High-Temperature Scale Formation of Fe–36% Ni Bicrystals in Air

Fe–36% Ni bicrystals were oxidized at temperatures from 1100 to 1250 K in air, both under and without tensile stress. The scales were divided into three categories: external scales, intragranular subscales, and intergranular subscales. A linear relationship exists beween the thickness of scales and subscales and the square root of oxidation time. Tensile stress significantly accelerates intergranular oxidation, but has essentially no influence on either the formation of intragranular subscales or that of external scales. The external scales are composed of -Fe₂O₃ and Fe₃O₄ after a longer oxidation time. The intergranular and intragranular subscales consist of Fe₃O₄ and FeO particles and an Ni-enriched matrix. Cellular or dendritic oxide nodules sometimes exist in the intragranular subscales, causing their frontier interfaces to significantly undulate.     

Kinetics of Fe-30% Ni-12.5% Co invar alloy during high temperature oxidation

The oxidation behavior of Fe-30% Ni-12.5%Co Invar alloy possessing low thermal expansion-high strength has been studied by exposing it in temperature ranges of 1000–1200 in an air atmosphere. The formed oxide scale consisted of two layers, an outer layer and an inner layer, and the oxidation mechanism showed uniform growth for all oxidation conditions. The growth rate of the scale had a parabolic relationship with oxidation time, and the estimated activation energy for the growth of the whole oxide layer was approximately 19.84 kcal/mol. The outer scale consisted of five oxide layers, whose outermost scale consisted of major phase CoFe₂O₄ containing a particulate Fe₂O₃ phase. The oxide scale of Fe-30 Ni-12.5% Co had different compositions and phases from the Fe-30%Ni alloy investigated in previous studies. Especaally, when the alloy was exposed for a longer oxidation time and at a higher temperature, the volume fraction ratio of CoFe₂O₄ to Fe₂O₃ was found to increase     

Influence of Zr or Hf additions to Fe-23Cr-5Al on the protectiveness of preformed Al2O3 scales against sulfidation

An Fe-23Cr-5Al alloy and those containing 0.17 w/o Zr or 0.12 w/o Hf were oxidized to form -Al₂O₃ scales in a flow of pure O₂ at 1300 K for specified periods up to 400 ks, and subsequently sulfidized at 1200 K in an H₂ –10% H₂S atmosphere without intermittent cooling. The protectiveness of the preformed scale was evaluated by the protection time after which a remarkable mass gain takes place owing to the rapid growth of sulfides. In general, the protection time increases as the scale thickens. Both additives increase the protection time to some degree by forming more structurally perfect scales. However, ZrO₂ particles on or near the outer surface of the scale on the Zr-containing alloy provide sites for sulfide formation. The scales formed on the grain boundaries of the Hf-containing alloy are ridged. The tops of the ridges are associated with cracks, which provide preferential sites for sulfide growth.     

Effect of creep on the oxidation characteristics of Fe-Si alloys at 973–1073 K

Studies of the simultaneous creep and oxidation of Fe-1Si and Fe-4Si alloys at a constant tensile stress of 16 N· mm^–2 at 973–1073 K have shown that scales formed at oxygen partial pressures of 20–1013 mbar were thicker by a factor of 2 than those formed on uncrept specimens. Scales on uncrept alloys comprised alternate layers of wustite and fayalite, whereas scales on crept alloys exhibited an additional external layer of magnetite. Only intergranular oxidation (fayalite) was observed in uncrept alloys, but crept alloys showed both intra- and intergranular oxidation (silica). Uniquely nodular scales were formed only on the Fe-4Si alloy on crept and uncrept specimens. Oxidized, uncrept Fe-1Si showed a fine-grained ferrite substrate which was absent in the crept alloy. It is believed that oxide growth stresses stimulated a recrystallization process.     

TEM Observations of the Initial Oxidation Stages of Nb-Ion-Implanted TiAl

Coupon specimens of TiAl were implanted with Nb ions at an acceleration voltage of 50 kV with a dose of 10²¹ ions m.^–2 They were then slightly oxidized during heating to 900 or 1200 K, or at 1200 K for 3.6 ksec (1 hr) in a flow of purified oxygen under atmospheric pressure. The implanted specimens and oxidized specimens were characterized and observed by AES, X-ray diffractometry, SEM, TEM, EDS, and EPMA. Implantation improves the oxidation resistance significantly by forming virtually -Al₂O₃ scales. The implanted layer is about 75 nm thick; the outer part of 30-nm thickness is -Ti phase and the rest of 45-nm thickness is amorphous. Heating to 900 K in O₂ results in partial crystallization of the amorphous layer to Ti₅Al₃O₂ (Z-phase) and to 1200 K results in oxide scales of 270 to 400 nm thickness consisting mainly of Al₂O₃. The fraction of Al₂O₃ in the scale increases toward the substrate. Oxidation at 1200 K for 3.6 ksec results in Al₂O₃-rich scales of about 400-nm thickness. The oxide grain size is very fine, about 80 nm in size, and becomes smaller toward the outer scale surface. This implies that implantation enhanced the nucleation of Al₂O₃ grains relative to the growth of TiO₂ grains. This finding and the formation of -Ti phase are thought to be responsible for the excellent oxidation resistance obtained.     

The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O₂ at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al₂O₃scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr₂O₃ and Al₂O₃.     

TEM studies of cross sections of oxidized Fe-25Cr-6Al-La alloy

The oxidation behavior of the alloy Fe–25%Cr–6%Al-RE (rich in lanthanum) was investigated by means of TEM analysis. The results show that after 2 hr oxidation of the alloy, in pure oxygen at 1200° C, La precipitated in the oxide scale at the top of -Al₂O₃ grains and at the grain-boundary regions in the form of tiny particles of hexagonal La₂O₃. These tiny particles prevented aluminum from diffusing toward the surface and suppressed lateral growth of the oxide scale. The rare-earth constituents accelerated the internal oxidation of the alloy during thermal cycling between 1200° C and room temperature. They appeared as particles of aluminum oxide and lanthanum oxide. Particles of cubic La₂O₃ precipitated in the alloy matrix near the oxide scale-metal interface in a direction parallel to grain boundaries.     

Transient oxidation in Fe-Cr-Ni alloys: A Raman-scattering study

Using Raman scattering we have investigated the oxidation, in air, of the Fe-Cr-Ni stainless steels Fe-25Cr-20Ni, Fe-25Cr-20Ni-3Zr, and Fe-24Cr-3Zr (wt.%) as a function of temperature in the range 300 to 1000°C. The Raman technique is very sensitive to, and provides a clear identification of, the oxides Fe₂O₃ and Cr₂O₃. However, the technique is insensitive to NiO, FeO, and does not give a clear identification of spinels. The Fe–Cr–Ni alloys form chromia scales at temperatures greater than 800°C. At lower oxidation temperatures, transient phases are observed. With a 1-h heat treatment at 300°C, we observe the formation of an unidentified scale; we speculate that it is either amorphous or consists of disordered spinel(s). Near 400°C we begin to observe hematite (Fe₂O₃). The intensity of the Fe₂O₃ signal increases with temperature to 600°C and then decreases, being largely replaced by the signal from Cr₂O₃. The thickness of the Cr₂O₃ scale increases with temperature up to 1000°C above which spallation becomes apparent. Spinel phases also apparently persist in the scale to 1000°C.     

The influence of yttrium additions on the oxidation resistance of a directionally solidified Ni-Al-Cr3C2 eutectic alloy

Isothermal oxidation of a directionally solidified Ni-Al-Cr₃C₂ eutectic alloy results in development of an external -Al₃O₃-rich scale. However, this scale breaks down after relatively short times at temperature and a less protective Cr₂O₃-rich scale is formed, together with substantial internal oxide in the alloy. In an attempt to maintain the external -Al₂O₃-rich scale and prevent damaging subscale oxidation, modified yttrium-containing directionally solidified alloys have been developed. The oxidation resistance of these alloys at 1000 and 1100°C in flowing air has been investigated and found to be considerably better than that of the corresponding yttrium-free alloy. At both temperatures an external -Al₂O₃-rich scale is produced and is retained for much longer periods than on the yttrium-free alloys during isothermal and thermal cycling oxidation. Some scale breakdown does occur during thermal cycling at 1100°C, but -Al₂O₃ is able to re-form as the surface oxide. However, although external -Al₂O₃-rich scales are retained for long periods on these alloys, some oxide penetration into the alloy beneath these scales does occur where coarse carbide fibers intersect the alloy surface. This is associated with relatively poor scale integrity at these intersections.     

The Oxidation of Fe-2 and 5 at.% Y Alloys at 600-800°C in Air

The oxidation of Fe-Y alloys containing 2 and 5at.% Y and pure iron has been studied at 600-800°Cin air. The oxidation of pure iron follows the parabolicrate law at all temperatures. The oxidation of Fe-Y alloys at 600°C approximatelyfollows the parabolic rate law, but not at 700 and800°C, where the oxidation goes through severalstages with quite different rates. The oxide scales on Fe-2Y and Fe-5Y at 700 and 800°C arecomposed of external pure Fe oxides containingFe₂O₃,Fe₃O₄, and FeO, with FeO being themain oxide and an inner mixture of FeO andYFeO₃. The scales on Fe-2Y and Fe-5Y at 600°C consist ofFe₂O₃,Fe₃O₄, andY₂O₃, with a minor amount of FeO.Significant internal oxidation in both Fe-Y alloysoccurred at all temperatures. The Y-containing oxidesfollow the distribution of the original intermetalliccompound phase in the alloys. The effects of Y on theoxidation of pure Fe are discussed.     

Intergranular oxidation and internal void formation in Ni-40% Cr alloys

Internal void formation and intergranular oxidation behaviour have been studied during the oxidation of two Ni-40Cr alloys in 1 atm oxygen at 1000° to 1200°C. The development of an external Cr₂O₂ scale causes vacancies to be generated in the alloy at the alloy-scale interface as chromium diffuses into the scale, and others to be generated in the alloy due to the different diffusion rates of chromium towards the interface and of nickel back into the bulk alloy. At 1200°C, internal void formation results from condensation of such vacancies at inclusions in the grains and at the grain boundaries. The intergranular oxidation observed at 1000°C, 1100°C and to a lesser extent. 1200°C results from preferential condensation of vacancies to form voids in the alloy grain boundaries. Significant depletion of chromium in the alloy adjacent to the scale facilitates the supply of oxygen from the scale and its penetration into the alloy grain boundaries to form intergranular oxide. Such intergranular oxide develops deep into the alloy following diffusion of this oxygen through a porous network in the oxide, which arises because of the vacancy condensation, and oxidation of chromium at the tip of the intergranular penetration.     

The Effect of Lanthanum on the Scales Developed on Thin Foils of Fe-20Cr-5Al at Very High Temperatures

A study has been undertaken of the effects oflanthanum on the oxidation of thin foils of Fe-20Cr-5Alin air at 1150°C. The addition of lanthanum causesthe time to breakaway to increase from about 24 hr for Fe-20Cr-5Al to over 400 hr. Oxidationof the lanthanum-containing alloy occurs in threestages. During the first stage, an-Al₂O₃ layer is establishedand thickens with time until the aluminum in the foil is depleted sufficiently for alayer of Cr₂O₃ to become stableand develop at the scale-alloy interface. This continuesto thicken at a relatively slow rate until breakawayoccurs. The main emphasis in the present paper has been anexamination and analysis of the scale established on thelanthanum-containing alloy in cross section in theanalytical transmission electron microscope (TEM), after an exposure period that coincides with thesecond stage of oxidation, prior to breakaway. The scaleat that time consists of three layers. The outer layeris composed of equiaxed Al₂O₃grains. The intermediate and inner layers consist of columnarAl₂O₃ grains and equiaxedCr₂O₃ grains, respectively.Numerous voids are observed in the oxide grainboundaries and at the intermediate-inner layerinterface. Lanthanum segregates in the oxide grain boundaries andits concentration increases toward the outermost surfaceof the scales. These results are consistent with thedynamic segregation model to account for the effects of reactive elements on thegrowth of Al₂O₃ scales.     

Oxidation of Fe-3 wt.% Cr alloy

The oxidation behavior and the oxide microstructure on Fe-3 wt. % Cr alloy were investigated at 800°C in dry air at atmospheric pressure. Two distinct oxidation rate laws were observed: initial parabolic oxidation was followed by nonparabolic growth. The change in the oxidation kinetics was caused by microchemical and microstructural developments in the oxide scale. Several layers developed in the oxide scale, consisting of an innermost layer of (Fe,Cr)₃O₄ spinel, an intermediate layer of (Fe,Cr)₂O₃ sesquioxide, and two outer layers of Fe₂O₃ hematite, each with different morphologies. Wustite (Fe_1–xO) and distorted cubic oxide (-(Fe,Cr)₂O₃) were observed during the iniital parabolic oxidation only.     

Criteria for the formation of protective Al2O3 scales on Fe-Al and Fe-Cr-Al alloys

The conditions for the formation of external alumina scales on binary Fe-Al alloys and the nature of the third-element effect due to chromium additions have been investigated by studying the oxidation at 1000 °C in 1 atm O₂ of a binary Fe-10 at.% Al alloy (Fe-10Al) and of two ternary Fe-Cr-10 at.% Al alloys containing 5 and 10 at.% chromium (Fe-5Cr-10Al and Fe-10Cr-10Al, respectively). An Al-rich scale developed initially on Fe-10Al was subsequently replaced by a multi-layered scale containing mixtures of Fe and Al oxides plus a large number of Fe-rich oxide nodules: internal aluminum oxidation was essentially absent from this alloy. Addition of 5 at.% chromium to Fe-10Al did not suppress the formation of nodules, but they were eventually healed by the growth of an alumina layer at their base, resulting in a significant reduction of the oxidation rate. Finally, the alloy with 10 at.% Cr formed continuous external alumina scales without any Fe-rich nodule. Thus, the addition of sufficient amounts of chromium to Fe-10Al produces a third-element effect as expected. However, the process found in this alloy system does not involve a prevention of the internal oxidation of Al. Instead, it shows a transition from the growth of mixed Fe- and Al-rich external scales directly to an external Al₂O₃ scale formation. An interpretation of this kind of mechanism involving a third-element is presented along with a prediction of the critical Al contents required to produce the various possible scaling modes on binary Fe-Al alloys.     

Observations of chromium-oxide films containing cerium and yttrium using transmission electron microscopy

Samples of a pure Fe-26%Cr alloy were sputter-coated with a 4-nm layer of CeO₂ or Y₂O₃, then oxidized at 900°C in 5×10^–3 torr of oxygen. The oxide scale was removed from the substrate by dissolution of the alloy substrate in a solution of iodine in anhydrous methanol and examined in, a transmission electron microscope to determine the microstructure of the oxide and the phases present in the scale. the phases CeCrO₃ (space group 62,Pnma and space group 221, ), YCrO₃ (space group 62,Pnma), and Y₂O₃ (space group 164, ) were found in the oxide scale in the initial stages of oxidation. The relevance of these species in the reactive-element effect is discussed.     

High-temperature oxidation resistance of sputtered micro-grain superalloy K38G

The oxidation of sputtered and cast superalloy K38G specimens was studied. The sputtered alloy was microcrystalline, with an average grain size <0.1 m. The mass gains of the sputtered alloy were much less than those of the cast alloy at 800, 900, and 1000°C up to 500 hr, and were even less than those of pack aluminide on the cast alloy. K38G is a chromia-forming cast nickel-base superalloy, so the oxide scale formed on it is composed of Cr₂O₃, TiO₂, Al₂O₃, and a spinel. The oxide scale formed on the sputtered alloy was Al₂O₃. This scale is thin, compact, and adherent. This result implied that micro crystallization reduced the critical aluminum content necessary to form alumina on the surface of this superalloy. No oxide spoliation, as typically observed for cast of aluminized alloys, occurred on the sputtered superalloy. The reduction of the critical aluminum content for the formation of alumina and the improvement of the spoliation resistance may be attributed to the microcrystalline structure formed during sputtering. The numerous grain boundaries favor outward aluminum grain-boundary diffusion, provide increased nucleation sites, and reduced stresses in the oxide scales.     

High-Temperature Oxidation Studies of Low-Nickel Austenitic Stainless Steel. Part II: Cyclic Oxidation

The cyclic-oxidation behavior of a low-nickel austenitic stainless steel (LNiSS) at 873 and 973 K has been investigated up to 500 cycles. A wide range of experimental techniques, including SEM, EDS, and XRD have been applied to examine the oxide scales. After cyclic oxidation at 873 and 973 K, the new LNiSS alloy showed good oxidation resistance. XRD and EDS analysis show that composition of oxide scales developed during cyclic oxidation at 873 and 973 K are the same. The oxide scales formed a cubic oxide-type M₂O₃ and a cubic spinel-type M₃O₄ adhered well to the substrate.     

Metallic yttrium additions to high temperature alloys: Influence on Al2O3 scale properties

In the present study, the effect of adding yttrium to alloys is investigated. The microstructure of the cast Fe-25Cr-4Al-0.5Y alloy used in the study shows that the vttrium is present in different shapes and sizes as the intermetallic phase, (Fe, Cr)₄(Al, Y), previously unreported in the literature. Upon oxidation in dry oxygen in the 1100–1200 °C temperature range, a columnar, fine-grained (0.5–1 m) -Al₂O₃ scale is formed which grows predominantly by inward oxygen grain-boundary transport. The intermetallic phase, during incorporation into the oxide scale, is converted into Y₃Al₅O₁₂, the chromium and iron from the intermetallic diffusing back into the metal matrix. The Y₃Al₅O₁₂ phase saturates the oxide scale with yttrium, which segregates to oxide grain boundaries. The microstructural features of the oxide scale resemble those of the scale formed on the yttria-dispersed alloy we investigated earlier. The improved adherence of the oxide scale is a consequence of yttrium doping, which facilitates the formation of a fine-grained scale in which oxide growth stresses can be relieved by diffusional plastic flow. Further, yttrium suppresses Al transport in the oxide scale and prevents Al₂O₃ nucleation within the scale, a process which can generate compressive stresses in the scale. The yttrium doping in the oxide scale is somewhat more efficient when it is present as a dispersoid in the metal.     

Analytical Electron-Microscopy Study of the Breakdown of α-Al2O3 Scales Formed on Oxide Dispersion-Strengthened Alloys

Alumina scales formed during cyclic oxidation at 1200°C on three Y₂O₃–Al₂O₃-dispersed alloys: Ni₃Al, -NiAl, and FeCrAl (Inco alloy MA956) were characterized. In each case, the Y₂O₃ dispersion improved the -Al₂O₃ scale adhesion, but in the case of Ni₃Al, an external Ni-rich oxide spalled and regrew, indicating a less-adherent scale. A scanning-transmission electron microscope (STEM) analysis of the scale near the metal–scale interface revealed that the scale formed an ODS FeCrAl showed no base metal-oxide formation. However, the scale formed on ODS Ni₃Al showed evidence of cracking and Ni-rich oxides were observed. The microstructures and mechanisms discussed may be relevant to a thermal-barrier coating with an Al-depleted aluminide bond coat nearing failure.     

The early stages of development of α-Al2O3 scales on Fe-Cr-Al and Fe-Cr-Al-Y Alloys at high temperature

The development ofthe oxides on Fe-14%Cr-4%Al, Fe-27%Cr-4%Al, and similar alloys containing 0.008% Y, 0.023% Y, and 0.8% Y has been investigated during the early stages of oxidation in 1 atm oxygen at 1000 and 1200°C. In all cases, a steady-state -Al₂O₃layer is established rapidly, after some initial formation of transient oxides rich in iron and chromium. For the yttrium-free alloys the steady-state situation is achieved more rapidly for the higher chromium-containing alloy and at the higher temperature. The amount of transient oxide formed is also determined by the specimen surface topography since the development of the -Al₂O₃ layer is less rapid at the base of alloy asperities than at a flat alloy-oxide interface. Following establishment of the complete -Al₂O₃layer, the oxide develops a convoluted oxide morphology at temperature, due to high compressive growth stresses in the oxide. These arise following reaction between oxygen ions diffusing inward down the oxide grain boundaries and aluminum ions diffusing outward through the bulk oxide. This results in lateral growth of the oxide and plastic deformation and movement of the alloy in a direction parallel to the alloy-oxide interface. The addition of yttrium to the alloys promotes the selective oxidation of aluminum. Also, the yttrium is incorporated into the growing oxide where it changes the mechanism of growth, reducing the production of the high compressive growth stresses and thus the development of the convoluted oxide morphology.