidation period, a healing layer of chromia was established onthe four alloys. The lower-aluminum alloys (2–4% Al) were observedto end up with Fe-rich oxide scales under the experimental conditions atall temperatures, whereas those containing aluminum in the range of6–8% formed -Al₂O₃ scales.     

Effect of Nanocrystallization on the Oxidation Behavior of a Ni–8Cr–3.5Al Alloy

Magnetron-sputter deposition was used to produce a Ni–8Cr–3.5Al(wt.%) nanocrystalline coating on substrates of the same alloy. Theoxidation behavior of the cast Ni–8Cr–3.5Al alloy and itssputtered coating were investigated at 1000°C in air. Complex,layered-oxide scales composed of Cr₂O₃ outer layer,mixed spinel NiAl₂O₄ and NiCr₂O₄middle layer, and -Al₂O₃ inner layer were formedon the Ni–8Cr–3.5Al nanocrystalline coating during 200-hroxidation, whereas Cr₂O₃, with some NiCr₂O₄external layer with internal Al₂O₃, formed on the castalloy. Because of the formation of this -Al₂O₃inner layer on the coating, the sputtered Ni–8Cr–3.5Al coatingshowed better oxidation resistance than the cast alloy. The effect ofnanocrystallization on oxide formation is discussed. It was indicated thatthe formation of this -Al2O3 inner layer was closely related to therapid diffusion of Al through grain boundaries in the nanocrystallinecoating and the relatively high Cr content in Ni–8Cr–3.5Al.     

The oxidation behavior of Fe-20Cr alloy foils in a synthetic exhaust-gas atmosphere

Oxidation tests of rare-earth-modified and Ti-modified Fe–20Cr alloy foils, which are under consideration for catalytic converter supports, were performed in a synthetic exhaust-gas atmosphere (N₂+H₂O+CO₂) between 900°C and 650°C. Between 900°C and 750°C, the rare earths had no effect on oxide growth rates while Ti increased growth rates. Oxide growth rates for the rareearth alloys at 800°C and 750°C are much lower than those found in the literature for oxidation of Fe–Cr alloys or pure Cr in O₂-rich atmospheres. The slow growth rates for the rare-earth alloys agree with literature data for oxidation of stainless steels containing >20% Cr in wet atmospheres and are caused by growth of an oxide scale only one grain thick. At temperatures 700°C, Fe–20Cr alloys grow massive Fe oxides; however, this can be suppressed by adding rare earths or Ti. To ensure good oxide adherence, free sulfur must be eliminated in the alloy by tying it up with a reactive-element addition. Both Ti and the rare earths can be used to tie up S, but the rare earths are more effective. For converter applications, the optimum alloy composition may contain rare earths for good oxide adherence and a small amount of Ti to suppress growth of Fe-rich oxides.     

Oxygen solubility and a criterion for the transition from internal to external oxidation of ternary alloys

Smith's model is expanded in order to derive expressions to quantitatively describe the oxygen-solubility behavior in ternary alloys as a function of alloy composition. Multicomponent-diffusion theory is used to establish a criterion for the onset of internal oxidation beneath the external scale when oxidizing conditions favor formation of the oxide of the least-noble metal in a ternary alloy. The oxygen-solubility model and the criterion are applied to the oxidation of Ni–Cr–Al alloys in 76 torr of oxygen at 1100 and 1200°C, predicting the minimum Al concentrations required to form a protective Al₂O₃ scale. It shows that sufficient Cr additions would significantly reduce the oxygen solubility and also alter the oxygen distribution in the ternary alloys, avoiding the oxygen supersaturation necessary for the onset of internal oxidation. These two factors make it easier to establish the protective Al₂O₃ scale.     

TEM Study of the Effect of Y on the Scale Microstructures of Cr2O3- and Al2O3-Forming Alloys

This study was undertaken to investigate and compare the effects of a yttrium addition on the oxide scale development of -Cr₂O₃- and -Al₂O₃-forming alloys under isothermal oxidation conditions. The alloys had a nominal composition (in wt.%) of Ni–30Cr, Ni–30Cr–0.5Y, Ni–30Cr–5Al, and Ni–30Cr– 5–Al–0.5Y. They were oxidized in air for 50 hr at 1000°C. The scale microstructures were characterized using cross-sectional transmission-electron microscopy combined with energy-dispersive X-ray spectroscopy. It was observed that the scale thickness decreases and the scale adherence increases due to the Y addition. The growth direction of -Cr₂O₃ scale changes from predominately outward to inward while countercurrent diffusion within -Al₂O₃ is replaced by inward diffusion due to Y modification. It is considered that the ability of Y to scavenge sulfur from the alloy and its segregation to the oxide grain boundaries primarily account for most of its beneficial effects.     

Microstructure and martensitic transformation of Ni–Fe–Ga–Si ferromagnetic shape memory alloys

The effects of the fourth element Si on the martensitic transformation and magnetic properties of Ni–Fe–Ga magnetic shape memory alloys were investigated. A complete thermoelastic martensitic transformation in Ni–Fe–Ga–Si alloys was observed in the temperature range of 218–285 K. The martensitic transformation temperatures of Ni–Fe–Ga alloys are obviously decreased by the substitution of Si for Ga element, that is, the substitution of 1 at.% Si for Ga leads to a decrease of martensitic transformation temperature of about 39.6 K. Moreover, the substitution of Si for Ga leads to a decrease of the saturation magnetic field and the magnetic anisotropy constant K₁ obviously.     

Oxidation of γ-Ni−Al−Si alloys at 1073 K

The formation and development of oxides in Ni–4Al and Ni–4Al–xSi (at.%, x=1, 3, 5) alloys at 5–9×10^–6 and 1 atm oxygen pressure at 1073 K have been studied. The oxidation rate increased with an increase of silicon content in the alloy at the early stage of oxidation, but decreased after longer time exposure due to formation of an intermediate layer composed of NiO and spinel (NiAl₂O₄ and Ni₂SiO₄) between the top NiO layer and the internal-oxidation zone. This intermediate layer became a barrier for releasing stress, generated by the volume expansion associated with oxidation of solute atoms, resulting in high dislocation density and severe distortion in the internal-oxidation zone for the Ni–Al–Si alloys. In Ni–4Al alloy where no complete intermediate-layer formation occurred, stress was easily released by an enhanced vacancy gradient, and therefore an enhanced vacancy-injection rate into the alloy, resulting in a higher oxidation rate than the situation where a sample was oxidized at an oxygen pressure associated with the dissociation of NiO.     

High-temperature oxidation behavior of a wrought Ni-Cr-W-Mn-Si-La alloy

An investigation was carried out to study the kinetics and products of oxidation of a wrought Ni–Cr–W–Mn–Si–La alloy at temperatures in the range of 950 to 1150°C. Oxidation kinetics were evaluated from measurements of weight change, metal loss, and internal penetration. Analytical electron microscopy, scanning electron microscopy, electron probe microanalysis, and X-ray diffraction were used to characterize the scale microstructure. Initially, La was observed to segregate within a surface layer of about 5 m thick, which promoted selective oxidation of Cr and Mn. Oxidation kinetics were found to follow a parabolic-rate law with an activation energy of about 232kJ/mol. During steady-state oxidation, the scale consisted of an inner adherent layer of -Cr₂O₃ modified by the presence of La and Si, and shielded by an outer layer of MnCr₂O₄. Most of the La was segregated to grain boundaries of the -Cr₂O₃ scale, however, Si was homogeneously distributed. It was concluded that the characteristic oxidation resistance of the alloy was related to the synergistic effects of Ni and Cr and to the effective minor additions of La, Si, and Mn; however, the useful life of the scale was limited by rupture and surface depletion in Cr, leading to accelerated internal oxidation.     

The Application of a Wedge-Shaped Sample Technique for the Study of Breakaway Oxidation in Fe–20Cr–5Al Base Alloys

The evolution of the breakdown of protective alumina scales on Fe–20Cr–5Alalloys has been studied by the use of taper-sectioned samples. The oxidationin air of two model alloys is compared with that of two commercial alloys,Kanthal APM and MA 956 at 1350°C. It is shown that the length of thebreakaway-oxidation region along the taper may be used to rank theperformance of the alloys and that the wedge geometry allows a detailedstudy to be made of the chemical processes involved in the degradation ofthe oxide scale just prior to breakaway oxidation.