The high-temperature oxidation resistance of iron-silicon-aluminum alloys

Silicon or chromium can be used as an oxygen getter in iron-aluminum alloys to prevent the internal oxidation of aluminum. This suppresses the formation of the iron oxide nodules that tend to destroy binary iron-aluminum alloys during high-temperature oxidation. Alloys of iron containing aluminum and silicon in varying proportions were heated in flowing air for 50 hr at 1093°C. Of the alloys tested, one containing 6% aluminum and 1 % silicon was the most resistant to oxidation.     

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 high-temperature oxidation behavior of Co-Re-Cr-based alloys

Co-Re-Cr-based model alloys have been developed for high-temperature applications beyond 1,200°C. The purpose of the present investigation is to gain an insight into the oxidation mechanisms of the model Co-Re-Cr alloys and to find ways to improve oxidation resistance of this class of materials. The first generation of this class of alloys showed a rather poor oxidation resistance during exposure to laboratory air. As a consequence of the lack of protectiveness of the oxide layer, the vaporization of rhenium oxide takes place during oxidation. It has been found that Si stabilizes the Cr₂O₃ scale, enhancing the oxidation resistance significantly.     

The high-temperature oxidation behavior of vanadium-aluminum alloys

The oxidation behavior in air of pure vanadium, V-30Al, V-30Al-10Cr, and V-30Al-10Ti (weight percent) was investigated over the temperature range of 700–1000° C. The oxidation of pure vanadium was characterized by linear kinetics due to the formation of liquid V₂O₅ which dripped from the sample. The oxidation behavior of the alloys was characterized by linear and parabolic kinetics which combined to give an overall time dependence of 0.6–0.8. An empirical relationship of the form: W/A=Bt + Ct^1/2 + D was found to fit the data well, with the linear contribution suspected to be from V₂O₅ formation for V-30Al and V-30Al-10Cr, and a semi-liquid mixture of V₂O₅ and Al₂O₃ for V-30Al-10Ti. The parabolic term is presumed related to the formation of a solid mixture of V₂O₅ and Al₂O₃ for V-30Al and V-30Al-10Cr, and TiO₂ for V-30Al-10TiThe addition of aluminum was found to reduce the oxidation rate of vanadium, but not to the extent predicted by the theory of competing oxide phases proposed by Wang, Gleeson, and Douglass. This was attributed to the formation of a liquid-oxide phase in the initial stages of exposure from which the alloys could not recover. Ternary additions of chromium and titanium were found to decrease the oxidation rate further, with chromium being the most effective. The oxide scales of the alloys were found to be highly porous at 900° C and 1000° C, due to the high vapor pressure of V₂O₅ above 800° C.     

The oxidation performance of modern high-temperature alloys

The high-temperature oxidation resistance of an alloy is a key design criterion for components in a variety of industrial applications, such as advanced gas turbines, industrial heating, automotive, waste incineration, power generation and energy conversion, chemical and petrochemical processing, and metals and minerals processing. The importance of correctly assessing the long-term oxidation behavior of high-temperature alloys is illustrated. As applications move to higher temperatures, new alloys are needed. In this paper, the oxidation performance of three newly developed alloys, an alumina-forming Ni-Fe-Cr-Al alloy, a γ′-strengthened Ni-Cr-Co-Mo-(Al+Ti) alloy, and a nitride-strengthened Co-Cr-Fe-Ni-(Ti+Nb) alloy is presented. Author’s note: All compositions reported in this article are in weight percent.     

The high-temperature internal oxidation and intergranular oxidation of nickel-chromium alloys

The development of internal oxides and intergranular oxides in dilute NiCr alloys, containing 1–5% Cr, in NiNiO packs and in 1 atm oxygen at 800–1100°C has been investigated. The internal oxide particles were relatively coarse and widely spaced and were Cr₂O₃, except for a narrow band adjacent to the surface where NiCr₂O₄ particles were also present. Several types of intergranular oxide were developed in the Ni/NiO packs, with preferential penetration being more extensive in the higher chromium-containing alloys at the lower temperatures. Discrete intergranular oxide particles were formed deep in the alloy beneath bands of Cr₂O₃ which developed over intersections of the alloy grain boundaries with the surface, or beneath continuous or discontinuous grain-boundary oxides near the surface, possibly due to the development of a relatively flat oxygen profile and a steep chromium gradient in the subjacent alloy. In the presence of a thickening NiO external scale, preferential intergranular oxidation was much less extensive than in the Ni/NiO packs as the rapid growth of the scale prevented development of Cr₂O₃-rich surface bands.     

A high-temperature oxidation-resistant Fe-Mn-Al-Si alloy

To develop satisfactory alloys without Cr or Ni for high-temperature application up to 1100C, three alloys based on Fe-10%Al-Si with differing fourth (or fifth) element additions were oxidized in air at 1100°Cfor 24 hr. A low carbon, Fe-30Mn-10Al-Si alloy exhibited excellent high-temperature oxidation resistance. The total weight gain for 24 hr oxidation in air at 1100°C was only 1.03 mg/cm ². After air oxidation for 6 days at 1100°C, no nodule formation or breakthrough oxidation occurred. Post-oxidation SEM and EDAX examination showed that a thin, compact, protective alumina scale formed on the alloy.Visiting Scientist (People's Republic of China).     

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.     

High-temperature oxidation-resistant hafnium-tantalum alloys

Alloy systems with oxidation resistance at temperatures in excess of 4000°F, yet with room temperature ductility that allows forming by techniques similar to those used for the refractory metals.     

On modeling the oxidation of high-temperature alloys

Oxidation of high-temperature alloys represents complex, strongly coupled, non-linear phenomena which include: (i) diffusion of oxygen in the alloy; (ii) an oxidation reaction in which the reaction product causes substantial permanent, anisotropic volumetric swelling; (iii) high-temperature elastic–viscoplastic deformation of the base alloy and the oxide; and (iv) transient heat conduction. We have formulated a continuum-level chemo-thermomechanically coupled theory which integrates these various nonlinear phenomena. We have numerically implemented our coupled theory in a finite-element program, and have also calibrated the material parameters in our theory for an Fe–22Cr–4.8Al–0.3Y heat-resistant alloy experimentally studied by Tolpygo et al. Using our theory, we simulate the high-temperature oxidation of thin sheets of FeCrAlY and show that our theory is capable of reproducing the oxide thickness evolution with time at different temperatures, the permanent extensional changes in dimensions of the base material being oxidized and the development of large compressive residual stresses in the protective surface oxide which forms. As an application of our numerical simulation capability, we also consider the oxidation of an FeCrAlY sheet with an initial groove-like surface undulation, a geometry which has been experimentally studied by Davis and Evans. Our numerical simulations reproduce (with reasonable accuracy) the shape-distortion of the groove upon oxidation measured by these authors. This example has obvious ramifications for delamination failure of a ceramic topcoat on a thermally grown oxide layer in thermal barrier coatings.     

The high-temperature strength of some Fe3Al alloys

The high-temperature strength of a Fe–25%Al–2%Nb alloy in both a solutionised state and a precipitated state has been determined for temperatures up to 900 °C and compared with that of a solution hardened Fe₃Al alloy. Some comparisons of these materials with previously reported Fe₃Al-based materials are also made. The Fe–25%Al–2%Nb material is capable of retaining good strength to temperatures above 800 °C, but rapid coarsening of the Laves precipitate particles at higher temperatures leads to strength loss at such temperatures. The presence of stable dispersed particles at the intermediate temperatures means that good strength can be retained to very low strain rates.     

The role of yttrium in high-temperature oxidation behavior of Ni-Cr-Al alloys

Oxidation of five nickel-chromium-aluminum alloys with yttrium additions of between 0.005 and 0.7wt.% has been studied in the temperature range 800–1200°C in oxygen at pressures of 1, 10, and 720 Torr. The yttrium additions improve the oxidation behavior of the nickel-chromium-aluminum alloys, and generally an addition of 0.1 or 0.2 wt.% yttrium gives the best improvement in the oxidation resistance. In this case, the oxidation kinetics indicate asymptotic approach toward zero scale growth with time. This is suggested to be caused by the formation of subgrains in the alloy which (a) provide enhanced diffusion of aluminum to the surface and (b) increase the number of oxide nucleation sites. Preferred oxidation of aluminum occurs, resulting in the formation of an -Al ₂O₃ layer. Additions of more than 0.3 wt.% yttrium result in preferential grain boundary oxidation and a convoluted alloy-oxide interface. This effect, key-on effect along with the formation of an aluminum and yttrium double oxide, produces increased oxide adherence for these alloys.Part of the work was carried out during the author's stay at Aerospace Research Laboratories, Wright-Patterson Air Force Base, Dayton, Ohio 45433, USA. It was sponsored in part by the Air Force Materials Laboratory, Research and Technology Division, AFSC, through the European Office of Aerospace Research, OAR, U.S. Air Force.     

显微组织对Cu—Cr—Ni合金高温氧化行为的影响

研究了两种单／双相Cu-Cr-Ni合金的高温氧化行为。结果表明,合金氧化动力学偏离抛物线规律,其瞬时抛物线速率常数随时间延长而降低。两种合金表面氧化膜的结构差别较大,单相合金表面形成－连续的Cr2O3层,双相合金表面氧化膜外层是一边疆的CuO层,Ni和Cr的氧化发生在合金内部,这种合金与氧化物共存的混合内氧化与经典的内氧化明显不同,氧化层最里面形成了一连续的CrO3膜,抑制了合金的进一步氧化。     

The high-temperature oxidation of iron-chromium-nickel alloys containing 0–30% chromium

The oxidation behavior of iron-chromium-nickel alloys containing 0–30% chromium has been determined for oxidation at 1000°C in static pure oxygen atmospheres. Particular emphasis has been placed on the correlation between the kinetics of oxidation and the morphologies and compositions of the scales produced. Maximum oxidation resistance was associated with the formation of chromic oxide scales on alloys containing greater than 20% chromium. The loss of an oxide species from these scales by volatilization may limit the usefulness of alloys protected by chromic oxide scales to a temperature less than 1000°C.Research supported by the Broken Hill Proprietary Co., Ltd., Postgraduate Research Scholarship.Unless otherwise specified, all compositions referred to in this paper are in weight percent.     

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.     

The role of nickel in the high-temperature oxidation of Fe-Cr-Ni alloys in oxygen

The oxidation of several largely austenitic Fe-Cr-Ni alloys in 1 atm oxygen at 800–1200°C has been studied thermogravimetrically, metallographically, and in detail by electron probe micro analysis. Fe-Cr-Ni alloys of this type are protected by Cr₂O₃-healed scale, which thickens slower than on the corresponding binary Fe-Cr and Ni-Cr alloys, presumably because nickel and iron ions dope the Cr₂O₃ more effectively together than singly and/or because the alloy composition and ability to absorb cation vacancies are such as to produce a smaller vacancy activity gradient or level in the scale, or voids within it. The scale adhesion, as on Ni-Cr alloys, is generally good after long times, at least partly due to the convoluted alloy-oxide interface, in some cases to large intergranular Cr₂O₃-rich stringers, and possibly to the general specimen mechanical properties. Nonprotective stratified scale development is relatively unusual and often produces nickel-rich, alloy-particle-containing nodules, as on Fe-Ni alloys. Careful selection of ternary and more complex alloys with appropriate alloy interdiffusion coefficients and oxygen solubilities and diffusivities should permit development of materials with the best compromise between ease of Cr₂O₃ establishment, avoidance of breakaway, and readiness of scale healing.     

Kinetics and mechanism of high-temperature oxidation of dilute cobalt-chromium alloys

Kinetics of oxidation of Co-Cr alloys containing 0.4%–15% Cr was studied as a function of temperature (1273–1573 K) and oxygen pressure (4 × 10²–10⁵Pa). The oxidation process was found to be approximately parabolic and faster than that for pure cobalt. The scales are double-layered and consist of a compact outer CoO layer and a porous inner layer containing CoO slightly doped by chromium and spinel CoCr₂O₄. The oxidation mechanism was investigated by means of platinum markers and the¹⁸O isotope. The scale on the alloys containing less than 1% Cr grows exclusively by outward diffusion of cobalt, while that on the alloys containing more chromium—with a significant contribution of inward oxygen transport from atmosphere. This transport is not a lattice diffusion, but proceeds presumably through microfissures resulting from the secondary process of perforating dissociation of the outer scale layer.     

Effect of water vapor on high-temperature oxidation of FeCr alloys

The suppression of protective chromia scale formation in water vapor containing service environments limits in many cases the upper application temperature of high-Cr martensitic and ferritic steels. The present paper discusses the mechanisms which are responsible for this technologically important effect, using results of oxidation tests with two types of FeCr model alloys in Ar-O₂, Ar-O₂-H₂O, and Ar(-H₂)-H₂O mixtures. The data shows that in atmospheres with a high ratio of water vapor to oxygen, Cr exhibits a higher tendency to become internally oxidized than in dry Ar-O₂, or e.g. air. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the scale and/or to enhance formation of volatile Cr species, the present results thus reveal that the presence of water vapor also affects the transport processes in the alloy, likely by incorporation of hydrogen.     

The high-temperature oxidation characteristics of alloys from the Nb-W-Cr system with C additions

The oxidation behavior of two alloys from the Nb-W-Cr system containing carbon has been studied. Selection of specific alloy compositions was based on the ternary isothermal sections of Nb-W-Cr at 1,000°C and 1,500°C. Oxidation experiments were conducted for 24 hours in air over a range of temperatures from 700°C to 1,400°C. Mass gain per unit area as a function of the temperature was used to determine the alloy’s oxidation resistance and the oxidation products were characterized by scanning electron microscopy/energy dispersive x-ray spectroscopy, and x-ray diffraction (XRD). The results have been compared with a previous study of the oxidation resistance of the alloys in the monolithic form. Beneficial effects have been observed when carbon is added to Nb-20W-5Cr alloy; however, the oxidation resistance of the alloy with higher chromium content was not improved by the addition of the modifier. The nature of oxides obtained from alloys with and without carbon additions has been observed to be similar based on the XRD results.