Gregory J. Yurek

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.

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 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.

Oxidation behavior of a fine-grained rapidly solidified 18-8 stainless steel

The cyclic oxidation behavior of a fine-grained, rapidly solidified 303 stainless steel was determined at 900 °C in pure oxygen. The rapidly solidified alloy exhibited superior resistance to oxidation compared with that of a wrought 304 stainless steel; its oxidation resistance was as good as that of a wrought 310 stainless steel, even though the latter alloy contained more Cr and Ni. The matrix of the rapidly solidified steel contained a uniform dispersion of fine MnS precipitates (0.2 to 0.5 μm), which were effective in inhibiting grain growth at elevated temperatures. The enhanced resistance to oxidation of the rapidly solidified alloy is attributed to two factors: (1) the formation and growth of protective Cr2O3 and SiO2 scales were promoted by the fine alloy grain size (5 to 8 =gmm) and by the presence of the MnS dispersion, and (2) the adherence of the scale was increased by the formation of intrusions of SiO2 from the external scale into the alloy, which formed around MnS precipitates and along closely-spaced alloy grain boundaries, and which acted to key the scale mechanically to the alloy.

Synthesis of a superconducting oxide by oxidation of a metallic precursor

A Eu-Ba-Cu alloy was oxidized completely in pure oxygen gas at 1 atm pressure at 300 °C. The alloy, which was produced by conventional melting and solidification techniques, and which was in-homogeneous, yielded an inhomogeneous oxidation product upon complete oxidation. After annealing in pure oxygen at a higher temperature (800 to 900 °C) for a sufficiently long time, the oxidation product became a high-temperature superconducting oxide (EuBa2Cu3077-x). The onset of super-conductivity occurred at 92 K. The diamagnetic screening at 10 K for a sample cooled in zero field was approximately 70 pct of the ideal value, and a substantial Meissner effect was observed.

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.

Generation and characterization of submicron aerosols of zinc oxide

An apparatus has been developed to produce submicron aerosols of zinc oxide by the condensation of supersaturated vapors in a tube furnace. The mass concentration and the size distribution of the aerosols were stable over a period of several hours. Various particle morphologies (chain aggregates, large blocky particles and fine spiked particles) were produced by varying the operating conditions. According to analyses of electron micrographs, the primary particle diameters obeyed a log-normal distribution with count median diameters between 0.008 and 0.040 µm with geometric standard deviations from 1.42 to 1.67.

Kinetics and mechanisms of the oxidation of cobalt at 600–800°C

Two-phase layered scales comprising CoO and Co3O4 formed on cobalt during oxidation at 600°, 700°, and 800°C and at oxygen partial pressures in the range 0.001–1 atm. The kinetics, which were obtained by thermogravimetric analysis, obeyed a parabolic rate law after an initial, non-parabolic stage of oxidation. The monoxide consisted of relatively large grains (∼10 μ) and the spinel comprised small grains (⪝3 μ) for all conditions of oxidation. Grain boundary diffusion of cations played a significant role in the growth of the spinel layer. Thermogravimetric data and the steady-state ratio of the oxide layer thicknesses were employed to calculate the rates of thickening of the individual oxide layers and the rate of oxidation of CoO to Co3O4.

Superconducting Yb2Ba4Cu7Ox produced by oxidizing metallic precursors

Abstract A new superconducting phase, Yb 2 Ba 4 Cu 7 O x ( a=3.81 A , b=3.86 A , c=50.45 A ; space group Ammm; Z =2), has been identified and analyzed, mainly by X-ray diffraction and transmission electron microscopy, in oxidized ribbons of silver-bearing metallic precursors. This structure is analogous to Yb 2 Ba 4 Cu 8 O x (2-4-8) described previously, but an excess CuO layer insertion occurs in every two unit cells of the normal 1-2-3 structure. A specimen in which this phase occupies 30% of the oxide volume shows T c =86 K ( ΔT c =4 K). In a transmission electron microscope, this “2-4-7” phase changes easily to the normal 1-2-3 structure by electron beam heating. Other defects in this structure are described.

Microstructures of the scales formed on zircaloy-4 in steam at elevated temperatures

The ZrO2scales formed on Zircaloy-4 PWR tubes during corrosion in steam in the temperature range of 1000–1300°C have been found to contain a metallic phase that is relatively rich in Sn. The precise composition of the metallic phase has not been determined. Most of the metallic phase is located in a line of metallic particles, which is oriented parallel to the alloy-scale interface and located near the center of the scale. The exact morphology of the scale on either side of the particle line has not been identified. The oxide between the metallic particles and the scale-steam interface contains very little Sn, except for a narrow zone adjacent to the scale-steam interface, which was formed in the beginning of the reaction. It appears that the scale between the metallic particles and the alloy-scale interface consists of thin columnar grains of ZrO2with a very fine metallic phase probably located at the ZrO2grain boundaries. The experimental evidence presently available indicates that the metallic phase exists in the scales at the reaction temperature. If the metallic particles were rich enough in Sn, then a liquid metallic phase would exist in the oxide during the reaction. Kinetic studies demonstrate that these particles move with respect to the scale-steam interface toward the center of the alloy during the course of the oxidation reaction. It appears that the presence of the line of metallic particles could, under certain conditions, markedly influence the mechanical properties of the oxide scales formed on Zircaloy-4.