Materials Week '97: Wednesday PM 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 Wednesday afternoon, September 17, 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 VI

Session Chair: Adrian P. Sutton, The University of Oxford, Oxford, UK

SOME APPLICATIONS OF MAZED MULTICRYSTAL FILMS IN INTERFACE RESEARCH: U. Dahmen, K.H. Westmacott, National Center for Electron Microscopy, LBNL, Berkeley, CA 94720

Polycrystalline thin films with extreme texture, containing only two, three or four different grain orientations, can be grown in a maze-like topology by heteroepitaxial deposition on single crystal substrates. Although the misorientation between grains is fixed, the inclination of the grain boundaries is variable and thus free to reach local equilibrium. Such mazed multicrystal films are excellent for fundamental studies of grain boundary behavior, especially in the important size range accessible only to electron microscopy. This talk will illustrate applications of this thin film geometry to the study of grain boundary phenomena such as faceting and dissociation reactions, segregation, second phase precipitation, color crystallography and the force balance at triple junctions. Research supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy under contract #DE-ACO3-76SFOOO98.

2:30 pm INVITED

ROLE OF GRAIN BOUNDARIES AND INTERFACES IN THIN FILM REACTIONS: K. Barmak, Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015

Interest in thin film reactions is driven by the importance of these reactions in technological applications. For example the formation of silicides and aluminides plays an important role in semiconductor metallization schemes. There is now growing evidence that accurate descriptions of these reactions must consider not only the growth stage, but also the nucleation stage for product phase formation. In this paper, experimental and theoretical studies of the role of interfaces and grain boundaries in nucleation and growth of product phases will be discussed. In addition, the evolution of grain structure of product phases and the similarities and differences of these structures to those classified in structure zone models for vapor-deposited films will be addressed.

3:00 pm INVITED

THIN FILM MICROSTRUCTURES PRODUCED BY NUCLEATION AND GROWTH TO IMPINGEMENT: H.J. Frost, C.V. Thompson^*, Thayer School of Engineering, Dartmouth College, Hanover, NH; ^*Department of Materials Science and Engineering, MIT, Cambridge, MA

Recent simulations of phase transformations involving nucleation and growth-to-impingement, in both two and three dimensions, have allowed substantial progress beyond the original analytic modeling of Johnson and Mehl, Avrami, and Kolmorgorov, in providing descriptions of the details of the topology and geometry which result from a wide variety of nucleation conditions and growth conditions. Careful experimental studies of the geometrical and topological properties of real microstructures have been far less common. Accurate studies of three dimensional structures are difficult because they require laborious cross-sectioning. In rare cases, a phase transformation within a thin film may be accurately described as a two-dimensional process. In this case the microstructure may be easily observed and compared directly with computer simulations, so as to quantitatively establish the conditions of the nucleation and growth processes. A classic example of this is provided by the experiments on the crystallization of films of amorphous silicides performed by Professor David A. Smith and co-workers. In this paper we will review both the experimental and the computer simulation literature describing the microstructures resulting from phase transformations involving nucleation and growth-to-impingement in thin films, focusing especially on the microstructures reported by Smith and coworkers.

3:30 pm BREAK

3:40 pm INVITED

MORPHOLOGY AND KINETICS OF CRYSTALLIZATION OF AMORPHOUS CoSi₂ THIN FILMS: K.N. Tu, J.M. Liang¹, L.J. Chen¹, L.T. Shi², Department of Materials Science and Engineering, University of California - Los Angeles, Los Angeles, CA 90095; ¹Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, China; ²IBM T.J. Watson Research Center, Yorktown Heights, NY 10598

The morphology and kinetics of crystallization of amorphous CoSi₂ thin films in the temperature range of 130 to 160°C were studied by using TEM, resistivity, and calorimetry techniques. The nucleation rate and growth rate of crystalline CoSi₂ particles were measured independently from TEM images. The rate of transformation was measured by resistivity changes, and the heat of transformation by differential scanning calorimetry. These data were combined to obtain an understanding of the kinetic behavior of the crystallization. An attempt to calculate the interfacial energy between the crystalline and amorphous CoSi₂ is given.

4:10 pm INVITED

STRUCTURE OF FUSED INTERFACES BETWEEN InP AND GaAs: L. Sagalowicz, A. Rudra, P.H. Jouneau, A. Sirbu, E. Kapon, Institute for Micro and Optoelectronics, Department of Physics, Ecole Polytechnique Fdrale de Lausanne, 1015 Lausanne, Switzerland

For some optoelectronic applications, it is necessary to grow (001) InP epitaxially onto (001) GaAs, with a cube on cube orientation relationship. There is a 3.9% mismatch between the two structures and geometrical theories predict that the easiest way to accomodate the misfit is through a square network of edge (Lomer) dislocations spaced by 104Å. Conventional epitaxial growth techniques lead to the development of threading dislocations. Wafer fusion is tested to see if it leads to a better confinement of the dislocations at the interface. TEM is used to study the structure of the interface and the dislocation content for various fusion conditions. high resolution transmission electron microscopy (HRTEM) reveals that the interface is made up either by edge dislocations, or by an amorphous intermediate layer for which the thickness is about 2 nm. It was determined by electron energy loss spectroscopy that this layer is made of indium oxide. In all cases, HRTEM and plan view electron microscopy indicate that the defects are confined at the fused interface.

4:40 pm

INTERPHASE INTERFACES AND THE FORMATION OF THE FERROMAGNETIC -PHASE IN MnAl-BASE PERMANENT MAGNET ALLOYS: D.P. Hoydick, W.A. Soffa, Department. of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261

Recently, new electron microscopy studies of the (hcp)'(B19)(L1₀) transformation in MnAl-base permanent magnet alloys have shown that the formation of the ferromagnetic -phase involves a diffusional nucleation and growth process akin to the so-called "massive transformation". These results contradict the conventional wisdom that has been promulgated for nearly 30 years which identified the transformation mechanism as a shear or martensitic mode. The mechanism of transformation will be discussed with an emphasis on the nature of the interphase interfaces and the attendant transformation twinning involved. Work supported by the National Science Foundation (DMR).

5:00 pm

THE ROLE OF GRAIN BOUNDARY MOBILITY IN THE DEVELOPMENT OF GOSS TEXTURE IN Fe-3%Si: P. Lin^1,2, G. Palumbo¹, E.M. Lehockey¹, K.T. Aust², ¹Ontario Hydro Technologies, 800 Kipling Avenue, Toronto, Canada M8Z 5S4; ²Department of Metallurgy and Materials Science, University of Toronto, Toronto, Canada M5S 3E4

Coincidence site lattice (CSL) grain boundaries have been shown to play an important role in the development of the Goss texture in the early stages of secondary recrystallization in Fe-3%Si. A "deficit" of these CSL boundaries, relative to the determined probability of occurrence during random growth, was determined for interfaces bounding Goss grains. This deficit is rationalized on the basis of preferential replacement of CSL boundaries by general (or higher S) interfaces due to the enhanced mobility of low S CSL grain boundaries. A simple statistical approach is applied to assess the relative mobility of CSL and non-CSL interfaces.

ECONOMIC VIABILITY OF EMERGING MATERIALS AND MANUFACTURING TECHNOLOGIES

Session Chairs: Sujit Das, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6205; S. Viswanathan, Oak Ridge National Lab., Oak Ridge, TN 37831-6083

ECONOMIC ANALYSIS OF STRUCTURAL SILICON NITRIDE COMPONENTS: J.M. Schoenung, Chemical and Materials Engineering, California State Polytechnic University, Pomona, CA 91768; B. Draskovich, Allied Signal Ceramic Components, Torrance, CA 90504

Economic analyses were conducted on five structural silicon nitride components, including stators, rotors, and cam roller followers. Multiple forming operations were considered, including slip casting, gelcasting, injection molding, and dry pressing. The economic analyses involved the development and refinement of process-based technical cost models using spreadsheet software. These TCMs were used to estimate the costs of fabrication under a wide variety of production scenarios and annual production volumes. Cost drivers were identified from various perspectives, for example, by cost element such as direct labor, by unit operation such as sintering, and by process inputs such as cycle time and process yield. Comparisons were made of the costs and cost drivers for the different components, and for the different shape forming operations. Sensitivity of cost to key input values was also investigated.

1:55 pm

TRIALS, TOOLS, AND TRICKS--SHARING 25 YEARS OF TECHNICAL-ECONOMIC ANALYSIS EXPERIENCE AT OWENS CORNING: Gabe Tincher, Owens Corning, Columbus, OH

For over 25 years, the technical-economic analysis group at Owens Corning has served as an internal consulting group conducting technical-economic analyses and competitive economics analyses of materials which compete in end-use markets and customer processes. This paper will include examples of some of the analytical tools used, share some of the approaches that have worked well, and discuss some of the trials and tribulations of the work. Case examples selected from the composites and building materials industries will be used to illustrate what has been learned and how these studies have been used in strategic decision making at Owens Corning.

2:20 pm

APPLIED ECONOMIC ANALYSIS FOR TECHNOLOGY DEVELOPMENT: Anthony E. Mascarin , Ted Hannibal, IBIS Associates, Inc., Wellesley, MA 02181-4003

The understanding of cost is a key factor in determining the commercial success of new technologies. Technical Cost Modeling (TCM) is a technique that provides understanding by developing a detailed economic simulation of manufacturing processes. TCM can provide economic assessments of technically competitive processes and the effects of alternative manufacturing scenarios. TCM has been applied to broad range of advanced materials programs to identify critical cost drivers in a dynamic fashion as the development of a cost effective manufacturing technology progresses. Elements critical to the component cost include, but are not limited to the component design, tolerances, automation, yield improvements, and learning effects. This paper reviews the general cost analysis methodology and presents sample results from advanced ceramics programs and discusses the relationships among critical cost factors such as cycle time, yield, finishing operations, and capital investment.

2:45 pm

A DESIGN COST MODEL FOR NEW PRODUCTS DEVELOPMENT: S.G. Shina, Mechanical Engineering Department, University of Massachusetts - Lowell, Lowell, MA 01701; Anil Saigal, Mechanical Engineering Department, Tufts University, Medford, MA 02155

A design cost model which is sensitive to many of the variables of the manufacturing process, in terms of the selection of components and part technologies, manufacturing automation, process quality, tooling investment and volume sensitivity is discussed. The model starts with an Activity Based Costing (ABC) model with additional capabilities dealing with each manufacturing process step in terms of cost, quality, automation level, tooling, machine setup and operator skills. The model can be used to study impacts on the cost of the new product due to reconfiguration of the manufacturing mix of existing products on the production floor. Alternate design and manufacturing process selection can be evaluated while changing estimated production volume and process flow. An example will be presented using CACI simulation package Simfactory 7 and a spreadsheet based design cost model, working interactively in order to evaluate new electronic product costs.

3:10 pm BREAK

3:20 pm

ASSESSING THE AFFORDABILITY OF CONTINUOUSLY REINFORCED METAL MATRIX COMPOSITES: D.M. Elzey, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903

A number of processing approaches are emerging for the manufacture of continuously reinforced metal matrix composites (cMMC's). To date, these new materials have been produced only in small quantities, making them expensive and limiting the availability of reliable experimental data. Consequently, it is difficult to assess the economic viability of these manufacturing routes and their capability to provide material having an acceptable combination of cost and performance. A recently developed method for assessing the affordability of emerging materials, known as QCM (Quality-Cost Modeling), allows quantitative comparison of competing manufacturing technologies and is being applied to selected cMMC processes. QCM is illustrated by application to slurry casting and plasma spray deposition processes for manufacturing continuously reinforced titanium matrix composites.

3:45 pm

THE USE OF PROCESS SIMULATION IN COST MODELING: W.M. Tilton, Summit Solutions, Inc., Colorado Springs, CO 80906

Process simulation can be used as a powerful tool for the cost/benefit analysis of manufacturing technologies. By careful assignment of cost values to resources and to the various elements of a manufacturing process, the cost associated with that process can be quickly evaluated under a variety of operational scenarios. Costs can be determined for each step in a manufacturing process, and can be aggregated to present a total cost picture. Bottlenecks can be identified, and the cost/benefit of options for their elimination can be determined by running excursions of the simulation. Modern simulation software supports the full range of statistical analysis and probability assignments, which enables cost/benefit evaluation of quality control issues and procedures. The use of full color, fully animated models supports common understanding of manufacturing processes by management and labor, resulting in greater likelihood of finding ways to improve the process, particularly important for emerging materials and alternative manufacturing technologies.

4:15 pm

MANUFACTURING BASED COST MODELING OF ADVANCED CERAMIC COMPONENTS: E.H. Kraft, Kyocera Industrial Ceramics Corp., Vancouver, WA 98661; J.M. Schoenung, Chemical and Materials Engineering, California State Polytechnic University, Pomona, CA 91768

A manufacturing cost model has been constructed for advanced ceramic components. The model has its origins in Excel spreadsheets used in quoting prototype and production quantities of ceramic parts. Input factors include run rates, material costs and lot quantities, part lot sizes, process step machine and labor times, yields, applicable machine capacities, and labor and machine cost rates. Tooling and engineering estimation is also included. Output includes total cost, unit cost, process step cost, and graphical display of unit operation cost. The model has been refined based on general industrial cost model experience, and the inputs improved using expanded manufacturing experience. It has been exercised to study manufacturing costs of industrial and heat engine components with production rates from a few prototypes to automotive volumes. Auxiliary models allow estimation of machine and building space requirements.

4:40 pm

SPREADSHEET MODEL OF COSTS OF AUTOMOBILE SHREDDING INDUSTRY: F.C. McMichael, A.L. Sterdis, Environmental Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

This paper describes a spreadsheet model of a typical automobile shredding facility. Automobile shredding facilities are an integral part of the existing U.S. automobile recycling infrastructure. Shredding facilities purchase automotive hulks and process them into marketable output products including ferrous, white metal and red metal scrap. The spreadsheet model discussed in this paper depicts the size reduction and separation processes and associated costs for the operations at a typical facility. Using estimates of cost and process efficiencies for each of the operations, the model calculates processing costs and composition of the output streams (ferrous, white metal, red metal and shredder residue). Process costs modeled include capital equipment, electricity, equipment maintenance and wear, and labor costs. Using scrap metal prices and estimated ASR disposal costs, along with these processing costs and output stream compositions, economic performance of the facility can be predicted. The results predict the consequences of hulk composition changes on the costs for hulk shredding and separation.

FATIGUE AND CREEP OF COMPOSITE MATERIALS: Session VI

Session Chairs: Sheldon Wiederhorn, Materials Science and Engineering Laboratory, National Institute of Standards & Technology, Gaithersburg MD 20899-0001; Leon L. Shaw, Department of Metallurgy and Materials Engineering, University of Connecticut, Storrs, CT 06269

CREEP FAILURE MECHANICS FOR CERAMIC MATRIX COMPOSITES: B.N. Cox, C. Argento, Rockwell Science Center, Thousand Oaks, CA 91360

In many current ceramic matrix composites (CMCs), the shielding of matrix cracks by unbroken fibers is compromised by fiber creep. Matrix cracks are therefore observed to grow subcritically. We will review experimental and theoretical studies of matrix cracks bridged by creeping fibers, emphasizing basic concepts. The processes of crack initiation and crack growth will be unified by micromechanical models. The question of whether to expect multiple or dominant single cracks is especially interesting and central to formulating life models. Failure maps will be discussed which show how micromechanical properties of the composite, length scales, notch and ply dimensions, and the steady state matrix cracking stress all influence failure modes.

2:30 pm

HIGH TEMPERATURE FLEXURAL CREEP BEHAVIOR OF REACTION-FORMED SILICON CARBIDE CERAMICS AND FIBER REINFORCED COMPOSITES: M. Singh, NYMA, Inc., NASA Lewis Research Center Group, Cleveland, OH 44135

Flexural creep behavior of reaction-formed silicon carbide (Hi-Nicalon^TM) fiber reinforced composites have been investigated from 1150 to 1350°C in air. These materials were fabricated by the melt infiltration of silicon in to porous carbonaceous preforms. Creep tests were carried out at 75, 150, and 200MPa for 100-200hrs. Microstructural characterization was carried out to identify the creep mechanisms. These results will be compared with literature data on reaction-bonded silicon carbide and CVI SiC/SiC composite materials.

2:50 pm

ROLE OF FIBER CREEP RATE IN THE CRACK GROWTH RATE OF SiC/SiC COMPOSITES: R.H. Jones, C.H. Henager, Jr., C.A. Lewinsohn, Pacific Northwest National Laboratory, PO Box 999, Richland, WA 99352

Optimization of the high-temperature structural properties of continuous fiber SiC/SiC composites requires a fundamental understanding of the relationship between fiber, matrix and interface properties and composite properties. Results of a study aimed at relating the fiber creep rate to the subcritical crack growth rate and fracture properties of SiC/SiC composites have demonstrated that the crack growth rate in a bulk composite is controlled by the fiber creep rate. This result was demonstrated for Nicalon-CG and Hi-Nicalon fiber reinforced material where a 100°C shift in the creep strength of the fibers resulted in a similar shift in the crack growth rate of the composite. The fiber creep rate of Hi-Nicalon S and Dow Corning's Sylramic fiber exhibit a 300°C improvement in the creep strength relative to Nicalon-CG. Crack growth rates of composite material made with these fibers are expected to exhibit similar increases. *Research supported by the Office of Basic Energy Sciences of the U.S. Department of Energy with Battelle Memorial Institute under Contract DE-AC06-75RLO 1830.

3:10 pm INVITED

THE MONOTONIC AND FATIGUE BEHAVIOR OF NICALON FIBER REINFORCED ALUMINA COMPOSITES: N. Miriyala, P.K. Liaw, C.J. McHargue, Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996; L.L. Snead, Oak Ridge National Laboratory, Oak Ridge, TN 37831

The mechanical behavior of a commercially available continuous fiber reinforced ceramic composite, namely a Nicalon fiber fabrics reinforced alumina composite, was investigated. Flexure specimens were subjected to monotonic and cyclic-fatigue loadings, at ambient temperature and 1000°C, with loading either parallel or normal to the fabric plies. The orientation of the laminate plies to the loading axis, and the exposure of the composite materials for prolonged times at the elevated temperature, resulted in significant differences in the monotonic and fatigue behavior. The damage mechanisms responsible for the observed effects will be the focus of the paper. Research supported by DOE under a subcontract from Lockheed Martin Energy Research Corporation (No. 11X-SV483V), and by the National Science Foundation, under contract No. EEC-9527527 with Mrs. Mary Poats as a contract monitor.

3:30 pm BREAK

3:50 pm INVITED

HIGH TEMPERATURE DESIGN WITH SILICON NITRIDE: Sheldon Wiederhorn, Materials Science and Engineering Laboratory, National Institute of Standards & Technology, Gaithersburg, MD 20899-0001

Abstract not available.

4:10 pm

ELASTIC STRAIN DISTRIBUTION IN THE PHASES OF A CuMo PARTICULATE REINFORCED METAL MATRIX COMPOSITE DURING CREEP DETERMINED BY NEUTRON DIFFRACTION: M.R. Daymond and M.A.M. Bourke, LANSCE, Los Alamos National Labs, NM 87545; D.C. Dunand & C. Lund, MIT, Cambridge, MA 02319

The macroscopic load bearing capability of a composite material is directly related to the strain partitioning occurring between the individual phases. To understand the creep behavior of a composite, and for model validation, we need a direct measure of these strains. Neutron diffraction offers in-situ determination of the elastic strains within a bulk specimen, without the ambiguity implicit in a surface x-ray diffraction measurement. We report the different strain partitioning occurring in a Cu-Mo particulate reinforced MMC for increasing applied tensile loads at room temperature, 300 and 350°C. The results are compared with finite element unit cell predictions, with good agreement.

4:30 pm

ELEVATED TEMPERATURE DEFORMATION BEHAVIOR OF THE INTERFACIAL REGION IN CONTINUOUS FIBER REINFORCED METAL-MATRIX COMPOSITES: I. Dutta, J.E. Funn, Department of Mechanical Engineering, Naval Postgraduate School, Monterey, CA 93943

This paper will present the results of ongoing studies of creep and stress relaxation behavior of the near-interface region in fiber reinforced metal-matrix composites (MMC). The studies are based on model single fiber composite (SFC) systems representing strongly bonded (W-Pb) and weakly bonded (Quartz-Pb) interfaces. The relative roles of interface diffusion and near-interface matrix creep in both strongly and weakly bonded systems will be discussed, and based on the experimental results, a constitutive law representing the dependence of the steady state strain rate of the interfacial region on temperature and the average interfacial shear stress will be proposed. Such a law will enable the modeling of multi-phase systems while accounting for interfacial strain accommodation by representing the interface as a separate entity with its own constitutive law, and will thus have an impact not only on the field of composites, but wherever there are interfaces between dissimilar materials. Supported by the National Science Foundation under contract # DMR-9423668.

4:50 pm

INDENTATION EXAMINATION OF METAL AND POLYMERIC MICROSTRUCTURE USING PRECISION STRAIN RATE SENSITIVITY: M. Van Prooijen, B.J. Diak, S. Saimoto, Department of Materials and Metallurgical Engineering, Queen's University, Kingston, Ontario, Canada, K7L 3N6

Microindentation testing is a very attractive materials testing technique due to its almost non-destructive nature of examination and position localized determination. However, hardness alone is not sufficient to describe the microstructure and we have implemented the measurement of thermodynamic response by precision displacement rate change to correlate strain rate sensitivity to microstructure. In order to distinguish the material response from the testing geometry, various materials, including brass, I.F. steel, aluminum alloys, silica modified polyester paints and ABS blends have been tested. For the case of polymeric material a unique master curve on the apparent activation work versus strain rare plot manifests itself independently of the test temperature and displacement rate used. The unique features of the crystalline dislocated case will be compared to those of the molecular polymeric one.

GRAIN REFINEMENT IN NON FERROUS METALS AND ALLOYS: General Abstract Session

Session Chairs: Harish Merchant, Gould Electronics, 35129 Curtis Blvd., Eastlake, OH 44095-4001; James G. Morris, University of Kentucky, 760 Anderson Hall, Dept. of Metallurgy, Lexington, KY 40506-0046

GRAIN REFINEMENT BY CONTINUOUS RECRYSTALLIZATION: Jian Li, S. Saimoto, Dept. of Materials & Metallurgical Engineering, Queen's University, Kingston, Ontario, Canada K7L 3N6

Recent studies have shown that continuous recrystallization rather than the conventional discontinuous one can result in grain size of about 1 µm. Our studies on nominal pure aluminum alloys have shown that the key element is Fe and its solute content has to be reduced below 1ppm (atomic) in order for grain refinement by continuous recrystallization to occur. To enhance rapid gettering of solutes at low temperatures, the material is pre-strained such that the dislocation density is high enough to reduce the diffusion distance of solutes but not sufficient to initiate recrystallization. This procedure have been used with success for AA3003 nickel and some stainless steels.

2:20 pm

AN APPROACH TO THE MICROSTRUCTURE ROLE IN THE REFINEMENT OF MONOPHASIC X-Zn ALLOYS: M.E. Noguez, G. Salas, T. Robert, J. Ramirez, Departmento de Metalurgia, Facultad de Quimica, UNAM, Mexico, D.F. 04510

Three casting refinement methods (mold rotation, chemical additions and cooling rate) have been used in the casting of Mg-5% Zn, Al-5% Zn and Cu-30% Zn alloys. A microstructural evaluation and a comparison among the results obtained for the same method in the three alloys and the three methods for the same alloy was made. The aim was to relate the effect of the refining method on the structure and on the properties including a corrosion behavior. Mold rotations were made from 100 to 500 rpm. A similar microstructural refinement and properties relation were found for the three alloys with the rotation. Chemical additions used were: for Al-Zn the commercial material Ti-B (6:1); for Mg-Zn the recommended hexacloroethane used in Mg-Zr alloys; and Bi for Cu-Zn, recommended for pure Cu which was found effective to a certain extent. Results with this method show dissimilarities discussed in text. Cooling rate from approximate 50°C/s to 1 to 2°C/s were used in all alloys. This method gave the expected results and relations. A final general discussion is made regarding the grain size and secondary dendrite arm spacings (DAS) and the common accepted relations DAS-Cooling rate and Grain size-properties, etc.

2:40 pm

SOLIDIFICATION PROCESSING FOR GRAIN REFINEMENT OF NON-FERROUS METALS AND ALLOYS: Goro Aragane, Yoshiaki Osawa, Susumu Takamori, Akira Sato, National Research Institute for Metals, Tsukuba, Ibaraki, 305 Japan

Ultrasonic vibration is applied to molten metals through steel horns coated with Al₂O₃ and SIALON horns during solidification, and effects of the ultrasonic vibration on the structures of Zn, Al, Al alloys and Cu ingots are investigated. A new process named "Rapid solidification with vigorous agitation" is developed to produce large ingots having fine grain structures. The molten Al-Si alloys are rapidly solidified with water cooled copper molds and at the same time stirred vigorously by graphite rods during falling down of the melt through the gap between the mold and the rod. The mean grain size of primary crystals in slurry obtained become smaller with increasing the intensity of cooling and the rotation speed of stirring rod. Primary and eutectic Si grains are not only small but also globular when the agitation is strong enough.

3:00 pm

PHENOMENON OF TEXTURE NONHOMOGENEITY IN GRAIN STRUCTURE OF METALS: O.B. Girin, Yu O. Proshenko, V.I. Bekarev, Dept. of Physics, State Metallurgical Academy of Ukraine, Prospekt Gagarina 4, Dnepropetrovsk 320635, Ukraine

Phenomenon of texture nonhomogeneity in grain structure of metals discovered by utilizing nonconventional techniques of X ray diffraction texture characterization and TEM is covered. The effect studied results from texture development and consists in that the grain size, substructure and defect density differ dramatically in the major axial, the minor axial and the random component of texture. On the average, the grain size and the defect density in the major axial component differ from those in the random component of texture. On the average, the grain size and the defect density in the major axial component differ from those in the random component by one order of magnitude. Features of grain structure evolution during texture development are addressed. It is demonstrated that the phenomenon at hand is to be taken into consideration in discussing the impact of grain size on metal properties.

3:20 pm

RECRYSTALLIZATION IN PARTICLE-CONTAINING ALUMINUM ALLOYS: L.M. Pawlowski, J.S. Vetrano*, I.M. Robertson and S.M. Bruemmer*, Dept. of Materials Science and Engineering, University of Illinois, 1304 W. Green, Urbana, IL 61801; *Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352

Additions of Sc and Mn have been added to solid solution Al-Mg alloys to study the influence of particle size, shape and type on deformation and recrystallization. Controlled heating experiments both in bulk samples and thin specimens heated in-situ in the transmission electron microscope have been performed to study these effects. Plate-like Al₆Mn particles are shown to pin grain boundaries and offer potential sites for nucleation of new grains. The presence of fine (<100 nm), coherent Al₃Sc particles, alternatively, inhibits recrystallization through pinning of dislocations and sub-grain boundaries. Processing of these alloys to produce microstructures suitable for superplastic deformation will be discussed. Work supported by the Materials Division, Office of Basic Energy Sciences, U.S. Dept. of Energy under Contract DE-AC06-76RLO 1830

HIGH CYCLE OF FATIGUE OF STRUCTURAL MATERIALS: Session IV: Fatigue Thresholds and Crack Growth

Section Chairs: Prof. Robert Wei, Lehigh University, Dept. of Mechanical Engineering, Bethlehem, PA 18015; Prof. Huseyin Sehitoglu, University of Illinois, Dept. of Mechanical Engineering, 1206 W Green, Urbana, IL 61801

PREFERRED LOCATIONS FOR FATIGUE CRACK INITIATION: P. Neumann, Max-Planck Institut fur Eisenforschung GmbH, Dusseldorf

Crack initiation in high cycle fatigue either at inclusions, at other faults with reduced strength or in localized slip bands. The size of inclusions and faults can be sufficiently reduced by process metallurgy in order to become irrelevant. Localized slip bands can be reduced in size by grain refinement. In a given microstructure they will form at those locations where stress concentrations are highest. Most metals are elastically anisotropic. Therefore most grain boundaries are elastically incompatible, act as stress raisers and therefore should be preferred locations for fatigue crack initiation. As an illustration 3-dimensional FEM calculations of the stress concentrations at the grain boundary in a bicrystal of NiAl will be presented. Strain controlled high cycle fatigue experiments in an austenitic and a ferritic stainless steel were performed in order to study the significance of grain boundaries for fatigue crack initiation. The relative frequency of cracks initiating at grain boundaries and grain boundary triple junctions increased with decreasing load amplitudes. In the high cycle i.e. low plastic strain regime (cycles to failure, N_f > 3.105) the most predominant initiation sites were found to be grain boundaries. In case of the austenitic steel these cracked boundaries were twin boundaries almost exclusively. In order to correlate the observation of individual events of crack initiation with local stress concentrations, the local orientation of more than 270 grains - cracked or uncracked - was measured by a modified electron channeling technique. In a simplified model these orientation data were used to calculate the local stress concentrations near each individual grain boundary. For the austenitic steel there is an excellent correlation between the values of the stress concentrations and the occurrence of crack initiation. Moreover the model explains the surprising but often observed phenomenon that cracks initiate only at every other boundary in a stack of lamellar twins.

1:25 pm INVITED

MICROSTRUCTURAL FRACTURE MECHANICS IN HIGH-CYCLE FATIGUE: E.R. de los Rios, Dept. of Mechanical Engr., Univ. of Sheffield, Sheffield S1 3JD U.K.; A. Navarro, Escuela Superior de Ingenieros de Sevilla, 41012 - Sevilla, Spain

Microstructural Fracture Mechanics principles are used to develop a model of crack growth in long life fatigue. In its simplest form microstructural modeling considers the material as a polycrystal of uniform grain size D, with a crack system divided into three zones: the crack, the plastic zone and the microstructural barrier zone. The crack may have various degrees of closure, introducing friction stresses on the crack flanks s₁, while the material's resistance to plastic deformation s₂ incorporates the work hardening characteristics of the material. The conditions for crack propagation establishes that the stress at the microstructural barrier s₃ should achieve the level of the barrier strength. The latter incorporates the effect of crystallographic orientation and the transfer of plasticity across barriers. The solution of the equilibrium equation allows for the calculation of the stresses sustained by the crack wake, plastic zone, barrier zone and elastic enclave, and the crack tip plastic displacement. Crack growth rate is calculated through a Paris type relationship in terms of f, i.e./dN = Cf_n.

1:50 pm INVITED

IN-SITU AFM OBSERVATIONS OF FATIGUE INDUCED SURFACE MICROPLASTICITY EVOLUTION: W. Gerberich, W. Geng, D. Kramer, S. Okerstrom, Depts. of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455

Evaluating the importance of surface microplasticity evolution to fatigue initiation is necessary for microplastic damage accumulation theories. Four point bending and uniaxial C-C fatigue testing are both employed in these experiments. Commercially pure and Grade IV titanium have been subjected to fully reversed cyclic loading and analyzed using atomic force and scanning electron microscopies. An in-situ AFM method has been developed here, which greatly improves the data quality from rough surfaces to make the results more reliable and repeatable. The evolution of persistent slip band spacing and height has been measured by observing the same position at intermittent periods over the fatigue life of the sample. As slip band spacings decrease, heights increase, however spacings have been observed to saturate with repeated cycling. How this fits into one damage accumulation model will be presented.

2:15 pm

FATIGUE INITIATION AS UNDERSTOOD FROM AFM OBSERVATIONS OF MICROPLASTICITY EVOLUTION: W. Geng, D. Kramer, S. Okerstrom and W. Gerberich, Depts. of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455

Four point bending and uniaxial fatigue testing have been used to study the evolution of surface microplasticity. Commercially pure grades of titanium have been subjected to fully reversed cyclic loading and analyzed using scanning electron microscopy and atomic force microscopy. The evolution of persistent slip band spacing and height has been followed by observing the same position at intermittent periods over the fatigue life of the sample. As slip band spacings decrease, heights increase; however spacings have been observed to saturate with repeated cycling, while heights initially continue to increase. This behavior is highly dependent upon the orientation of the individual grains with respect to the applied stress axis. Grains that do not show signs of initial surface slip may activate with further cycling as existing slip bands saturate. The magnitude of surface microplastic damage is used along with a critical step size criterion to predict the number of cycles required for fatigue crack initiation.

2:35 pm

ON THE CRACK GROWTH MECHANISM IN THE THRESHOLD REGIME OF FATIGUE CRACKS: F.O. Riemelmoser, Erich-Schmid-Institt fur Festkorperphysik, Leoben, Austria

The conventional explanation of the unusual behavior of fatigue cracks in the threshold regime is that the crack grows intermittently and cleavage-like. However, the experimental results of Hong and Laird (1991) and Thompson (1996) show that the patterns on the surface of a specimen broken in the threshold regime do not necessarily, stem from a cleavage like process. Therefore we questioned whether the drop in the crack growth curve in the threshold regime can be understood when only the dislocation-nature of plasticity is taken into account. Note that the crack growth rate near the threshold is just in the order of some Burgers vectors. We simulate crack tip plasticity by means of a discrete dislocation model. For large stress intensity ranges the crack growth curve of the dislocation model runs into the Paris-line, but for small DK the crack growth rate is smaller than predicted by the Paris relation. The reason is that discrete nature of plasticity. The dislocation model also predicts a minimum striation spacing and A dislocation pattern which is in accordance with measured ones. In the presentation the intermittent model and the dislocation mode are compared.

2:55 pm INVITED

GRAIN SIZE INFLUENCE ON THE GROWTH OF LONG FATIGUE CRACKS: Basic Physics and Experimental Data: A.K. Vasudevan, Office of Naval Research, Arlington, VA 22217; K. Sadananda, Naval Research Labs, Washington, DC 20375

We observe that fatigue damage has to be represented by two driving force parameters like DK(cyclic) and K_max (static). Of these two driving forces, K_max has a strong dependence on microstructural and environmental variations. This is because K_max is directly related to the breaking of the atomic bonds at the crack tip resulting in crack extension. In this paper, we illustrate the basic physics of the problem, current interpretations and some modifications into newer ones.

3:20 pm BREAK

3:35 pm

BEHAVIOR OF SMALL SURFACE CRACKS IN HIGH-CYCLE FATIGUE OF A Ti-8Al ALLOY: IMPLICATIONS OF CRACK SHAPE AND CLOSURE: K.S. Ravichandran, Department of Metallurgical Engineering, 412 WBB, The University of Utah, Salt Lake City, UT 84112

Fatigue crack growth behavior of surface cracks were investigated in Ti-8Al alloy. The variations in crack aspect ratio induced by grain boundaries, as well as closure behavior were studied. Aspect ratios were determined from crack compliance data and the surface crack length data, collected continuously during the fatigue tests. The variations in compliance, surface length and the calculated aspect ratios as a function of crack growth were found to be sensitive to the size of the surface crack relative to the grain size of the material. Fractographic analyses of fracture surfaces were performed to correlate the growth behavior to fracture micro-mechanisms and crack aspect ratios. It is shown that when surface cracks exhibit crack shape variations, conventional methods of calculation of small-crack data, performed with an assumption of a/c=1, can result in errors in DK calculation and hence scatter in crack growth data. As an alternative, the basis for the correlation of small crack data in terms of compliance is explored. The closure behavior of small cracks and its role in explaining the relatively faster growth rates of surface cracks are also discussed.

3:55 pm INVITED

FATIGUE CRACK GROWTH--A MODELING PERSPECTIVE: H. Sehitoglu, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801

The fatigue crack advance in metallic materials at the micro-scale occurs as a result of localized plastic flow. Depending on the orientation of the crack plane with respect to the crystallographic orientation, the flow bands can be inclined at different angles with respect to crack growth direction. The orientation angles decide the plastic strain magnitudes, the residual stresses and the crack opening profiles and hence the crack growth rates. Despite this background of local slip at the micro-level, the current models of fatigue crack growth have relied on a description of crack tip stress fields assuming plastic strain description independent of orientation. Consequently, a quantitative description of the local orientation effects on fatigue crack growth has not been proposed. The present work addresses this issue. In the second part of the work, the irregularity of fatigue crack surfaces on the fatigue crack advance will be considered. A model is developed that accounts for the sliding and plastic flow of asperities. During this process, the contact area, contact load changes non-linearly with separation distance between crack surfaces. This nonlinear response develops even in the presence of `pure' elastic deformations. The presentation will provide an overview of micro-contact and fatigue crack closure analysis.

4:20 pm INVITED

THE INFLUENCE OF DEFORMATION-INDUCED MARTENSITE ON FATIGUE CRACK GROWTH IN METASTABLE AUSTENITIC STEELS: J.W. Morris, Jr., Department of Materials and Minerals Engineering, University of California, Berkeley, California 94720; Z. Mei, Hewlett Packard Co., 1501 Page Mill Rd., Palo Alto, CA 94303

The research reports an investigation of the influence of the mechanically induced martensitic transformation on the fatigue crack growth rate in 304-type austenitic stainless steels. The steels 304L and 304LN were used to test the influence of composition, the testing temperatures 298K and 77K were used to study the influence of test temperature, and various load ratios were used to determine the influence of the mean stress. It was found that decreasing the mechanical stability of the austenite by changing composition or lowering temperature reduces the fatigue crack growth rate and increases the threshold stress intensity for crack growth. However, this beneficial effect diminishes as the ratio increases, even though increasing the load ratio increases the martensite transformation. Several mechanisms that may affect this phenomenon are discussed, including the perturbation of the crack-tip stress field, crack deflection, and the work hardening characteristics and relative brittleness of the transformed material. The perturbation of the stress field seems the most important; by modifying previous models we develop a quantitative analysis of the crack growth rate that provides a reasonable fit to the experimental results.

4:45 pm

A REEXAMINATION OF THE EFFECTS OF OVERLOADS AND UNDERLOADS: K. Sadananda, Code 6323 Naval Research Laboratory, Washington, D.C. 20375; K.V. Jata, WL/MLLM, WPAFB, Dayton, OH 45433; A.K. Vasudevan, Office of Naval Research, Arlington, VA 22217; B. Biner, Ames National Lab, MD #208, Ames, IA 50011

It is well known that application of overloads or underloads during crack growth results in deceleration or acceleration of crack growth. Understanding and predicting these effects are necessary for fatigue life prediction of a component in service that is subjected to variable amplitudes. The retardation and acceleration effects have been, in the past, attributed to crack closure. Since our recent analysis indicated that crack closure effects are negligible, we provide an alternative explanation to account for the observed effects.

5:05 pm INVITED

CHARACTERIZATION OF FATIGUE CRACK INITIATION FROM CORROSION PITS IN Al 2024 -T3: S.I. Rokhlin, H. Nagy, Dept. of Industrial Systems & Engineering, The Ohio State University, Columbus, OH 43210

It is well known that corrosion pitting has strong effect on fatigue life of Al alloys. The statistical and empirical analyses have shown deviations from the experimental results especially at small pit sizes. This is mainly due to the poor understanding of the crack initiation from the pits. In the current work, fatigue crack initiation from actual corrosion and simulated pits has been studied. The effect of pit size was investigated using direct fractographic observations and nondestructive measurements. Ultrasonic, acoustic emission and micro-radiographic techniques were used to monitor crack initiation and propagation. The results were analyzed using fracture mechanical models including those for small cracks.

5:30 pm

ROLE OF MICROCRACKS IN HIGH CYCLE FATIGUE DAMAGE OF AN AL-Si COMPOSITE: E.Y. Chen¹, L. Lawson², M. Meshii³, ¹Physical Metallurgy Laboratory, GE Corporate Research & Development, P.O. Box 8, Schenectady, NY 12301; ²226 Interstate Parkway, Bradford, PA 16701; ³Department of Materials Science and Engineering, Northwestern University, Evansotn, IL 60208

Advanced Al-SiC composites are considered potential candidates for replacing monolithic metals in high cycle fatigue (HCF) applications such as aircraft wing skins and automotive engine connecting rods. To assess chair aptitude in such instances, this study examines the role of microcracks in the HCF damage and critical crack formation process of a X2080 Al -15 vol.% SiCp composite. Microcracks are important in fatigue since their growth (or lack of growth) greatly determines fatigue strength. In the low cycle fatigue (LCF) of this composite, the microcrack regime can dominate for over 60% of the fatigue life. In HCF, this is still often the case. While microcracks may initiate within the first 10% of the life, most retard immediately afterwords and microcrack development can exceed 70% of the life. These and other topics such as microcrack growth rates, coalescence, and fatal crack formation in HCF will be discussed in comparison to those obtained under LCP conditions. These results will emphasize the implication of microcracks when designing for fatigue strength and reliability inspectability.

5:50 pm

CORROSION FATIGUE CRACK PROPAGATION IN ALUMINUM ALLOYS: X. Zheng, R. Wang, Dept. of Materials Science and Engr., Northwestern Polytechnic University, Xi'an Shaanxi Province 710072, China

Fatigue tests were carried out to measure the corrosion fatigue crack propagation (CFCP) rates of widely used aluminum alloys in 3.5% NaCl solution environment. Test results of CFCP rates of aluminum alloys were analyzed by using the following equation: da/dN = B_cf[DK-DK_cf]² where B_cf and DK_cf are defined as the CFCP coefficient and threshold, respectively. Test results and analysis show that the corrosive environment increases the value of B_cf but has no remarkable effect on the value of DK_thcf of aluminum alloys at various stress ratios. Therefore, the corrosive environment increases the CFCP rates in the intermediate region, i.e. the range of da/dN = 10^-5 -10^-3 mm/cycle, but has little effect on the CFCP rates in the near-threshold region. Test results and analysis also show that DK_thcf decreases and the CFCP rates, especially in the near-threshold region increases with increasing stress ratio. The test results of CFCP rates of aluminum alloys obtained at two loading frequencies f= 10Hz and f= 1Hz have no appreciable difference in the intermediate region. The increase of B_cf in 3.5% NaCl environment could be attributed to the change of FCP mechanism in the intermediate region, and the effects of 3.5% NaCl environment on DK_thcf and thus the CFCP rates in near-threshold region may be thought due to the crack closure and the crack tip blunting included by the corrosion of the metal at the crack tip.

HIGH TEMPERATURE DEFORMATION: General Abstract Session

Session Chair: Rajiv S. Mishra, University of California, Davis, CA

GRAIN SIZE AND TEMPERATURE DEPENDENCE OF SUPERPLASTIC DEFORMATION BEHAVIOR IN AN Al-Mg ALLOY UNDER NEAR-ISOSTRUCTURAL CONDITIONS: D.H. Bae, A.K. Ghosh, Dept. of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109

To understand superplastic deformation mechanisms, stress-strain rate behavior of a fine grain processed Al-Mg alloy has been characterized by conducting an improved multi-step strain rate change test method to preserve near isostructural conditions during the test over a temperature range of 400-550°C. The grain size of the alloy was varied in the range of 8-30µm by varying thermomechanical processing conditions. A sigmoidal relationship between stress and strain rate is observed unequivocally in each case. Activation energy (Q), stress exponent (n) and grain size exponent (p) are calculated as a function of strain rate. In high strain rate region, n is 4.55, p is 0.37 and activation energy is close to that of lattice self diffusion. These results are quantitatively in agreement with coupled dislocation glide-climb process, except for a small grain size dependence which has not been clearly identified in the literature. In low and intermediate strain rate region, using an effective stress versus strain rate relationship, n is 1.7, p is 2.07 and activation energy is somewhat higher than that of grain boundary diffusion. In this strain rate region, grain boundary sliding accommodated by the climb of dislocations within the mantle region is found to provide good agreement.

2:20 pm

HOT TORSION OF CREEP RESISTANT RAPIDLY SOLIDIFIED ALLOYS FVS 1212 AND FVS 0812 (8009): D. Shimansky, H.J. McQueen, Mechanical Engineering, Concordia University, 1455 de Maisonneuve Blvd. W., H-549-34, Montreal, Quebec H3G 1M8, Canada

Isothermal hot torsion tests were performed on the rapidly solidified (RSP) aluminum alloys FVS1212 (Al-12.4Fe-1.2V-2.3Si) and FVS 0812 8009) over the T range of 300 to 600°C and at strain rates from 0.1 to 4.0 s^-1. Due to deformation heating and possible localized dynamic. recrystallization, stress peaks feature prominently in the - curves. Fracture strain increased with rising T, decreasing ; ductility is low compared to conventional Al alloys. Flow stresses decreased gradually with increasing deformation T and declining . The activation energies were much higher than those of pure Al due to the high volume fraction of very fine nearly spherical Al₁₃(Fe,V)₃Si dispersoids which present major obstacles to dislocation motion and which are very resistant to coalescence. In the (sinh)ⁿ function, there is a linear correlation between coefficient and slope s in the Arrhenius plots and an inverse correlation between n and ; as a net result of these relations activation energy is independent of for which the optimum value is 0.02 MPa^-1. Comparisons are made to high T deformation of other RSP alloys.

2:40 pm

APPLICATION OF SURFACE DISPLACEMENT MAPPING TO DUCTILITY ISSUES IN -TiAl BASED ALLOYS: N.E. Biery, M. DeGraef, T.M. Pollock, Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15213-3890

Gamma TiAl-based alloys are good candidates to replace nickel-based alloys in some turbine engine components. However, the constraints that should be imposed on the design of turbine components to account for the lower ductility of these materials are not completely clear. One of the major concerns is the ability of these alloys to blunt stress concentrators through plastic deformation. To study this behavior we have developed a fast and inexpensive technique to measure surface strains. We will show that strain contours at the root of sharp notches in -based alloys agree well with FE solutions if the local microstructure is taken into account. The local strain at the notch root is considerably higher than that obtained in bulk tensile tests, suggesting that smooth bar testing underestimates the ability of these alloys to blunt stress concentrators.

3:00 pm BREAK

3:10 pm

MECHANICAL BEHAVIOR OF DEFENSE WASTE PROCESSING FACILITY SURROGATE GLASS AT ELEVATED TEMPERATURES: Brian M. Powers, Eric M. Taleff, University of Texas at Austin, Austin, TX 78712-1085

An experimental study of high-temperature mechanical properties was conducted on Defense Waste Processing Facility (DWPF) surrogate glass. DWPF glass is currently used to vitrify nuclear wastes for safe disposition. The DWPF surrogate glass used in this investigation differs from DWPF glass only in the use of nonradioactive materials as surrogates for radioactive waste products. The disposal process involves pouring molten DWPF glass, during which pour instabilities cause void formation. These voids result in stress concentrations as the molten glass hardens, often leading to crack formation. The mechanical properties of DWPF surrogate glass have been studied experimentally in order to better understand the fracture process. Mechanical tests have been conducted in a range of temperatures, up to 1050°C, through which the viscoelastic behavior of the DWPF surrogate glass varies greatly. The data accumulated are expected to lead to better mechanical integrity in future DWPF glass castings, and thus safer disposition of nuclear wastes.

3:30 pm

FRACTURE TOUGHNESS OF COPPER-BASE ALLOYS FOR FUSION ENERGY APPLICATIONS: D.J. Alexander, S.J. Zinkle, A.F. Rowcliffe, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6151

The fracture toughness of several high-strength copper alloys was measured from room temperature to 250°C to determine their suitability for structural applications in fusion energy applications. Oxide-dispersion strengthened copper alloys and precipitation-hardened copper-nickel-beryllium alloys showed a significant reduction in toughness at elevated temperatures (250°C). This decrease in toughness was much larger than would be expected from the relatively modest changes in the tensile properties. However, copper-chromium-zirconium alloys strengthened by precipitation showed only a small decrease in toughness at the higher temperatures. The embrittled alloys showed a transition in fracture mode from transgranular microvoid coalescence at room temperature to intergranular with localized ductility at high temperatures. The Cu-Cr-Zr alloy maintained the ductile microvoid coalescence failure mode at all test temperatures.

MICROSTRUCTURE EVOLUTION, CHARACTERIZATION, AND MODELING: Session VI: Solid-Solid Systems III

Session Chair: R.D. Doherty, Department of Materials Engineering, Drexel University, Philadelphia, PA 19106

INTERFACE MIGRATION IN STRESSED SYSTEMS FAR FROM EQUILIBRIUM: William C. Johnson, Department of Materials Science and Engineering, Thornton Hall, University of Virginia, Charlottesville, VA 22903-2442

Both misfit strains and applied stresses can influence the migration of interphase interfaces during diffusional phase transformations through the dependence of the interfacial compositions (boundary condition for diffusion) on stress. If the interface is unable to achieve local thermodynamic equilibrium, significant changes in the motion of the interface and composition profiles, in addition to those induced by stress, are possible. Numerical calculations based on a finite-difference scheme are presented to show the effects of stress and deviation from local equilibrium on interface migration owing to diffusion in ternary alloys. This work is supported by NSF under Grant DMR-9496133.

2:35 pm

PARTICLE LIMITED GRAIN GROWTH: EXPERIMENTS AND SIMULATIONS IN THREE DIMENSIONS: Roger D. Doherty, Kang Li, Kishore T. Kashyap, and Le Chun Chen, Dept. of Materials Engineering Drexel University; Philadelphia, PA 19104; Michael P. Anderson, Exxon Research and Engineering Company, Route 22 East, Clinton Township, Annandale, NJ 08801

Earlier simulations of the particle limited grain size in normal grain growth in three dimensions have shown a much smaller size, R=4r/f^1/3, than the standard Zener model's prediction of R=4r/3f. The difference appears to arise from the ability of particles to remove grain curvature unlike the standard model's balancing of the pressure for grain growth, 2g/R, against the Zener drag of fg/r. R and r are mean grain and particle radii, f the volume fraction of particles, and g is the grain boundary energy. Experiments to test this result at low values of f in an Al-Fe alloy showed firstly a strong dependence on starting grain size and, at small starting grain sizes, a ready transition to abnormal grain growth. These experiments suggested a need for simulation exploring the role of initial grain size and conditions giving rise to abnormal grain growth. On the basis of these experiments and the supporting simulations a new model for particle limited grain growth and particle activated abnormal grain growth are proposed.

3:05 pm INVITED

A STOCHASTIC THEORY OF GRAIN GROWTH IN TWO DIMENSIONS: C.S. Pande, R.A. Masumura, Materials Science and Technology Division, Naval Research Laboratory, Washington, DC 20375

A stochastic method of modeling two-dimensional grain growth is proposed. It is shown that von Neumann's law leads to a Fokker-Planck equation for the grain size distribution, which is then solved in a series form. The solution is seen to give a Rayleigh or Hillert distribution in the two limits corresponding to the random or deterministic component for boundary motion being dominant, respectively. The predictions of the theory are shown to be in good agreement with experimental and simulation results.

3:35 pm

SECOND PHASE INHIBITED GRAIN GROWTH: B.R. Patterson, S. Basu, Department of Materials and Mechanical Engineering, University of Alabama at Birmingham, Birmingham, AL 35294-4461

Pore inhibited grain growth during sintering of both nickel-doped and undoped tungsten powder has been studied using stereological measurements. In particular, the effects of doping level and temperature have been examined with respect to the amount of grain boundary-pore contact and the tendency for pore mobility. The results of these studies are compared to second phase grain growth inhibition in other materials.

4:05 pm

CHARACTERIZATION OF DISLOCATION WALL SPACING DISTRIBUTIONS: A. Godfrey, D.A. Hughes, Center for Materials and Applied Mechanics, Sandia National Laboratories, Livermore, CA 94550

Quantitative characterization of dislocation boundary spacing distributions as a function of increasing deformation is desirable for the further understanding of microstructural evolution and as input to constitutive models. These measurements are complicated by the restriction in the TEM to viewing only the traces of dislocation boundaries which can have different morphologies, e.g. sheet-like or round, depending on boundary type. To understand the consequences of this limitation, a model has been developed whereby a volume containing a series of planes of variable spacing/orientation can be constructed. Sections may then taken through this volume and various stereological techniques applied to determine the optimum method for characterizing the spacing distributions (as opposed to just determining their average values). The fundamental question of how to define a spacing for two non-parallel planes is discussed with respect to the typically observed features of deformation microstructures. This method is then applied to experimentally observed dislocation microstructures and the results discussed.

4:35 pm

SOLID STATE DENDRITIC GRAIN BOUNDARY PRECIPITATES: M.V. Kral, G. Spanos, Naval Research Laboratory, Code 6324, Physical Metallurgy Branch, Washington, DC 20375-5343

An isothermally transformed Fe-1.34%C-13.1% Mn alloy was deeply etched and examined by Scanning Electron Microscopy (SEM) to reveal cementite precipitate morphologies. Experimental observations of grain boundary cementite precipitates indicate that grain boundary films in this alloy commonly develop by growth of fern-like or dendritic precipitates within the grain boundary that eventually impinge and/or overlap. Alternatively, growth of cementite sideplates and laths does not appear to occur dendritically. Further study of these precipitates by Transmission Electron Microscopy (TEM) showed that many of the dendritic cementite precipitates are monolithic crystals, suggesting that their growth occurs by diffusional instabilities, while others may have formed by sympathetic nucleation. A discussion of the mechanisms responsible as well as implications regarding their generality in other alloy systems will be included.

MICROSTRUCTURE PROPERTY RELATIONSHIPS IN / TITANIUM ALLOYS: Session III: Mechanical Properties

Session Chair: Neville Moody, Sandia National Labs, P.O. Box 969, Division 8312, Livermore, CA 94551-0969

RELATIONS BETWEEN MIICROSTRUCTURE AND PROPERTIES IN TITANIUM ALLOYS: Anthony W. Thompson, ESRC, University of California, Berkeley, CA 94720

The role of microstructural variables in controlling mechanical properties of alpha-beta titanium alloys has been well explored. For tensile, toughness and fatigue properties, there are a number of generalizations which appear appropriate; the same is true to a lesser extent for environmental properties. However, there are several unsolved problems in these relationships, which can largely be characterized as examples of the "two ductile phases" problem. The current status of these problems is the basis for recommendations of additional research.

2:40 pm

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TITANIUM CASTINGS: G. Wegmann, J. Albrecht, G. Lütjering, Technical University Hamburg-Harburg, D-21071 Hamburg, GERMANY; K.D. Folkers and Ch. Liesner, Titan-Aluminum-FeinguSS GmbH, D-59909 Bestwig, GERMANY

The demand for the use of titanium for aircraft applications has been increased over the past years by the necessity for weight reductions for improved fuel economy. Due to the high cost of titanium, the use of investment cast parts receives an increasing interest due to the large cost saving potential of this technology in manufacturing the part. Contrary to forged material, however, the possibilities to optimize the properties via microstructural control are limited for cast parts to purely thermal treatments. This paper discusses the possibilities to modify the lamellar structure of cast Ti-alloys by heat-treatment in the + phase field, followed by controlled cooling. By this treatment, the -phase can be hardened by the precipitation of very fine -lamellae, resulting in a so-called "bi-lamellar" microstructure. It was shown that bi-lamellar microstructures have improved yield stress, creep and fatigue resistance as compared to conventional lamellar (as-cast) microstructures. The influence of the processing parameters, i.e. +-annealing temperature and subsequent cooling rate as well as the cooling rate during casting, on the mechanical properties of cast Ti-6Al-4V and Ti-6242, will be discussed on the basis of the microstructural parameters.

3:00 pm

PROPERTIES OF ALPHA-BETA TITANIUM ALLOYS AT THE SUBMICRON SCALE: N.R. Moody, C. Cadden, Sandia National Laboratories, Livermore, CA 94550; A.W. Thompson, University of California, Berkeley, CA 94720; D. Allen, University of Florida, Gainesville, FL 32611

The full potential for the use of alpha-beta titanium alloys is limited by our understanding of how the properties of each microstructural phase contribute to the properties of the alloy. We are therefore using nanoindentation to measure the elastic modulus and hardness of individual phases in alpha-beta titanium alloys at the submicron scale. This technique has been used to study the variation in elastic modulus and hardness across a weld zone of a titanium aluminide alloy. It has also been used to study the effects of hydrogen on the elastic modulus and hardness of individual phases in another alloy of similar composition. These results will be discussed in this presentation and compared with properties of the parent alloys to help define the contributions of constituent phases to alloy properties. This work supported by U.S. DOE Contract DE-AC04-94AL85000.

3:20 pm

RHEOLOGICAL ASPECTS OF THE SUPERPLASTICITY OF Ti-6Al-4V: Michael L. Meier, Amiya K. Mukherjee, Department of Chemical and Materials Science, University of California at Davis, Davis, CA 95616

The full potential for the use of alpha-beta titanium alloys is limited by our understanding of how the properties of each microstructural phase contribute to the properties of the alloy. We are therefore using nanoindentation to measure the elastic modulus and hardness of individual phases in alpha-beta titanium alloys at the submicron scale. This technique has been used to study the variation in elastic modulus and hardness across a weld zone of a titanium aluminide alloy. It has also been used to study the effects of hydrogen on the elastic modulus and hardness of individual phases in another alloy of similar composition. These results will be discussed in this presentation and compared with properties of the parent alloys to help define the contributions of constituent phases to alloy properties. This work supported by U.S. DOE Contract DE-AC04-94AL85000.

3:40 pm

THE FIBRE-MATRIX INTERFACIAL REGION OF TITANIUM MATRIX COMPOSITES AND ITS EFFECT ON FATIGUE CRACK GROWTH RESISTANCE: S.V. Sweby, P. Bowen, and M. Strangwood, The University of Birmingham, School of Metallurgy and Materials, Elms Road, Edgbaston, Birmingham,B15 2TT, UK

Ti-6Al-4V (wt.%) continuously reinforced with SM1140+ or SM1120 fibers has been characterized in the as-processed condition and after heat treatment above the -transus. The nature of the interface between matrix and fibers, as well as the matrix microstructure, has been characterized by SEM and TEM. These results have been related to fiber push-out tests for material in both conditions, which, with single fiber strength results, have been used to rationalize variations in fatigue crack growth resistance as a function of heat treatment.

NON-LINEAR EFFECTS IN MATERIALS SCIENCE: Session IV: Surfaces and Interfaces

Session Chair: F.G. Yost, Sandia National Laboratories, Albuquerque, NM 87185

STICK SLIP RELATIVE MOTION BETWEEN CONTACT BODIES: James C.M. Li, Materials Science Program, Department of Mechanical Engineering, University of Rochester, Rochester, NY 14627

The stick/slip phenomena are described and its source analyzed. High speed data acquisition for velocity variations during slip and atomic force microscopy observations for the impression profile during stick provide sufficient clues to reveal the origin of non-uniform relative motion. The roles played by the mechanical properties of the materials and stiffness of the loading system are emphasized. Quantitative understanding of the of horizontal and normal force variations and the time and distance periods between sticking is possible in some situations such as scratching. Some environmental effects are described and analyzed also. This work was supported by NSF through DMR9623808 monitored by Dr. Bruce MacDonald.

2:30 pm INVITED

ROUGH SURFACES GENERATED BY NON-LINEAR TRANSPORT: J. Adin Mann, Jr.*, Wojbor A. Woyczynski, Case Western Reserve University, Dept. of Chemical Engineering* and Department of Statistics, Cleveland, OH 44106

Kardar, Parisi and Zhang (KPZ) in their 1986 paper provided a non-linear governing equation for the instability of growth of a solid-fluid interface. Their results and contributions from other authors have provided a theory for understanding interfacial instability in many situations ranging from the melt/solid interface to the roughness of the ocean floor and includes chemical vapor deposition (CVD) processing. The physical reasoning that KPZ use in interpreting the constitutive coefficients that appear in their govern equation is flawed. We show that the KPZ equation can be obtained for CVD by a mass balance at the interface that includes chemical reactions at the surface that occur as part of the growth mechanism. The assumption of an overly simplified reaction mechanism and that the flux from the vapor phase is ballistic leads to the KPZ equation where now each constitutive coefficient is well defined. Moreover, extensions to KPZ become obvious and a rich set of non-linear governing equations result. KPZ assumes that the surface transport is pure diffusion. However, under certain conditions we know that trapping can occur in such a way that the diffusion mechanism must be generalized. We are exploring an approach using a "fractional order" Laplacian operator to augment the Laplacian operator of the diffusion reaction equation. The physical ideas of this extension will be discussed.

3:00 pm INVITED

THE MORPHOLOGICAL STABILITY OF ALLOY THIN FILMS: J.E. Guyer, P.W. Voorhees, Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208

We consider the stability of a growing alloy thin film to both changes in the morphology of the film surface and the compositional uniformity of the film. The instability is a result of the elastic stress generated by the lattice mismatch between the film and substrate, and the composition modulations in the film. The composition modulations are due to the morphological instability in the surface of the growing film; there is no bulk diffusion in the film. We find that for some lattice-matched m-v alloys the composition modulations themselves can induce a morphological instability in the film surface. Due to the compositionally generated stresses, the morphological instability in the film surface can be in the form of traveling surface waves. The relationship between composition modulations induced by the morphological instability of the film surface and the more classical explanation for these composition modulations, spinodal decomposition, will also be discussed. This work was supported by the NSF-MSREC under award DMR9120521.

3:30 pm BREAK

3:40 pm INVITED

ATOMIC MODELING OF CRITICAL PROCESSES IN THE NON-LINEAR CRACK TIP ZONE OF CRYSTALLINE SOLIDS: R.G. Hoagland, School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920

Because of the analytical intractabilities posed by the non-linear properties of the cores of crack tips and dislocations, elasticity offers little help in describing the details of the energetics of crack extension and dislocation emission. Atomistic modeling, however, provides a useful alternative for studying some of the key features of crack tip behavior and the factors that distinguish a brittle material from an intrinsically tough material. In this paper, we describe some of the non-linear properties within crack tip and dislocation cores that derive from such calculations. We also examine the energetics of crack tip evolution and show that the Peierls-like criterion posed by Rice et al. consistently predicts the behaviors of atomic models of crack tips in terms of defining the competition between dislocation emission and crack extension. This work was supported by the Division of Materials Science, Office of Basic Energy Sciences, U.S. Department of Energy through Grant DEFG-6-87ER45287.

4:10 pm

OSTWALD RIPENING IN ELASTICALLY STRESSED SOLIDS: N. Akaiwa, M.E. Thompson, P.W. Voorhees, Department of Materials Science and Engineering, Northwestern University Evanston, IL 60208; National Research Institute for Metals, Japan

A two-phase microstructure can evolve in time even when the phase compositions and volume fractions are nearly at their equilibrium values. Such an evolution process is called Ostwald ripening, or coarsening. For stress-free systems, the evolution of the microstructure is driven by a decrease in the total interfacial energy of the two-phase mixture. In a wide array of two-phase solid systems, however, the ripening process is frequently accompanied by elastic stress. In these cases, ripening is driven by a decrease in the sum of the elastic and interfacial energies. This change can give rise to qualitatively new phenomena, such as inverse Ostwald ripening wherein small particles grow at the expense of large particles, large-scale particle migration and particle shape bifurcations. A discussion of these phenomena as well as the results of recent calculations on the kinetics of ripening using large numbers of elastically and diffusionally interacting particles will be presented. This work was supported by the NSF under award DMR-9322687.

4:40 pm

NON-LINEAR SOLUTE SEGREGATION DURING IRRADIATION: E.P. Simonen, S.M. Bruemmer, Pacific Northwest National Laboratory, P.O. Box 999 MS P8-5, Richland, WA 99352

Radiation-induced segregation of solute to grain boundaries occurs during irradiation of alloys because of discriminating interaction between point-defect flow and solute. Segregation is predicted from simultaneous solutions to transport equations for point defects and solute. Inverse-Kirkendall diffusion in irradiated stainless steel is an example of nonlinear segregation phenomena. Solutions to five rate equations, two for defects and three for solute species, are solved. Stable solutions result and are in agreement with measured composition profiles near irradiated grain boundaries. Point defect concentrations are controlled by production rates, mutual annihilation rates and annihilation by migration to grain boundaries. The mathematical formalism follows that of the well-known preditor-prey problem. This work was supported by the Materials Sciences Branch, BES,U.S. Department of Energy, under Contract DE-AC06-76RLO 1830.

5:00 pm

NON-LINEAR CORROSION INTERACTIONS AT TIPS OF STRESS CORROSION CRACKS: E.P. Simonen, C.H. Windisch and R.H. Jones, Pacific Northwest National Laboratory, P.O. Box 999 MS P8-15, Richland, WA 99352

Aqueous corrosion of metals under stress can result in stress corrosion cracking Corrosion at the crack tip is influenced by production of corrosion products chemical reactions and transport between the occluded crack environment and the bulk aqueous solution. The production and destruction of aqueous species are calculated by numerically solving a set of non-linear equations as a function of time and position inside a crack. Stable solutions are obtained that provide estimates of crack tip chemistries during stress corrosion cracking. Multiple ionic species are accounted for as well as distinct reactions at crack walls and crack tips with unique material chemistry. Crack chemistries in Ni, Fe and stainless steel have been modeled and compared with measured cracking responses. This work was supported by the Materials Sciences Branch, BES, U.S. Department of Energy, under Contract DE-AC06-76RLO 1830.

5:20 pm

APPLICATION OF FUZZY CLUSTERING IN THE ANALYSIS OF MISORIENTATION CRYSTALLOGRAPHY: Mary Ann Egan*, P. Krishnamoorthy*, Krishna Rajan, Departments of Computer Science* and Materials Science, Renssalaer Polytechnic Institute, Troy, NY 12180-590

The clustering of crystallographic orientation data has long been studied especially in the field of texture analysis. More recently, the concept of mapping misorientation information has been developed using the fonnalism of Rodrigues-Frank vectors. In this presentation we utilize a non-linear analysis termed fuzzy clustering to study misorientation data in Rodrigues-Frank space. Fuzzy clustering is a clustering mechanism that allows for ambiguity in the data and provides the corresponding degrees of support. In this presentation, we will describe fuzzy clustering techniques and the usefulness of this method. Some results are presented in the framework of determining a probability distribution function in Rodrigues-Frank space.

P/M CURRENT RESEARCH AND INDUSTRIAL PRACTICES: Session II: Industrial Practices

Session Chair: Gregg M. Janowski, Department of Materials and Mechanical Engineering, University of Alabama-Birmingham, AL35294-4461

INDUSTRIAL APPLICATIONS OF IN SITU AND DENSIFICATION OF ADVANCED MATERIALS: F.D.S. Marquis, J.A. Puszynski, College of Chemical, Physical, and Materials Science and Engineering, South Dakota School of Mines, Rapid City, SD 57701

The in situ synthesis and densification of advanced materials is of considerable scientific and technological interest because it makes possible the formation of near net shape parts of ceramics, intermetallic, composites, and other low defect tolerant materials. These advanced materials have shown a significant potential for future economic impact in diverse applications in military, electronic, aerospace, chemical, and automobile industries. Recent advances on in situ processing techniques, such as Self Propagating High Temperature Synthesis (SHS), and Shock Induced Synthesis (SIS) in single- and multi-phase materials, will be presented. SHS has received considerable attention because of very short synthesis times, simplicity, energy efficiency, and versatility of combustion reactors. SIS offers considerable advantages, such as extremely short-term thermal exposures, with consequent minimization of oxidation and retention of extremely fine metastable microstructures. Current and potential industrial applications will be discussed.

2:30 pm INVITED

POWDERS FOR MIM THROUGH MECHANICAL ALLOYING: J.W. Newkirk, *G.M. Brasel, and J.A. Sago, Department of Metallurgical Engineering University of Missouri-Rolla, Rolla MO; *Megamet Industries, Inc., St. Charles, MO

Mechanical alloying (MA) produces unique powders which have many drawbacks for conventional powder metallurgy. However, these drawbacks for PM are potential advantages for MIM. The powders can also exhibit elevated levels of impurities, especially oxygen. This study benchmarks for a MIM feedstock and MIM parts the MA powder characteristics, rheological characteristics, and mechanical test results. The contamination of MA powders during milling is examined to see if reduction methods add complexity and cost to debinding and sintering. MA parameters of milling time, control agent, and powder reactivity are discussed to arrive at a MIM processing window for MA powders.

3:00 pm INVITED

SYNTHESIS OF SiC POWDER THROUGH CARBOTHERMIC REDUCTION ENHANCED BY HIGH ENERGY BALL MILLING: Leon L. Shaw, Rui-Ming Ren and Zhen-Guo Yang, Department of Metallurgy and Materials Engineering, University of Connecticut, Storrs, CT 06269

Silicon carbide (SiC), an important material for engineering applications, can be produced through many approaches. However, most of the industrial SiC powders are manufactured in the Acheson process by carbothermic reduction of SiO₂ at temperatures ranging from 2000 to 23000°C for 30 hours. The overall chemical reaction for the Acheson process is: SiO₂+3 C = SiC +2 CO () and the commercial product from this process has a large grain size and is contaminated with oxygen. In this study, the possibility of enhancing the aforementioned carbothermic reduction by high energy ball milling has been explored. Effects of ball milling conditions including the milling speed, the mill temperature, the milling time, and the ball-to-powder weight ratio on the formation of nano- and micron-sized SiC powders have been investigated. It has been found that the kinetics of the carbothermic reduction can be enhanced dramatically by high energy ball milling. With proper milling conditions 100% conversion of SiO₂ to SiC can be achieved at temperatures as low as 14000°C, a dramatic improvement over the current Acheson process. Mechanisms for the enhancement of the carbothermic reduction by high energy ball milling will be discussed in terms of thermodynamics and reaction mechanisms. A patent disclosure resulting from this investigation is currently under preparation.

3:15 pm

ADVANCES IN NONDESTRUCTIVE EVALUATION FOR THE PLASTIC SHAPING OF CERAMIC POWDER SUSPENSIONS: C.H. Schilling and J.N. Gray, Center for Nondestructive Evaluation, Department of Materials Science and Engineering, and Department of Mechanical Engineering, Iowa State University, Ames, IA

Although significant, recent advances have been made in the synthesis of ultrafine, nonclay ceramic powders, experience indicates that, in comparison to clays, these powders require much more stringent control of packing-structure heterogeneities in order to prevent drying- and sintering cracks. NDE methods, coupled with fluid- and soil-mechanics modelling, are needed that provide a better fundamental understanding of the role of critical processing variables on the evolution of green microstructures forming from a colloidal suspension. This paper reviews recent advances in this area, with an emphasis on ultrasonic- and radiographic studies linking green-microstructure development with colloidal processing variables.

3:40 pm

INDUSTRIAL POWDER MAKING AND ATOMIZATION RESEARCH ADVANCES: I.E. Anderson, M.G. Osborne, and J. Ting, Ames Laboratory, Ames, Iowa

When compared to chemical precipitation, mechanical milling, and electrolytic deposition, for example, melt atomization emerges as the most versatile approach available for metal powder production. Melt atomization permits the direct manufacture of powders of pure metals and alloys with effective control of many physical and metallurgical properties of the powder. The diversity of approaches possible for liquid metal atomization will be presented in terms of the physical principals and of several commercially important examples. The three general types of atomization processes; 1) two fluid atomization, 2) centrifugal atomization, and 3) single fluid atomization, will be included in this review. Three of the current research directions in the field of liquid metal atomization will also be summarized, including refinement of powder size, control of powder size distribution, and processing of reactive and high temperature materials. Support from DOE-BES-MSD under contract no. W-7405-Eng-82 is gratefully acknowledged.

4:05 pm INVITED

RECENT STUDIES ON POWDER PROCESSED NITROGENATED STAINLESS STEEL: Frank Biancaniello, NIST, Gaithersburg, MD 20899

Nitrogenated steel was invented at the Carnegie Institute in 1912. Over the years nitrogenated stainless steels have found a nitch in several applications but never achieved their full potential due to standard casting defects (inclusions, segregation, etc.) and problems in maintaining consistent nitrogen content. In this study, inert gas atomized and HIP consolidated high nitrogen stainless steels were tested and found to exhibit high strength, hardness, ductility, and corrosion resistance simultaneously. Processing conditions and property enhancements will be discussed.

4:30 pm INVITED

ARE P/M STAINLESS STEELS, "STAINLESS": Kenneth Moyer, Magna-Tech P/M Labs, 4 Greenbriar Lane, Cinnaminson, NJ 08077

Recently there has ensued considerable discussion pertaining to the nature of the "stainless" of P/M stainless steels. Specifications, based on appearance of the surface after immersion in various corrosive solutions, are being considered as a measure of the corrosiveness of the various P/M grades. Most of the criteria being considered are based on the quantity of rust on the surface of the parts. Certainly no one is going to purchase stainless steels that contain rust on the surface after a given period of time. However, is the appearance of rust the true measure of the corrosiveness of the material, or is the rust appearance masking the true corrosiveness of the material? Some evidence is available that suggests that the corrosion resistance of P/M stainless steels are not too different from wrought stainless steels, if measured on a weight change basis. The purpose of this work is to demonstrate that P/M stainless steels are really corrosion resistant if they are sintered correctly. It will be demonstrated that some alloys will not show evidence of surface rusting, but on a weight change basis, they are really less corrosion resistant. A hypothesis will be presented that explains why traditional alloys appear to be corroded whereas other modifications of these alloys appear otherwise. The conclusion is that these modified alloys will not indicate that hey are corroded, however, in fact, the modified alloys will have sustained a greater weight change than the conventional alloys.

THERMOMECHANICAL PROCESSING AND MECHANICAL PROPERTIES OF HYPEREUTECTOID STEELS AND CAST IRONS: Session II: Properties and Performance of Ultrahigh-Carbon Steels and Cast Irons

Session Chair: George Mayer, ARO, Washington, D.C.; Bruce Bramfitt, Bethlehem Steel Corporation, Bethlehem, PA 19016

RAILROAD RAILS AND FROGS: APPLICATIONS FOR HYPEREUTECTOID STEELS: Roger K. Steele, Metallurgical Consulting Services Inc., Vernon, CT 06066

Currently, railroad rails and some switch frogs are manufactured from pearlitic eutectoid carbon steel. This steel can be hardened by on-line heat treatment in large tonnages to 400 BHN at little cost beyond that of basic rail. Although hypereutectoid steels can match the strength and ductility properties of the eutectoid steels (both pearlitic and bainitic), it is not clear that large quantities can be manufactured at high strength levels as economically as can the eutectoid steels. Also the issue of welding into very long rail strings is problematic. However, switch frogs which currently can utilize a cast austenitic manganese steel insert may be a potential application for hypereutectoid steels because no connective welding is necessary. The high impact and surface traction that can occur during wheel traversal of the flangeway gap has focused attention on high strength frog materials such as weld deposited maraging steels as well as low carbon, high strength bainitic steels The extraordinarily high strength coupled with modest ductility achievable with tempered martensitic hypereutectoid steels is attractive for this type of application.

1:50 pm

APPLICATION OF HYPEREUTECTOID STEEL FOR RAILS IN HEAVY HAUL TRACKS: Masaharu Ueda, Nippon Steel Corp., Kitakyushu-City, Japan

Currently, railroad rails and some switch frogs are manufactured from pearlitic eutectoid carbon steel. This steel can be hardened by on-line heat treatment in large tonnages to 400 BHN at little cost beyond that of basic rail. Although hypereutectoid steels can match the strength and ductility properties of the eutectoid steels (both pearlitic and bainitic), it is not clear that large quantities can be manufactured at high strength levels as economically as can the eutectoid steels. Also the issue of welding into very long rail strings is problematic. However, switch frogs which currently can utilize a cast austenitic manganese steel insert may be a potential application for hypereutectoid steels because no connective welding is necessary. The high impact and surface traction that can occur during wheel traversal of the flangeway gap has focused attention on high strength frog materials such as weld deposited maraging steels as well as low carbon, high strength bainitic steels The extraordinarily high strength coupled with modest ductility achievable with tempered martensitic hypereutectoid steels is attractive for this type of application.

2:10 pm

MECHANICAL BEHAVIOR OF ULTRAHIGH STRENGTH, ULTRAHIGH-CARBON STEEL WIRE AND ROD: Donald R. Lesuer, Chol K. Syn, Lawrence Livermore National Laboratory, P.O. Box 808, L-342, Livermore, CA 94550; Oleg D. Sherby, Stanford University, Dept. Materials Science & Eng., Stanford, CA 94305

Ultrahigh-carbon steels (UHCSs) can achieve very high strengths in wire or rod form. These high strengths result from the mechanical work introduced during wire and rod processing. These strengths have been observed to increase in a linear manner with carbon content. In wire form, tensile strengths greater than 5000 MPa are predicted for UHCS containing 1.8%C. In this presentation we will discuss the influence of processing (including rapid transformation processes used during wire patenting) and microstructure on the mechanical behavior of UHCS wire. The tensile properties of as-extruded rods are described as a function of extrusion temperature and composition. For spheroidized steels, yield and ultimate strength are a function of grain size, interparticle spacing and particle size. For pearlitic steels, yield and ultimate strength were found to be functions of colony size, carbide size and plate spacing and orientation. Alloying additions (such as C, Cr, Si, Al and Co) can influence the effect of processing on these microstructural features. For both spheroidized and pearlitic steels, fracture was found to be a function of carbide size and composition.

2:30 pm

MECHANICAL BEHAVIOR OF A HYPEREUTECTOID STEEL EXHIBITING A DAMASK SURFACE PATTERN: Eric M. Taleff¹, Chol K. Syn², Donald R. Lesuer², Oleg D. Sherby³, ¹The University of Texas at Austin, Austin, Texas, ²Lawrence Livermore National Laboratory, Livermore, CA; ³Stanford Univ., Stanford, CA; D.K. Kim, The Goodyear Tire & Rubber Co., Akron Ohio 44309; W. Daniel Whittenberger, NASA-Lewis Research Center, Cleveland OH 44135

Hypereutectoid steel plates have been produced by industrial processing methods, including hot and warm rolling, to exhibit a surface Damask marking. This marking is similar to those exhibited by the famous Damascus Steels, which were also of hypereutectoid composition. The microstructure of this material contains fine spheroidized carbides and a discontinuous carbide network along former austenite grain boundaries, which give rise to the Damask pattern. Tension tests conducted at room temperature reveal tensile strengths of up to 1,100 MPa and tensile ductilities beyond 8% in this modern Damascus Steel. The rolling process used to produce the plate material results in a slight directionality in strength and ductility.

2:50 pm

MECHANICAL PROPERTIES AND STRUCTURAL SUPERPLASTICITY IN ULTRAHIGH- CARBON STEELS AND BORON ALLOY TOOL STEELS PRODUCED BY RAPID SOLIDIFICATION AND POWDER METALLURGY: G. Frommeyer, U. Giegel, H.J. Speis, J.A. Jimenez, Max Planck Institut fur Eisenforschung GmbH, Dusseldorf, Germany

Ultrahigh carbon and boron tool steels with fine grained microstructures consisting of equiaxed - -matrix grains (3 to 5 µm in size) and a fine dispersion of special carbides or borides (1 to 3 µm in size) were prepared by rapid solidification technology - argon melt atomization - and consolidated by extrusion or hot isostatic pressing. These steels exhibit structural superplasticity in the temperature regime from 800 to 1050°C depending upon their composition. The paper presents the mechanical properties and structural superplasticity of selected ultrahigh carbon alloy tool steels with carbon contents up to 2.5 mass%, 8-10 mass% vanadium, 5-10 mass% chromium and molybdenum, and ultrahigh boron alloy steels with boron contents up to 2.5 mass%, 25 mass% chromium, and 10 mass% nickel. The superior strength properties, wear and corrosion resistance favor these steels to be used for a large variety of applications. The superplastic properties enable near net shape forming of cutting tools, dies etc. with complex geometry.

3:10 pm BREAK

3:30 pm

TENSILE ELONGATION BEHAVIOR OF FINE-GRAINED Fe-C ALLOYS AT ELEVATED TEMPERATURE: Woo-Jin Kim, Hong-Ik University, Seoul, 121-791, Korea

Tensile elongation behavior of fine-grained Fe-C alloys has been investigated as a function of cementite volume fraction, degree of microstructural refinement, and the Zener-Hollomon parameter. The strain rate-stress relationships and creep strengths of Fe-C alloys with carbon contents from 1.3 to 5.25wt.%C are found to be similar when grain size is similar. Room-temperature strength, however, is found to be a function of the volume percent of cementite. Superplastic ductility of ingot-processed alloys increases with carbon content (cementite volume fraction) but starts to decrease after 2.1%C. The increase of tensile ductility from 1.3 to 2.1%C is attributed to reduction of dynamic grain growth rate with increase of fine cementite particles with carbon contents, whereas the decrease of tensile ductility above 2.1%C is due to the increase in number of coarse cementite particles and to an increase in the area of cementite/cementite grain boundaries. Superplastic ductility of Fe-C alloys with carbon contents higher than 2.1%C can be significantly enhanced when powder-processing routes are utilized instead of ingot-processing routes. Tensile elongation behavior of cementite-based alloys is revealed to be different from that of iron-based alloys when compared as a function of the Zener-Hollomon parameter.

3:50 pm

PROCESSING AND SUPERPLASTICITY OF STAINLESS STEEL CLAD ULTRAHIGH CARBON STEEL: Glenn S. Daehn¹, Oleg D. Sherby, Stanford University, Dept. Materials Science & Eng., Stanford, CA 94305; ¹The Ohio State University, Dept. of Materials Science & Eng., 116 W. 19th Ave., Columbus, OH 43210-1179

Lamination of a non-superplastic material to a superplastic one can result in a composite that behaves superplastically if the majority of the composite strength resides in the superplastic material. This concept is used in the specific case of laminating ferritic stainless steel to an ultrafine grained ultrahigh carbon steel to produce composites that can be superplastically formed at temperatures near 775°C. In this work we have developed processing routes that can: provide ultrafine grained structures in the ultrahigh carbon steel, give effective lamination, include diffusion barriers and result in composites that have been superplastically formed. It appears that with modest development this concept could be used to build large high-strength, stainless-clad structures.

4:10 pm

HYPEREUTECTOID STEELS AND CAST IRONS: Hong Li, University of Michigan, Ann Arbor, MI 48109

The research is concerned with the study of structural refinement and superplasticity in hypo-eutectic low alloy white cast irons. Three white cast irons, containing 2.2, 2.6 and 3.0%C were investigated. in order to obtain a fine grained structure, thermomechanical processing methods were used. The mechanism of carbide refinement occurring during both hot working and warm working were analyzed using optical, scanning electron and transmission electron microscopy. Mechanical tests both in tension and in compression for fine grained cast irons were performed at elevated temperatures. It was concluded that white cast iron ingots containing up to 3.0%C are capable of undergoing extensive hot forging, hot rolling and warm rolling in the range from 700°C to 1000°C. Thermomechanical processing produced a fine grained superplastic structure in cast irons. The fine-grained cast irons exhibit good superplasticity and high strain rate sensitivity in both tension and compression in the temperature range of 650-770°C. Tensile elongation to failure of 220%, 150% and 80% were obtained for 2.2%C, 2.6%C and 3.0%C cast irons, respectively.

4:30 pm

ULTRAHIGH CARBON STEEL AND DUCTILE IRON RESEARCH IN SPAIN: O.A. Ruano¹, M. Carsi¹, C. Bertrand², J. Ibanez¹, F. Penalba³, O.D. Sherby⁴, ¹Centro Nacional de Investigaciones Metalurgicas, Madrid, Spain; ²SIDENOR I+D, Vizcaya, Spain; ³INASMET, San Sebastian, Spain; ⁴Stanford University, Stanford, CA

Ultrahigh carbon steel (UHCS) and ductile iron research and development work is reported on studies at three different research and development laboratories in Spain. Emphasis has been placed on studies of the ductility and strength at high strain rates and high temperatures by means of torsion testing. These studies are to simulate the processing steps in the manufacture of contemporary steels. It is shown that UHCS is slightly weaker and as ductile as low carbon steels at high strain rates (10 to 26 s^-1) in the range of 1000 to 1200°C. The conditions under which the deleterious proeutectoid carbide network in UHCS can be avoided has been assessed. Fast cooling after hot working leads to a pearlitic structure and the room temperature hardness of pearlite is shown to be principally a function of the temperature of working and secondarily of the dilute alloy additions. Spot welding studies of a 1.5%C steel sheet have been made. A strength of 725 MPa was obtained for the welded sample after annealing at 800°C for 5 minutes and furnace cooling. Ductile iron research has lead to development of austempered ductile iron with a structure consisting of fine bainite with nodular graphite. Ultimate tensile strengths over 800 MPa with elongations to failure of over 10% elongation were obtained.