Anthony J. Garratt-Reed

The reactive element effect in commercial ODS FeCrAI alloys

Two commercial oxide dispersion strengthened alumina-forming FeCrAl alloys, Inco alloy MA956 and Kanthal alloy APM, were studied in order to look at the effect of reactive elements on their oxidation behaviour. MA956 has a distribution of Y2O3−Al2O3 particles, while APM has a ZrO2—AI2O3 distribution. Isothermal oxidation at 1000°C and 1200°C showed a reduced oxidation rate for both alloys compared to that of an undoped FeCrAl alloy. In short-term cyclic tests at 1200°C, both alloys exhibited excellent scale adhesion. Using scanning transmission electron microscopy with X-ray energy dispersive spectroscopy, both Y and Zr, respectively, were found to segregate to the oxide grain boundaries and the metal-scale interface after oxidation at 1000°C and 1200°C. These experimental observations are discussed with regard to a new theory to explain the reactive element effect.

Controlled biosynthesis of greigite (Fe 3 S 4 ) in magnetotactic bacteria

We report the presence of intracellular greigite crystals in two different types of rod-shaped single-celled magnetotactic bacteria collected from sulfide-rich sites

The development of some dual-phase steel structures from different starting microstructures

The development of dual-phase structures with different morphologies has been studied in detail by intercritical annealing specimens of a steel containing 0.11 pct C and 1.6 pct Mn with different microstructures before annealing. The kinetics of formation of the two-phase structure at the annealing temperature and the redistribution of substitutional solute elements were measured in specimens quenched from the intercritical annealing temperature. The structure before annealing was either banded ferrite-pearlite, homogenized ferrite-pearlite, lath martensite, spheroidal cementite dispersed in ferrite, or austenite. No measurable partitioning of silicon or molybdenum, present in the steel in small concentrations, was found. However, close to equilibrium partitioning of manganese occurred on annealing specimens with either a ferrite-pearlite or a lath martensite structure, but during the separation of ferrite from austenite in step-quenched specimens there was no partitioning. Surprisingly, measurements of manganese concentrations using an electron beam of 1 nm diameter at intervals of 5 nm or less revealed the presence of narrow spikes in the concentration profile at many ferriteaustenite interfaces in specimens with a ferrite-pearlite or martensite starting structure as well as in those step-quenched from austenite. In some instances, a minimum in the concentration profile was found in ferrite, adjacent to a maximum at an interface. Thus, adsorption of manganese at ferriteaustenite interfaces produces concentrations in excess of the concentrations indicated by the equilibrium diagram. The probable diffusion processes controlling the kinetics of transformation in the different microstructures are identified.

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.

Copper association with iron sulfide magnetosomes in a magnetotactic bacterium

Greigite (Fe3S4) and pyrite (FeS2) particles in the magnetosomes of a many-celled, magnetotactic prokaryote (MMP), common in brackish-to-marine, sulfidic, aquatic habitats, contained relatively high concentrations of copper which ranged from about 0.1 to 10 atomic per cent relative to iron. In contrast, the greigite particles in the magnetosomes of a curved magnetotactic bacterium collected from the same sampling site did not contain significant levels of copper. The ability of the MMP to biomineralize copper within its magnetosomes appeared to be limited to that organism and dependent upon the site from which it was collected. Although the chemical mechanism and physiological function of copper accumulation in the magnetosomes of the MMP is unclear, the presence of copper is the first evidence that another transition metal ion could be incorporated in the mineral phase of the magnetosomes of a magnetotactic bacterium.

Precipitation and Recrystallization in Some Vanadium and Vanadium-Niobium Microalloyed Steels

Static precipitation and recrystallization following hot compression of austenite and the interactions between the two processes have been studied in a set of aluminum-killed HSLA steels containing 0.1 pct carbon, [0.016 - 0.026] pct nitrogen and 0.1 or 0.2 pct vanadium. Two steels containing both vanadium (0.1 and 0.2 pct) and niobium (0.03 pct) were included for purposes of comparison. The compression and the static tests were all carried out isothermally at temperatures between 800 and 900 °C. The course of recrystallization was followed by measurements of the rate of softening and by optical metallography of specimens quenched from the test temperature after different times. Precipitation was studied by measurements of the rate of hardening, by transmission electron microscopy of thin foils, carbon and aluminum extraction replicas, and by X-ray dispersion and energy-loss spectroscopy from individual precipitates.The temperature of the nose of theC-curve for precipitation in vanadium steels is much lower than that in niobium steels, as is the temperature, TR, below which no recrystallization occurs in short times. Precipitation occurs both at austenite grain boundaries and in the grains (matrix precipitation). The former starts early and the precipitates grow rapidly to an approximately constant size; the matrix precipitates grow more slowly and are responsible for the observed hardening of the austenite. The relevance of various models proposed for the retardation and arrest of recrystallization of austenite are discussed.In the steels containing vanadium and niobium the precipitates contain both heavy elements: (V,Nb) (C,N). The Nb/V ratio in the matrix precipitates is different than in the parent austenite. The grain-boundary precipitates, however, contain the same Nb/V ratio as the parent austenite. The rate of hardening exhibits a reverseC-curve behavior, being more rapid than in the corresponding vanadium steels at higher temperatures and about the same at lower temperatures.

Analytical electron-microscopy study of the breakdown of α-Al2O3 scales formed on oxide dispersion-strengthened alloys

Alumina scales formed during cyclic oxidation at 1200°C on three Y2O3–Al2O3-dispersed alloys: Ni3Al, β-NiAl, and FeCrAl (Inco alloy MA956) were characterized. In each case, the Y2O3 dispersion improved the α-Al2O3 scale adhesion, but in the case of Ni3Al, an external Ni-rich oxide spalled and regrew, indicating a less-adherent scale. A scanning-transmission electron microscope (STEM) analysis of the scale near the metal–scale interface revealed that the scale formed an ODS FeCrAl showed no base metal-oxide formation. However, the scale formed on ODS Ni3Al showed evidence of cracking and Ni-rich oxides were observed. The microstructures and mechanisms discussed may be relevant to a thermal-barrier coating with an Al-depleted aluminide bond coat nearing failure.

Biomineralization of Iron Sulfides in Magnetotactic Bacteria from Sulfidic Environments

Magnetotactic bacteria contain intracellular iron mineral inclusions termed magnetosomes (Balkwill et al., 1980) which impart a permanent magnetic dipole moment to the cell resulting in its alignment and navigation in magnetic fields (Blakemore, 1975, 1982; Frankel, 1984). Various methods have been used to determine the mineral phase of the magnetosomes including Mossbauer spectroscopy, x-ray powder diffraction, selected area/micro-electron diffraction, and energy dispersive x-ray analysis (Frankel et al., 1979; Towe and Moench, 1981; Sparks et al., 1990). The particles in almost all magnetotactic bacteria have been shown to consist of the mineral magnetite (Fe3O4) (Frankel et al., 1979; Towe and Moench, 1981; Matsuda et al., 1983; Mann et al., 1987; Bazylinski et al., 1988), sometimes admixed with hydrous ferric oxide, a precursor to Fe3O4 precipitation (Frankel et al., 1983; Bazylinski et al., 1988).

Resolving composition variations at interfaces by STEM

Abstract The scanning transmission electron microscope (STEM) equipped with an energy dispersive X-ray detector is discussed with particular emphasis on the use of this instrument for analyzing composition variations at interfaces with a resolution below 10 nm. In order to achieve a spatial resolution for compositional analysis at this level care must be taken in the preparation of the sample, the sample geometry relative to the detector, and the operating characteristics of the instrument. In addition the issues of beam broadening and counting statistics are of great importance. These parameters, and a number of illustrative examples drawn from materials science, are discussed.

Spatial resolution for compositional analysis in STEM

Abstract Spatial resolution for compositional analysis in STEM depends upon instrumental parameters as well as the characteristics of the sample being observed. In an effort to characterize the importance of these parameters in defining spatial resolution for microanalysis, experiments on materials of scientific interest have been performed and the results tested against a model. Experiments have been performed involving the depletion of Cr at grain boundaries is sensitized stainless steels and the segregation of Fe to grain boundaries in heat-treated MgO. The experimental composition profiles have been determined as a function of sample thickness and incident probe size. These results have been modelled by describing the electron intensity as a Gaussian distribution that broadens with depth in the sample. The impact that increasing sample thickness and increasing electron probe size have on spatial resolution for microanalysis is discussed with reference to the experimental data obtained and the model used.