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1.
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. 相似文献
2.
On the transient oxidation of a Ni-15Cr-6Al alloy 总被引:2,自引:0,他引:2
Stages in the development of a protective -Al2O3 scale on a Ni-15Cr-6Al (wt.%) alloy have been examined. It is shown that prior to the formation of a continuous -Al2O3 layer, a transient stage of oxidation occurs that consists of a rapid uptake of oxygen with conversion of a thin surface layer of alloy to predominantly spinel and the subsequent development of a discrete layer of Cr2O3. It is also shown that during the transient period of oxidation metastable phases of aluminum oxide are formed which transform to -Al2O3 upon incorporation into the external oxide scale. 相似文献
3.
The oxidation of presulphidized Ni-Cr alloys has been studied by taking into account the influence of the two distinct oxidation mechanisms described in part I of this article. Sulphur enters the Cr2O3 scale (in Ni-34Cr alloys) mainly as S2– species, which at high temperatures increases the VCr content, and hence the oxidation kinetics. Sulphur is randomly distributed in the scale, except at the inner oxide-alloy interface, where intergranular microsulphides are analyzed in the oxide-scale zone. In the case of NiO, NiCr2O4, Cr2O3 oxide multilayers (in a Ni-20Cr alloy), sulphur in the S2– state is distributed in the oxide layers or at Si-precipitate interfaces. Such a distribution leads to crack formation, especially during cooling. 相似文献
4.
Alloys of Co-21 wt. % Cr-3 vol. % Y
2
O
3
have been prepared by a mechanical alloying method, and oxidized in oxygen at 100 Torr in the temperature range 900–1200°C. The general effects of the dispersed oxide phase are similar to those reported for nickel-base alloys: the selective oxidation of chromium to form a continuous protective Cr
2
O
3
scale is promoted; the rate of growth of Cr
2
O
3
is reduced compared to dispersoid-free alloys; the adhesion of the Cr
2
O
3
is greatly improved; and the scale-forming reaction is probably at the scale-metal interface in the alloys containing the dispersoid, whereas it is at the scale-oxygen interface in dispersoid-free alloys. This last point has not been positively demonstrated. The improvement in adhesion is of particular significance, since the scales on cobalt-base alloys are prone to spallation, and it has been possible to study the mechanism of adhesion in more detail. It appears that in dispersoid-free material the metal recedes from the scale-metal interface, leaving the scale supported on the tops of metal peaks but this does not happen in the alloy containing the dispersoid, either because the growth direction of the scale has been changed, or because of changes in the substrate grain size. In general, the observations support the model proposed in an early study for the oxidation of Ni-20 wt.% Cr alloys containing oxide dispersions.This work has been supported by the Naval Air Systems Command under Contract No. N00019-71-C-0079. 相似文献
5.
This study reports the effects of up to 4 at.%rhenium addition on the cyclicoxidation behavior of-NiAl + -Cr alloys having a basecomposition (in at.%) Ni-40Al-17Cr. Tests were conductedin still air at 1100°C for up to 250 1-hr cycles.The ternary alloy (without rhenium addition) exhibitedpoor cyclic-oxidation resistance, undergoing extensivescale spallation and internal oxidation. Additions of rhenium considerably improved the oxidationbehavior, reducing the extent of both scale spallationand internal oxidation. These beneficial effectsincreased with increasing rhenium content. Rhenium additions improved cyclic-oxidation resistanceby both decreasing the solubility of chromium in the phase and causing the interdendritic -Crprecipitates in the alloy microstructure to become more spheroidized and disconnected. Theseeffects aided in preventing both interdendritic attackand the dissolution of the -Cr precipitates fromthe subsurface region of the alloy. The maintenance of -Cr precipitates at the alloy-scaleinterface decreased the extent of scale spallation byproviding a lower coefficient of thermal-expansion (CTE)mismatch between the alloy and theAl2O3-rich scale. 相似文献
6.
Ni3Al+5% Cr and Ni3Al+5% Cr+0.3% Y (wt.%) microcrystalline coatings were produced using a close-field, unbalanced magnetron-sputter deposition (CFUMSD) technique. Isothermal and cyclic-oxidation tests were carried out to assess the oxidation resistance of the coatings. The results showed that Al2O3 formed on the coatings as the main oxidation products, with the formation of - and -Al2O3 scales at 900 and 1200°C, respectively. The spallation resistance of the Al2O3 scales formed on the coatings was superior to the oxide scales formed on cast Ni3Al. After oxidation, interfacial voids were observed on the oxide–metal interface of the cast alloy while no voids were found on the coating surfaces. On the basis of the enhancement of Al diffusion, because of the high density of grain boundaries in the coatings, oxidation mechanisms were proposed. 相似文献
7.
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. 相似文献
8.
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. 相似文献
9.
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. 相似文献
10.
The isothermal oxidation of Co-Cr-Al alloys, containing 10–30 % Cr and 1 or 4.5% Al in 1 atm flowing oxygen at 1000 and 1200°C has been studied by thermogravimetric methods, optical metallography, electron probe microanalysis, and scanning electron microscopy. The addition of 1 % Al to Co-10% Cr and Co-15% Cr has little effect on the over-all oxidation rate, although there is increased internal oxidation and the outer-inner scale thickness ratio is decreased. The oxidation rate is controlled largely by Co2+ ion diffusion out through the entire scale, with oxygen gas transport across voids, spinel blocking effects and doping in the inner layer probably playing subsidiary roles. With Co-15 %-Cr-1%Al, limited healing by Cr2O3 increases progressively with time at the alloy-oxide interface. An addition of 1 % Al to Co-30 %Cr assists the formation of an initially protective Cr2O3-rich surface layer by internally oxidizing, thereby allowing more of the chromium to diffuse to the surface and form an external scale. This Cr2O3 layer tends to lift and crack open, enabling CoO-rich scales to form on the exposed alloy. Co-15%Cr-4.5% Al produces a protective -Al2O3 layer on certain surface regions, sometimes with an overlying Cr2O3 layer and internal -Al2O3 particles in the underlying alloy. In other regions, rapidly growing CoO-rich nodules develop from the outset, or after early lifting and fracture of the -Al2O3 scale. Generally, the presence of 28% Cr and 4.5% Al is sufficient to ensure an external scale of -Al2O3, the chromium acting as an oxygen getter. If such scale fractures, healing is very rapid. 相似文献
11.
The influence of externally located platinum on the isothermal stability of -Al2O3 scales formed at high temperatures has been examined. It has been observed that a nickel-base alloy forms an external scale of -Al2O3 during oxidation at 1200°C, but this scale breaks down isothermally, enabling a faster-growing Cr2O3-rich scale to develop. However, in the presence of platinum metal alongside the specimen in the furnace hot zone, the breakdown of the -Al2O3 scale is postponed for a substantial period of time. It appears that platinum, as the volatile species PtO2, is incorporated into the growing -Al2O3 scale where it either influences the stress relief mechanism at temperature or reduces oxidation growth stress generation and thus significantly enhances the isothermal stability of the scale. 相似文献
12.
Oxidation kinetics of a parent Fe-5Cr-4Al alloy subjected to two types of anneals were investigated at temperatures ranging from 1000°C to 1320°C. The alloy annealed at 850°C exhibited a rapid transient oxidation stage associated with growth of nodules containing iron oxides and internal precipitation of -Al2O3 in the alloy beneath these nodules. The nodules nucleated and grew from sites located in the regions of the alloy grain boundaries during the period of rapid alloy grain growth. Nodular growth virtually ceased when a continuous -Al2O3 film formed at the nodule-alloy interface. The alloy subjected to anneal at 1000°C and at the reaction temperature to stabilize the alloy grain size tended upon oxidation to form a protective -Al2O3, layer by parabolic kinetics at temperatures to 1250°C. If this alloy was oxidized in stages at 1000°C, a protective -Al2O3 scale was formed up to 1320°C. The temperature coefficient of the parabolic oxidation kinetics was consistent with diffusion processes at boundaries of the -Al2O3 grains playing an essential role during growth of this protective oxide layer. 相似文献
13.
The influence of microstructure of the two-phase alloyTi–46Al–1Cr–0.2Si on the oxidation behavior in air between600 and 900°C was studied. The oxidation rate, type of scale, and scalespallation resistance were strongly affected by the type of microstructure,i.e., lamellar in as-cast material and duplex after extrusion at1300°C. The oxidation rate was affected by the size and distribution ofthe 2-Ti3Al phase, being faster for the extrudedmaterial with coarse 2-Ti3Al. The type of oxide scaledetermines the spalling resistance. Cast material developed a uniform scalethat spalled off after short exposure times at 800 and 900°C when a criticalthickness was reached. The extruded material presented a heterogeneous scalewith predominant thick regions formed on -TiAl-2-Ti3Algrains and thin scale regions formed on -TiAl grains. Thistype of scale could permit an easier relaxation in the matrix of stressesgenerated by both thermal-expansion mismatch between scale and alloy andoxide growth, resulting in a higher spallation resistance. 相似文献
14.
-FeAl coatings containing various Cr contents of 6.5–45 wt.%were produced with a closed-field, unbalanced magnetron sputter (CFUMS)deposition technique. Cyclic oxidation tests at 1100°C in air for100 1-hr cycles and isothermal exposures at 1000°C in pure O2 for100 hr were carried out with the coatings and an as-cast FeAlspecimen. All of the coatings showed good scale-spallation resistanceduring cyclic oxidation and the coating with 6.5 wt.% Cr exhibited thelowest oxidation rates in both cyclic and isothermal oxidationexposures. After oxidation, fine-grain ridge-type oxide scales formed onthe coatings, while the oxide scale formed on the cast FeAl showed alarge quantity of -Al2O3 blades and large interfacial voids on thebase–alloy surface. The transformation from to -Al2O3was accelerated due to the presence of Cr in the coatings. The fasttransformation considerably reduced oxidation rates, suppressed fastoutward Al diffusion for the growth of a -Al2O3 scale, and preventedthe formation of interfacial voids that played a major role in causing thescale spallation. 相似文献
15.
The effect of grain size on the oxidation of 9 wt.% Cr-1 wt.% Mo steel (with prior-austenite grain sizes of 90, 210, and 360 m) has been studied at 973 K in air. After the initial stages of oxidation (up to 60 min), the fine-grain specimen (90 m) suffered heavy spallation. A similar spallation took place after 120 min in the case of the specimen with intermediate grain size (210 m), and after 360 min in a coarse-grain (360 m) specimen. This increase in the duration for the onset of pronounced spallation with grain size has been attributed to the smaller area of the grain boundaries which are the locations of higher growth stresses leading to spallation. This dependence of spallation characteristics on the alloy grain size has been confirmed by the incidence of higher acoustic-emission activity during AE monitoring. Scanning electron microscopy (SEM) has confirmed the occurrence of pronounced spallation of the oxides formed in the areas adjoining the grain boundaries. 相似文献
16.
The oxidation behavior of nickel-matrix/aluminum-particle composite coat–ings was studied using thermogravimetric (TG) analysis and long-term furnace exposure in air at 1000°C. The coatings were applied by the composite-electrodeposition technique and vacuum heat treated for 3 hr at 825°C prior to oxidation testing. The heat-treated coatings consisted of a two-phase mixture of (Ni)+ (Ni3Al). During short-term exposure at 1000°C, a thin -Al2O3 layer developed below a matrix of spinel NiAl2O4, with -Al2O3 needles at the outer oxide surface. After 100 hr of oxidation, remnants of -Al2O3 are present with spinel at the surface and an inner layer of -Al2O3. After 1000–2000 hr, a relatively thick layer of -Al2O3 is found below a thin, outer spinel layer. Oxidation kinetics are controlled by the slow growth of the inner Al2O3 layer at short-term and intermediate exposures. At long times, an increase in mass gain is found due to oxidation at the coating–substrate interface and enhanced scale formation possibly in areas of reduced Al content. Ternary Si additions to Ni–Al composite coatings were found to have little effect on oxidation performance. Comparison of coatings with bulk Ni–Al alloys showed that low Al -alloys exhibit a healing Al2O3 layer after transient Ni-rich oxide growth. Higher Al alloys display Al2O3-controlled kinetics with low mass gain during TG analysis. 相似文献
17.
The hot-corrosion resistance of two /- eutectic alloys, Ni-23.1 Nb-4.4 Al and Ni-19.7Nb-6Cr-2.5 Al, has been studied using three techniques: (1) a salt-coated test, in which the specimens are coated with a thin layer of Na2SO4 and oxidized; (2) the Dean rig test, in which salt is evaporated into the gas stream and allowed to condense continuously on the specimen; and (3) a sulfidation/oxidation test, in which the specimens were presulfidized for a short time in a H2/H2S mixture and then oxidized. The salt-coated test suggested that these alloys are generally quite resistant to sodium-sulfateinduced corrosion, although localized severe attack was sometimes encountered. The morphology of the corrosion products suggested that the mechanism of the attack was sulfidation/oxidation, and that salt fluxing mechanisms were relatively unimportant. In the Dean rig test it was shown that sodium chloride greatly increased the severity of attack, as has been shown earlier for alumina-forming superalloys. 相似文献
18.
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. 相似文献
19.
Sulfidation behavior of Fe-25Cr-20Ni-1.5Al2O3 in simulated coal combustion/gasification environments
The effect of minor addition of -Al2O3 dispersoids on the sulfidation behavior of Fe-25Cr-20Ni was investigated over a range of pO2, 1.13×10–20 to 1.18×10 ****–22 atm. at constant pS2=1.22×10–8 atm. Fe-25Cr-20Ni and Fe-25Cr-20Ni 1.5 Al2O3 with and without preformed oxide scales were exposed to bioxidant gas mixtures H2/H2O/H2S/Ar at 700° C. Both isothermal and cyclic exposures were included. Scales were characterized by a combination of several surface analytical tools. A remarkable improvement in sulfidation resistance is observed in Fe-25Cr-20Ni-1.5Al2O3 under the conditions investigated here. This is attributed to the ability of the alloy to form and maintain a predominantly Cr2O3 scale with reduced Fe-diffusion and content. Possible scientific reasons for such improvement are discussed. The base alloy, Fe-25Cr-20Ni, fails to develop and retain such a Cr2O3 scale and undergoes sulfidation within a few minutes of exposure. The scale breakdown process by sulfidation is explained qualitatively. Experimental evidence suggests that sulfur in the environment enhances Fe-diffusion and content in the scale. 相似文献
20.
High-resolution SIMS and analytical TEM evaluation of alumina scales onβ-NiAl containing Zr or Y 总被引:1,自引:0,他引:1
High-resolution SIMS and TEM have been used to evaluate growth processes and interfacial segregation occurring in -Al2O3 scales grown at 1200°C on -NiAl containing zirconium or yttrium.18O/SIMS shows that the extent of aluminum diffusion occurring during -Al2O3 growth is reduced by the presence of these alloying elements, which are seen by SIMS imaging as oxide particles within the scale. STEM/EDS of the same oxide scales show that zirconium and yttrium also segregated to the oxide-alloy interface to the extent, respectively, of 0.15 and 0.07 of a monolayer and to oxide grain boundaries (0.2 monolayer). The complementary information provided by SIMS, TEM, and STEM provides a better understanding of the role of reactive elements in modifying scale-growth processes. 相似文献