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1.
Anwar Ul-Hamid 《Corrosion Science》2004,46(1):27-36
In the present study the isothermal oxidation behaviours of Ni-10Cr-5Al, Ni-20Cr-5Al and Ni-30Cr-5Al alloys were investigated. The alloys were oxidised in air for 50 h at 1000 °C. Analytical transmission electron microscopy was used to characterize the morphology, structure and composition of the oxide scale. The oxide formed adjacent to the alloy was α-Al2O3 such that the higher was the Cr content of the alloy the easier was its formation. The Ni-30Cr-5Al alloy formed a complete layer of α-Al2O3 in the initial stages of oxidation through ‘oxygen gettering’ by Cr. A decrease in scale thickness and an increase in scale adherence were observed with an increase in Cr content from 10 to 30 wt.%. 相似文献
2.
A Ni3Al-based alloy, the composition of which was Ni-16.0% Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025%B, was cyclically oxidized in the temperature range of 900 to 1100°C in air for up to 500 hr. The alloy displayed good cyclic oxidation resistance up to 1000°C, with little scale spallation. It, however, lost cyclic oxidation resistance during oxidation at 1100°C after about 200 hr, displaying large weight losses due to serious scale spallation. NiO, α-Al2O3, NiAl2O4 and ZrO2 were formed. The oxide scales consisted primarily of an outer Ni-rich layer which was prone to spallation, and (Al, Cr, Zr, Mo, Ni)-containing internal oxides which were adherent due mainly to the formation of (Al2O3, ZrO2)-containing oxides that keyed the oxide scale to the matrix alloy. 相似文献
3.
The allotropic phase transformation behavior of Al2O3 scale formed on Ni–22Al–30Pt (in at.%) with and without 0.5Hf was investigated during short-term (i.e., 3?min dwell) cyclic oxidation at 1,150?°C in air. Hafnium addition did not appear to affect the oxidation rate in the early oxidation cycles, but it did delay the phase transformation from the metastable θ-Al2O3 structure to the stable α-Al2O3. Small dimples, which corresponded to α-Al2O3 grains, started to form on the Hf-free alloy after only three oxidation cycles; whereas, no apparent morphological change of the oxide scale surface was observed on the Hf-modified alloy. The transformation to α-Al2O3 was found to initiate at scale/alloy interface on the Hf-free alloy, but it initiated at gas/scale interface on the Hf-modified alloy. Depth profiling using glow discharge optical emission spectroscopy revealed that Hf enriched at the scale/alloy interface due to Hf rejection associated with the formation of an Al-depleted γ-layer, which has a low Hf solubility. Higher positive strain energy due to Hf solution in the metastable Al2O3 was inferred to be the main contributor to the delayed the transformation. 相似文献
4.
The cyclic oxidation behaviour of a Co32Ni21Cr8Al0.6Y (wt.%) alloy with and without the addition of 0.2 wt.% dysprosium was investigated at 800 and 1100 °C in static laboratory air. The Dy-containing alloy showed a faster θ- to α-alumina transformation and significantly less weight gain than Dy-free alloy. Under cyclic oxidation at 1100 °C, Dy addition produced a continuous and protective Al2O3 scale. The Dy-free alloy exhibited poor oxidation resistance. Scale spallation led to the development of a complex oxide scale and internal precipitation: (Al,Cr)2O3 on the surface, followed by a Al2O3 layer, then (Al,N) precipitates alone beneath the external scale. 相似文献
5.
X.H Wang 《Corrosion Science》2003,45(5):891-907
The isothermal oxidation behavior of bulk Ti3AlC2 has been investigated at 1000-1400 °C in air for exposure times up to 20 h by means of TGA, XRD, SEM and EDS. It has been demonstrated that Ti3AlC2 has excellent oxidation resistance. The oxidation of Ti3AlC2 generally followed a parabolic rate law with parabolic rate constants, kp that increased from 4.1×10−11 to 1.7×10−8 kg2 m−4 s−1 as the temperature increased from 1000 to 1400 °C. The scales formed at temperatures below 1300 °C were dense, adherent, resistant to cyclic oxidation and layered. The inner layer of these scales formed at temperatures below 1300 °C was continuous α-Al2O3. The outer layer changed from rutile TiO2 at temperatures below 1200 °C to a mixture of Al2TiO5 and TiO2 at 1300 °C. In the samples oxidized at 1400 °C, the scale consisted of a mixture of Al2TiO5 and, predominantly, α-Al2O3, while the adhesion of the scales to the substrates was less than that at the lower temperatures. Effect of carbon monoxide at scale/substrate was involved in the formation of the continuous Al2O3 layers. 相似文献
6.
Microstructural development during high-temperature oxidation of Ti2AlC below 1300 °C involves gradual formation of an outer discontinuous TiO2 layer and an inner dense and continuous α-Al2O3 layer. After heating at 1400 °C, an outer layer of mixed TiO2 and Al2TiO5 phases and a cracked α-Al2O3 inner layer were formed. After heating to 1200 °C and cooling to room temperature, two types of planar defect were identified in surface TiO2 grains: twins with (2 0 0) twin planes, and stacking faults bounded by partial dislocations. Formation of planar defects released the thermal stresses that had generated in TiO2 grains due to thermal expansion mismatch of the phases (TiO2, α-Al2O3 and Al2TiO5) in the oxide scale. After heating to 1400 °C and cooling to room temperature, crack propagation in TiO2 grains resulted from the thermal expansion mismatch of the phases in the oxide scale, the high anisotropy of thermal expansion in Al2TiO5 and the volume changes associated with the reactions during Ti2AlC oxidation. An atomistic oxidation mechanism is proposed, in which the growth of oxide scale is caused by inward diffusion of O2? and outward diffusion of Al3+ and Ti4+. The weakly bound Al leaves the Al atom plane in the layered structure of Ti2AlC, and diffuses outward to form a protective inner α-Al2O3 layer between 1100 and 1300 °C. However, the α-Al2O3 layer becomes cracked at 1400 °C, providing channels for rapid ingress of oxygen to the body, leading to severe oxidation. 相似文献
7.
《腐蚀工程科学与技术》2013,48(6):247-256
AbstractThe isothermal oxidation behaviour of Ni-7% Al and Ni-12·5% Al in 1atm oxygen at 800, 1000 and 1200°c has been studied by thermogravimetric methods, optical metallography, electron probe microanalysis and scanning electron microscopy. The ease of formation of an initially complete, protective, external α-Al2O3 scale is greater the higher the alloy aluminium content and the higher the temperature. Once developed, this external α-Al2O3 scale tends to fail mechanically in localised regions, the subsequent oxidation behaviour depending upon the composition of the exposed alloy. Under conditions favouring breakaway, Ni-7% Al yields rapid oxidation rates as NiO-rich nodules are formed on the exposed alloy substrate, although the rate declines later as a healing α-Al2O3 layer develops at their bases. If the α-Al2O3 fails on Ni-12·5% Al, the aluminium content of the revealed alloy is still high enough to ensure rapidproduction of a new external α-Al2O3 layer. 相似文献
8.
The oxidation behavior of Ni–Fe–41.5at.%Al alloys with different Fe/Ni ratios was investigated in air at 1000 °C in order to clarify the effect of Fe on the phase transformation of Al2O3 scale, using in-situ high-temperature X-ray diffraction by means of synchrotron radiation. The oxidation mass gain of alloys after 25 h of oxidation generally decreased with increasing Fe content; however, the initial oxidation mass gain was significantly decreased by increasing alloy Fe content. In-situ X-ray diffraction analysis indicated that higher alloy Fe contents promoted rapid formation of the stable α-Al2O3, while lower Fe in the alloy maintained the metastable Al2O3 for longer time oxidation. The effect of Fe on promoting α-Al2O3 formation can be explained by the initial formation of α-Fe2O3, whose structure is isomorphous with α-Al2O3. The additional effect of Fe on the growth rate of α-Al2O3 is also discussed. 相似文献
9.
J. W. Byeon J. Liu M. Hopkins W. Fischer N. Garimella K. B. Park M. P. Brady M. Radovic T. El-Raghy Y. H. Sohn 《Oxidation of Metals》2007,68(1-2):97-111
Ti2AlC ternary carbide is being explored for various high temperature applications due to its strength at high temperatures,
excellent thermal-shock resistance, and high electrical conductivity. A potential advantage of Ti2AlC over conventional Al2O3-forming materials is the near-identical coefficient of thermal expansion (CTE) of Ti2AlC and α-Al2O3, which could result in superior spallation resistance and make Ti2AlC a promising option for applications ranging from bondcoats for thermal barrier coatings to furnace heating elements. In
this study, isothermal and cyclic oxidation were performed in air to examine the oxidation behavior of Ti2AlC. Isothermal oxidation was performed at 1000, 1200 and 1400 °C for up to 25 h and cyclic oxidation consisted of 1,000 1-hour
cycles at 1200 °C. Characteristics of the oxide scale developed in air, including mass change, residual stress in the α-Al2O3 scale, phase constituents and microstructure, were examined as functions of time and temperature by thermogravimetry, photostimulated
luminescence, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy via focused ion beam in
situ lift-out. Above a continuous and adherent α-Al2O3 layer, a discontinuous-transient rutile-TiO2 scale was identified in the oxide scale developed at 1000 and 1200 °C, while a discontinuous-transient Al2TiO5 scale was identified at 1400 °C. The continuous α-Al2O3scale thickened to more than 15 μm after 25 h of isothermal oxidation at 1400 °C, and after 1,000 1-hour cycles at 1200 °C,
yet remained adherent and protective. The compressive residual stress determined by photoluminescence for the α-Al2O3 scale remained under 0.65 GPa for the specimens oxidized up to 1400°C for 25 hours. The small magnitude of the compressive
residual stress may be responsible the high spallation-resistance of the protective α-Al2O3 scale developed on Ti2AlC, despite the absence of reactive element additions. 相似文献
10.
The influence of silicon on the oxidation of Fe-14% Cr and Fe-28% Cr has been studied at high temperature, with particular emphasis on the development and nature of the healing SiO2 layer. In general, silicon is a less effective addition than aluminium to these alloys in improving oxidation resistance because SiO2 grows at a lower rate than α-Al2O3. Hence, silicon is a less successful oxygen secondary getter and development of a complete healing layer of SiO2 is less rapid than that of α-Al2O3 on a corresponding aluminium-containing alloy. Nonetheless, the addition of only 1% Si to Fe-28% Cr causes a marked reduction in the overall oxidation rate, particularly by facilitating development of the Cr2O3 scale. Precipitates of SiO2 form at the alloy/scale interface. These grow inwards and laterally until they eventually link up to establish a continuous healing layer at the interface after several hundred hours exposure at 1000°C. Similar features are observed for Fe-14% Cr-3% Si but the healing SiO2 layer develops after a much shorter time for Fe-14% Cr-10% Si, due to the high silicon availability. In every case, the healing layer has been shown to be amorphous SiO2. Although this phase is very protective during isothermal oxidation, it is a site of weakness during cooling and scale spallation is very extensive from specimens where the SiO2 is continuous, with failure occurring cohesively within that layer. Ion implantation of silicon into Fe-14% Cr and Fe-28% Cr gives a reduced oxidation rate due to facilitation of a more rapid establishment of a Cr2O3 scale. Similar implantation of yttrium into the ternary alloys assists in development of the silicon-containing oxide layer, possibly associated with an influence on the nucleation of the oxide precipitates in the early stages of exposure. 相似文献
11.
Oxide-dispersed Fe-28at.% Al-2%Cr alloys were produced by a powder metallurgy technique followed by hot extrusion. A variety of stable oxides were added to the base alloy to assess the effect of these dopants on the oxidation behavior at 1200°C in air and O2. An Al2O3 dispersion flattened the α-Al2O3 scale, but produced none of the other reactive element effects and had an adverse influence on the long-term oxidation behavior. A Y2O3 dispersion improved the alumina scale adhesion relative to a Zr alloy addition at 1200 and 1300°C. However, the Y2O3 dispersion was not as effective in improving scale adhesion in Fe3Al as it is in FeCrAl. This inferior performance is attributed to a larger amount of interfacial void formation on ODS Fe3Al. 相似文献
12.
Yuri Kitajima Shigenari Hayashi Takumi Nishimoto Toshio Narita Shigeharu Ukai 《Oxidation of Metals》2010,73(3-4):375-388
Rapid formation of an α-Al2O3 scale on Fe–50 at.%Al by pure metal thin coatings of Ni, Al, Ti, Cr or Fe was investigated, and the effects of those elements on Al2O3-scale evolution were assessed. The oxidation behavior of samples with and without coatings could be divided into two groups: the samples with/without Ni and Al, and those with Ti, Cr and Fe. The mass gains of samples coated with Al and Ni were almost the same as that of non-coated Fe–50 at.%Al alloy. The mass gains of samples coated with Ti, Cr, and Fe were much lower than that of the Fe–50 at.%Al alloy. A stable α-Al2O3 scale was found to develop from the beginning of oxidation on the samples coated with Ti, Cr and Fe. However metastable θ-Al2O3 remained after long-time oxidation of non-coated and Ni- and Al-coated samples. The direct α-Al2O3 scale formation on the samples with Cr or Fe coatings was speculated to be due to sympathetic nucleation of α-Al2O3 on the surface of Al-supersaturated Fe2O3 for Fe-coated sample, and composition changes from (Cr,Al)2O3 to (Al,Cr)2O3 for the Cr-coated sample. Initial formation of an oxide having a corundum structure was inferred to provide a nucleation site for precipitation of α-Al2O3 without prior formation of a metastable Al2O3 scale. 相似文献
13.
γ-NiCrAl alloys with relatively low Al contents tend to form a layered oxide scale during the early stages of oxidation, rather than an exclusive α-Al2O3 scale, the so-called “thermally grown oxide” (TGO). A layered oxide scale was established on a model γ-Ni–28Cr–11Al (at.%) alloy after isothermal oxidation for several minutes at 1100°C. The layered scale consisted of an NiO layer at the oxide/gas interface, an inner Cr2O3 layer, and an α-Al2O3 layer at the oxide/alloy interface. The evolution of such an NiO/Cr2O3/Al2O3 layered structure on this alloy differs from that proposed in earlier work. During heating, a Cr2O3 outer layer and a discontinuous inner layer of Al2O3 initially formed, with metallic Ni particles dispersed between the two layers. A rapid transformation occurred in the scale shortly after the sample reached maximum temperature (1100°C), when two (possibly coupled) phenomena occurred: (i) the inner transition alumina transformed to α-Al2O3, and (ii) Ni particles oxidized to form the outer NiO layer. Subsequently, NiO reacted with Cr2O3 and Al2O3 to form spinel. Continued growth of the oxide scale and development of the TGO was dominated by growth of the inner α-Al2O3 layer. 相似文献
14.
The development of internal oxides, intergranular oxides and internal voids in Ni-15.1Cr-1.1Al and Ni-28.8Cr-1.0Al during oxidation in 1 atm oxygen at 1000° to 1200°C has been studied. In both cases, the formation of an external Cr2O3-rich scale causes vacancies to be generated in the alloy due to the different diffusion rates of chromium towards the alloy-scale interface and of nickel back into the bulk alloy. At 1000°C, condensation of these vacancies at the alloy grain boundaries facilitates formation of intergranular oxides while, at 1200°C, the vacancies condense to give voids in the grains and grain boundaries. Internal oxides are formed at both temperatures. The internal and intergranular oxides are mainly α-Al2O3, although some Cr2O3-rich oxides are produced near the alloy-scale interface. Possible mechanisms for the development of the internal and intergranular oxides in these alloys are discussed and related to the observed oxide morphologies and compositions. 相似文献
15.
《Metallography》1989,22(1):57-67
A Co-32Ni-21Cr-8Al-0.5Y alloy coating was plasma sprayed on Hastelloy X. The microstructure of the coating layer consists of γ phase solid solution, γ′ phase, and Y-rich intermetallic phase. This coating exhibits excellent oxidation and sulfidation resistance after exposure in air and in sodium sulfate at 1,000°C for 60 h, due to the formation of α-Al2O3 oxide scale. However, the presence of chloride in the sodium sulfate leads to rupture of the aluminium oxide scale, and this results in the precipitation of chlorides and sulfides within the coating layer. 相似文献
16.
The effects of pure metal coatings, including Ni, Fe and Cr, on long-term oxidation kinetics, surface morphology and structure were studied. Ni-50Al alloy and Ni-coated, Fe-coated and Cr-coated samples were pre-oxidized at 900 °C in air. They were then oxidized isothermally at 1,000 °C in air. The bare Ni-50Al alloy oxidized rapidly during the initial stage of oxidation due to the formation of θ-Al2O3, but the oxidation rate decreased after α-Al2O3 had developed. Oxidation of the Ni-coated sample was slow from the beginning of oxidation even though the θ-Al2O3 was predominated for a longer oxidation time. No θ-Al2O3 developed on the Cr and Fe-coated samples, but the oxidation rates of these samples were much faster than those of bare and Ni-coated samples. Cross-sectional images revealed that the grain size of α-Al2O3, which formed on Cr and Fe-coated samples, was smaller than those of bare and Ni-coated samples. These metal coatings affected the microstructure of α-Al2O3 and they showed a strong effect on the growth rate of α-Al2O3 in the steady-state oxidation stage. 相似文献
17.
采用SEM和XRD等分析手段对NiAl-31Cr-2.9Mo-0.1Hf-0.05Ho定向共晶合金的高温氧化行为进行研究。结果表明,在900~1100°C下合金表面生成连续的Al2O3氧化膜,从而使合金具有良好的抗氧化性能;在1150°C下合金表面的Al2O3氧化膜破裂,氧化增重升高。定向凝固工艺细化合金的组织以及微量稀土元素Ho的加入,均有利于在合金表面形成连续的Al2O3氧化膜。在氧化过程中,表面氧化膜存在着θ-Al2O3→α-Al2O3的相变过程,从而导致1000°C和1050°C氧化增重反常现象的出现。 相似文献
18.
Y2O3-dispersed NiAl was produced by a powder-metallurgy process. By adding Y as an oxide dispersion (OD), problems with NiYx formation and internal oxidation were avoided. Short-term isothermal and cyclic-oxidation performance at 1200°–1500° C was compared to cast NiAl alloys with and without Zr. Results indicate that the Y2O3 addition was beneficial to scale adhesion and significantly modified the α-Al2O3 scale microstructure, similar to a Zr alloy addition. However, at 1400 and 1500° C, neither the Y2O3 or Zr additions changed the scale-growth rate, eliminated the formation of voids at the metal-scale interface or prevented scale spallation. These similarities in performance suggest that similar mechanisms occur when the reactive element is added as either an OD or an alloy addition. 相似文献
19.
《Intermetallics》2007,15(9):1217-1227
The isothermal oxidation kinetics of molybdenum silicide based alloys with composition (in at.%) as 76Mo–14Si–10B (MSB), 77Mo–12Si–8B–3Al (MSB3AL), and 73.4Mo–11.2Si–8.1B–7.3Al (MSB7.3AL) processed by reaction hot pressing of elemental powders, have been investigated in the temperature range of 700–1300 °C in dry air for 24 h. The microstructures of all the alloys have shown the presence of α-Mo, Mo3Si, Mo5SiB2 and SiO2 or α-Al2O3 phases. The oxidation kinetics and the resulting scale characteristics depend on the alloy composition and temperature of exposure. While all the three alloys show unabated loss of mass causing pest disintegration at 700 °C, the MSB3AL and MSB7.3AL alloys undergo large mass loss in the range of 800–900 °C as well. The loss in mass has been attributed primarily to volatilization of MoO3 as well as spallation. The oxide scales formed in the range of 700–800 °C show SiO2 and MoO3, while those formed at 900 °C or above contain Mo, MoO2 and SiO2. In addition, α-Al2O3 or mullite has been found in the oxide scales of MSB3AL and MSB7.3AL alloys. The oxidation resistance of the Mo–Si–B alloys can be enhanced in the range of 700–800 °C by pre-oxidation treatment at 1150 °C to form a protective scale of B2O3–SiO2. 相似文献
20.
In this work, NiCoCrAlY coatings were deposited on a new Ni-base alloy, IC-6. The oxidation kinetic curves of alloy IC-6, K17 and NiCoCrAlY coatings on alloy IC-6 at 900-1100 °C were obtained. The results indicated that the oxide scales consisted of α-Al2O3, NiAl2O4, NiO, as well as a small amount of NiMoO4 and MoO2. These scales occurred after alloy IC-6 exposure at 900 °C for 100 h. The weight loss occurred when alloy IC-6 were exposed at 1050 and 1100 °C due to the formation of volatile MoO3. After the NiCoCrAlY coating was deposited, the scales mainly contained α-Al2O3, when the specimens were oxidized at 900 °C, and α-Al2O3and Cr2O3 at 1050 °C. The formation of α-Al2O3 and Cr2O3 scales on NiCoCrAlY coating was directly responsible for improving oxidation resistance of the alloy IC-6. 相似文献