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Mechanically reliable scales and coatings

Description: As the first stage in examining the mechanical reliability of protective surface oxides, the behavior of alumina scales formed on iron-aluminum alloys during high-temperature cyclic oxidation was characterized in terms of damage and spallation tendencies. Scales were thermally grown on specimens of three iron-aluminum composition using a series of exposures to air at 1000{degrees}C. Gravimetric data and microscopy revealed substantially better integrity and adhesion of the scales grown on an alloy containing zirconium. The use of polished (rather than just ground) specimens resulted in scales that were more suitable for subsequent characterization of mechanical reliability.
Date: July 1, 1995
Creator: Tortorelli, P.F. & Alexander, K.B.
Partner: UNT Libraries Government Documents Department

The effect of aging at 343/degree/C on type 308 stainless steel welds

Description: Three nominally 25-mm (1-in) thick shielded metal-arc welds were prepared from 304L base plate with 308 filler material, to obtain three different ferrite levels (4, 8, and 12 %). Portions of these welds were then aged at 343/degree/C for 3000, 10000, and 20000 hours. Charpy V-notch and tensile specimens were taken from the welds. The tensile results were similar for all the specimens and showed little effect of aging on either the yield or ultimate tensile strengths. The Charpy impact properties of the higher ferrite content materials were significantly degraded by these agings, with larger decreases in the impact energy with increased aging time. The microstructures of the welds were examined by metallography and transmission electron microscopy, and the fracture surfaces were examined by scanning electron microscopy. The changes in the mechanical properties and the fractography are discussed in light of the observed changes in the microstructure. 3 refs., 3 figs., 2 tabs.
Date: January 1, 1989
Creator: Alexander, D.J.; Alexander, K.B. & Nanstad, R.K.
Partner: UNT Libraries Government Documents Department

Multivariate statistical analysis of spectrum lines from Si{sub 3}N{sub 4} grain boundaries

Description: It is well known that the high-temperature properties of polycrystalline Si{sub 3}N{sub 4} ceramics are strongly influenced by the nanometer-scale glassy phase at the grain boundaries. The authors have recently analyzed the variation of the near-edge fine structure (ELNES) of the Si-L{sub 2,3} edges using a combination of TEM spectrum-line acquisition with an imaging filter and multivariate statistical analysis. The glassy phase at the Si{sub 3}N{sub 4} grain boundaries is easily damaged by the fine probes usually used in scanning transmission electron microscopy to acquire ELNES data. Thus an alternative method using a Gatan imaging filter (GIF), called TEM spectrum-line analysis, was used. This technique will be used to correlate variations in grain boundary chemistry and bonding with the observed performance of Si{sub 3}N{sub 4} ceramics.
Date: April 1, 1997
Creator: Rice, P.M.; Alexander, K.B. & Anderson, I.M.
Partner: UNT Libraries Government Documents Department

Monte Carlo simulation of spatial resolution for electron backscattered diffraction (EBSD) with application to two-phase materials

Description: The study of texture and grain boundary misorientation in multiphase materials has been greatly benefited from the recent automation of the electron back-scattered diffraction (EBSD) technique. With this technique, each phase in a multiphase material can be individually sampled and analyzed. This is of great significance and interest in the study of thin films, inclusions and multiphase alloys. Spatial resolution, which depends on experimental conditions such as beam energy and specimen tilt, and the material being studied, is critical in order to determine the orientation of different phases in multiphase materials. The Monte Carlo (MC) method has been effectively used to investigate spatial resolution in single phase materials. In this paper, the MC simulation is modified and applied to two-phase geometries, specifically an Al/Au specimen, and a 750 nm thick Au film on a SiO{sub 2} substrate.
Date: October 1, 1997
Creator: Ren, S.X.; Kenik, E.A. & Alexander, K.B.
Partner: UNT Libraries Government Documents Department

Practical factors affecting the quantification of elemental concentrations in beam-sensitive ceramics by EFTEM

Description: In order to obtain high spatial resolution microchemical data from beam-sensitive materials, alternative techniques which avoid the use of fine probes, such as energy-filtered imaging techniques must be used. Robust quantification procedures are currently under development. The quantification procedure which yields images whose intensities are proportional to the elemental concentrations has already been determined. The net core-loss intensity images, S{sub i}, can be corrected for diffraction contrast by normalization with low loss images, whereas thickness variations are corrected by normalization with t/{lambda} maps. This procedure has been shown to give the correct value for the concentration ratio between two phases for a single elemental map. Determining concentration ratios between two elements in the same phase can actually be simpler, in theory, than the above procedure since it only requires dividing the net core-loss intensity images of two elements and correcting for the difference in cross-section. However, there are a few practical factors that must be considered in practice. The partial ionization cross-section {sigma}{sub i}({Delta},{beta},{alpha}) is not only a function of the slit width, {Delta}, and the collection half angle, {beta}, but is also a function of the convergence angle {alpha}. Thus, to calculate elemental concentration ratios it is necessary to acquire the pre- and post-edge images for both core-loss edges of interest with a constant convergence angle and incident current density. To avoid saturating the CCD for strong lower-loss edges, or collecting too little signal for weak higher-loss edges, it is necessary to vary the acquisition times for images of different elements. The resultant images are then normalized to constant acquisition time. Directly ratioing two net core-loss intensity maps tends to result in noisy elemental concentration ratios particularly in regions in which the concentration of one of the elements is low.
Date: April 1, 1997
Creator: Rice, P.M. & Alexander, K.B.
Partner: UNT Libraries Government Documents Department

The microstructure and mechanical reliability of alumina scales and coatings

Description: Alumina scales on iron-aluminides (Fe{sub 3}Al-based) and NiCrAl- based alloys were characterized in order to develop the knowledge to control the oxidation performance of alloys by controlling the microstructure and microchemistry of their scales. Plasma-deposited amorphous alumina coatings on iron-aluminides were used to study phase transformations, transport processes in the scales, and S segregation to the scale/metal interface. It was found that during heat treatment in absence of oxidation, amorphous coatings first transform to {gamma}-Al{sub 2}O{sub 3} and eventually {alpha}-Al{sub 2}O{sub 3} nucleates at the scale/metal interface. Sulfur from the Zr- free alloy segregates to the scale/metal interface during heat treatment. Thermally grown scales on Zr-doped iron-aluminides were compared to those formed after oxidation of a specimen with an alumina coating. Microstructural and gravimetric results showed that the primarily amorphous alumina coating promoted the nucleation and growth of metastable alumina phases, which resulted in more rapid oxidation. The thermally grown oxide was found on top of the coating. The NiCrAl-based alloys formed columnar alumina scales underneath a layer of mixed oxides. Segregation of alloying elements like Y, Hf, and At was found at both oxide grain boundaries and scale/metal interfaces.
Date: September 1, 1997
Creator: Alexander, K.B.; Pruessner, K. & Tortorelli, P.F.
Partner: UNT Libraries Government Documents Department

Processing and properties of FeAl-bonded composites

Description: Iron aluminides are thermodynamically compatible with a wide range of ceramics such as carbides, borides, oxides, and nitrides, which makes them suitable as the matrix in composites or cermets containing fine ceramic particulates. For ceramic contents varying from 30 to 60 vol.%, composites of Fe-40 at. % Al with WC, TiC, TiB{sub 2}, and ZrB{sub 2} were fabricated by conventional liquid phase sintering of powder mixtures. For ceramic contents from 70 to 85 vol.%, pressureless melt infiltration was found to be a more suitable processing technique. In FeAl-60 vol.% WC, flexure strengths of up to 1.8 GPa were obtained, even though processing defects consisting of small oxide clusters were present. Room temperature fracture toughnesses were determined by flexure testing of chevron-notched specimens. FeAl/WC and FeAl/TiC composites containing 60 vol.% carbide particles exhibited K{sub Q} values around 20 MPa m{sup 1/2}. Slow crack growth measurements carried out in water and in dry oxygen suggest a relatively small influence of water-vapor embrittlement. It appears therefore that the mechanical properties of iron aluminides in the form of fine ligaments are quite different from their bulk properties. Measurements of the oxidation resistance, dry wear resistance, and thermal expansion of iron aluminide composites suggest many potential applications for these new materials.
Date: December 31, 1996
Creator: Schneibel, J.H.; Subramanian, R.; Alexander, K.B. & Becher, P.F.
Partner: UNT Libraries Government Documents Department

Iron aluminide-titanium carbide composites: Microstructure and mechanical properties

Description: Composites of intermetallics and carbides (with binder contents less that 50 vol.%) are considered as potential candidates for applications requiring high wear resistance in corrosive environments. Intermetallics, especially aluminides, provide the corrosion resistance, and the high hardness of the carbide phase contributes to increased wear resistance of the composites. In this study, cost effective and simple processing techniques to obtain FeAl-TiC composites, over a wide range of binder volume fractions, are demonstrated. Binder volume fractions range from 0.15 to 0.7 (18 to 75 wt. % binder). Two techniques - liquid phase sintering of mixed powders and pressureless melt infiltration of TiC preforms was found to be very successful for obtaining fully dense composites with binder volume fractions from 0.15 to 0.3 (18 to 34 wt. %), whereas for higher binder contents liquid phase sintering of mixed powders was the best approach. Mechanical properties of these composites including the 3-point bend strength, fracture toughness and hardness are presented.
Date: September 1, 1996
Creator: Subramanian, R.; Schneibel, J. H. & Alexander, K. B.
Partner: UNT Libraries Government Documents Department

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

Description: 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.
Date: October 1, 1996
Creator: Pint, B. A.; Alexander, K. B. & Tortorelli, P. F.
Partner: UNT Libraries Government Documents Department

The Effect of Platinum on the Growth and Adhesion of alpha-Al{sub 2}O{sub 3} Scales

Description: Cast PtAl and NiPtAl alloys were used as model materials to study the effect of Pt on the oxidation behavior of aluminide bond coats. The addition of Pt improves alpha-Al{sub 2}O{sub 3} scale adhesion in cyclic testing at 1150{degrees}C and 1200{degrees}C. However, in comparison with aluminides which contain reactive elements (e.g. Zr and Hf), the beneficial effect of Pt was not as great. SEM and TEM analysis of the oxides indicated little effect of the Pt on scale microstructure. Using TEM/EDS analysis, Pt was not detected as a segregant to the alumina grain boundaries grown on PtAl, whereas Zr and Hf have been routinely detected. Also, no sulfur was detected at the PtAl/Al{sub 2}O{sub 3} interface. While Pt is somewhat beneficial in cast aluminides, Pt in an aluminide bond coat on a single crystal Ni-base superalloy substrate also may provide additional benefits to the overall oxidation behavior.
Date: December 1997
Creator: Dickey, E. C.; Pint, B. A.; Alexander, K. B. & Wright, I. G.
Partner: UNT Libraries Government Documents Department

Grain boundary segregation of cation dopants in {alpha}-Al{sub 2}O{sub 3} scales

Description: A Fe-20at.%Cr-10%Al matrix was dispersed with a wide range of different oxides in order to study the effect of oxygen-active dopants on the high-temperature growth and adhesion of {alpha}-Al{sub 2}O{sub 3} scales. Effect of these various cation dopants on the alumina scale microstructure was correlated with dopant ion segregation to the {alpha}-Al{sub 2}O{sub 3} grain boundaries using analytical electron microscopy. Elements such as Mn and V showed little effect on the oxide scale and were not observed to segregate. Elements such as Y and Gd resulted in finer, more columnar {alpha}-Al{sub 2}O{sub 3} grains and were segregated to scale grain boundaries. However, Ti, Ta, Ca, and Nb also were found to segregate but had a lesser effect on scale morphology. This indicates that cation segregation to scale grain boundaries is not a sufficient condition to achieve beneficial oxidation effects. The driving force for segregation in growing alumina scales is discussed.
Date: December 1996
Creator: Pint, B. A. & Alexander, K. B.
Partner: UNT Libraries Government Documents Department

Sulfur segregation to oxide/metal interfaces: a comparison of thermally grown and plasma deposited Al{sub 2}O{sub 3}

Description: Segregation of S to oxide/metal interfaces was studied using scanning Auger microscopy after removing the oxide film in ultrahigh vacuum. Two types of alumina films were investigated: one formed from oxidation at 1000 C on Fe-28at.%Al-5at.%Cr; the other was deposited on the same alloy via plasma synthesis, where Al was codeposited with O to form an amorphous Al2O3 film, 0.2 or 0.8 {mu}m thick. Subsequent heat treatment of the deposited film at 1000 C caused it to slowly transform to {alpha}-Al2O3, and the transformation took place at the film/alloy interface. S segregated to the interface during heat treatment. The amount increased with heat treatment time but was much less than that with scales formed by oxidation. Not all the film/alloy interfaces contained S; the behavior was different from the uniform S coverage found under thermally grown scales. S segregation to the film/alloy interface seems to be controlled by availability of interfacial sites rather than bulk diffusion rates.
Date: September 1, 1996
Creator: Hou, P. Y.; Wang, Z.; Pruessner, K.; Alexander, K. B. & Brown, I. G.
Partner: UNT Libraries Government Documents Department

Interfacial segregation in oxide scales on NiCrAl-based alloys

Description: Previous studies addressing the segregation of reactive elements in protective oxide scales and their beneficial effect on scale adhesion have primarily concentrated on primary alumina-formers. In this study the isothermal oxidation behavior of three NiCrAl alloys, which form complex oxide scales was studied in air at 1,473 K for 100 hrs. The resulting oxide scales were characterized by scanning electron microscopy (SEM) and cross=sectional transmission electron microscopy (TEM) techniques. Segregation at internal interfaces was analyzed in the STEM mode on a Philips CM200 FEG-TEM, operated at 200 kV and equipped with a Link ultrathin-window EDS detector. The probe size of the electron beam was about 1.5 nm. Spectra were recorded with grain boundaries oriented parallel to the electron beam. The oxide scale on Rene N5 ({approximately}5 {micro}m) consists of columnar {alpha}-Al{sub 2}O{sub 3} at the bottom ({approximately}4 {micro}m) and a mixed layer of spinel (Ni, Co, Ta) (Al,Cr){sub 2}O{sub 4} and {alpha}-Al{sub 2}O{sub 3} ({approximately}1 {micro}m) at the top. On Ni-7Cr-6.5Al+Y, the scale ({approximately}9 {micro}m) consists mainly of columnar Cr-doped {alpha}-Al{sub 2}O{sub 3}. Ni-10Cr-10Al+Y forms an oxide scale ({approximately}8 {micro}m) that consists mainly of columnar Cr-doped {alpha}-Al{sub 2}O{sub 3}. The results are consistent with previous results on primary alumina-formers.
Date: April 1, 1997
Creator: Pruessner, K.; Alexander, K.B.; Pint, B.A.; Tortorelli, P.F. & Wright, I.G.
Partner: UNT Libraries Government Documents Department

Interfacial composition analysis of silicon nitride whisker reinforced oxynitride glass systems

Description: Scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy and parallel-detection electron energy-loss spectroscopy was used to analyze the composition at interfaces in {beta}-Si{sub 3}N{sub 4} whisker reinforced oxynitride glass systems.
Date: June 1, 1995
Creator: Sun, E. Y.; Alexander, K. B.; Becher, P. F. & Hwang, S. L.
Partner: UNT Libraries Government Documents Department

Properties of boron/boron-nitride multilayers

Description: Boron-Nitride films are of interest for their high hardness and wear resistance. Large intrinsic stresses and poor adhesion which often accompany high hardness materials can be moderated through the use of a layered structure. Alternate layers of boron (B) and boron-nitride (BN) are formed by modulating the composition of the sputter gas during deposition from a pure B target. The thin films are characterized with TEM to evaluate the microstructure and with nanoindentation to determine hardness. Layer pair spacing and continuity effects on hardness are evaluated for the B/BN films.
Date: June 1, 1996
Creator: Jankowski, A. F.; Wall, M. A.; Hayes, J. P. & Alexander, K. B.
Partner: UNT Libraries Government Documents Department

Intermetallic bonded ceramic matrix composites

Description: A range of carbide and oxide-based cermets have been developed utilizing ductile nickel aluminide (Ni{sub 3}Al) alloy binder phases. Some of these, notably materials based upon tungsten and titanium carbides (WC and TiC respectively), offer potential as alternatives to the cermets which use cobalt binders (i.e. WC/Co). Samples have been prepared by blending commercially available Ni{sub 3}Al alloy powders with the desired ceramic phases, followed by hot-pressing. Alumina (Al{sub 2}O{sub 3}) matrix materials have also been prepared by pressurized molten alloy infiltration. The microstructure, flexure strength and fracture toughness of selected materials are discussed.
Date: July 1995
Creator: Plucknett, K. P.; Tiegs, T. N.; Alexander, K. B.; Becher, P. F.; Schneibel, J. H.; Waters, S. B. et al.
Partner: UNT Libraries Government Documents Department

Ceramic composites with a ductile Ni{sub 3}Al binder phase

Description: Composites using B-doped ductile Ni{sub 3}Al alloys were produced with both non-oxide (WC, TiC) and oxide (Al{sub 2}0{sub 3}) ceramic powders. Typical powder processing techniques were used to fabricate materials with ceramic contents from 0-95 vol. %. The microstructural morphology of the composites depends primarily on the wetting behavior between the alloys and the ceramic powders. The non-oxide ceramic powders wet well and the Ni{sub 3}Al alloys form a semi-continuous intergranular phase. On the other hand, the Ni{sub 3}Al alloys do not wet the oxide powders well and tend to form discrete ``islands`` of the metallic phase. Wetting in these materials can be improved by the addition of non-oxide particles, such as TiC. Results on the mechanical properties showed ambient temperature flexural strength similar to other Ni-based hardmetals. In contrast to the WC-Co materials, the flexural strength is retained to temperatures of at least 800 C. The fracture toughness and hardness were found to be equal or higher than comparable Co-based hardmetal systems. Initial corrosion tests showed excellent resistance to acid solutions.
Date: June 1, 1995
Creator: Tiegs, T.N.; Alexander, K.B.; Plucknett, K.P.; Menchhofer, P.A.; Becher, P.F. & Waters, S.B.
Partner: UNT Libraries Government Documents Department

Improving the Oxidation Resistance in Advanced Single Crystal Nickel-Based Superalloys for Turbine Applications

Description: The focus of this project was the examination of the role of yttrium and other alloying elements on the microstructure and oxidation performance of improved single crystal nickel-based superalloys for advanced turbine applications. The microstructure and microchemistry of both base and modified alloys and their surface oxides have been measured with state-of-the-art microanalytical techniques (atom probe field ion microscopy) and then correlated with identifying the partitioning behavior of the elemental additions in these superalloys before and after burner rig and engine-test oxidation performance. The overall technical goals included; (1) identifying the partitioning behavior of the elemental additions in these superalloys before and after burner rig and engine tests and the effect on the misfit energy between the phases in the alloys; (2) examining the oxidation performance of these newly-developed alloys; (3) identifying the influence of pre-oxidation processing on the subsequent oxidation performance; and (4) relating the microstructural and microchemical observations to the observed performance of these superalloys. The comparison of the base and modified alloys will produce a better understanding of the interaction between chemistry, structure, and performance in superalloys. In addition, it will lead to optimized alloys with improved performance including enhanced durability in the operating environments at the elevated temperature required to improve energy efficiency. The availability of alloys capable of higher temperature operation will minimize the need for expensive coatings in extreme temperature applications.
Date: July 1, 1999
Creator: Alexander, K.B.; Kenik, E.A.; Miller, M.K.; Lin, L.S. & Cetel, A.D.
Partner: UNT Libraries Government Documents Department

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

Description: 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.
Date: December 1997
Creator: Wright, I. G.; Pint, B. A.; Lee, W. Y.; Alexander, K. B. & Pruessner, K.
Partner: UNT Libraries Government Documents Department