D. F. Mitchell

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 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.

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.

Thickness determination of oxide films on iron by electron back-scattering Mssbauer spectroscopy

Magnetite films in the range 265–4520 Å have been grown on natural iron substrates and subsequently investigated by electron back-scattering Mössbauer spectroscopy. In particular, the percentage, P, of the total spectrum area contributed by the oxide has been determined as a function of oxide thickness, d. It was found that d up to ≈ 3000Å may be expressed (to an accuracy of ∼ 5%) by d (Å)=−1.95 × 103 ln (1–0.01 P). The experimental data have been compared with the theoretical predictions of both Huffman and Bainbridge. Good agreement between experiment and Huffmans predictions of P is obtained using values of the electron attenuation coefficient, μ, of 1.10 × 104 cm2 g−1 for the 7.3 keV electrons and 1.73 × 104 cm2 g−1 for the 5.4 keV electrons. A good fit of our data to Bainbridges expression requires a somewhat lower effective μ, value of 0.8 × 104 cm2 g−1. The experimental P value for the thickest oxide (4520 Å) is lower than the theoretical predictions, probably as a result of a neglected mechanism recently identified by Tricker, Ash, and Cranshaw.

The effect of ceria coatings on the high-temperature oxidation of iron-chromium alloys☆

Abstract The effect of sputtered ceria coatings (0.2–30 nm thick) on the oxidation of Fe25Cr alloys at 1100 °C has been studied using the 18 O/secondary ion mass spectrometry technique. Samples were oxidized either in 16 O 2 only or sequentially first in 16 O 2 and then in 18 O 2 for periods up to 20 h. Oxidation rates were reduced with increasing ceria thickness up to 4 nm; there was no significant further reduction in rate for thicknesses of 4 nm or more. The secondary ion mass spectrometry sputter profiles indicated that scales were mainly Cr 2 O 3 ; iron was present in the outer part of the oxide layer at levels of 1% or less. With increasing ceria coating thickness the maximum in the cerium sputter profile moved from the vicinity of the alloy-oxide interface towards the gas-oxide interface. When Fe25Cr was sequentially oxidized, 18 O was found to have diffused towards the alloy-oxide interface. The secondary ion mass spectrometry data suggest that when ceria is located within the scale there has been a change in mechanism from predominantly cation to predominantly anion diffusion.

Interfacial segregation during oxidation in O2 at 1173 K of CeO2-coated Fe-20Cr alloys

The interfacial segregation of sulphur and carbon during the oxidation in 1 torr O2 at 1173 K of Fe-20Cr alloy, which was either free of Ce, alloyed with 0.078 wt.% Ce, or sputter-coated with a 4 nm-thick CeO2 layer, was studied using polyatomic SIMS. Oxidation periods were up to 19 hr. During oxidation, sulphur and carbon accumulated at the alloy-oxide interface region of both uncoated and coated alloys. The amount of segregated sulphur did not vary appreciably with time, whereas carbon increased with time. The total amount of segregants was similar for both uncoated and coated alloys, although the scales formed on the sputter-coated alloy maintained adhesion and were about 10 times thinner than those on the uncoated alloys.

Low-temperature oxidation of Fe-24 wt.%Cr

Ferritic polycrystalline Fe-24 wt.% Cr was oxidized in pure oxygen at 190 ≤ T≤490° C and pressures in the range 5.3×10−2–13.3 Pa for periods of up to 5 hr. The reaction proceeded in three stages. An initial period of accelerating rate was accompanied by oxide island nucleation and growth. Following island coalescence the rate was approximately logarithmic at low temperatures and somewhat slower than parabolic at high temperatures. Rate control during this period was thought to be due to mass transport through the oxide grain boundaries left by the island impingement process. During these first two stages the oxide formed was γ-M2O3 with possibly some spinel. The final stage of reaction involved the appearance of α-M2O3 on the outer oxide surface and a substantial slowing of the oxidation rate due to the low diffusivity in this phase.

Rutherford backscattering study of high temperature oxidation of Y implanted Fe-24Cr

We have investigated the effect of Y ion implantation on the high-Temperature oxidation of Fe-24Cr using Rutherford-backscattering spectroscopy, secondary ion mass spectroscopy, and analytical electron microscopy. Results indicate that implantation of Y has a very large effect on the growth rate of the oxide compared to metals alloyed with Y. Analytical tools have been applied to determine the spatial distribution of Y, the microstructure of the oxide, and contribution of oxygen transport to the oxidation process. Results are compared with those of recent cation-diffusion measurements in Cr2O3 and Cr2O3 doped with Y2O3.

Segregation of cerium in chromia scales

Abstract The oxidation rate of Fe-Cr alloys at 900-1100°C is significantly decreased by the presence of a thin (~40A) sputter coating of Ce02. 18O/SIMS experiments show that Ce found within the scale changes the mechanism of Cr2O3 growth from predominantly cation to predominantly anion diffusion, both of which are considered to occur along oxide grain boundaries. For oxides > 1000 A thick, EDX, EELS, TEM and electron diffraction data indicate that Ce segregates to form particles of CeCr03 which likely retard grain boundary diffusion of Cr cations. Analogous results are observed for Y203-coated and Y-implanted alloys.