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Cross-Roll Flow Forming of ODS Alloy Heat Exchanger Tubes For Hoop Creep Enhancement

Description: Mechanically alloyed oxide dispersion strengthened (ODS) Fe-Cr-Al alloy thin walled tubes and sheets, produced via powder processing and consolidation methodologies, are promising materials for eventual use at temperatures up to 1200 C in the power generation industry, far above the temperature capabilities of conventional alloys. Target end-uses range from gas turbine combustor liners to high aspect ratio (L/D) heat exchanger tubes. Grain boundary creep processes at service temperatures, particularly those acting in the hoop direction, are the dominant failure mechanisms for such components. The processed microstructure of ODS alloys consists of high aspect ratio grains aligned parallel to the tube axis, a result of dominant axial metal flow which aligns the dispersoid particles and other impurities in the longitudinal direction. The dispersion distribution is unaltered on a micro scale by recrystallization thermal treatments, but the high aspect ratio grain shape typically obtained limits transverse grain spacing and consequently the hoop creep response. Improving hoop creep in ODS-alloy components will require understanding and manipulating the factors that control the recrystallization behavior, and represents a critical materials design and development challenge that must be overcome in order to fully exploit the potential of ODS alloys. The objectives of this program are to (1) increase creep-strength at temperature in ODS-alloy tube and liner components by 100% via, (2) preferential cross-roll flow forming and grain/particle fibering in the critical hoop direction. The research program outlined here is iterative in nature and is intended to systematically (a) examine and identify post-extrusion forming methodologies to create hoop strengthened tubes, which will be (b) evaluated at ''in-service'' loads at service temperatures and environments. In this 12th quarter of performance, program activities are concluded for Task 2 and continuing for Tasks 3, 4 and reported herein. Two sets of MA956 tubes rotary cross-rolled at rolling angles of {Beta}=2{sup ...
Date: September 30, 2006
Creator: Kad, Bimal K.
Partner: UNT Libraries Government Documents Department

Directed light fabrication of iron-based materials

Description: Directed light fabrication (DLF) is a process that fuses gas delivered metal powders within a focal zone of a laser beam to produce fully dense, 3-dimensional metal components. From a computer generated solid model, deposition ``tool paths`` are constructed that command the laser movement to fabricate near net shape parts a layer at a time. Among potential candidate systems to study, iron-based alloys are particularly attractive for rapid prototyping. To evaluate the processing parameters in the DLF process, studies have been performed on the microstructural development in 1-dimensional and 2-dimensional Fe-based components. For example, continuous microstructural features are evident, implying a continuous liquid/solid interface during processing. In addition, solidification cooling rates have been determined based upon secondary dendrite arm spacings in Fe-25wt. % Ni and 316 stainless steel. Cooling rates vary from 10{sup 1}{minus}10{sup 5} K s{sup {minus}1}, and the solidification behavior has been simulated using macroscopic heat transfer analyses.
Date: January 1, 1995
Creator: Thoma, D.J.; Charbon, C.; Lewis, G.K. & Nemec, R.B.
Partner: UNT Libraries Government Documents Department

Reduction in Defect Content of ODS Alloys

Description: The work detailed within this report is a continuation of earlier work carried out under contract number 1DX-SY382V. The earlier work comprises a literature review of the sources and types of defects found principally in Fe-based ODS alloys as well as experimental work designed to identify defects in the prototype ODS-Fe{sub 3}Al alloy, deduce their origins and to recommend methods of defect reduction. The present work is an extension of the experimental work already reported and concentrates on means of reduction of defects already identified rather than the search for new defect types. This report also includes results gathered during powder separation trials, conducted by the University of Groningen, Netherlands and coordinated by the University of Liverpool, involving the separation of different metallic powders in terms of their differing densities. The scope and objectives of the present work were laid out in the technical proposal ''Reduction in Defect Content in ODS Alloys-III''. All the work proposed in the ''Statement of Work'' section of the technical proposal has been carried out and all work extra to the ''Statement of Work'' falls within the context of an ODS-Fe{sub 3}Al alloy of improved overall quality and potential creep performance in the consolidated form. The outturn of the experimental work performed is reported in the following sections.
Date: May 15, 2001
Creator: Ritherdon, J.
Partner: UNT Libraries Government Documents Department

Strain measurements in thermally grown alumina scales using ruby fluorescence

Description: We have measured strains in alumina scales thermally grown on Fe-Cr- Al alloys by exploiting the strain dependence of the ruby luminescence line. Measurements were done on Fe-5Cr-28Al and Fe-18Cr-10Al (at.%, bal. Fe) oxidized between 300-1300 C with periodic cycling to room temperature. Significantly different levels of strain buildup were observed in scales on these alloys. Results on similar alloys containing a dilute reactive element (Zr or Hf) are also presented. We observe that scales on alloys containing a reactive element (RE) can support higher strains than scales on RE-free alloys. With the luminescence technique, strain relief associated with spallation thresholds is readily observed. In early stage oxidation, the evolution of transition phases is monitored using Raman and fluorescence spectroscopies. The fluorescence technique also provides a sensitive probe of early stage formation of {alpha}-Al{sub 2}O{sub 3}. It appears that, in presence of Cr{sub 2}O{sub 3} or Fe{sub 2}O{sub 3}, the {alpha}-alumina phase can form at anomalously low temperatures.
Date: December 31, 1996
Creator: Veal, B. W.; Natesan, K.; Koshelev, I.; Grimsditch, M.; Renusch, D. & Hou, P. Y.
Partner: UNT Libraries Government Documents Department

High-temperature oxidation/sulfidation resistance of iron-aluminide coatings

Description: Iron aluminides containing > 20-25 at. % Al have oxidation and sulfidation resistance at temperatures well above those at which these alloys have adequate mechanical strength. Accordingly, these alloys may find application as coatings or claddings on more conventional higher-strength materials which are generally less corrosion-resistant at high temperatures. To this end, iron-aluminide coatings were prepared by gas tungsten arc and gas metal arc weld-overlay techniques. Specimens were cut from weld deposits and exposed to a highly aggressive oxidizing-sulfidizing (H2S-H2-H2O-Ar) environment at 800 C. All the weld overlayers showed good corrosion behavior under isothermal conditions, including a gas metal arc-produced deposit with only 21 at. % Al. Rapid degradation in corrosion resistance was observed under thermal cycling conditions when the initally grown scales spalled and the rate of reaction was then not controlled by formation of slowly growing Al oxide. Higher starting Al concentrations (> {approximately} 25 at. %) are needed to assure overall oxidation-sulfidation resistance of the weld overlays, but hydrogen cracking susceptibility must be minimized in order to physically separate the corrosive species from the reactive substrate material.
Date: April 1, 1996
Creator: Tortorelli, P.F.; Wright, I.G.; Goodwin, G.M. & Howell, M.
Partner: UNT Libraries Government Documents Department

A model for the yield strength anomaly in FeAl

Description: A phenomenological model is used to explain the yield strength anomaly in FeAl. The model incorporates hardening by thermal vacancies at intermediate temperatures, and dislocation creep at elevated temperatures. Since the vacancy concentration increases exponentially with temperature, the model predicts an exponential increase in strength with temperature. This increase is terminated by onset of dislocation creep. The model captures the experimentally observed strain rate dependency of the yield stress at high temperatures and yields an activation enthalpy for vacancy formation which is in excellent agreement with a previously measured value.
Date: December 31, 1996
Creator: Baker, I. & George, E.P.
Partner: UNT Libraries Government Documents Department

Influence of strain rate and temperature on the mechanical behavior of iron aluminide-based alloys

Description: Iron aluminides are receiving increasing attention as potential high temperature structural materials due to their excellent oxidation and sulfidation resistance. Although the influence of strain rate on the microstructure/property relationships of pure iron and a variety of iron alloys and steels has been extensively studied, the effect of strain rate on the stress-strain and deformation response of iron aluminides remains poorly understood. In this paper the influence of strain rate, varied between 0.001 and 10{sup 4} s{sup {minus}1}, and temperature, between 77 & 1073{degree}K, on the mechanical behavior of Fe-40Al-0.1B and Fe-16.12Al-5.44Cr-0.11Zr-0.13C-1.07Mo-006Y, called FAP-Y, (both in at.%) is presented. The rate sensitivity and work hardening of Fe-40Al and the disordered alloy based on Fe-16% Al are discussed as a function of strain rate and temperature.
Date: April 1, 1995
Creator: Gray, G.T.
Partner: UNT Libraries Government Documents Department

Reduction in Defect Content in ODS Alloys

Description: The work detailed within this report is a continuation of earlier work that was carried out under contract number IDX-SY382V. The earlier work comprised a literature review of the sources and types of defects found principally in Fe-based ODS alloys together with a series of experiments designed to identify defects in ODS Fe{sub 3}Al material and recommend methods of defect reduction. Defects found in the Mechanically Alloyed (MA) ODS Fe{sub 3}Al included regions of incomplete MA, porosity, intrusions and fine-grained stringers. Some defects tended to be found in association with one another e.g. intrusions and fine-grained stringers. Preliminary powder separation experiments were also performed. The scope and objectives of the present work were laid out in the technical proposal ``Reduction in Defect Content in ODS Alloys--II'' which formed the basis of amendment 3 of the current contract. The current studies were devised in the context of the preceding work with a view to extending and concluding certain experiments while exploring new avenues of investigation of defect control and reduction where appropriate. All work proposed was within the context of achieving an ODS Fe{sub 3}Al alloy of improved overall quality and potential creep performance (particularly) in the consolidated, release condition. The interim outturn of the experimental work performed is also reported.
Date: February 1, 2000
Creator: Ritherdon, J. & Jones, A.R.
Partner: UNT Libraries Government Documents Department

Quantitative assessment of pore development at Al2O3/FeAl interfaces during high temperature oxidation

Description: Alloys of commercial grades that do not contain a reactive element, such as yttrium, often develop pores at the scale/alloy interface. The accumulation and growth of these pores greatly weaken scale adhesion. The purpose of this study is to evaluate pore development in Fe-40at% Al and determine the change in pore volume with oxidation time. Experimental results are then compared to a theoretical calculation where all vacancies are allowed to condense as voids. After removing the oxide scales that formed after various times of oxidation at 1000 C in oxygen, the alloy surface was analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM) to determine the size and depth of interfacial pores. Results are discussed in light of possible mechanisms involved in pore formation at scale/alloy interfaces.
Date: April 24, 2001
Creator: Hou, Peggy Y.; Van Leiden, C.; Niu, Y. & Gesmundo, F.
Partner: UNT Libraries Government Documents Department

Melting and casting of FeAl-based cast alloy

Description: The FeAl-based intermetallic alloys are of great interest because of their low density, low raw material cost, and excellent resistance to high-temperature oxidation, sulfidation, carburization, and molten salts. The applications based on these unique properties of FeAl require methods to melt and cast these alloys into complex-shaped castings and centrifugal cast tubes. This paper addresses the melting-related issues and the effect of chemistry on the microstructure and hardness of castings. It is concluded that the use of the Exo-Melt{trademark} process for melting and the proper selection of the aluminum melt stock can result in porosity-free castings. The FeAl alloys can be melted and cast from the virgin and revert stock. A large variation in carbon content of the alloys is possible before the precipitation of graphite flakes occurs. Titanium is a very potent addition to refine the grain size of castings. A range of complex sand castings and two different sizes of centrifugal cast tubes of the alloy have already been cast.
Date: November 1, 1998
Creator: Sikka, V.K.; Wilkening, D.; Liebetrau, J. & Mackey, B.
Partner: UNT Libraries Government Documents Department

Growth of alumina scale on Fe-Cr-Al : a multiprobe study.

Description: The use of local probes at specific spatial locations provides a clear picture of the scale growth. The thermal evolution of the oxide scale on 72Fe-18Cr-10Al-0.15Hf (at.%) has been investigated using a number of spectroscopic techniques. Well defined regions ({approx}70 mm in diameter) were probed by a battery of techniques as a function of oxidation at different temperatures (Between 800 C--1100 C). This study provides information about the dependence of scale evolution on fluctuations in the local composition and/or grain structure of the starting material. Results suggest that properties of the starting material, which were not investigated in this study, strongly influence the scale evolution, even to the stage of mature scales.
Date: June 3, 1998
Creator: Veal, B. W.
Partner: UNT Libraries Government Documents Department

Mechanically reliable surface oxides for high-temperature corrosion resistance

Description: Corrosion is widely recognized as being important, but an understanding of the underlying phenomena involves factors such as the chemistry and physics of early stages of oxidation, chemistry and bonding at the substrate/oxide interface, role of segregants on the strength of that bond, transport processes through scale, mechanisms of residual stress generation and relief, and fracture behavior at the oxide/substrate interface. Because of this complexity a multilaboratory program has been initiated under the auspices of the DOE Center of Excellence for the Synthesis and Processing of Advanced Materials, with strong interactions and cross-leveraging with DOE Fossil Energy and US industry. Objective is to systematically generate the knowledge required to establish a scientific basis for designing and synthesizing improved protective oxide scales/coatings (slow-growing, adherent, sound) on high-temperature materials without compromising the requisite properties of the bulk materials. The objectives of program work at Argonne are to (1) correlate actual corrosion performance with stresses, voids, segregants, interface roughness, initial stages of oxidation, and microstructures; (2) study such behavior in growing or as-grown films; and (3) define prescriptive design and synthesis routes to mechanically reliable surface oxides. Several techniques, such as Auger electron spectroscopy, X-ray diffraction, X-ray grazing incidence reflectance, grazing-angle X-ray fluorescence, optical fluorescence, and Raman spectroscopy, are used in the studies. Tne project has selected Fe-25 wt.% Cr-20 wt.% Ni and Fe-Cr-Al alloys, which are chromia- and alumina-formers respectively, for the studies. This paper presents some of the results on early stages of oxidation and on surface segregation of elements.
Date: May 1, 1995
Creator: Natesan, K.; Veal, B.W.; Grimsditch, M.; Renusch, D. & Paulikas, A.P.
Partner: UNT Libraries Government Documents Department

Tensile properties of Fe-16 at. % Al alloys

Description: A newly developed melting method for Fe-16 at. % Al alloy (FAPY) is described. Tensile data on the air-induction-melted (AIM) and vacuum-induction-melted (VIM) heats of FAPY after identical processing are presented. Optical, scanning electron micrographs (SEM), and microprobe analysis were carried out to explain the lower room-temperature ductility and more scatter in the data for the AIM material as opposed to the VIM material.
Date: February 1, 1995
Creator: Sikka, V. K.
Partner: UNT Libraries Government Documents Department

Cross-Roll Flow Forming of ODS Alloy Heat Exchanger Tubes for Hoop Creep Enhancement

Description: Mechanically alloyed oxide dispersion strengthened (ODS) Fe-Cr-Al alloy thin walled tubes and sheets, produced via powder processing and consolidation methodologies, are promising materials for eventual use at temperatures up to 1200 C in the power generation industry, far above the temperature capabilities of conventional alloys. Target end-uses range from gas turbine combustor liners to high aspect ratio (L/D) heat exchanger tubes. Grain boundary creep processes at service temperatures, particularly those acting in the hoop direction, are the dominant failure mechanisms for such components. The processed microstructure of ODS alloys consists of high aspect ratio grains aligned parallel to the tube axis, a result of dominant axial metal flow which aligns the dispersoid particles and other impurities in the longitudinal direction. The dispersion distribution is unaltered on a micro scale by recrystallization thermal treatments, but the high aspect ratio grain shape typically obtained limits transverse grain spacing and consequently the hoop creep response. Improving hoop creep in ODS-alloy components will require understanding and manipulating the factors that control the recrystallization behavior, and represents a critical materials design and development challenge that must be overcome in order to fully exploit the potential of ODS alloys. The objectives of this program are to (1) increase creep-strength at temperature in ODS-alloy tube and liner components by 100% via, (2) preferential cross-roll flow forming and grain/particle fibering in the critical hoop direction. The research program outlined here is iterative in nature and is intended to systematically (1) examine and identify post-extrusion forming methodologies to create hoop strengthened tubes, which will be (2) evaluated at ''in-service'' loads at service temperatures and environments. This research program is being conducted in collaboration with the DOE's Oak Ridge National Laboratory and the vested industrial partner Special Metals Corporation. In this ninth quarter of performance, program activities are continued for ...
Date: April 10, 2006
Creator: Kad, Bimal K.
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

Study of the reactive element effect in ODS iron-base alumina-formers

Description: Iron aluminide (Fe{sub 3}Al) and FeCrAl compositions were dispersed with 15 different oxides in order to study the effect of oxygen- active dopants on high-temperature growth and adhesion of {alpha}- Al{sub 2}O{sub 3} scales. In these model-type, oxide dispersion strengthened (ODS) systems, the chemical effects of various cation dopants were compared to the baseline effect of an Al{sub 2}O{sub 3} oxide dispersion. By conducting isothermal and cyclic oxidation tests and by characterizing the oxidation product, effects on scale adhesion, growth rate and microstructure were evaluated. The dopants were categorized based on effectiveness in modifying the alumina scale. An Al{sub 2}O{sub 3} dispersion yielded some improvement in oxidation behavior apparently by strengthening the relatively weak substrate. However, the type of improvements in adhesion and change in growth mechanism associated with addition of reactive elements such as Y were not achieved. In general, due to the weaker substrate and the inherently faster interfacial void formation, the dispersions were less effective in ODS Fe{sub 3}Al than in ODS FeCrAl.
Date: May 1, 1996
Creator: Pint, B. A.
Partner: UNT Libraries Government Documents Department

Effect of initial composition distribution on the phase transformation behavior in the Fe-Cr-Ni system

Description: A finite-difference implicit numerical model was used to study the diffusion-controlled {alpha}{minus}{gamma} (ferrite-to-austenite) solid-state phase transformation in the Fe-Cr-Ni system. The influence of a nonuniform initial composition distribution was examined in order to assess the impact that nonuniform solute profiles resulting from solidification may have on subsequent transformation behavior in weldments and castings. A significant impact on the transformation kinetics and transformation path was found in some cases. Factors that affect the degree of influence are discussed.
Date: December 31, 1995
Creator: Vitek, J.M. & David, S.A.
Partner: UNT Libraries Government Documents Department

Stress determination in thermally grown alumina scales using ruby luminescence

Description: By exploiting the strain dependence of the ruby luminescence line, we have measured the strain in alumina scales thermally grown on Fe-Cr- Al alloys. Results are compared and found to be reasonably consistent with strains determined using x rays. Oxidation studies were carried out on alloys Fe - 5Cr - 28Al and Fe - 18Cr - 10Al (at.%). Significantly different levels of strain buildup were observed in scales on these alloys. Results on similar alloys containing a ``reactive element`` (Zr or Hf) in dilute quantity are also presented. Scales on alloys containing a reactive element (RE) can support significantly higher strains than scales on RE-free alloys. With the luminescence technique, strain relief associated with spallation thresholds is readily observed.
Date: June 1, 1996
Creator: Renusch, D.; Veal, B. W.; Koshelev, I.; Natesan, K.; Grimsditch & Hou, P. Y.
Partner: UNT Libraries Government Documents Department

Processing and properties of low-aluminum alloy FAPY

Description: This paper deals with the melting, processing, properties, and microstructure of three commercially melted heats of Fe-16 at. % Al alloy FAPY. All of the heats were air-induction melted (AIM), two at Hoskins Manufacturing Company (Hamburg, Michigan) and one at United Defense (Anniston, Alabama). One ingot from each of the heats was used for testing at the Oak Ridge National Laboratory. A 127-mm.-long section from each ingot was used for determining properties and microstructure in the as-cast, cast and hot-processed, and cold-rolled conditions. The fine-grained sheet showed 20% elongation at room temperature.
Date: July 1, 1996
Creator: Sikka, V.K.; Howell, C.R.; Hall, F. & Valykeo, J.
Partner: UNT Libraries Government Documents Department

A Parametric Analysis of Solidification in Y(Fe,Ni,Cr)-Nb-C Alloys

Description: A parametric analysis is presented which summarizes the amount of total ({gamma}/NbC + {gamma}/Laves) and individual {gamma}/NbC and {gamma}/Laves constituents which form during solidification of {gamma}{sub (Fe,Ni,Cr)} alloys with variations in nominal Nb and C contents. Calculated results are presented for Fe base alloys and Ni base alloys. The results provide a quantitative rationale for understanding the relation between alloy composition and solidification microstructures and should provide useful insight into commercial alloys of similar composition.
Date: February 22, 1999
Creator: DuPont, J.N. & Robino, C.V.
Partner: UNT Libraries Government Documents Department

Ab-initio based calculations of vacancy formation and clustering energies including lattice relaxation in Fe{sub 3}Al

Description: Vacancy formation and clustering significantly affect structural properties of transition-metal aluminides. Ab-initio quantum mechanical total-energy calculations using a full-potential linear combination of muffin-tin orbitals (LMTO) technique provide a convenient method of studying relevant characteristics such as changes in density of states, and charge redistribution around defects. Augmented with Hellmann-Feymann forces, LMTO allows calculations of relaxation geometries and relaxation energies. The authors have performed such calculations for vacancies and antisite substitutional point defects in Fe{sub 3}Al with DO{sub 3} crystallographic structure. There are two limiting factors complicating calculations of defect formation energies directly from ab-initio calculations. The first is that a single defect, due to the lattice periodicity necessitated by the use of ab-initio total energy techniques, cannot be considered as an isolated defect, even in the maximum computable simulation cell. Unlike previous calculations, which did not find a dependency on the size of the simulation cell, the calculations have shown a significant difference in results for 32- and 16- atom cells. This difference provides information about vacancy clustering since it can be explained by a relatively small attractive interaction energy {approximately} 0.2 eV between two vacancies located in adjacent simulation cells and separated by the lattice constant distance (5.52 {angstrom}). By comparing the internal energies for two configurations of 30 atom cells (32 atom--2 vacancies), the authors were able to estimate that the attractive interaction between two vacancies could reach 1.2 eV. The second complication is the fact that chemical potentials of elements cannot be directly extracted from the total energy calculations for the compound. To deal with this problem, they considered two possible approximations and compared results, which were found to be quite similar for iron vacancies.
Date: March 1, 1999
Creator: Muratov, L.S.; Cooper, B.R. & Wills, J.M.
Partner: UNT Libraries Government Documents Department

Analysis of ripple formation in single crystal spot welds

Description: Stationary spot welds have been made at the (001) surface of Fe-l5%Ni-15%Cr single crystals using a Gas Tungsten Arc (GTA). On the top surface of the spot welds, very regular and concentric ripples were observed after solidification by differential interference color microscopy. Their height (typically 1--5 {micro}m) and spacing (typically {approximately} 60 {micro}m) decreased with the radius of the pool. These ripples were successfully accounted for in terms of capillary-wave theory using the fundamental mode frequency f{sub 0} given by the first zero of the zero-order Bessel function. The spacing d between the ripples was then equated to v{sub s}/f{sub 0}, where v{sub s} is the solidification rate. From the measured ripple spacing, the velocity of the pool was deduced as a function of the radius, and this velocity was in good agreement with the results of a heat-flow simulation.
Date: October 1, 1997
Creator: Rappaz, M.; Corrigan, D. & Boatner, L. A.
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