J. A. Little

Oxidation behavior of several chromia-forming commercial nickel-base superalloys

Several commercially available Ni-base superalloys were exposed isothermally in air at temperatures between 750° and 1000°C and also under cyclic conditions at 1000°C. The kinetics of oxidation were determined and the scales were analyzed by electron microscopy and X-ray diffraction. Thin adherent chromia-rich scales formed on the alloys at 750°C after 1000 hr. Although Waspaloy showed the lowest weight gain in this test, it also showed the deepest internal corrosion due to oxidation of the grain-boundary carbides. At temperatures up to 1000°C the external scales were also chromia-rich but there was greater internal corrosion. Titanium in the alloys oxidized, diffusing through the chromia scale to form faceted rutile (TiO2) grains at the surface as well as forming TiO2 and TiN internally. The amount of rutile at the oxide surface increased with temperature and alloy Ti concentration. Alumina formed as discrete internal oxides below the chromia scale, although Astroloy when oxidized isothermally at 1000°C developed a semicontinuous internal layer of alumina due to its higher Al content. Under cyclic conditions Astroloy formed a thicker, less-protective scale of transition oxides probably due to its lower Cr content.

Oxidation behaviour and protection of carbon/carbon composites

The oxidation behaviour of carbon/carbon composite materials and graphite (in cube form), in flowing air, has been studied in the temperature range 500 to 1100 °C. Gasification for unprotected samples occurred at temperatures around 500 °C. SiC coatings offered only limited protection below their intrinsic protection range due to the diffusion of oxygen along microcracks. Diffusional control was more significant for thicker coatings. However, the use of boron oxide applied on an underlayer of SiC, gave good protection for extended periods at temperatures up to 1000 °C, due to microcrack sealing. The use of borate coatings, both with and without an SiC underlayer, was limited by the volatility of the borate.

Glass sealants for carbon-carbon composites

SiC coatings were chemically vapour deposited (CVD) on a carbon-carbon (C/C) composite. These coatings contain thermal mismatch cracks which require sealing between 600 and 1100 °C. The volatility, wettability and moisture sensitivity of binary and multicomponent borosilicate glasses were investigated, and these glasses were found to provide sealing over some or all of this temperature range. The volatilization rate is enhanced by the presence of water vapour and results in the loss of the B2O3 constituent from the glass. The C/C composite surface showed poorer wetting than the CVD SiC surface, and both surfaces showed improved wetting for glasses of higher B2O3 content. The moisture sensitivity of the binary borosilicate glasses was proportional to the B2O3 content. Room-temperature moisture had little effect on the multicomponent glasses.

Sintering of pre-mullite powder obtained by chemical processing

The crystallization and sintering behaviour of a premullite powder which had been synthesized from aluminium sulphate [Al2(SO4)3· 16H2O] and colloidal silicon dioxide have been studied. Calcination of the mixture at 860 °C for 12 h gives a very active powder (surface area − 188 M2g−1) in the form of spine] and mullite forms via this spinel phase. The non-mullitized powder can be reactively sintered at 1500–1550°C to 97%–99% density in 3–5 h with a very fine microstructure.

High-temperature oxidation of Fe-2 1 4 Cr-1Mo in oxygen

The oxidation of a 214Cr-1Mo steel in dry flowing oxygen has been studied in the temperature range 550–700°C for periods of up to 100 hr. A detailed low-resolution microstructural investigation revealed a layered oxide consisting of a very fine-grained and finely pored innermost layer of doped spinel, a central columnar-grained relatively coarsely pored layer of magnetite, and an outer fine-grained hematite layer with fine pores and covered with whiskers of α-Fe2O3. This structure is compared with previous results on Fe and model Fe-Cr alloys, as are the kinetics of the oxidation reaction.

The effects of boron additions on the oxidation of Fe-Cr alloys in high-temperature steam: Analytical results and mechanisms

The oxide films formed on iron-chromium alloys in superheated steam have been investigated using a variety of microanalytical techniques, most notably electron energy loss spectroscopy. The addition of boron dopants leads to the rapid formation of a microcrystalline film of composition (Cr)xB1}-x2O3,which resists further oxidation. Analysis of the near-edge structures associated with each core-loss edge after different oxidation times allows us to postulate various mechanisms for the observed behavior.

Fresnel contrast analysis of composition changes and space charge at grain boundaries in mullite

Abstract Grain boundaries in two mullite specimens have been characterised using Fresnel contrast analysis in the transmission electron microscope. A ratio of the scattering potential differences ΔV between the grains and the grain boundaries in the two specimens was obtained without needing to know the absolute specimen thicknesses of the regions examined. The characterisation of a glassy intergranular film in one specimen using energy dispersive X-ray spectroscopy then allowed the magnitude of ΔV at a crystalline boundary in the second specimen to be determined. For the first time, this scattering potential difference was interpreted in terms of contributions to the contrast from space-charge layers as well as changes in electron scattering factor and density.

Degradation of oxidized SiC-Si3N4 in molten aluminium

Silicon nitride-bonded silicon carbide ceramic tubes are used to provide indirect immersion heating in non-ferrous foundries. Samples of such tubes have been oxidized for varying lengths of time and then immersed in liquid aluminium. Long-term prior oxidation renders the ceramic susceptible to wetting by aluminium with subsequent degradation of mechanical properties. Samples not oxidized before immersion were stable in the liquid metal.

Hot-salt oxidation characteristics of titanium aluminides

The oxidation and hot-salt corrosion of a Ti–46%Al, 1.8%Nb, 1.9%Cr (at%) alloy were investigated in the temperature range 700–800°C. Oxidation tests were carried out using thermogravimetric analysis and show parabolic oxidation after 30 h for salt-coated and uncoated samples. Oxide-scale morphologies were examined using scanning electron microscopy and X-ray diffraction for various oxidation times. The X-ray data reveal the presence of rutile, alumina and NaTiO2 as surface oxides. For quantitative analysis, the cross-sections of the specimens were analysed by energy dispersive X-ray analysis.

Microstructural characterization of surface layers formed on a low alloy steel during erosion oxidation in a fluidized bed

Low alloy steel specimens were subjected to erosion-oxidation in a simulated fluidized bed environment in the range 100–600° C. Transmission electron microscopy was carried out on the surfaces which experienced particle impacts, with specimens prepared by backthinning. At low temperatures (≤ 200° C) a dense, protective layer of bed particle fragments formed on the steel surface due to fragmentation of particle asperities and their subsequent comminution to a very fine size (< 10 nm). At intermediate temperatures thin oxide films developed on the wear scar surfaces; these were predominantly magnetite with a fine grain size. There was rapid material loss with particle impacts removing the oxide and some metal below, but the thin oxide rapidly regrew due to the fine grain size, absence of a haematite layer and the mechanical damage during particle impact. At high temperatures the oxide became sufficiently thick to be mechanically protective. Erosion occurred within the fine grained surface haematite layer, while there was grain growth in the lower magnetite layer.