Materials Week '97: Thursday AM Session Abstracts

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following sessions will be held Thursday morning, September 18, during Materials Week 1997. To view other programming planned for the meeting, go to the technical program contents page.

BOUNDARIES AND INTERFACES IN MATERIALS: THE DAVID A. SMITH SYMPOSIUM: Session VII

Session Chair: David B. Williams, Lehigh University, Bethlehem, PA 18015

GRAIN BOUNDARIES IN THE HIGH TEMPERATURE CUPRATE SUPERCONDUCTORS: P. Chaudhari, IBM, Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598

The electromagnetic properties of grain boundaries in the cuprate materials have greatly influenced the rate and the direction of progress in this field. This talk will review the properties of grain boundaries: both their deleterious and useful aspects. Their impact on critical currents, on processing of materials, superconducting devices such as SQUIDS, and on understanding the mechanism of superconductivity are presented.

9:00 am INVITED

GRAIN BOUNDARIES AND HETERO-INTERFACES IN Tl-BASED HIGH TEMPERATURE SUPERCONDUCTORS: Chris Grovenor, Annette Bramley, John O'Connor¹, Chris Eastell, Jo Moore, Department of Materials and ¹Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PH, UK

Many kinds of internal interfaces can influence the properties of both bulk and thin film samples of high temperature superconductors. We have studied the growth modes of Tl-2212, 2223 and 1223 thin films on different substrates and buffer layers by cross sectional TEM, and correlated the superconducting microwave performance with interfacial reaction characteristics and lattice defect concentrations. In addition, we have investigated the microstructure of Tl-1223 tapes by TEM, making observations on grain boundary structure, preferential alignment effects between grains and at 1223/silver interfaces and intercalation defects; again linking these observations to the macroscopic superconducting properties.

9:30 am INVITED

GRAIN BOUNDARIES AND THEIR ROLE IN THE DEVELOPMENT OF HIGH T_c SUPERCONDUCTING MATERIALS FOR LARGE-SCALE APPLICATIONS: Susan E. Babcock, Materials Science and Engineering and Applied Superconductivity Center, University of Wisconsin-Madison, 1500 Engineering Drive, Madison, WI 53706

Elucidation of structure-property relationships for individual grain boundaries, exploration of percolative current paths through textured materials, and engineering of the grain boundary microstructure and texture all remain critical issues in the development of the known high T_c superconductors. Professor Smith's research has influenced all of these topics both directly, through his work on superconducting materials, and indirectly through his insights into the general behavior of grain boundaries. His contributions to the study of high T_c superconductors will be reviewed in the context of current research and understanding of these materials.

10:00 am BREAK

10:10 am INVITED

EFFECT OF / INTERFACES ON CREEP BEHAVIOR OF SINGLE COLONY TITANIUM ALLOYS AT ROOM TEMPERATURE: S. Suri, G.B. Viswanathan, T. Neeraj, D-H. Hou¹, J.M. Scott² and M.J. Mills, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210; ¹Micron Technology Inc., Boise, ID 83706; ²Wright Laboratories, MLLM Materials Directorate, Wright Patterson AFB, OH 45433-6533

An important limitation in the application of titanium alloys for structural applications is their tendency to creep significantly at low homologous temperatures and at stress levels well below the yield strength. Several studies in the literature have shown that in two-phase alloys with a Widmanstatten structure, creep strain is affected by colony size and by sliding at / interfaces. In this investigation, the structure of / interfaces for a near- alloy, Ti-5Al-2.5Sn-0.4Fe, has been determined for the first time using diffraction contrast and high resolution TEM. In addition, the processes of slip transmission from to phases has been studied in detail via TEM investigation of single colony crystals which have been oriented and deformed to activate particular slip systems in the a matrix. The strong anisotropy of the constant strain rate and creep properties of these colony crystals will be discussed in terms of the observed mechanisms of slip transmission and interfacial sliding. Research supported by the Air Force Office of Scientific Research under grant #F49620-95-1-0153.

10:40 am INVITED

FINITE ELEMENT ANALYSIS OF PLASTIC YIELDING IN Fe-3%Si BICRYSTALS: R.H. Wagoner, Z.C. Yao¹ and Q. Wu², Dept. of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210; ¹National Steel Corp., Technical Research Center, 1745 Fritz Drive, Trenton, MI 48183; ²Department of Welding Engineering, Beijing Aeronautical Manufacturing Technology Research Institute, P.O. Box 863, Beijing, China

Over twenty years ago, R.E. Hook and J.P. Hirth observed the active slip systems for a range of Fe-3%Si bicrystals subjected to small compressive strain. These configurations have been stress-analyzed using anisotropic elastic finite element modeling (FEM) based on the commercial program ABAQUS. Nine bicrystals orientation pairs were investigated, all with the compression axis parallel to the grain boundary plane. The simulated stress patterns, when interpreted in terms of a critical resolved shear stress (CRSS) criterion, predicted the slip systems identified as "primary", "secondary", and "elastic incompatibility" types. Not only were the observed slip systems found to have the highest RSS at the boundary, but the front and back surfaces of the crystals were consistently differentiated by the FEM and the experiments. Only the slip systems labeled "second order" by Hook and Hirth had low RSS's. Elastic-plastic transition in bicrystals was also simulated to reveal the propagation of plastic deformation for these cases.

11:10 am

AN ATTEMPT TO CONTROL THE INTERFACIAL STRENGTH IN FeAl/TiB₂ COMPOSITES: J.H. Schneibel, R. Subramanian, Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6115

Composites consisting of a B2 iron aluminide matrix and 40vol.% of TiB₂ particles were processed by liquid phase sintering. Thermodynamic calculations suggest that by adding B or Ti, respectively, the iron aluminide melt may be enriched in B at the expense of Ti, or in Ti at the expense of B. This in turn may lead to segregation of either element at the FeAl/TiB₂ interfaces. Room temperature tests show slight increases in the flexure strengths (up to 1400 MPa) and the fracture toughness for composites microalloyed with B. Interfacial segregation of B may have contributed to this result. This research was sponsored by the Division of Materials Sciences, U.S. Department of Energy under contract number DE-AC05-96OR22464 with Lockheed Martin Energy Research Corp.

11:20 am

DEVELOPMENT OF A PREFERRED GRAIN BOUNDARY STRUCTURE IN AlCu INTERCONNECTS: David P. Field, TexSEM Laboratories, 226W 2230N, Provo, UT 84604

Analysis of AlCu interconnect lines indicates development of a preferred grain boundary structure in very narrow lines (<0.75 µm). This structure is observed to evolve because of grain boundary migration and preferential growth of given orientations during the post-patterning anneal. Experimental evidence is presented showing the evolution of interconnect microstructure subsequent to patterning of non-passivated lines. It is proposed that the structure develops because of surface energy minimization of both the top and sides of the lines, in addition to grain boundary energy considerations. Implications for the reliability of interconnect lines are discussed.

11:40 am INVITED

THE RELATION BETWEEN MECHANICAL AND ELECTRICAL PROPERTIES OF NANO-SCALE METALLIC CONTACTS: D.B. Williams, Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015-3195

Nano-scale metallic contacts have been made by several groups and their electrical and mechanical properties have been measured. This paper will first review these fascinating new results in the science of interfaces and the controversies that have arisen. It will then describe dynamical simulations of these contacts which model simultaneously their mechanical evolution and their electronic conductance. It will be shown that the jumps in the electronic conductance that are observed experimentally are almost always a consequence of a structural rearrangement inside the contact arising from a mechanical instability. If time permits we will also mention our most recent work on modeling electro-migration in these contacts.

HIGH CYCLE OF FATIGUE OF STRUCTURAL MATERIALS: Session V: High Cycle Fatigue II

Session Chairs: Prof. T.S. Srivatsan, University of Akron, Dept. of Mechanical Engineering, Auburn Science Center, Akron, OH 44325-3903; Prof. Don Koss, Pennsylvania State University, Dept. of Materials Science & Engineering, 209 Steidle Bldg., University Park, PA 16802-7501

HIGH-CYCLE FATIGUE AND FRACTURE BEHAVIOR IN SQUEEZE CAST 356 Al/SiCW MMCS AND 390 Al/SiCW MMCS: D.A. Koss*, Sang-Beom Kim, Department of Material Science and Engineering, Penn State University, University Park, PA 16802; *On leave from Daewoo Heavy Industries Ltd.

Specimens of 356 Al/SiCw and 390 Al/SiCW metal matrix composites, produced by squeeze casting, were tested under fully reversed high cycle fatigue conditions. A staircase method was employed to determine the mean fatigue strength of the material at ten million cycles. Presence of 17% SiC whiskers in 356 Al alloy increases mean fatigue strength at 10⁷ cycles by 30%. However, in 390 Al alloy, the increase of mean fatigue strength by the presence of 17% SiCw is relatively low compared with that of 356 Al. Fracture surface analysis of both composites shows that fatigue cracks are initiated primarily at regions which are characterized by a low volume fraction of SiCw. The influence of such local "discontinuity" on the fatigue life is modeled. In addition, it has been found that there are large differences between the tensile yield strength and the compressive yield strength in both composites. This result is indicative of the presence of thermally induced residual stresses. The difference of the fatigue strength increase in 356 Al/SiCw and in 390 Al/SiCw is interpreted in terms of a combination of local "discontinuity" and thermal residual stresses. This research is supported by General Motors Corp.

8:25 am INVITED

THE EFFECT OF SOLIDIFICATION RATE ON THE GROWTH OF SMALL FATIGUE CRACKS IN A CAST 319 ALUMINUM ALLOY: M.J. Caton, J.W. Jones, Dept. of Materials Science and Engr., Univ. of Michigan, Ann Arbor, MI; J.E. Allison, Ford Research Laboratories, MD 3182 SRL, Dearborn, MI 48121

Increased use of cast aluminum alloys in automotive applications has necessitated development of a better understanding of fatigue crack growth behavior in these materials. Of particular interest in materials containing casting porosity is the fatigue behavior of small cracks. This paper will describe the influence of solidification rate and microstructure on fatigue crack growth of small cracks in 319 Al, a common Al-Si-Cu alloy used in automotive castings. A replication technique capable of detecting cracks as small as 10 mm in length was used to monitor the growth of small, naturally-initiated fatigue cracks and establish da/dN vs. DK curves. A comparison between these results and those from large cracks will be made. The paths which these fatigue cracks take through the dendritic microstructure will be described.

8:50 am

THE EFFECT OF SOLIDIFICATION RATE ON THE MICROSTRUCTURE AND FATIGUE PROPERTIES IN CAST 319 AL: J.M. Boileau, J.E. Allison, Ford Research Laboratories, MD 3182 SRL, Dearborn, MI 48121

As the automotive industry increases its use of cast aluminum components, the need for more detailed information relating the impact of casting practice on mechanical behavior also increases. Therefore, a study characterizing the influence of solidification rate on the microstructure and fatigue properties in a cast 319 Al alloy was conducted. A wedge-shaped casting was developed with a dendrite arm spacing range of 10 - 100 mm and a porosity range of 0.01 - 1%. To decouple the combined influence of dendrite arm spacing and porosity, hot isostatic pressing was performed on several wedges to eliminate the porosity; all castings were then subsequently heat-treated to a T7 condition. Multiple fatigue tests were conducted at selected stress levels so that valid statistical comparisons could be made on the effect of dendrite arms spacing and of porosity. Extensive metallographic and fractographic characterization was performed to understand the influence of porosity on fatigue life. This data, along with the microstructural features initiating failure in these cast Al alloys, will be discussed.

9:10 am

RELATION OF PORE SIZE AND LOCATION TO FATIGUE FAILURE IN Al ALLOY A356 CAST SPECIMENS: M.E. Seniw, M.E. Fine, M. Meshii, Dept. of Materials Science and Engr., Northwestern Univ., Evanston, IL 60628; E.Y. Chen, GE Corporate R&D, Schenectady, NY 12301

Because of weight and cost savings motor vehicle manufacturers are making more extensive use of cast aluminum alloys to replace forged and cast iron or steel components. The presence of pores in certain locations may reduce the fatigue life. This is of special concern for safety critical components. This study is an examination of the influence of porosity morphology and location on the fatigue life of die cast aluminum A356 alloy specimens. The castings are first x-rayed to locate pores. Fatigue specimens were then machined from the cast test bars so that pores of various sizes are located various distances from the nearest free surface in the gage length surface. After fatigue failure the pore resulting in crack formation is carefully measured in size and distance from the surface. A statistical relation between pore size and distance from the free surface and fatigue life under constant amplitude loading was established. Distance from the surface was found to be the most important variable. This work was supported by the NIST under Contract No. 70NNANB9H0916.

9:30 am

CYCLIC STRESS RESPONSE, CYCLIC STRAIN RESISTANCE AND FRACTURE BEHAVIOR OF AN Al-Zn-Mg-Cu ALLOY, INFLUENCE OF TEMPERATURE: T.S. Srivatsan, S. Anand, Dept. of Mechanical Engineering, The University of Akron, Akron, OH 44325

The design of structural components for the newer generation of civilian and military aircraft demands satisfactory performance from the material under conditions of cyclic stress amplitude and strain amplitude control, and an extended service life. In this connection, a study has been made to understand the influence of test temperature on cyclic stress response characteristics, cyclic strain resistance, fatigue life and fracture behavior of a high strength aluminum alloy 70SS. Test specimens of the alloy were cyclically deformed over a range of strain amplitudes, giving less than 10⁴ cycles to failure, at both ambient and elevated temperatures. In this presentation, the cyclic stress response characteristics, cyclic stress-strain characteristics, cyclic strain resistance and resultant low-cycle fatigue properties and fatigue fracture characteristics of the alloy will be highlighted in light of competing and mutually interactive influences of cyclic plastic strain amplitude, concomitant response stress, intrinsic microstructural effects and matrix deformation characteristics.

9:50 am INVITED

HIGH CYCLE FATIGUE BEHAVIOR OF PARTICLE REINFORCED ALUMINIDE COMPOSITES: J.E. Allison, Ford Research Laboratories, Ford Motor Company, Dearborn, MI 48124; J.W. Jones, Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48105

The high cycle fatigue response of aluminum alloys can be significantly altered by the addition of high modulus particle reinforcements. This talk will review the current understanding of the influence high modulus particles have on cyclic deformation, fatigue crack initiation and fatigue lifetime of unnotched samples of aluminum alloys. Fatigue behavior in these materials can be described in terms of combinations of classical composite strengthening (direct strengthening) and changes in matrix microstructure and deformation characteristics (indirect effects) which arise from the presence of the reinforcement. Accelerated aging and refinement of slip length are two particularly important indirect phenomena affecting high cycle fatigue behavior of DRA. The improvements in stress-controlled fatigue performance that generally result from reinforcement can be understood using this framework, as can strain controlled fatigue results and elevated temperature fatigue response.

10:15 am

HIGH CYCLE FATIGUE STRENGTH OF AUSTEMPERED DUCTILE CAST IRON: S.K. Putatunda, P. Prasad Rao, Rachan Rao and Nathan Clark, Departments of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI 48202

Austempered Ductile Cast Iron (ADI) has attracted considered interest in recent years because of its excellent properties such as high strength with good ductility, good wear resistance and fracture toughness. It is therefore, considered as an economic substitute for steel in several structural applications especially for the automotive industry. In the investigation, the influence of austempering time and temperature on the high cycle fatigue strength of an alloyed ADI (1.5% Ni and 0.3 % Mo) was investigated. The effect of the carbon content and the volume fraction of austenite on the high cycle fatigue strength of ADI was also studied. The crack growth mechanism in fatigue was also examined.

10:35 am

THE EFFECTS OF SURFACE OXIDE FILMS ON THE FATIGUE BEHAVIOR OF ALUMINUM-LITHIUM ALLOYS: A. Inchekel, Adv. Engr. Consulting Services, Wichita, KS 67277; J.E. Talia, Dept. of Mechanical Engr., Wichita St. Univ., Wichita, KS 67260

The fatigue behavior of oxide film coated Al-Li alloy samples was investigated. Adherent anodic oxide films of varying thickness were applied electrolytically onto the surface of the specimens. Tensile tests shown that the flow stresses increased as the oxide film thickness increases. However, fatigue life tests presented a reduction in life of the samples at a critical thickness of oxide and an increase in the life at a larger thickness of oxide coatings. These results are more pronounced during high cycle fatigue testing. A mechanism, based on dislocation structures changes in the surface and subsurface region introduced into the material by the presence of oxide films, is proposed.

10:55 am INVITED

THE EFFECTS OF SCRATCHES AND SHOT PEENING ON THE HIGH CYCLE FATIGUE CRACK GROWTH OF ALUMINUM ALLOY 2024-T3: J.E. Talia, Dept. of Mechanical Engineering, Wichita State Univ., Wichita, KS 67260; M. Talia, Dept. of Mechanical Engineering, Univ. of Kansas, Lawrence, KS 66045

An experimental study was performed to determine the crack growth rates of scratched-then-shot peened Aluminum alloy 2024-T3 specimens under high cycle fatigue conditions. Tests were conducted at a constant stress amplitude with different stress ratios. As expected, the crack growth rate of a scratch specimen increased with increasing the scratch depth. However, glass shot peening significantly reduced such crack growth rates. The rates of scratched-then-shot peened specimens were similar to those of as-received specimens at low and intermediate stress intensity factors' ranges. The increase of the positive stress ratio augments the crack growth rates of specimens. Nevertheless, the change of the negative stress ratio resulted in a different effect. In addition, it was found that the scratch exhibited small crack growth behavior. This behavior is related to a crack's path change occurring before the crack propagates across the specimens. An analytical model based on the stress intensity factor was developed for scratched samples.

11:20 am

FATIGUE BEHAVIOUR OF ADI-SOME SPECIFIC FEATURES: J. Svéjcar, Technical University of Brno-Pisek Institute of Materials Engineering, 616 69 Brno, Czech Republic

Ausformed ductile iron (ADI), with the structure of upper, transition, and lower bainite, exhibits anomalous behaviour in comparison with other materials (e.g., the form of Heigh diagram, decrease of the fatigue limit with increasing yield point). Microfractographical analyses and additional experiments, the results of which are presented in this paper, were carried out with the aim to explain this anomalous behaviour.

11:40 am

FATIGUE CRACK GROWTH BEHAVIOR OF SELECTED ALUMINUM ALLOYS-VARIABLE AMPLITUDE LOADING: N. Ranganathan, E.N.S.M.A., Teleport 2, BP 109 Futuroscope Cedex, France

The different kinds of loading studied are: single overload, block programs and transport aircraft wing loading spectra. Firstly, the micromechanisms and the associated effects are studied based on single overload tests (with an overload ratio of 2) covering a large ÆK range from near threshold to medium ÆK ranges (Paris law regime). It is shown from tests conducted in vacuum that the slip behavior (planar or multiple slip) can strongly affect the delay after an overload, the best behavior is obtained for the underaged alloy. In air all the studied microstructures show a similar delay after an overload and the micromechanisms are similar. In the Aluminum Lithium alloy, 8090 T651, which exhibits a mixed (planar and multiple slip) behavior in air, the delay is higher in air than in vacuum for tests at R=0.1. This peculiar behavior is shown to be associated to the activation of a large slip band along a (111) plane while the conditions at the crack-tip prevailing after the overload are not sufficient to sustain this slip activity. In the case of flight simulation tests, the longest life is observed for the 2024 alloy while the shortest life is obtained for the 7075 T351 alloy. In the 2024 alloy (for which the fracture surface analysis was carried out in detail) the global fracture surface appearance is again governed by the K_max value and the most predominant R ratio as for the Block programmed tests. In the Al-Li surface covered by a new kind of striations not observed under constant amplitude loadings. Under such conditions the crack growth is accelerated four folds as confirmed from equivalent block load tests. In vacuum, no such change of micromechanism is observed for variable amplitude tests in the Al-Li alloy and the crack growth life is three times as high as that for the 2024 alloy.

P/M CURRENT RESEARCH AND INDUSTRIAL PRACTICES: Session III: Current Research II

Session Chairperson: Burton R. Patterson, Department of Materials and Mechanical Engineering, University of Alabama-Birmingham, AL 35294-4461

METAL POWDER RESEARCH AT NIST: Stephen D. Ridder, NIST, B156, Bldg. 231, Gaithersburg, MD 20899

Powder Metal processing research at NIST includes expert system control of Inert Gas Atomization (IGA), nitrogenated stainless steel powder processing, advanced sensors for diagnostics and control, mechanical alloying, and thermal spray deposition. Recent IGA studies have focused on the measurement and control of process pressures, temperatures, and flow rates of metal and gas and quantifying the effects of controllable parameters on the metal powder particle size. Instrumentation has been developed that allows real-time measurement and control of the gas to metal mass flow ratio, the atomizing gas jet pressure ratio, and the resulting metal powder size distribution. An Expert System driven computer control system incorporating these sensors provides the operator real-time graphical indications of the atomizers performance and allows for the construction of control strategies of arbitrary complexity. In addition to these IGA studies, new DC and RF plasma spray systems are being installed to extend the sensor and expert system control capabilities to include thermal spray processing. This technology, currently used to provide thermal barrier and wear resistance coatings for many high-performance/high-cost products, will require new advanced sensors and control for improved coating reliability and reduced overall cost before it will be accepted in high production industries such as automotive and consumer electronics.

9:00 am INVITED

UNDERSTANDING THE BEHAVIOR OF FULLERENES IN METALS WHEN PROCESSED BY P/M METHODS: E.V. Barrera, Xiayang Sheng, J. Sims and *V. Provenzano, Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX 77005; *Physical Metallurgy, Division of Materials Science and Technology, Code 6320, Naval Research Laboratory, Washington, DC 20375-5343

With the recent awarding of the 1996 Nobel Prize in Chemistry to the discovery and understanding of fullerenes, it is evident that the capability of developing new materials with these molecules is dependent on the ability to process them in various material systems. This talk will focus on the processability of fullerenes in metals, polymers, and ceramics. The focus will be on powder processing methods but will review the range of processing where the fullerene powder has been used. Recent investigations directed toward the use of nanotubes will also be discussed. Fullerenes are seen as a potential dispersoid, reinforcement or second phase in the numerous material systems being explored. This research is supported by a NSF grant: DMR-935705, Texas Higher Education Coordinating Board grant: TATP 3604-056, and an Office of Naval Research grant: N00014-97-1-0391.

9:20 am INVITED

MODELING AND CONSOLIDATION OF NANOCRYSTALLINE CUTTING TOOLS FOR MACHINING TITANIUM: Ram B. Bhagat, Penn State University, Engineering Science and Mechanics Department, 227 Hammon Bldg., University Park, PA 16802

A key issue pertaining to the consolidation of nanocrystalline powders is the grain growth to micrometer size. Addition of grain growth inhibitors such as VC and Cr3C2 has been reported to be useful during the liquid-phase sintering of nanocrystalline WC-Co powder to produce ultra fine grained material, but not truly nanograined material (grain size < 100 nm). This paper deals with modeling and rapid solid-state consolidation techniques (microwave sintering and vacuum hot pressing) for fabricating nanograined cermet (WC-10 wt.% Co) wherein the maximum temperature is kept much below the eutectic temperature of the WC-Co system (~13200°C). In conjunction with the results of the modeling, the processing conditions are optimized to fabricate fully dense samples with minimum grain growth. The fully dense nanograined WC-10 wt.% Co samples (with no dopants) have higher hardness and fracture toughness than those hitherto reported for conventionally processed material of identical composition. Nanograined ball end mills (WC-10 wt.% Co) have been fabricated for machining titanium; initial results appear very promising for improved cutting performance and enhanced tool life.

9:50 am INVITED Talk by the Secretary of the P/M Committee

THE DEVELOPMENT OF A LIQUID-PHASE-SINTERED, Cu-BASED P/M MATRIX FOR AN SiC REINFORCED COMPOSITE MATERIAL: B. Wang, G.M. Janowski, Department of Materials and Mechanical Engineering, The University of Alabama at Birmingham, Birmingham, AL 35294-4461

A liquid-phase sintered copper-based alloy was developed as a matrix for SiC particle reinforced composite materials. The concentrations of elemental additives (Al, Si, and Mg), sintering temperature, green density, and sintering atmosphere were varied in order to achieve high density and strong matrix/particle bonding. Two satisfactory alloys were developed: Cu-7 Al-2 Mg-2 Si and Cu-8 Al-2 Mg-2 Si (in wt.%), with the latter material having a higher strength (yield strength of 280 MPa compared to 218 MPa). Although the uncombined Mg was likely not present after vacuum sintering, its presence in the elemental powder blends was key in achieving a strong bond between the SiC particles and Cu matrix. A comparison of the particle cracking of these Cu-based composite materials will be made with similar Al-based composites. This research was supported by Alabama-NSF EPSCoR.

10:10 am INVITED

A STUDY OF THE BRAIN GROWTH BEHAVIOR ON NANOCRYSTALLINE Cu AND Cu-Fe ALLOYS PREPARED BY MECHANICAL ALLOYING: Steven C. Axtell, Department of Mechanical Engineering, Center for Materials Research and Analysis, University of Nebraska-Lincoln, Lincoln, NE 68588-0656

Research at the University of Nebraska has focused, in part, on the characterization of the grain growth behavior of nanocrystalline alloys prepared by mechanical alloying. The grain growth behavior of nanocrystalline materials is of interest due to its technological importance in the consolidation of these materials and its scientific importance in determining whether the behavior is similar to that of larger-grained materials. Isochronal and isothermal annealing experiments have been performed on Cu and Cu-Fe alloys using in situ x-ray diffraction. The variation in the diffracting particle size and the grain growth exponent, n, in the normal grain growth equation as a function of grain growth of nanocrystalline Cu prepared by mechanical alloying decreases with increasing temperature and increases with the addition of Fe.

10:40 am INVITED

NEW PROCESS CONTROL AGENTS FOR MECHANICAL ALLOYING AND BALL MILLING: Frank Biancaniello, NIST, Gaithersburg, MD 20899

Prior to about 1970, fine metal powders (<100=B5m) were produced by grinding coarse metal powders in a conventional ball mill. Lubricants such as kerosene or fatty acids were employed to eliminate the cold welding (agglomeration) of the particles during the milling process, thus achieving a higher yield of the fine powders. However, these lubricants, which are presently referred to as process control agents (PCA), often contaminate the powders leading to degradation of the alloy properties following powder consolidation. In the late 1960's, researchers at Inco discovered that metal powders could undergo repeated welding, fracturing, and rewelding when processed in a dry high-energy ball mill using stearic acid (SA) as a PCA. In this study, three new PCA's are investigated and compared with SA. X-ray diffraction analysis results indicate that these new PCA's are as efficient or more so than SA and leave little or no contaminating residue when heated at relatively low temperatures during degassing followed by subsequent consolidation at higher temperatures.

11:10 am INVITED

POWDER METALLURGY AT LOS ALAMOS NATIONAL LABORATORY THEN AND NOW: S.R. Bingert and H. Sbeinberg, P.O. Box 1663, MS G770, Los Alamos National Laboratory, Los Alamos, NM 87545

Powder metallurgy has been an active part of Los Alamos programs for over fifty years. Although not all state-of-the-art, the capabilities represent one of the widest and most unusual collections of processing equipment in the country, most of which has been used for fabrication of components for internal requirements and for research and development, or demonstrations of processing technology. The past, present, and future operational capabilities of the section will be presented and include: powder making, powder characterization, reactive powder handling, powder conditioning, hot and cold uniaxial pressing, hot and cold isostatic pressing, extrusion, powder rolling, sintering and heat treatment facilities, atmospheric and low pressure plasma spray, flame spray, wire arc spray, alloy development, rapid and directional solidification, and consolidation and micromechanics modeling. The team has worked on a large variety of materials over the course of the LANL history including most metals, many oxides, composites, and unusual materials. Some select projects and results from past to present programs will be presented.

RELIABLE OXIDE SCALES AND COATINGS FOR HIGH-TEMPERATURE INDUSTRIAL ENVIRONMENTS

Session Chairs: K. Natesan, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439; R.J. Gottschall, U.S. Dept. of Energy, Division of Materials Sciences, 19901 Germantown Road, Germantown, MD 20874-1290

OPENING REMARKS: Organizers and Session Chairs

8:45 am KEYNOTE

NECESSARY PROPERTIES/PERFORMANCE CHARACTERISTICS OF PROTECTIVE SURFACE OXIDES FOR USE IN ENERGY AND UTILITY APPLICATIONS: J. Stringer, Electric Power Research Institute, P.O. Box 10412, Palo Alto, CA 94303

In utility systems, alloys for use in components exposed to high temperatures in use ultimately depend generally rely on adherent, slowly-growing, protective scales formed by the oxidation of one of the alloy constituents. The properties required of this protective oxide will be determined by what is frequently a very complex service environment. Other processes taking place during periods when the equipment is not operating may also produce effects which may affect the protectiveness of the oxide when the surface is again heated to operating temperature. These effects have to be considered in the context of how long the component is required to remain operational, what the most probable mode of failure is, and with what degree of confidence the lifetime may be predicted.

9:30 am

MICROSTRUCTURAL BASIS FOR OXIDE SCALE AND COATING FAILURES: K.B. Alexander, K. Prüßner, B.A. Pint, P.F. Tortorelli, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6156; K. Natesan, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439

Electron microscopy techniques are applied to the microstructural and chemical characterization of the alumina scale/coating, oxide-metal interface, and substrate for Fe-Al, FeCrAl(Y), and NiCrAlY alloys. These observations are used to establish links between oxidation performance and microstructural factors. Differences between alumina scales and coatings after high-temperature exposure help identify factors related to the dynamics of the oxide growth process as they affect surface oxide reliability and failure.

10:00 am

ANALYSIS OF STRESSES IN OXIDE LAYERS: J.K. Wright, R.L. Williamson, Idaho National Engineering and Environmental Laboratory, 2151 N. Boulevard St., Idaho Falls, ID 83415-2218; R.M. Cannon, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Mail Stop 8-200, Berkeley, CA 94720; M. Grimsditch, B.W. Veal, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439

Analysis of residual stresses is important to understanding the driving forces for crack propagation and spallation of protective oxides. Finite elemental analysis is used to calculate thermally-induced stresses when alumina layers on alloys are cooled from high temperatures. The effects of specimen geometry, surface roughness, and substrate properties on the magnitude and distribution of stresses in the oxide and alloy are modeled and the stresses are compared with photostimulated luminescence measurements.

10:30 am

RELATING RELIABILITY OF SURFACE OXIDES TO INTERFACIAL PROCESSES: P.Y. Hou, R.M. Cannon, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Mail Stop 62-203, Berkeley, CA 94720

The adherence of oxide scale on alumina-forming alloys is examined in terms of segregation of sulfur, interface roughness, and substrate composition. These factors are related to observed failure modes in the context of energy considerations for crack propagation at and near oxide-metal interfaces. Correlations between materials parameters and spallation resistance are being used to develop preliminary capabilities for predicting appropriate conditions for scale adherence.

11:00 am

MAINTAINING SCALE/COATING INTEGRITY IN THE PRESENCE OF SALT DEPOSITS AND OTHER ENVIRONMENTAL IMPURITIES: N.S. Bornstein, United Technologies Research Center, East Hartford, CT 06108

The use of alumina-forming alloys in high-temperature industrial environments present the additional complications of the effects of alkali metal and other contaminants on the integrity of the oxide scales that proffer oxidation protection and coating durability. This work examines the critical factors associated with maintaining oxide scale integrity in complex, application-relevant environments.