Effect of alloy grain size and silicon content on the oxidation of austenitic Fe-Cr-Ni-Mn-Si alloys in pure O2

Austenitic Fe-18Cr-20Ni-1.5Mn alloys containing 0, 0.6, and 1.5 wt.% Si were produced both by conventional and rapid solidification processing. The isothermal and cyclic oxidation resistance of the alloys were studied at 900°C in pure O₂ to elucidate the role of alloy microstructure and Si content on oxidation properties. The conventionally-processed, large-grained alloy that contained no silicon formed Fe-rich nodules during oxidation. The nodule formation was effectively eliminated by either reducing the alloy grain size by rapid solidification or by adding Si to the alloy. The lowest weight gains were achieved when a continuous silica layer formed between the alloy and the external chromia scale. The formation of the continuous silica layer required a ombination of fine alloy grain size and high Si content. The presence of S in the alloy was found to be detrimental to oxide scale adherence when the silica layer was continuous.     

The effect of niobium ion implantation on the oxidation behavior of aγ-TiAl-based intermetallic

The effect of niobium implantation (2×10¹⁶ and 10¹⁷ ions/cm²) on the oxidation behavior of a -TiAl-based intermetallic alloy Ti–48Al–2Cr in air at 800°C has been examined. Isothermal studies with exposure times up to 200 hr and cyclic-oxidation tests up to 800 hr revealed a negligible effect for the low implantation dose. However, a remarkable decrease in oxidation rates was observed for the material implanted with 10¹⁷ Nb ions/cm². The improvement in the oxidation resistance appeared to be similar to that obtained by alloying of the intermetallic with a few atomic percent niobium. Possible mechanisms for the effect of niobium on the oxidation resistance of TiAl-base intermetallics are discussed in view of the results obtained for the implanted material.     

Comparison of the effects of small additions of silicon or aluminum on the oxidation of iron-chromium alloys

A study has been undertaken of the oxidation behavior of Fe-26 wt.% Cr-1 wt.% Al and Fe-26 wt.% Cr-1 wt.% Si at 800° and 1000°C in oxygen, in order to determine the usefulness of the two tertiary elements in facilitating the development of the Cr₂O₃ external scale. The research has also permitted a comparison of the modes of internal oxidation of these elements, with a view to ascertaining the ease of establishment of the tertiary element oxides as healing layers at the scale/alloy interface. It has been shown that aluminum is the more effective addition in this respect, due to formation of a higher population density of internal oxide nuclei in the early stages. However, in the 1% Al alloy, the precipitates penetrate inward, to considerable depths, as continuous platelets, making development of a complete healing layer difficult. In practice, a higher aluminum concentration is necessary for the closely spaced precipitates to coalesce to form the healing layer, but the process then occurs rapidly. The initial internal oxide nuclei in the 1% Si alloy have a much smaller population density and are restricted to a location very close to the surface. Thus, a healing layer can be established, but the large interparticle spacing makes this a very slow process. Even at a higher silicon concentration, it takes a significant period to be completed. The effects are discussed and accounted for, particularly in terms of the relative stabilities of the various oxides.     

The influence of superficially applied oxide powders on the high-temperature oxidation behavior of Cr2O3-forming alloys

The effects of superficially applied CeO₂, mixed rare earth oxides, Co₃O₄, and Cr₂O₃ powders on the isothermal and cyclic oxidation of Ni-Cr alloys and the effects of CeO₂ and MgO powders on the isothermal oxidation of Fe-25 wt.% Cr have been studied over the temperature range 940–1150°C in pure oxygen and dry air. The rates of oxidation of both the Ni- and Fe-base alloys were markedly reduced by the application of CeO₂ powder. The presence of CeO₂ also improved the scale adherence and resulted in marked changes in the oxidation morphology. The presence of Co₃O₄ or Cr₂O₃ powders on Ni-Cr alloys or MgO on Fe-Cr also produced changes in the oxidation morphology but did not decrease the rate of oxidation. These results are interpreted in terms of the influence of the oxide powders on the development of scale microstructure and their effectiveness in decreasing grain boundary transport in Cr₂O₃.This paper is based in part on the Ph.D. thesis of G. M. Ecer (1975) and in part on the M.S. thesis of R. B. Singh (1977).     

On the high-temperature oxidation of nickel

This paper summarizes on some of the extensive experimental data and corresponding models suggested to account for the oxidation mechanism of Ni in the temperature range 500-1400 °C. In addition it reports on in-house experimental data from investigations related to the oxidation of high-purity Ni from 500 to 1300 °C in the oxygen pressure range 1×10⁻⁴-1 atm based on TG, measurements of surface kinetics, two-stage oxidation, scanning electron microscopy, atomic force microscopy, secondary ion mass spectroscopy etc. The main part of this paper focuses on the more complex models suggested to account for experimental observations of the oxidation kinetics and the oxide morphology below 1000 °C.     

Mechanism of the influence of microadditions on the oxidation resistance of high-temperature nickel alloys

The effect of rare earth metals (yttrium, cerium, praseodymium) on the oxidation resistance in cast high-temperature nickel alloys ZhS6K-V1 and ZhS6U-VI is considered from the standpoint of reducing defect concentrations in the oxides and changing the parameters of their crystal lattice.     

The influence of ion-implanted yttrium on the selective oxidation of chromium in Co-25 wt.% Cr

Specimens of Co-25 wt.% Cr, Co-25 wt.% Cr-1 wt.% Y, and yttriumimplanted Co-25 wt.% Cr alloy were oxidized at 1000°C in 1 atm O₂. The implantation dosage ranged between 10¹⁶ to 10¹⁸ ions/cm². The unimplanted binary alloy oxidized to a duplex Co-rich scale, but the Y-containing ternary alloy formed a continuous Cr₂O₃ layer. When the implantation dosages were lower than a nominal 10¹⁸ ions/cm², the alloy failed to develop a similar continuous Cr₂O₃ layer as that observed with the Y-containing alloy. A temporarily stable external Cr₂O₃ scale was formed on the most heavily implanted specimen (1×10¹⁸ Y⁺/cm²). This Cr₂O₃ scale consisted of very fine-grained oxide, which is permeable to the outward transport of Cr and Co. Internal oxidation pretreatment of the ion-implanted specimens converting the Y metal to its oxide prior to the oxidation experiment, can enhance the development of an external Cr₂O₃ scale, but this scale is also unstable. Results suggest that the selective oxidation of chromium in an ordinarily non-Cr₂O₃ -forming alloy can be due to the reactive element oxides acting as preferential nucleation sites on the alloy surfaces, but the subsequent growth of these scales may require a continuous supply of reactive elements in the alloy.     

Morphological and microchemical phenomena in the high-temperature oxidation of binary Al-Li alloys

The high-temperature oxidation behavior of binary Al-Li alloys has been characterized by scanning electron microscopy and secondary ion mass spectrometry in order to understand the mechanism of rapid oxidation in these alloys and to correlate the oxide morphology to its microchemistry. The oxide scale developed on polished specimens during short exposures in air at 530°C shows characteristic nodules that usually nucleate at grain boundaries. Examination of the alloy surface after removal of the oxide layer shows that the initial growth of the oxide nodules occurs laterally in addition to thickening normal to the oxide/alloy interface. Microchemical analysis of the oxide film with a scanning ion microprobe reveals a thick Li-oxide layer at the oxide/gas interface indicating preferential oxidation of Li at the free surface; the rest of the oxide film is composed of both Al- and Li-rich oxides, probably Li₂O and LiAlO₂ The presence of trace impurities (K, Na, F, and Cl) in the oxide scale was also detected. A microstructural model for the development of the oxide film in the Al-Li system is presented on the basis of both morphological and microanalytical data obtained in this study; this new model is compared with existing models.     

The high-temperature oxidation resistance of Co-Al alloys

Most Ni and Co-base alloys used for high-temperature service rely on the production of a compact, stable Cr₂O₃ scale for their oxidation resistance. However, as operating temperatures have risen above 900–950° C, the loss of Cr₂O₃ as the volatile CrO₃ has led to an inadequate life span of these alloys, particularly in rapidly flowing, turbulent gas streams. As a result of this, it has been necessary to examine the possibility of using Al₂O₃ as the protective scale. Al₂O₃ has a lower growth rate than Cr₂O₃, it is nonvolatile, and, unlike Cr-containing systems, it is less likely to form compound oxides such as spinels. In this study, the amount of Al which must be present in the Co-Al system to form a continuous layer of Al₂O₃ in the temperature range 800–1000° C has been determined. The quantity was found to rise from about 7–10 wt. % at 800° C to 10–13 wt. % at 900° C and 13 wt. % at 1000° C. Notice has also been taken of the abilities of the alumina-forming alloys to re-form a protective oxide in the event of spalling, blistering, or any other disruptions of the scale, and some cyclic-oxidation checks have been conducted on the Co13Al alloy at 900 and 1000° C.This work has been partly supported by the Science Research Council and one of us (G.N.I.) wishes to thank them for the award of a Science Research Council Research Studentship     

The use of thin layer activation in high-temperature cyclic oxidation studies

Thin layer activation (TLA) has been applied to a study of the high-temperature cyclic oxidation behaviour of pure and yttrium-implanted chromium, taking advantage of the high area selectivity of the technique.On pure chromium, two different sample areas, i.e. corners and the central part of a large surface area, were selected for activation. Due to this area selectivity, it was possible to distinguish the spallation behaviour of the oxide scale formed on flat from that formed on highly curved surfaces. In particular, the TLA data showed that the oxide scale formed during high-temperature cyclic oxidation near corners and edges was more prone to cracking and spallation.In the case of yttrium-implanted specimens, activation was performed only at the centre of the implanted zone, avoiding the distorting effect of the non-implanted parts of the sample. The area selectivity of TLA made it possible to study more accurately the beneficial effect of ion implantation on the cyclic oxidation behaviour than by conventional thermogravimetry. Due to the complementary character of TLA with respect to the conventional thermogravimetry, a more complete and better understanding of the cyclic oxidation performance of materials can be obtained.     

The influence of reactive-element coatings on the high-temperature oxidation of pure-Cr and high-Cr-content alloys

The influence of various reactive-element (RE) oxide coatings (Y₂O₃, CeO₂, La₂O₃, CaO, HfO₂, and Sc₂O₃) on the oxidation behavior of pure Cr, Fe–26Cr, Fe–16Cr and Ni–25Cr at 900°C in O₂ at 5×10^–3 torr has been investigated using the¹⁸O/SIMS technique. Polished samples were reactively sputtercoated with 4 nm of the RE oxide and oxidized sequentially first in¹⁶O₂ and then in¹⁸O₂. The effectiveness of each RE on the extent of oxidation-rate reduction varied with the element used. Y₂O₃ and CeO₂ coatings were found to be the most beneficial, whereas Sc₂O₃ proved to be ineffective, for example, for the oxidation of Cr. SIMS sputter profiles showed that the maximum in the RE profile moved away from the substrate-oxide interface during the early stages of oxidation. After a certain time the RE maximum remained fixed in position with respect to this interface, its final relative position being dependent on the particular RE. The position of the RE maximum within the oxide layer also varied with the substrate composition. For all coatings¹⁸O was found to have diffused through the oxide to the substrate-oxide interface during oxidation, the amount of oxide at this interface increasing with increasing time. The SIMS data confirm that for coated substrates there has been a change in oxidegrowth mechanism to predominantly anion diffusion. The RE most probably concentrates at the oxide grain boundaries, generally as the binary oxide (RE) CrO₃. Cr³⁺ diffusion is impeded, while oxygen diffusion remains unaffected.     

The high-temperature oxidation of Ni-20%Cr alloys containing various oxide dispersions

The oxidation behavior of Ni-20%Cr alloys containing approximately 3 vol.% Y₂O₃, ThO₂, and A1₂O₃ as dispersed particles has been examined in the temperature range 900 to 1200° C in slowly flowing oxygen at 100 Torr. The results show that the oxidation behavior of the Y₂O₃-, ThO₂-, Al₂O₃-, and Ce0₂-containing alloys is very similar and that some anomalies in the behavior of the ThO₂-containing alloy might be explained by the slower rate of chromium diffusion in this coarse-grained alloy. Two Al₂O₃-containing alloys were studied. One with a relatively coarse dispersoid size behaved in a manner analogous to a dispersion-free Ni-30% Cr alloy at 1100°C. The other alloy contained a dispersion of fine Al₂O₃ particles and behaved exactly like the Y₂O₃-containing alloy at 1000 and 1100°C, but at 1200° C oxidized at a faster rate. It has been shown that the adherent scales on dispersion-containing alloys have a stabilized fine grain size, whereas the nonadherent scales on dispersion-free alloys undergo grain growth.This work has been supported by the Naval Air Systems Command under Contract No. N00019-72-C-0190.     

Anomalous high-temperature oxidation in the zirconium-copper system

The high-temperature oxidation of the Zr-3 mass% Cu alloy and Zr₂Cu in oxygen is characterized by selective oxidation of zirconium while the excess of copper is accumulated at the alloy-oxide interface forming the Zr₈Cu₅ phase. The oxidation of Zr₂Cu at elevated temperatures shows an anomalous decrease of the oxygen consumption rate in the temperature range 890-975 °C. The oxide layer consists of monoclinic ZrO₂ mainly, with preferentially oriented crystallites in depth region at 900 °C and tetragonal ZrO₂ on the surface below 600 °C, and small amounts of CuO and Cu₂O. The reaction kinetics obeys a parabolic rate law. The activation energy of 117.5 and 54.4 kJ/mol has been estimated for the oxidation of the Zr-3 mass% Cu alloy and Zr₂Cu, respectively.     

The influence of borate coatings on the high-temperature oxidation of iron

Coatings of di-sodium tetraborate and of calcium metaborate have been found to protect iron against oxidation in the temperature range 750°–1050°C in oxygen. Protection is due to the compound iron boro-ferrite (4 FeO.Fe₂O₃.B₂O₃) that forms as a non-coherent, granular, blocking layer along the scale/metal interface, which effectively decreases the interfacial area for iron ion diffusion. Void formation, associated with the layer of complex oxide, may possibly enhance the blocking effect.     

The transition from internal oxidation to continuous-film formation during oxidation of dilute Ni-Si alloys

The oxidation of Ni-2.05Si and Ni-4.45Si was studied in oxygen over the range of 600°–1000°C for 18 hr. The oxidation kinetics did not follow a parabolic rate law, bur rather a power law of the form (w)ⁿ=kt was followed. The value of n ranged from 2.7 to 4.9 for Ni-2.05Si and from 3.0 to 6.4 for Ni-4.45Si. The low-silicon alloy exhibited extensive internal oxidation, whereas the higher-silicon alloy did not internally oxidize. In general, NiO containing little or no silicon formed as an exterior layer on both alloys. The internal oxidation zone in Ni-2.05Si was highly irregular in thickness, and in some areas there was no internal oxidation. The higher-silicon alloy formed a continuous layer of a silicon-rich oxide. X-ray diffraction did not detect silica (amorphous), and no evidence of Ni₂SiO₄ was observed, although EDAX analysis suggests that small amounts of the silicate might have formed. Theaverage thickness of the internal oxidation zone was found to agree well with calculated values based on oxygen solubility and diffusivity data. No enrichment of silicon occurred in the internal oxidation zone. Calculated values, 0.033 and 0.038 (depending on the model used), of the mole fraction of silicon required for the transition from internal oxidation to continuous silica film formation agreed well with experimental data obtained in both this study and with others reported in the literature.     

表面处理对锆合金性能的影响

介绍了锆合金的表面处理技术,讨论了各种表面处理技术对于锆合金表层结构和合金元素分布的影响及改善锆合金性能的机理,提出了进一步研究的建议.     

The effect of aluminum ion implantation on the oxidation of an Fe-6 at.% Al alloy at 1173 K

Samples of an Fe-6 at.% Al alloy were implanted with doses of 5×10¹⁶–3×10¹⁷ Al⁺ ions/cm² at 100 kV, and subsequently oxidized at 1173 K in O₂ gas at 27 kPa. By comparison with the behavior of unimplanted specimens, no effect on oxidation was observed if the dose of implant was less than 1×10¹⁷ Al⁺ ions/cm². At doses of 1×10¹⁷ Al⁺ ions/cm² or higher, a change in the oxide scaling morphology was effected from a duplex -Fe₂O₃--Al₂O₃ scale to an -Al₂O₃-enriched film with a dispersion of -Fe₂O₃ nodules. Oxygen uptake by the high-dose samples was due principally to nodule formation and weight gains after 45 hr were lower by 50–70% as compared with the unimplanted material. Nucleation and growth of the nodules occurred only within the first 5 hr of reaction, but their presence acted as an upper limit on the benefits provided by implantation of aluminum. Nodule formation was shown to be characteristic of the oxidation properties of the alloy material, not an extraneous feature introduced by the implantation process itself. Surface preparation prior to implantation had little effect on oxidation: high-dose electropolished samples behaved similarly to mechanically polished samples when implanted to comparable doses.     

The influence of thermal cycling on the oxidation and oxide spallation of a 1%Cr–0.5%Mo low-carbon steel

The oxidation and oxide spallation of 1%Cr–0.5%Mo low carbon steel disks in dry oxygen was studied isothermally at 800°C (1073 K) and in thermal cycling between 800 and 600°C (1073 and 873K) followed by cooling at rates from 3 to 100°C/min. Mostly parabolic oxidation kinetics were observed. Thin scales (10 ) were more prone to spalling than thicker scales (20 ). The thickness and growth imperfections of an inner scale layer enriched in chromium, molybdenum, and silicon strongly influenced the probability of cohesive failure exceeding that of adhesive failure of the scale. Cohesive failures in the bulk scale during thermal cycling were probably nucleated at voids and microcracks produced in the initial isothermal period of scale growth. The number of segmented scale layers that became detached during cycling was governed by the number of parallel rows of voids in the scale and not necessarily by the number of cycles.     

The influence of silicon on the oxidation properties of iron

Five Fe—Si alloys containing between 0·06 and 5·3 w/o have been oxidized in a gaseous mixture of 20 w/o oxygen in argon. The oxidation temperatures were 500°, 625°, 800°, 900° and 1000°C. The reaction kinetics have been studied gravimetrically and the oxidized samples examined microscopically, with an electron microprobe and with X-ray methods. The detrimental effect of silicon on the oxidation rate of iron is most pronounced at high temperatures and silicon contents. For comparative purposes, the oxidation of two chromium-nickel alloyed steels has been studied at 800° and 1000°C.     

Factors affecting the high-temperature oxidation behavior of some dilute nickel- and cobalt-base alloys

Oxidation of nickel- and cobalt-base alloys, containing small additions of a higher valent second metal, in oxygen or air at high temperatures results in the formation of relatively complicated scale morphologies which change subtly with increasing additions of the second element and its characteristics. The various factors that can influence the oxidation behavior of such alloys are assessed and correlated with the oxidation kinetics and scale morphology types. For very dilute alloys the increase in oxidation rate compared with that of the corresponding pure metal (nickel or cobalt) is largely due to doping of the external oxides. However, once the solubility limit of the second metal in this oxide is exceeded, additional increases in second metal content of the alloy can either increase further or decrease the oxidation rate. The exact behavior depends on the relative interplay of factors such as internal oxide formation and coalescence, blocking effects of incorporated internal oxide or pores in the scale, short-circuit paths through the scale, doping, and the relative diffusion rates of the two metals in the scale. Probable rate-determining steps for oxidation of different alloy composition ranges are proposed.