Kathleen B. Alexander

Solidification of YBa2Cu3O7−δ from the melt

Abstract Nucleation and growth of 123 from the melt via a peritectic reaction into domains of aligned platelets is studied. Analysis of the microstructure of well-formed domains indicates that there is no orientation difference between adjacent platelets within a domain, suggesting that a domain grows from a single nucleus. The platelet boundaries are found to be filled-in with secondary phases that correspond to the liquid phase at high temperature, suggesting that constitutional supercooling effects may be operative. Samples quenched from temperatures considerably below the peritectic temperature contain only a few crystals, indicating the presence of a large nucleation barrier. The above observations, coupled with extensive microstructural examination of quenched solid-liquid interfaces, suggest that the 211 size, distribution and volume fraction not only control the growth rate of 123 along the fast growth ab -plane (by supply of yttrium), but also the growth rate along the slow growth c -direction since the nucleation barrier is reduced at 211/123 intersections. At high cooling rates there is a distinct change in the nucleation and growth processes. Structures characteristic of sympathetic or autonucleation and spherulitic growth are observed. These structures are distinct from the single crystal nature of well-formed domains. The growth mechanism which results in the formation of 123 domains and the final microstructure within a single domain, also explains the observed non-weak-link characteristics for current flow along the a , b - and c -directions, as determined by direct transport and magnetization measurements.

Ceramic composites with a ductile Ni3Al binder phase

Abstract Composites of B-doped ductile Ni3Al alloys with both non-oxide (WC, TiC) and oxide (Al2O3) ceramic powders were produced by hot-pressing. The Ni3Al alloys wet the non-oxide ceramic powders well and form a semi-continuous intergranular phase. However, the Ni3Al alloys do not wet the oxide powders well and tend to form discrete “islands” of the metallic phase. Mechanical property testing showed the flexural strength is retained to temperatures of at least 800 °C. The fracture toughness and hardness were found to be equal to or higher than comparable Co-based hardmetal systems. Initial corrosion tests showed excellent resistance to acid solutions.

Exploring Spatial Resolution in Electron Back-Scattered Diffraction Experiments via Monte Carlo Simulation

A Monte Carlo model was used to simulate specimen-electron beam interactions relevant to electron back-scattered diffraction (EBSD). Electron trajectories were calculated for a variety of likely experimental conditions to examine the interaction volume of the incident electrons as well as that of the subset of incident electrons that emerge from the specimen, i.e., back-scattered electrons (BSEs). The spatial resolution of EBSD was investigated as functions of both materials properties, such as atomic number, atomic weight, and density, and experimental parameters, such as specimen thickness, tilt, and incident beam accelerating voltage. These simulations reveal that the achievable spatial resolution in EBSD is determined by these intrinsic and extrinsic parameters.

Oxidation behavior of platinum-aluminum alloys and the effect of Zr doping

The isothermal and cyclic oxidation behavior of PtAl and PtAl+Zr was studied followed by postoxidation microstructural and microchemical analyses. Their isothermal oxidation performance at 1200 degree sign C was similar to that of NiAl and NiAl+Zr. In short (1-h) cycles, the cyclic oxidation resistance of undoped PtAl was found to be substantially better than NiAl. However, with longer (100-h) cycles, little improvement in the metal consumption rate was observed relative to NiAl. The addition of Zr to PtAl significantly improved cyclic oxidation performance in both short- and long-cycle tests. Spatially resolved energy dispersive spectroscopy indicated Zr segregation to both the metal-oxide interface and Al{sub 2}O{sub 3} grain boundaries. (c) 1999 Materials Research Society.

The MBE growth and optical quality of BaTiO{sub 3} and SrTiO{sub 3} thin films on MgO

High quality epitaxial BaTiO{sub 3} and SrTiO{sub 3} have been grown on MgO; stabilized at a one unit cell height; and grown to film thicknesses of 0.5--0.7 {mu}m. These relatively thick films remain adherent when thermally cycled between growth temperatures and room temperature, are crack free with high optical quality, and have both in-plane and out-of-plane X-ray rocking curves of 0.3--0.5{degree}. These films have been grown using molecular beam epitaxy (MBE) methods starting with the TiO{sub 2} layer of the perovskite structure. The TiO{sub 2}-layer/MgO interface uniquely satisfies electrostatic requirements for perovskite heteroepitaxy and provides the template structure that leads to the high quality films that are obtained. Wavelength dependence of optical loss has been characterized between 475 nm and 705 nm with loss coefficients < l dB/cm being obtained at the He-Ne wavelength.

Some effects of metallic substrate composition on degradation of thermal barrier coatings

Comparisons have been made in laboratory isothermal and cyclic oxidation tests of the degradation of oxide scales grown on single crystal superalloy substrates and bond coating alloys intended for use in thermal barrier coatings systems. The influence of desulfurization of the superalloy and bond coating, of reactive element addition to the bond coating alloy, and of oxidation temperature on the spallation behavior of the alumina scales formed was assessed from oxidation kinetics and from SEM observations of the microstructure and composition of the oxide scales. Desulfurization of nickel-base superalloy (in the absence of a Y addition) resulted in an increase in the lifetime of a state-of-the-art thermal barrier coating applied to it compared to a Y-free, non-desulfurized version of the alloy. The lifetime of the same ceramic coating applied without a bond coating to a non-desulfurized model alloy that formed an ideal alumina scale was also found to be at least four times longer than on the Y-doped superalloy plus state-of-the-art bond coating combination. Some explanations are offered of the factors controlling the degradation of such coatings.

Transformation behavior in Al2O3ZrO2 ceramic composites

Abstract Neutron powder diffraction was used to investigate the tetragonal-to-monoclinic transformation of ZrO2 in an Al2O3 ZrO2 ceramic composite containing 40 vol% tetragonal ZrO2. The neutron-diffraction data were analyzed using the Rietveld refinement technique, which allowed determination of the extent of the transformation, as a function of temperature. The onset transformation temperature determined for this sample was 130 K. Below this temperature, the fraction of the monoclinic phase continued to increase to about 9 vol% at 80 K and remained constant for temperatures below 80 K. The calculated thermal expansion, using the refined lattice parameters, was found to be in excellent agreement with dilatometry data, confirming that the sharp increase in the thermal expansion upon cooling resulted from the tetragonal-to-monoclinic phase transformation in ZrO2.

Auger electron spectroscopy analysis of SiC-whisker surfaces and SiC-whisker/alumina interfaces

Auger electron spectroscopy (AES) has been used to examine as-received and oxidized silicon carbide whiskers and their respective whisker/matrix interfaces after fabrication into SiC-whisker-reinforced alumina composites. As-received whisker surfaces exhibited a 2--3 nm-thick near-surface region that was C-rich. Oxygen was detected at the outer surface, but diminished to near zero within 25 nm of the surface. Oxidized whiskers had 60 nm-thick SiO{sub 2} surface layers, which was in agreement with the transmission electron microscopy observations. The whisker/matrix interfaces in both composites consisted of thin ({lt}0.5 nm) layers of a C--Si--O noncrystalline material. The thick SiO{sub 2} layers on the oxidized whiskers were ejected from the interfaces during hot-pressing. It was concluded that (i) the higher toughness of the composite fabricated with as-received SiC whiskers may be related to the higher C and lower O in its SiC{sub {ital w}}/Al{sub 2}O{sub 3} interfaces, and (ii) interface composition cannot be reliably predicted using the surface composition of free whiskers prior to fabrication.

The effect of various oxide dispersions on the oxidation resistance of Fe{sub 3}Al

Oxide-dispersed Fe-28at.%Al-2%Cr alloys were produced by a powder metallurgy technique followed by hot extrusion. Yttria and ceria were added to the base alloy to assess the effect of these dopants on the oxidation behavior. The amount of dopant was varied from 0.05-0.5 at.% Y in a series of Y{sub 2}O{sub 3}-dispersed alloys. isothermal and cyclic oxidation testing was conducted at temperatures from 800{degrees} to 1300{degrees}C. A CeO{sub 2} addition was detrimental to the oxidation behavior. The Y{sub 2}O{sub 3} improved the {alpha}-Al{sub 2}O{sub 3} scale adhesion relative to an undoped alloy, but was not as effective as similar additions to an oxide-dispersed FeCrAl alloy.

An Oxygen Potential Gradient as a Possible Diffusion Driving Force

Oxygen-active elements such as Y, Zr and Hf are added to alumina-forming alloys to improve the adhesion of the external α-Al 2 O 3 scale formed during high temperature oxidation. During oxidation at 1000°-1500°C, many such elements are observed to diffuse from the alloy through the scale and form oxide particles at the scale-gas interface. Once nucleated, the volume fraction of these particles increases with oxidation temperature and time. The continued increase in volume fraction with time suggests that diffusion is not merely driven by a concentration gradient of the oxygen-active elements. Pre-coating with a dense, 1 μm-thick, plasma-deposited alumina layer prior to oxidation did not prevent the nucleation and growth of these types of particles at the gas interface of the coating. The driving force for this diffusion phenomenon is attributed to the oxygen potential gradient across the metal-oxide-gas system and the high oxygen affinity of these elements.