R. J. Hussey

The influence of grain-boundary segregation of Y in Cr2O3 on the oxidation of Cr metal

The oxidation behavior at 900°C of pure Cr and Cr implanted with 2×1016 Y ions/cm2 was studied. The kinetics of oxidation were measured thermogravimetrically and manometrically. The mechanisms of oxide growth were studied using18O-tracer oxidation experiments, and the composition and microstructure of the oxide scales were characterized by TEM and STEM. Segregation of Y cations at Cr2O3 grain boundaries was found to be the critical factor governing changes in the oxidation behavior of Cr upon the addition of Y. In the absence of Y, pure Cr oxidized by the outward diffusion of cations via grain boundaries in the Cr2O3 scale. When Y was present at high concentration in the scale, as when Cr implanted with 2×1010 Y ions/cm2 was oxidized, anion diffusion predominated. It is concluded that strain-induced segregation of Y at grain boundaries in the oxide reduced the cation flux along the grain boundaries. The rate of oxidation was reduced because the grain-boundary diffusivity of cations became lower than the grain-boundary diffusivity of the anions, which then controlled the rate of oxidation. Changes in the relative rates of Cr3+ and O2− transport, as well as a solute-drag effect exerted by Y on the oxide grain boundaries, resulted in changes in the microstructure of the oxide.

The influence of implanted yttrium on the mechanism of growth of Cr/sub 2/O/sub 3/ on Cr

One of the apparent effects of Y on the growth of Cr/sub 2/O/sub 3/ scales, based on inert marker studies, is a change in growth mechanism from predominant cation diffusion to predominant anion diffusion. The interpretation of inert marker studies has always been open to question, and a recent study of the mechanisms of growth of ..cap alpha..-Al/sub 2/O/sub 3/ scales using /sup 18/O tracer techniques has demonstrated that the results of inert marker studies can be misleading. Results of /sup 18/O tracer oxidation experiments conducted with Y-implanted and Y-free Cr are reported in this paper, which demonstrate that Y implantation does change the mechanism of oxide growth.

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.

Effect of carbon on cavity formation during the high-temperature oxidation of Ni

Ni containing 0.00002 to 0.003 wt.% carbon was oxidized in 1 atm O2 at 700 to 1270°C for 1 min to 20 hr. Cavity formation in the metal and oxide and at the oxide-metal interface was affected by the carbon content. Extensive cavitation developed at the grain boundaries of Ni containing as little as 0.0001 wt.% carbon but no cavitation occurred in decarburized Ni. Metal cavitation is dependent on the local concentration of carbon segregated at the Ni grain boundaries rather than on the overall carbon content. The cavities arise from hot deformation or creep of the metal substrate induced by the oxidation process. The cavities in the oxide formed on decarburized Ni remain near the oxide-metal interface; on Ni containing appreciable carbon the oxide cavities migrate outward by a dissociative mechanism assisted by the gaseous transfer of oxygen across the cavities with CO-CO2 acting as the carrier.

The effect of CeO2 coatings on the oxidation behaviour of Fe-20Cr alloys in O2 at 1173 K

Abstract The influence of 4 nm thick CeO 2 coatings on the oxidation resistance of Fe-20Cr at 1173 K in 1 torr O 2 for periods up to 19 h was examined using 18 O/SIMS, TEM and SEM/EDX. Finely dispersed CeO 2 became incorporated into the oxide scale and promoted the formation and retention of an adherent layer, which grew at a rate ∼10 times lower than that of the base alloy. Sequential oxidation experiments in 16 O 2 and 18 O 2 confirmed a change in growth mechanism for the coated alloy from predominantly cation to predominantly anion diffusion. Ceria likely present at oxide grain boundaries blocks cation diffusion outwards allowing inward oxygen transport to predominate.

Thermal oxidation of single-crystal aluminum at 550C

Oxygen transport during oxide growth on (110), (111), and (100) Al crystals at 550°C is investigated by18O/SIMS combined with kinetic measurements and SEM and TEM observations. Starting with an electropolished surface, the experimental evidence suggests oxide growth by oxygen anion transport via local pathways through an outer amorphous Al2O3 layer and oxygen incorporation at the periphery of the underlying laterally growing γ-Al2O3 islands. The kinetics, island morphology and epitaxy are sensitive to substrate orientation. This oxide growth behavior is compared with oxide formation on a sputter-cleaned and annealed (111) Al surface.

Comparison of the kinetics ofhigh-temperature oxidation of Fe as influenced by metal purity and cold work

Abstract The oxidation of annealed and cold-worked Fe of varying purity was investigated at 500and 550°C in Oa at latm. The apparent parabolic rate constant for cold-worked Fe is high initially, drops steeply, and approaches a constant value at long times. Annealed Fe oxidizes slowly and develops voids between oxide and metal. The faster oxidation of cold-worked Fe is caused not only by extra vacancy sinks in the metal suppressing void formation at the Fe304-Fe interface but because diffusion through the oxide is rapid initially. Three values of the oxidation “rate constant” can be distinguished —for cold-worked Fe, for annealed Fe with oxide in good contact, and for annealed Fe with separated oxide. Impurities affect the oxidation of cold-worked Fe by increasing the amount of cold work, and of annealed Fe by modifying the separation between oxide and metal.

The influence of grain-boundary segregation of Y in Cr2O3 on the oxidation of Cr metal. II. Effects of temperature and dopant concentration

The oxidation behavior of pure Cr and Cr implanted with Y was studied as a function of temperature (900 and 1025°C) and ion-implantation dose (1×1015 and 2×1016 Y ions/cm2). The microstructures of the Cr2O3 scales were affected by both of the variables studied. Yttrium ions segregated at the grain boundaries in the Cr2O3 scales formed on the implanted alloys and the concentration of Y at the grain boundaries decreased with a decrease in the dose of implanted Y. The mechanism of growth of the Cr2O3 scales was altered by the presence of the Y ions at the Cr2O3 grain boundaries only when a critical concentration of Y at the grain boundaries was exceeded.

The growth and structure of oxide films formed on Fe in O2 and CO2 at 550C

A study has been made of the structure of oxide layers formed at different times on abraded Fe oxidized in 1 atm O2 and CO2 at 550°C. A duplex Fe3O4 layer was formed and the inner layer was considered to grow by an oxide dissociation mechanism. The growth of both layers has been explained by a model, which correlates the overall kinetics with oxide grain growth. Derived values of the parabolic rate constant for lattice diffusion have been used to calculate self-diffusion coefficients, which were in good agreement with literature values for Fe diffusion in Fe3O4, but were very much larger than the values for either Fe or O in α-Fe2O3.

Characterization and growth of oxide films

Abstract Surface-analytical techniques are useful to characterize oxide films and to study growth processes on metals and semiconductors. This paper will summarize work at the National Research Council of Canada on the high temperature oxidation of nickel, chromium, FeCrAl alloys (with and without yttrium additions), β-NiAl and silicon. The application of secondary ion mass spectrometry, reflection high energy electron diffraction, and transmission electron microscopy is emphasized. Also considered is the thermal oxidation of III–V semiconductors, indium phosphide and gallium arsenide. X-ray photoelectron spectroscopy provides additional useful information on the chemical composition of the oxides. The often complementary information provided by the various techniques leads to (i) a better understanding of oxidation processes and of oxide growth mechanisms on an atomic scale, (ii) interfacial segregation phenomena, and (iii) the role of reactive elements like yttrium in modifying transport processes in oxides.