Materials Week '97: Tuesday 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 Tuesday afternoon, September 16, during Materials Week 1997. To view other programming planned for the meeting, go to the technical program contents page.

ALLOY MODELING AND DESIGN: Session IV: Physical, Chemical, and Mechanical Properties of Alloys II

Session Chair: Dr. M. Asta, Sandia National Laboratories, Livermore, CA 94551

SELECTIVE VARIANT GROWTH OF COHERENT PRECIPITATES UNDER EXTERNAL CONSTRAINTS: D.Y. Li, L.Q. Chen, Department of Materials Science and Engineering, The Pennsylvania State University, State College, PA 16802

An anisotropic distribution of coherent precipitate variants may result in anisotropic behavior of multiphase materials. The anisotropic distribution or the selective variant growth of the coherent precipitate could be obtained by constrained aging. The present study demonstrates that the selective variant growth results from the anisotropic coupling between the applied stress/strain and the local strain caused by the lattice mismatch. Under a certain constraint, the coupling energy is different for differently oriented precipitate variants. As a result, the variant growth becomes selective and this, in turn, modifies the material's performance. Selective variant growth of Ti₁₁Ni₁₄ precipitates in Ti-51.5 at %Ni alloy was investigated as a particular example. It was demonstrated that the selective variant growth of Ti₁₁Ni₁₄ precipitate can be predicted based on the symmetry analysis and the elastic energy calculation. The research was conducted in combination with TEM analysis and computer simulation. A positive corroboration between the theoretical analysis and the experiment was found.

2:30 pm

MECHANICAL BEHAVIOR ANALYSIS OF PST TiAl INTERMETALLIC COMPOUNDS WITH FINITE ELEMENT METHODS: Leilei Zhang, David G. Atteridge, Oregon Graduate Institute of Science and Technology, Department of Materials Science and Engineering, P.O.Box 91000, Portland, OR 97291-1000

Finite element methods was used to simulate the mechanical behavior of PST TiAl intermetallic Compound during tensile test under room temperature. Different lamellar orientation to the axis of the tensile stress was studied. The relationship between the lamellar orientation and mechanical properties agree with the experimental results. With the experimental results of PST TiAl under high temperature, grain rotation mechanism was proposed for the deformation characteristic of TiAl intermetallic compounds. It will help people to understand the deformation characteristic of TiAl and improve its room temperature ductility. That will improve the wide use of TiAl intermetallic compounds.

3:00 pm

MICROSTRUCTURAL DESIGN OF HIGH STRENGTH ALUMINUM ALLOYS: J.F. Nie, B.C. Mudle, Department of Materials Engineering, Monash University, Clayton, Victoria 3168, Australia

A common feature of high strength (_y > 450 Mpa) and ultra-high strength (_y > 700 Mpa) aluminum alloys is that maximum strength and hardness are achieved through precipitation hardening involving predominantly plate-shaped precipitates formed on rational habit planes {100} and {111} in an -Al matrix. However, there is currently little detailed quantitative understanding of the strengthening mechanisms operative and the relationship between the form and distribution of the strengthening precipitate phases and the observed tensile strength. The development of structural alloys remains largely empirical, and there is a need for an improved theoretical basis for alloy design. This presentation will include a review of those microstructures associated with maximum strength in this class of alloys and attempt to identify those microstructural parameters important in optimizing precipitation hardening or dispersion strengthening. It will also review recent attempts to model the effects of precipitate shape and orientation on yield strength, using appropriate versions of the Orowan equation and models of precipitation strengthening developed for rationally oriented plate or rod-shaped precipitates.

3:30 pm BREAK

3:40 pm INVITED

FERROMAGNETIC SHAPE-MEMORY COMPOUNDS WITH HEUSLER-TYPE STRUCTURE: K. Inoue, Department of Materials Science and Engineering, University of Washington, Box 352120, Seattle, WA 98195-2120; M. Taya, Mechanical Engineering, University of Washington, Box 352600, Seattle, WA 98195-2600; M. Kitamura, Electrical and Electronic Engineering, Utsonomiya University, Utsonomiya, Japan

There is a strong interest in the development of high performance materials for various applications including actuators for aircraft, and biomedical devices, in which large controllable strains and rapid response are most desired to have as important properties besides mechanical properties such as appreciable specific strength and high durability. Shape memory (SM) materials and piezoelectric ceramics are considered to be candidates for actuator materials but they only exhibit either large controllable strains or rapid response. We have shown effects of applied magnetic field on displacive phase transformation for off stoechiometric Ni₂MnGa-based compounds, which is indicative of potentiality of magnetically induced SM effects. We will report on displacive phase transformation and magnetic properties of ferromagnetic Ni₂MnGa-based compounds with and without quaternary additions (FE, Co, Ge, Si, Al). Effects of elemental additions on transformation temperature and magnetic properties will be presented, and relationships between phase stability and magnetic properties will be discussed.

4:20 pm

MAGNETISM AND POINT DEFECTS IN INTERMETALLIC FeRh: Luke S.J. Peng, Gary S. Collins, Department of Physics, Washington State University, Pullman, WA 99164-2814

Stoichiometric FeRh is a highly-ordered B2 intermetallic that is antiferromagnetic below about 400 K and ferromagnetic above. There is considerable interest in large changes in unit-cell volume and electrical conductivity and large magnetostrictive effects that take place at the AF-F transition. We are beginning a study of magnetism and defects in FeRh using 57Fe Mossbauer spectroscopy. In the AF phase, annealed, stoichiometric FeRh has a large hyperfine field (27.28(2) T at 20 K) and narrow lines indicating very little lattice disorder. In the present talk will be described measurements just beginning on annealed samples with 48 and 52 at .% Fe, for which there will be either 4 at .% of Rh or Fe antisite atoms on the Fe or Rh sublattices, respectively. Observed hyperfine-field-shifts due to antisite defects in the AF phase are expected to distinguish between two alternative magnetic structures that have been proposed. Supported in part by the NSF under grant 96-12306 (Metals Program).

4:50 pm

ATOMIC SCALE STUDIES OF POINT DEFECTS IN INTERMETALLICS: Gary S. Collins, Department of Physics, Washington State University, Pullman, WA 99164-2814

Over the past five years we have been using the method of perturbed angular correlation of gamma rays (PAC) to study properties of point defects in ordered intermetallics such as NiAl, CoAl and PdIn. Defects such as lattice vacancies and antisite atoms produce localized charge-density disturbances that modify the nuclear quadrupole interaction in nearby probe atoms. Using the 111 In/Cd probe, good signal-resolution is obtained for defects out to several atomic shells. Measurements on quenched NiAl, CoAl and PdIn have helped determine that the high-temperature equilibrium defect in each system is the Schottky vacancy-pair. The temperature dependence of site fractions of probe atoms with neighboring vacancies has yielded formation enthalpies and defect concentrations. We are now beginning measurements at high-temperature that will serve to test the methodology used for analysis of measurements on quenched samples and to provide information about diffusion. An overview of this work will be provided.

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

Session Chair: Alexander H. King, SUNY at Stony Brook, Stony Brook, NY 11794

STUDIES OF BOUNDARIES AND INTERFACES ON THE ATOMIC SCALE BY ATOM PROBE FIELD ION MICROSCOPY: G.D.W. Smith, Department of Materials, University of Oxford, Oxford, UK

David Smith was the first person to interpret correctly the image contrast observed from grain boundaries in the field ion microscope (FIM). This paper will begin with a short review of the contributions which he made to the understanding of grain boundary dislocations, and to the FIM observation of the atomic scale topography of interfaces. The second part of the paper will consist of an overview of atom probe FIM studies of the fine-scale chemical composition of grain boundaries and interface phases. The contributions of this technique to the understanding of interfacial chemistry will be illustrated by examples from ferrous and non-ferrous metals, semiconductor materials, and oxide superconductors.

2:30 pm INVITED

ATOMISTIC STUDIES OF GRAIN BOUNDARIES AND HETEROPHASE INTERFACES: EXPERIMENTS AND SIMULATIONS: David N. Seidman, Northwestern University, Department of Materials Science and Engineering, Evanston, IL 60208-3108

This talk is a review of our research on both grain boundaries (GBs) in single-phase binary metal alloys and ceramic/metal (C/M) heterophase interfaces. The emphasis is on obtaining a detailed atomistic picture of solute-atom segregation as obtained from both experiments, theory, and simulations. Heavy use is made of atom-probe field-ion microscopy to determine directly solute-atom segregation at GBs, whose five macroscopic degrees of freedom are measured by transmission electron microscopy; thereby systematically exploring the eight-dimensional GB phase space. Our experimental work is both complemented and supplemented by Monte Carlo simulations of solute-atom segregation for a wide range of twist and tilt boundaries. The chemistry of pristine C/M interfaces, as well as ones at which a segregant is present, are studied by atom-probe and Z-contrast microscopies. Local density functional theory (LDFT) and molecular dynamics studies are presented and compared with the experimental observations. This research is supported by the Department of Energy/Basic Energy Sciences and the National Science Foundation/Division of Materials Research.

3:00 pm INVITED

TRANSMISSION ELECTRON MICROSCOPY OF INTERFACES IN ENGINEERING CERAMICS: K.M. Knowles, Department of Materials Science and Engineering, The University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK

Interphase and grain boundaries in ceramic materials can be much more complex chemically than in metallic materials. For example, covalently bonded materials such as silicon nitride tend to contain thin (Å 1nm wide) amorphous grain boundary phases left behind after liquid phase sintering at high temperature. Fibre-reinforced glass ceramics, of interest as light, potentially damage tolerant materials, tend to possess Å0.1µm wide interphase regions between the matrix and the nanocrystalline fibres. In these interphase regions there are a variety of different possible chemical species, the precise details of which will be a sensitive function of heat treatment for a given combination of matrix and fibre. Electronic ceramics such as zinc oxide and strontium titanate internal barrier layer capacitors are further examples where interfacial phenomena dictate device properties. In this talk, we will use examples from a number of different engineering ceramics to illustrate the variety of ceramic interfaces that can arise and show how a combination of transmission electron microscopy techniques can be used to gain insight into the structure, chemistry and materials properties of the ceramic interfaces at near-atomic level.

3:30 pm BREAK

3:40 pm INVITED

THE EFFECT OF INTERFACES IN SOLID-STATE REACTIONS BETWEEN OXIDES: Matthew T. Johnson, Paul G. Kotula, C. Barry Carter, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455

The structure of an interface is an important factor in determining its response to different applied forces. This basic premise, which underlies much of the work of the late David A. Smith, has led to continuing studies on internal interfaces in polycrystalline materials. In ceramic oxides, the bonding is mixed covalent and ionic, which means that local changes in density can occur; this factor has important consequences for the movement of internal interfaces in these materials. Solid-state reactions occur by the movement of heterophase boundaries. This movement can be driven by either chemical or electrochemical driving forces. Through the use of pulsed-laser deposition (PLD), thin-films of NiO, In₂O₃ and Fe₂O₃ have been deposited onto monocrystalline bulk substrates of a-Al₂O₃ and MgO to produce initially planar phase boundaries. This geometry of a thin film on a bulk substrate has proved to be ideal for studying some of the fundamental processes occurring in solid-state reactions as they relate to the interfaces. In the case of the NiO/Al₂O₃ system, the effect of interfaces (or substrate orientation) on the reaction kinetics has been studied. The substrate orientation controls the overlayers and therefore the structure of the interfaces. In the case of the In₂O₃/MgO and Fe₂O₃/MgO systems, interfacial stability has been studied when the systems were reacted both without and under the influence of an electric field. The electric field provides a driving force for mass transport that affects both the reaction and the interfacial stability. Following the classical approach used by David Smith, the systems were all characterized using transmission electron microscopy (TEM); the TEM images were complemented by those obtained using a field-emission scanning electron microscope (FESEM).

4:10 pm INVITED

INTERFACE STRUCTURE-PROPERTY RELATIONSHIPS IN DIRECTIONALLY SOLIDIFIED EUTECTICS (DSEs) OF OXIDES: Elizabeth C. Dickey*, Vinayak P. Dravid, M.R. Notis+, C.E. Lyman+, and Alexandre Revcolevschi++, Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208; +Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015; ++Université de Paris, France

When properly grown, DSEs exhibit aligned fibrous/lamellar microstructure which contain numerous crystallographically identical heterophase interfaces amenable to extensive analysis. Over the last decade we have investigated a large number of lamellar DSEs, both for their structure as well as properties as reflected in crack propagation or phase transformations. The presentation will cover the connection between all length scales of interface microstructure and its influence on residual stresses and crack propagation behavior in DSEs. This presentation covers a collaborative effort which spans a full decade, three generations of mentor-student relationships and the Atlantic ocean. One of the authors (VPD) in this long list was fortunate to have late Prof. David A. Smith as his PhD committee member. He was a clear beneficiary of David's advice and words of wisdom, which he carried with him to Northwestern.

4:40 pm

DISLOCATION BEHAVIOR AT OXIDE/METAL INTERFACES UNDER NANOSCALE CONTACTS: D.E. Kramer, W.W. Gerberich, Dept. of Chemical Engineering & Materials Science, 151 Amundson Hall, Washington Avenue SE, Minneapolis, MN 55455

Metal single crystals with native oxide films exhibit unusual loading behavior when probed by nanoindentation. Initially, loading is elastic, up to stresses that approach the theoretical shear strength of the metal. The initiation of plastic deformation manifests itself in the form of a yield excursion or "pop in" in the load/displacement curve. The presence of mechanical deformation near the oxide/metal interface and the oxide thickness are shown excursion behavior. These interactions have been investigated on electropolished Fe-3% Si single crystals. Thicker oxide films require greater loads to initiate plastic flow. A mechanical deformation layer at the oxide / metal interface results in plastic deformation prior to, or the elimination of, the excursion event. A model for dislocation nucleation at the oxide/metal interface is discussed in light of the results.

5:00 pm

EDS, EELS, AND AUGER STUDIES OF METAL-CERAMIC INTERFACES: R.Y. Hashimoto, E.S.K. Menon, M. Saunders, A.G. Fox, Center for Materials Science and Engineering, Department of Mechanical Engineering, Naval Postgraduate College, Monterey, CA 93943

Metal-ceramic interfaces are important in electronics packaging applications. We have studied the copper-alumina and aluminum-alumina systems using transmission electron microscopy (TEM), energy-dispersive x-ray spectroscopy (EDS), electron energy-loss spectroscopy (EELS) and Auger electron spectroscopy (AES). The interfaces were created under vacuum by diffusion bonding of 100mm metal foils pressed between polished alumina substrates (99.98% purity) for several hours at approximately 90% of the metal melting temperature. The presence and distribution of impurities is investigated by EDS, EELS, and AES. Of particular importance is the role of silicon (the major impurity in commercially available alumina) which previous studies have suggested should migrate to the interface during bonding thus influencing its thermomechanical behavior. The energy-loss near edge structure (ELNES) of the EELS spectra is also considered in an attempt to characterize the interfacial chemistry. The ultimate aim is to relate the mechanical properties of the interface to the chemical properties.

EXTRACTION AND PROCESSING OF TITANIUM: Session IV: Process Characterization

Session Chairs: Dr. B. Mishra, Department of Metallurgical & Materials Engineering, Colorado School of Mines, Golden, CO 80401; Dr. D.L. Olson, Department of Metallurgical & Materials Engineering, Colorado School of Mines, Golden, CO 80401

TEMPERATURE DISTRIBUTION IN A KROLL BATCH REACTOR FOR TITANIUM SPONGE PRODUCTION: CH RVS Nagesh, CH Sridhar Rao, DMRL, Kanchanbagh, Hyderabad, Andhra Pradesh 500058, India; N.B. Ballal, P.K. Rao, Department of Metall. Engrng. & Matl. Sc., Indian Institute of Technology, Powai, Bombay 400 076, India

Titanium sponge is produced in batches by the magnesiothermic reduction of titanium tetrachloride at high temperature (Kroll Process). The temperature distribution within the reactor speak about the state and progress of the reduction reactions and sponge formation. The reaction temperature is controlled by suitable TiCl₄ feed rate and external cooling of the reactor. Experiments were carried out to measure the temperature distribution within the reactor by introducing TiCl₄ on a magnesium surface of 1.3m². Different TiCl₄ feed rate conditions ranging from 120-320 kg/hr.m² were used in the experiments. The experimental temperature profiles were compared with the theoretical temperature profiles determined by FEM analysis of heat conduction. The temperature data and results of thermodynamic computations carried out in the temperature range 1000-1500 K were analyzed discussed.

2:40 pm

THE DIRECT RESISTANCE ROTARY HEATING FURNACE (DRHF): L.D. Smillie, M.D. Heydenrych, Mattek- CSIR, Division of Materials Science & Technology, P.O. Box 395, Pretoria 0001, South Africa

The DRHF is a continuous gas-solid reactor which can maintain an extremely tight control on the constituents of the gas atmosphere at high temperature. With a wide field of applications such as the reduction of metal oxides, it is also suited for the low temperature regeneration of activated carbon in the gold industry. The furnace is a high temperature reactor which can operate up to 1500°C at the hot face and can be designed for very low heat losses. The DRHF is unique in that it can maintain high temperatures for endothermic reactions in a controlled gas atmosphere. The furnace contains no electrical elements to generate heat, but uses the inherent electrical properties of the material to be processed, hence the name Direct Resistance Heating Furnace.

3:05 pm

MECHANISM OF MAGNESIOTHERMIC REDUCTION OF TICl₄ BY AN ELECTRONICALLY MEDIATED REACTION (EMR): T.H. Okabe, T. Uda, E. Kasai, Y. Waseda, Research Center for Metallurgical Process Engineering, Institute of Advanced Materials Processing, Tohoku University, 2-1-1 Karahira, Aobaku, Sendai 980-77, Japan

The mechanism of magnesiothermic reduction of TiCl₄ has been discussed in the framework of electronically mediated reaction (EMR). Feed material, TiCl₄, and reductant magnesium were charged into different locations in molten MgCl₂ at 1073°K. Current flow between these feed and reductant locations was monitored when shortening these electronically isolated sites. Electrochemical potentials of each site were also measured by interrupting the current during the electronically mediated reaction. Large current, more than 5 amperes, was detected during reaction and its reproducibility was well-confirmed. This result shows titanium metal can be produced by EMR without direct physical contact between feed and reductant. Morphological characteristics and location of titanium deposition seems to depend on reaction pathway. The present results strongly suggest the concept of EMR plays an important role in the Kroll process which occurs via electron transfer through an electronically conductive medium, e.g., reactor wall and titanium deposit.

3:30 pm BREAK

3:45 pm

LOW COST PRODUCTION OF TiAl AUTOMOTIVE VALVES USING COLD WALL INDUCTION MELTING AND PERMANENT MOLD CENTRIFUGAL CASTING: A. Choudhury, M. Blum, ALD Vacuum Technologies GmbH, Rockinger Strasse 12, D- 635268 Erlensee, Germany; P. Busse, ACCESS eV., D- 52072, Aachen, Germany; D. Lupton, M. Gorywoda, W.C. Heraeus GmbH, D- 63450, Hanau, Germany

Due to the low density and high temperature strength -titanium aluminide is an excellent candidate for automotive exhaust valve applications. Lighter weight valvetrain components allow either improved performance or reduction of valve spring loads that reduce noise and friction, thereby improving fuel economy. The cost of TiAl-valves must, of course, be competitive. Existing production routes developed for aircraft industry applications are very complex and expensive and hence not appropriate for an economical mass production of TiAl-valves. For this reason, a joint project carried by three research institutes and five industrial companies along the manufacturing chain has developed a new manufacturing process under the MaTech program established by the German Federal Ministry for Research and Technology. The cost saving process consist of melting and alloying using a cold crucible furnace and centrifugal casting. In a preheated permanent mold in a single step. The new process as well as first results obtained during the running in period of the pilot plant will be presented.

4:10 pm

LEACHING OF ILMENITE WITH SULFURIC ACID BY MICROWAVE IRRADIATION: J. Peng and C. Liu, Department of Metallurgy, Kunming University of Science and Technology, Kunming, Yunnan 650093, P.R. China

4:35 pm

CHARACTERISTICS OF TEMPERATURE INCREASE OF TITANIUM MINERALS AND COMPOUNDS BY MICROWAVE IRRADIATION: J. Peng and C. Liu, Department of Metallurgy, Kunming University of Science and Technology, Kunming, Yunnan 650093, P.R. China

FATIGUE AND CREEP OF COMPOSITE MATERIALS: Session IV

Session Chairs: Linda S. Schadler, Dept. of Materials Science and Engineering, Rennselaer Polytechnic Institute, Troy NY 12180; John J. Lewandowski, Dept. of Materials Science & Engineering, Case Western Reserve University, Cleveland, OH 44106

DESIGNING METAL MATRIX COMPOSITES WITH DISCONTINUOUS REINFORCEMENT FOR HIGH CREEP STRENGTH: R.S. Mishra, A.K. Mukherjee, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616

Metal matrix composites (MMCs) with discontinuous reinforcement are attractive for high temperature applications because of their creep strength. An additional benefit of discontinuously reinforced MMCs is the ease of fabrication through a number of processing routes. Although the increase in creep strength because of the second phase reinforcement is well documented, the scientific issues related to creep deformation are not so well understood. Also, the magnitude of creep strengthening depends on the nature of reinforcement and the matrix material. By using a 'dislocation creep mechanism map' it is possible to predict the level of creep strengthening in aluminum matrix composites. The role of interparticle spacing on the creep strength of various aluminum and titanium matrix composites is discussed through a detailed analysis.

2:30 pm INVITED

EFFECT OF MATRIX MICROSTRUCTURE ON FATIGUE BEHAVIOR OF AN Al-2080/SiC COMPOSITE: Christoph Andres*, J. Wayne Jones*, and John E. Allison+, *Department of Materials Science, The University of Michigan, MI; +Ford Research Laboratory, Ford Motor Company, Dearborn, MI

The main focus of this study is the influence of the matrix precipitate structure on the fatigue behavior of powdermetallurgy 2080-Al and 2080-Al reinforced with SiC particles. The materials were subjected to either T6 or T8 heat treatments. The thermo-mechanical T8 heat treatment was utilized to achieve uniformly dispersed S' precipitates. A certain number of T8 and T6 samples were further aged to induce an overaged structure. This overaging led to a lower fatigue strength compared to the peakaged counterparts. The fatigue behavior of a 2080-Al with four different SiC particle volume fractions (0, 10, 20, 30 vol.%) and two different particle sizes of @ 4 mm (FEPA grade F - 1000), @ 10 mm (FEPA grade F-600) was studied. At a given SiC particle volume fraction peakaged and overaged T6 conditions showed higher fatigue strengths than the T8 conditions despite coarser and more inhomogeneously distributed precipitates. TEM studies have been conducted in order to understand the effect of precipitate size and precipitate spacing on the fatigue behavior.

2:50 pm INVITED

EFFECT OF REINFORCEMENT ON CRACK INITIATION AND EARLY CRACK GROWTH OF A P/M PROCESSED Al-2080/SiC COMPOSITE: Christoph Andres*, J. Wayne Jones*, and John E. Allison+, *Department of Materials Science and Engineering, the University of Michigan, Ann Arbor, MI; +Ford Research Laboratory, Ford Motor Company, Dearborn, MI

The low cycle and high cycle fatigue crack initiation and small crack growth process of a particle reinforced aluminum matrix composite Al-2080/SiC has been evaluated. A controlled matrix microstructure was achieved by using a thermo-mechanical heat treatment (T8), which led to a homogeneous distribution of S'-precipitates. Four different SiC particle volume fractions (0, 10, 20, 30 vol.%) and three different particle sizes of @ 4 mm (FEPA grade F-1000), @ 10 mm (FEPA grade F-600) and @ 36 mm (FEPA grade F-280) have been investigated. The influence of these composite variables on fatigue crack initiation will be examined in detail. The experimental results show for a given loading condition, larger particle sizes, and higher volume fractions tend to favor particle-cracking. In many cases large iron-or silicon-rich intermetallic inclusions were responsible for crack nucleation. To understand the crack initiation and propagation behavior surface replication and SEM studies have been conducted and will be reviewed.

3:10 pm INVITED

HIGH TEMPERATURE CREEP OF AN Al-7005 COMPOSITE REINFORCED WITH Al₂O₃ PARTICULATES: Yong Li and Terence G. Langdon, Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453

A creep investigation was conducted over temperature from 573 to 773 K on an Al-7005 metal matrix composite reinforced with 20 volume per cent of Al₂O₃ particulates. The creep data, which extend over six orders of magnitude of strain rate, show curvatures in the logarithmic plots of strain rate versus stress, thereby indicating the existence of a threshold stress. By incorporation of a threshold stress into the analysis, it is shown that the true stress exponent of the composite is close to 4.4 and the true activation energy is close to ~ 120 kJ mol^-1: these values are in reasonable agreement with earlier reports for an Al-5 wt.% Zn alloy. Therefore, the results indicate that the creep mechanism in the Al7005 composite is essentially identical to that reported in dilute Al-Zn alloys.

3:30 pm BREAK

3:50 pm INVITED

INFLUENCE OF MICROSTRUCTURE ON THE HIGH CYCLE FATIGUE BEHAVIOR OF Al-BASED METAL MATRIX COMPOSITES: Don Lesuer, Chol Syn, T.G. Nieh, L-342, Lawrence Livermore National Laboratory, Livermore, CA 94551

It is generally recognized that the high cycle fatigue behavior of metal matrix composites (MMCs) can be superior to that of the matrix alloy. For composites in which defect-related failures can be avoided, these improved fatigue properties are related to the increases in strength and stiffness that are produced in the MMC. In this paper we explore the influence of microstructure on the high cycle fatigue response of particle-reinforced aluminum alloys. Experimental work has been performed to establish the cyclic stress - strain behavior and resulting stress - life response for a 6090/SiC/25p-T6 composite. The material exhibited fatigue properties that are not dominated by defects and thus approach the inherent fatigue response of the composite. The results have been compared to the fatigue behavior of other particle-reinforced aluminum alloys. This work has resulted in a quantitative understanding of the influence of microstructure on the high cycle fatigue response of particle reinforced metals.

4:10 pm

HIGH-TEMPERATURE RUPTURE OF PARTICULATE REINFORCED AND UNREINFORCED 2124 UNDER MULTIAXIAL STRESS STATES: Ahmadali Yousefiani, Farghalli A. Mohamed, James C. Earthman, Materials Science and Engineering, Dept. of Chemical and Biochemical Engineering, University of California, Irving, CA 92697

Creep rupture experiments were performed on 2124 Al under three different stress states (uniaxial tension, biaxial shear and triaxial tension) in both an unreinforced and a 10 vol.% SiC_p reinforced condition. Rupture times are compared for the three states with respect to the maximum principal stress, von Mises effective stress, and the principal facet stress. The results of this comparison along with microstructural observations regarding the cavitation behavior of the alloy are discussed in reference to the effect of the presence of the reinforcing particles on observed high temperature damage.

4:30 pm

CREEP BEHAVIOR OF A METAL MATRIX COMPOSITE: I.M. Daniel, Northwestern University, Evanston IL; H.J. Chun, Yonsei University, Seoul Korea

The creep characteristics of a silicon carbide/ aluminum (SiC/Al) unidirectional composite were measured under transverse tensile loading over a temperature range from 204°C (400°F) to 288°C (550°F). It was found that the minimum creep strain rate of the composite can be described by an Arrhenius type power law relation similar to the one used for the unreinforced matrix. This creep rate for the composite is less sensitive to stress amplitude and temperature than that of the matrix material. The increased creep resistance of the composite is attributed to redistribution of the stresses in the matrix and to matrix stress relaxation around the relatively rigid fibers. Creep tests were also conducted during thermal deformation cycling. The latter was found to increase creep significantly above that under isothermal conditions, even at temperatures equal to or higher than the peak cyclic temperature. The creep behavior was also analyzed by means of a micromechanical model based on the average field theory. The measured creep strains at various stress amplitudes and at various temperatures were in favorable agreement with predictions.

4:50 pm INVITED

FATIGUE CRACK GROWTH ALONG GRAPHITE/EPOXY INTERFACE: James Ryan, J.K. Shang, Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801

Effect of graphite surface-modification on fatigue crack growth resistance of graphite-epoxy interface was examined using flexural peel specimens. Edge surfaces of pyrolytic graphite were treated in an oxygen plasma for various times and subsequently bonded to a toughened epoxy to form flexural peel specimens. Fatigue crack growth rates were measured in the plasma-treated and untreated specimens as a function of strain energy release rate. Fatigue crack resistance of plasma-treated specimens was notably higher, with the fatigue threshold doubled at an optimal treatment time. X-ray photoelectron spectroscopy, electron microscopy and surface profilometry studies indicated that both chemistry and morphology of the graphite surface were changed by the plasma treatment. High temperature annealing was used to restore the original surface chemistry and fatigue experiments were then performed on annealed specimens to separate chemical and morphological effects. Chemical modification turned out to be secondary, contributing less than 30% to the overall improvement from the plasma treatment.

GEORGE R. IRWIN SYMPOSIUM ON CLEAVAGE FRACTURE: Session IV: Fracture Mechanisms

Session Chairs: A.W. Thompson, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720; A.R. Rosenfield, Rosenfield and Rosenfield, Columbus, OH 43212

2:00 pm INVITED

OXIDE FILM EFFECTS ON CLEAVAGE: W.W. Gerberich, University of Minnesota, Minneapolis, MN 55455; N.R. Moody, Sandia National Laboratories, Livermore, CA; M.D. Kriese, University of Minnesota, MN

When cleavage nucleates from crack tips containing oxide layers or when time-dependent, subcritical crack growth occurs by a brittle cleavage or intergranular process, what is the role of an oxide film? The mechanism of stress corrosion cracking by a film rupture process has been postulated for some time. In addition, film-induced cleavage has been proposed more recently by Sieradzki and Newman. While either or both of these may exist, is there yet an even more general effect which must be considered? That is, either along with these mechanisms or in some cases in lieu of them, must we consider how oxide films affect dislocation emission which then becomes involved in shielding effects on cleavage or time-dependent failure? What we do know is that oxide films greatly affect the yield load occurring under a sharp indenter. Might this not also affect yield behavior at a crack tip since both represent sharp stress concentrators? For example, indentation induced yield excursions can vary by an order of magnitude in Fe-3wt%Si and tungsten with different oxide films. Plans for sorting this out are underway using companion experiments undergoing cleavage as well as indentation. Results on NiAl single crystals will be reported.

2:30 pm INVITED

SURFACE FILM SOFTENING AS A PROBLE OF CLEAVAGE FRACTURE: R. Gibala, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136

We have examined experimental data on the phenomenon of surface film softening of quasi-brittle metals and intermetallics in the context of cleavage fracture and its interaction with the intrinsic plasticity of these materials. Surface film softening is manifested as increased plasticity and, in many instances, as reduced flow stresses of a substrate material due to enhanced dislocation generation associated with film-substrate constraint under applied stress. Nominally ductile refractory metals such as Nb and Ta can be substantially ductilized by surface films to the point of exhibiting appreciable free strain. More brittle metals, such as Mo or W exhibit film-enhanced plasticity, some evidence of free strain, and associated changes in fracture mechanisms. Intermetallic alloys and compounds, e.g. NiAl, FeAl, and MoSi₂, can exhibit film-enhanced plasticity but usually without significant changes in fracture behavior.

3:00 pm

SHEAR-INDUCED CLEAVAGE FRACTURE: Kwai S. Chan, Southwest Research Institute, San Antonio, TX 78238

Experimental evidence shows that shear-induced cleavage fracture is prominent in many intermetallic alloys and in-situ composites, as well as in other brittle materials. In materials with a plastically deformable matrix, the characteristics of fracture generally include planar slip, slipband decohesion, and cleavage crack formation. In contrast, shear cracks with wing-tip cleavage cracks dominate in plastically nondeformable materials. In this article, the phenomenon of shear-induced cleavage fracture is revisited. Theoretical analyses of the driving force for the fracture process are summarized to illustrate the salient features of shear-induced cleavage fracture and to derive the critical condition for crack instability. Applications of the brittle fracture theory to treating shear-induced cleavage in TiAl alloys, Nb-Cr-Ti alloys, rock salt, alumina scale on overlay coating, and thermal barrier coating are presented with experimental verifications.

3:20 pm BREAK

3:30 pm

CLEAVAGE MECHANISM IN VANADIUM ALLOYS: G.R. Odette, E. Donahue, G.E. Lucas, Department of Mechanical Engineering, University of California Santa Barbara, Santa Barbara, CA 93106-5070

The effect specimen geometry, loading rate and irradiation on the ductile-to-brittle transition in a V-4Ti-4Cr alloy were evaluated and modeled. Confocal microscopy-fracture reconstruction and SEM were used to characterize the sequence-of-events leading to cleavage, as well as the CTOD at fracture initiation. This alloy undergoes normal stress-controlled transgranular cleavage below a transition temperature that depends primarily on the tensile properties and constraint. Thus an equivalent yield stress model is in good agreement with observed effects of loading rate and irradiation hardening. Predicted effects of specimen geometry based on a critical stress-area criteria and FEM simulations of crack tip fields were also found to be in agreement with experiment. Some interesting characteristics of the fracture process are also described.

3:50 pm

A NEW TOOL FOR TOPOGRAPHIC AND CRYSTALLOGRAPHIC ANALYSIS OF CLEAVAGE FRACTURE SURFACES: C.O.A. Semprimoschnig, O. Kolednik, R. Pippan, Erich-Schmid-Institut für Festkörperphysik der Österreichischen Akademie der Wissenschaften, A-8700 Leoben, Austria

A new experimental technique is presented for the combined topographic and crystallographic analysis of cleavage fracture surfaces. The technique combines an automatic surface reconstruction system and the electron back scatter diffraction technique. The automatic surface reconstruction system uses stereo-image pairs (digital fractographs) taken in the scanning electron microscope to determine the 3-dimensional shape of a fracture surface region. The orientation of cleavage planes or lines on these planes with respect to a certain specimen coordinate system can be interpolated. The electron backscatter diffraction technique provides the crystallographic orientation of single grains or subgrains on the fracture surface with respect to the same coordinate system. The combination of both techniques enables us to measure crystallographic indices of cleavage planes as well as indices of directions on the planes. The technique can be also applied to detect a possible plastic deformation during the propagation of a cleavage crack. Different examples for the application of the new technique are presented.

4:10 pm

A FRACTOGRAPHIC ANALYSIS OF THE BRITTLE TRANSGRANULAR FRACTURE OF THE AUSTENITIC PHASE OF HIGH NITROGEN STAINLESS STEELS: J. Ivan Dickson, Ecole Polytechnique, Département de métallurgie et génie des matériaux, CP 6079, Montréal, Québec H3C 3A7, Canada; Jean-Bernard Vogt, Université Physique, B,t. C6, 59655 Villeneuve d'Ascq, France

The study consists of a crystallographic analysis of the brittle fracture planes of a Cr Mn austenitic s.s.(0.9%N) and of the austenitic phase of a / duplex s.s.(0.6%N). After fracture at low temperature, evidence of pure cleavage on {111} and {100} is pointed out according to the presence of the large and flat primary facets. The most frequently observed {111} facets contain numerous slip lines suggesting an important role of plasticity in the nucleation of cleavage cracks. "Non pure" cleavage (appearance of river lines on primary facets) and alternative cracking on {111} microfacets resulting in a {110} average orientation are also reported. Hydrogen embrittlement leads to cleavage on {100} planes for the a.s.s with a restricted contribution of plasticity. In the d.s.s., cleavage occurs on {100} planes after fracture at room temperature in the phase without any other orientations.

4:30 pm

BRITTLE FRACTURE IN HIGH-STRENGTH AUSTENITIC STAINLESS STEELS: Yu I. Chumljakov, I.V. Kireeva, E.I. Litvinova, V.I. Kirillov, and N.S. Surikova; Siberian Physical-Technical Institute, Revolution sq. 1, 634050, Tomsk, Russia

On austenitic stainless steels single crystals with stacking fault energy _st =0.02 J/m² the role of high deforming stress level reached by hardening with nitrogen, hydrogen, precipitate nitride particles has been investigated. Friction level, deformation mechanism-slip/twining have been shown to be essential factors for brittle fracture of high-strength crystals, and "ductile-brittle" transition. 1) Regardless of the certain way of achieving high strength and crystal orientation, there are critical values of deforming stresses _cr>=G/100 (G-shear modulus), under which at temperature below Tcr=300K brittle fracture by cleavage occurs on {111} planes. At T>_cr in high strength fcc crystals "ductile-brittle" transition is observed, which is typical for bcc crystals. 2) Decrease of deforming stress level _cr<G/100 leads to appearance of orientational dependence of fracture mechanism-in <111> twining at T<300K and brittle fracture, in <100>-slip and ductile fracture. 3) The role of hydrogen in fracture of high strength crystals has been investigated.

4:50 pm

INFLUENCE OF TiN PARTICLES AND MICROSTRUCTURE ON CLEAVAGE FRACTURE IN SIMULATED HAZs: L.P. Zhang, C.L. Davis, and M. Strangwood, School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom

Thermally stable TiN particles can effectively pin austenite grain boundaries in HAZs, improving toughness, but can act as cleavage initators. The effects of prior austenite grain size and matrix microstructure on the cleavage fracture of simulated HAZs in steel with two Ti levels (0.045 and 0.1 wt% have been investigated using two peak temperatures (T_p) and three cooling times (t_8/5) using a Gleeble 1500TCS. Coarse (1-6µm) and fine (35-400 nm) TiN particles were identified, whose mean size increase with increasing Ti content at constant number density. Higher T_p (1350(C vs. 1100°C) for the same cooling time gave austenite grain growth and decreased toughness significantly. However, increasing cooling time from 8 to 90s changed the matrix microstructure from bainite to ferrite-pearlite with little variation in toughness, although further investigation is required at lower Ti levels.

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

Session Chairs: Prof. Robert Ritchie, Univ. of California-Berkeley, Dept. of Materials Science & Eng., 282 Hearst Mining Bldg., Berkeley, CA 94720; Dr. Ted Nicholas, USAF Wright Laboratory, Wright-Patterson AFB, Dayton, OH 45433

ISSUES IN HCF OF MATERIALS IN GAS TURBINE ENGINES: Ted Nicholas, USAF Wright Laboratory, Wright-Patterson AFB, Dayton, OH 45433

HCF failures in materials used in rotating components of gas turbine engines have often been found to be attributable to fatigue loading on materials which have sustained damage from other sources. Damage can be present from initial material or manufacturing defects, or can develop during service operation. IN-service damage, while not catastrophic by itself, can degrade the HCF resistance of the material so that the fatigue runout stress, plotted on a Goodman diagram, is reduced. In studying failures in military and civil engines, three major sources of in-service damage have been identified which can alter the HCF resistance individually or in conjunction with one another: low cycle fatigue (LCF), foreign object damage and fretting. Experiments are being conducted to evaluate the type and intensity of damage which causes a reduction in the HCF behavior of a common compressor blade titanium alloy, Ti-6Al-4V. Some recent results on the effects of LCF on the Goodman diagram are presented.

1:25 pm INVITED

A 2.5 kHz LOADING STAGE FOR THE SEM AND DESCRIPTION OF EXPERIMENTS USING IT TO STUDY HCF PHENOMENA: D.L. Davidson, Southwest Research Institute, P.O. Box 28510, San Antonio, TX 78228-0510

A loading stage for the SEM that cycles at a resonant frequency of about 2.5 kHz and that is capable of high levels of meanload will be described. This system was constructed to allow the surfaces of specimens to be observed under high resolution conditions during simulated operating conditions of gas turbine blades at ambient temperature in the compressor section. Fatigue cracks have been started from notches machined by section. Fatigue cracks have been started from notches machined by focused ion beams and under fretting fatigue conditions in Ti-6Al-4V. The initiation and growth of cracks will be described and some results of measurements of crack tip micromechanics parameters are given.

1:50 pm INVITED

ENSURING DAMAGE TOLERANCE OF TITANIUM ALLOYS UNDER LOADING SPECTRA CONTAINING HIGH CYCLE FATIGUE: J.M. Larsen, B.D. Worth, J.R. Jira, D.C. Maxwell, Materials Directorate, Wright Laboratory (WL/MLLN), Wright-Patterson AFB, OH 45433; The University of Dayton Research Institute, Dayton, OH 45419-0128

Damage tolerance methods are widely used to assure reliability of fracture critical components in advanced turbine engines. Although low cycle fatigue often controls component lifetimes, high cycle fatigue of airfoils and airfoil attachment regions is a growing concern. An overview is presented of the role of damage tolerance methods available for life management of titanium alloy turbine engine components that experience loading spectra containing low- and high-cycle fatigue. While crack growth rate behavior may be a dominant factor affecting component lifetime under low cycle fatigue, the threshold crack growth condition is crucial under high cycle fatigue conditions. Key factors controlling fatigue crack growth thresholds in titanium alloys used in turbine engines are discussed, including effects of alloy composition, microstructure, realistic loading spectra and crack size. In addition, the influence of realistic statistical variation in properties of current materials is examined in light of damage tolerance requirements.

2:15 pm INVITED

MICROMECHANISMS OF FATIGUE IN STRUCTURAL AEROENGINE ALLOYS: S. Dubey, V. Sinha, M. Foster, C. Mercer, D. DeLuca*, W.O. Soboyejo, Dept. of Materials Science and Engineering, The Ohio State University, 2041 College Rd., Columbus, OH 43210-1179; *Pratt & Whitney, government Engines and Space Propulsion, P.O. Box 109600, West Palm Beach, FL 33410-9600

The micromechanisms of fatigue crack initiation and crack growth in nickel and titanium base alloys are discussed in this paper. Following an initial review of the existing applications of aerospace alloys, the effects of microstructure, thickness and stress ratio on the fatigue crack growth behavior of Ti-6Al-4V are discussed. The micromechanisms of fatigue crack nucleation and growth will be identified in Ti-6Al-4V with equiaxed and Widmanstatten microstructural morphologies. The trends in the fatigue crack growth rate data will be rationalized using crack closure concepts. The micromechanisms of cyclic crack-tip deformation in single crystal and polycrystalline ingot metallurgy nickel base superalloys will then be discussed within the context of recent evidence of crack-tip deformation obtained from crack-tip TEM analysis. The implications of the results will be discussed for the future modeling of fatigue crack initiation and fatigue crack growth.

2:40 pm

EFFECTS OF MICROSTRUCTURE ON THE SUBSURFACE CRACK INITIATION OF Ti-6Al-4V ALLOYS: O. Umezawa, K. Nagai, National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305, Japan; H. Yokoyama, T. Suzuki, Dept. of Mechanical Engineering, Kogakuin University, Shinjyuku, Tokyo, Japan

The materials having various microstructures such as acicular- or equiaxed-type were obtained by heat-treatment or hot-working for Ti-6Al-4V alloys. The high-cycle fatigue test with an R ratio equal to 0.01 was done at 77 K. Subsurface crack initiation was detected in longer-life range such as over 10⁵ cycles for each material. The subsurface crack initiation site reveals a facet or facets which was identified as the phase. The size of subsurface crack initiation site was evaluated. The dependence of initiation site size on the maximum stress can be explained by a threshold condition assumption.

3:00 pm BREAK

3:15 pm INVITED

OVERLOAD EFFECTS IN FATIGUE CRACK PROPAGATION-A REVIEW: Arthur McEvily, University of Connecticut, 97 N. Eagleville Rd., Storrs, CT 06269

Paul Paris has made a number of major contributions to the field of fatigue crack growth. Among these are the use of the stress intensity factor as a correlating parameter for determining the rate of fatigue crack growth, the introduction of the Paris-Erdogan law, the study of the threshold behavior of fatigue cracks, the early recognition of the importance of Elber's discovery of crack closure, and the discovery (with Hermann) that two opening events could be detected following an overload. The present paper is chiefly concerned with this last topic, but in a sense each of the other topics also plays a role. A brief review of past work on overload effects will be given, and the present state of our understanding of the physical factors involved as well as quantitative analysis of these overload effects will be discussed.

3:40 pm INVITED

FATIGUE THRESHOLD MAPS OF PWA 1480 SUPERALLY SINGLE CRYSTAL IN AIR AND VACUUM AT ROOM TEMPERATURE: R.L. Holtz, Geo-Centers, Inc., 10903 Indian Head Highway, Suite 502, Ft. Washington, MD 20744; M.A. Imam, K. Sadananda, Materials Science & Technology Div., Naval Research Laboratory, Washington, DC 20375

The fatigue crack growth thresholds of PWA 1480 superalloy single crystal were measured in air and in vacuum at room temperature for stress ratios 0.1, 0.5 and 0.7. The DK versus K_max maps of the fatigue crack growth thresholds are correlated with microstructure features and dislocation densities near the crack-tip. The relative importance of crack closure is examined.

4:05 pm INVITED

THRESHOLD BEHAVIOR OF A NICKEL-BASE SUPERALLOY AT VERY HIGH FREQUENCY AND HIGH R-RATIO: W.W. Milligan, S.A. Padula, Dept. Of Metallurgical and Materials Engineering, Michigan Technological Univ., Houghton, MI 49931

Initial experiments on fatigue crack propagation and threshold behavior of a nickel-base turbine disk alloy at frequencies up to 1,000 Hz will be presented. Effects of R-ratio and frequency, as well as grain size, will be discussed. Experimental issues related to closed-loop servohydraulic testing of metals at 1,000 Hz, including the challenge of measuring crack length, will be included. We thank the Air Force of Scientific Research for sponsoring the work.

4:30 pm

HIGH-CYCLE FATIGUE CRACK INITIATION OF ULTIMET ALLOY: E.Y. Shen, P.K. Liaw, C.R. Brooks, Dept. Of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996; D.L. Klarstrom, Haynes International, Inc., Kokomo, IN 46904; J.Y. Huang, Institute of Nuclear Energy Research, Chlaan Village, Lungtan, Taiwan

ULTIMET alloy is a cobalt-base (approximately 55 wt. % Co, 26 wt. % Cr and 9 wt. % Ni) superalloy which has excellent wear and corrosion resistance. IN this paper, preliminary results of fatigue testing will be presented. Specifically, fatigue crack initiation in high-cycle, tension-tension loading at 25°C has been studied. By using different techniques (scanning electron microscopy, atomic force microscopy, scanning tunneling microscopy, laser extensometer, krak gage), the crack initiation behavior has been examined. The effect of microstructure on crack initiation is discussed.

4:50 pm

THE CRYSTALLOGRAPHY OF FATIGUE CRACK INITIATION IN TWO AUSTENITIC Fe-Ni SUPERALLOYS: C.R. Krenn, J.W. Morris, Jr., Center for Advanced Materials, Lawrence Berkeley National Laboratory and Dept. of Materials Science and Mineral Engineering, Univ. of California, Berkeley, CA 94720; Z. Mei, Hewlett Packard Company, 1501 Page Mill Road, Palo Alto, CA 94303

Fatigue crack initiation in the austenitic Fe-Ni superalloys Incoloy-908 and A-286 is examined using local crystallographic orientation measurements. Preliminary results are consistent with sharp transgranular initiation and propagation occurring almost exclusively on {111} planes in Incoloy-908 but on a variety of low index planes in A-286; ongoing research will be presented. Initiation in each alloy occurred both intergranularly and transgranularly and was often associated with blocky surface oxide and carbide inclusions. Taylor factor and resolved shear stress and strain crack initiation hypotheses were found to convincingly describe preferred crack initiation sites in either alloy. Subsurface inclusions are thought to play a significant role in crack initiation. This work was supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00098.

5:15 pm INVITED

FATIGUE STRENGTH OF TiN AND TiCN CERAMIC COATED 1Cr-1Mo-0.25V STEEL: C.M. Suh, K.Y. Kim, S.G. Do, Department of Mechanical Engr., Kyungpook National University, Teagu City, South Korea

In order to clarify the effect of ceramic coating films on the fatigue strength, and crack propagation properties of material, fatigue tests (R=0.1, R=-1) were carried out in room air, using the round plain specimens and compact tension (CT) specimens of 1Cr-1Mo-0.25V steel coated with TiN and TiCN by physical vapor deposition (PVD). It was observed that the scatter band of fatigue life at low fatigue strengths was wider than that of fatigue life at high fatigue strengths. The obvious improvement of fatigue life was confirmed in TiCN coated specimens for the region of low fatigue strengths, as compared with uncoated and TiN coated specimens. It was explained that the increase of fatigue life in the TiCN coated material was attributed to the retardation of crack initiation due to the restriction of surface plastic deformation in the substrate with hard coating layer. Also, the fatigue strength of 107 cycles of ceramic coated material was increased about 15 - 21% higher than that of base material.

5:40 pm

ON A PHENOMENOLOGICAL MODEL OF FRETTING FATIGUE DESTRUCTION: J.Z. Shtilerman & V.I. Iogansen, Scientific Research Institute, St. Petersburg, Russia

We considered an element (sample or detail) subjected to cyclic deforming with presence of fretting. Our model is based on two main hypotheses: 1. The work in fixed size of the element during the cycle of deforming is a criterion of fretting fatigue destruction, but the work of normal and tangent surface forces must be taken into account together with the work of stress in material (i.e. elastic energy). 2.The fretting influence area depth depends essentially on the element's stress amplitude. The simple expressions of "equivalent" (with fretting) stress and fretting fatigue factor (which is similar to the reduced factor of stress concentration) have been obtained. The model constants' values of high-strength rotor steel have been estimated. Calculations based of this model have confirmed directly well-known experimental fretting fatigue limit dependencies of various parameters that characterize the element's material, the contact conditions, the absolute dimensions etc. Therefore, the adequacy of proposed model has been proven.

MECHANICAL BEHAVIOR OF BULK NANO-MATERIALS: Session III

Session Chair: Shankar M. Sastry, Washington University, Campus Box 1185, One Brookings Drive, St. Louis, MO 63130

MECHANICAL BEHAVIOR OF BULK NANOSTRUCTURED METALS: W.W. Milligan, A.N. Fisher, J.E. Carsley, E.C. Aifantis, Michigan Technological University, Houghton, MI 49931-1295

Mechanical behavior of iron-copper alloys and tungsten alloys processed by attritor milling and consolidated by HIP or rapid forging will be discussed. Fully dense metals with grain sizes varying between 40 nm and 1700 nm were produced and tested in tension and compression. Behavior similar to that of metallic and polymeric glasses, including a strong tension/compression asymmetry, shear banding, and an apparent pressure dependence of the yield criterion, were all observed. Experiments at 77K, elevated temperature, and at various strain rates will be presented. The support of the National Science Foundation and the Air Force Office of Scientific Research are gratefully acknowledged.

2:30 pm

MECHANICAL ALLOYING, CONSOLIDATION, AND MECHANICAL PROPERTIES OF INTERSTITIAL AND SUBSTITUTIONAL ELEMENT ADDITION TO IRON ALLOYS: J. Rawers, D. Smith, D.Cook1, U.S. Dept. of Energy, Albany (Oregon) Research Center, 1450 Queen Ave. SW, Albany, OR 97321; 1Old Dominion University, Norfolk, VA

Alloying iron with interstitial and substitutional alloying elements affects mechanical properties differently. In this study interstitial (carbon and nitrogen) and substitutional (Al, Cr, Nb, and Ti) was mechanically alloyed into iron powder. The milled powder was subsequently compacted (hot-pressed) and several mechanical properties evaluated (hardness, strength). Powder characterization based on grain size, X-ray diffraction, and Mossbauer analysis, showed that a finer nanostructure results from alloying with interstitials. Characterization of compacts showed is little effect on the compacts due to alloy additions. Tensile and compression strengths were also found to be relatively independent of alloy addition, but characteristic of compacted ceramics.

2:55 pm

EFFECT OF B ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MECHANICALLY ALLOYED TiAl ALLOYS: H.H. Chung, N.J. Kim, Center for Advanced Aerospace Materials, Pohand University of Science and Technology, San 31, Hyojadong, Pohang, 790-784, Korea

TiAl alloys have been produced by mechanical alloying in an attritor mill using pre-alloyed powders. The mechanically alloyed (MA) powders were consolidated by vacuum hot pressing (VHP) at 1000°C for 2h. To prevent the coarsening of grain size during consolidation, B was added to the alloys. The hot pressed billets were subjected to various heat treatments and mechanical properties were measured by compression testing at room temperature. Yield strengths of MA alloys ranged from 736 MPa to 2720 MPa depending on the heat treatment. Heat treated materials showed considerable compressive ductility without any microcracking detected. Effects of microstructure on the mechanical properties of the MA materials are discussed.

3:20 pm BREAK

3:35 pm INVITED

HIGHEST STRENGTH PILE-UP PREDICTIONS FOR NANOMATERIALS: R.W. Armstrong, Department of Mechanical Engineering, University of Maryland, College Park, MD 20742-3035

The dislocation pile-up basis originally proposed for the Hall-Petch linear relation of flow strength versus the reciprocal square root of average grain diameter is pushed to the theoretical limit of nanometer grain sizes, say, as was proposed for bcc and fcc materials (R.W. Armstrong, "Strength Properties of Ultrafine Grain Metals", in ULTRAFINE GRAIN METALS, Syracuse University Press, 1970, 1-25). For the smallest pile-ups, the dislocation line tension is an important consideration that leads to a fall-off from the H-P extrapolation for bcc metals but increase in strength for fcc materials. Stress jumps needed for yielding at smaller pile-up lengths are smeared out, apparently, for real material grain size distributions and orientations. Intrinsic obstacle strengths, plastic instability, and inclusion effects are added complexities to consider.

4:05 pm

MECHANICAL CHARACTERIZATION OF RAPIDLY STRAIN HARDENING ALLOYS MP35N AND 70/30 BRASS ìINFLUENCE OF COLD WORK AND STRESS STATE ON THE DEFORMATION RESPONSE: E.A. El-Danaf, Ebrahim M. Shaji, S.R. Kalidindi, R.D. Doherty, Department of Materials Engineering, Drexel University, Philadelphia, PA 19104

MP35N (35% Co, 35% Ni. 10% Mo, 20% Cr) exhibits a remarkable combination of ultrahigh strength, high ductility, high fracture toughness. The alloy was found to show a 50% reduction in area before failure in tension tests of cold drawn and aged samples. 70/30 Brass was shown to exhibit similar strain hardening behavior. Although the materials start with grain sizes of the order of microns, during deformation, twinning reduces the slip length to the nanoscale. Our study includes determining: (i) strain hardening rate versus stress (or strain) for different stress states in simple compression plane strain compression and shear of annealed MP35N and 70/30 Brass. (ii) magnitude of secondary hardening as a function of prior cold-work level for different stress states for MP35N. (iii) influence of cold-work and aging on tensile ductility of cold drawn MP35N. (iv) propensity of alloy to exhibit extensive macro-scale shear banding in deformed plus aged condition.

4:30 pm

MECHANICAL PROPERTIES OF NANOCRYSTALLINE Ni: B.R. Elliot, J.R. Weertman, Northwestern University, Evanston, IL 60208

The influence of improved processing on the internal structure of nanocrystalline Ni has led to improved mechanical properties. Correlations between the internal structure (including grain size, pore size distribution, and impurities) and results of a variety of mechanical property measurements will be presented (including microhardness, tensile, and compression tests). Some comparison will also be made between samples produced by traditional inert gas condensation (IGC) and the newer jet blown arc IGC. Possible deformation mechanisms will be discussed in light of the structure and property measurements. *Research supported by Department of Energy Grant # DE-FG-02-86ER45229.

MICROSTRUCTURE EVOLUTION, CHARACTERIZATION AND MODELING: Session IV: Solid-Solid Systems I

Session Chair: G. Spanos, Naval Research Laboratory, Code 6324, 4555 Overlook Ave. SW, Washington, DC 20375-5000

EXPERIMENTALLY DETERMINED NUCLEATION AND GROWTH RATES DURING RECRYSTALLIZATION: B.R. Patterson, S. Grandhi, M.J. Papo, Department of Materials and Mechanical Engineering, University of Alabama at Birmingham, Birmingham, AL 35294-4461

The effect of the amount of cold work on nucleation and growth rates during recrystallization has been studied using Al-0.23% Cu. Nucleation rates were determined from estimation of the number of grains per unit volume as a function of time. Both a site saturated nucleation component and a component with a continuously increasing nucleation rate were observed, with the magnitudes of both components increasing with the amount of cold work. Growth rates, computed from the Cahn-Hagel equation, were found to remain constant throughout recrystallization, with higher rates at greater amounts of cold work.

2:30 pm

COMPUTER SIMULATION OF MICROSTRUCTURAL EVOLUTION IN Ni-RICH TiNi SHAPE MEMORY ALLOYS: D.Y. Li, L.Q. Chen, Department of Materials Science & Engineering, The Pennsylvania State University, University Park, PA 16802

Microstructural evolution in a coherent two-phase mixture is driven by the minimization of the total free energy which includes the bulk chemical free energy, the interfacial energy and the elastic energy generated by coherency strains, as well as by external stresses or strains. A particular case, Ti₁₁Ni₁₄ precipitation in TiNi shape memory alloys, was investigated using computer simulations based on a continuum field model in which the orientational difference between precipitate variants is distinguished by structural field variables whereas the compositional difference between the precipitate and the matrix is described by a concentration field variable. The temporal variation of the field variables is determined by numerically solving the time-dependent Ginzburg-Landau equations and the Cahn-Hilliard diffusion equation. Precipitate morphologies and interactions under strain-constraints were studied. Influences of a strain gradient on the morphology, migration, and the variant distribution was also analyzed. Work supported by the U.S. Office of Naval Research.

3:00 pm

COMPUTER SIMULATION OF MORPHOLOGICAL EVOLUTION AND STRESS-INDUCED RAFTING OF PHASE IN Ni-Al SUPERALLOYS: D.Y. Li, L.Q. Chen, Department of Materials Science & Engineering, The Pennsylvania State University, University Park, PA 16802

The morphological evolution and coarsening kinetics of phase particles in Ni-Al superalloys under applied external stresses were investigated. A computer simulation model based on a diffuse-interface kinetic field model was employed. In this model, a two-phase microstructure is described by a concentration field and a three-component long-range order parameter field. The temporal evolution of the field variables was determined by solving the non-linear Ginzburg-Landau and Cahn-Hilliard equations. The elastic inhomogeneity was taken into account using the effective medium approximation recently proposed by Khachaturyan. It is shown that an applied stress results in rafting structures whose orientations depend on the magnitude and direction of the applied stress. The effect of intrinsic coherent stress and the applied stress on the two-phase equilibria will be discussed. Work supported by the U.S. Office of Naval Research.

3:30 pm

MICROSTRUCTURE DEVELOPMENT IN Ni-BASE SUPERALLOY IN738LC: Ercan Balikci, A. Raman, R. Mirshams, Louisiana State University, Baton Rouge, LA 70820

IN738LC is one of the recently developed Ni-base superalloys, having some prominent features for high temperature applications. Like the other superalloys, IN738LC owes these exceptional features to its fcc Ni-rich matrix strengthened by gamma prime, L12 Ni3(Al, Ti), precipitate phase. The volume fraction of the precipitate phase in this alloy is about 40-43%. A standard heat treatment is generally applied to IN738LC, which is a solution treatment of 1120 C/2h/AC or AAC and a subsequent aging treatment at 850°C/24h/FC. In this present study it was observed that this solution treatment neither produces a single phase solid-solution matrix, nor do the aging treatments subsequent to this solution treatment change the microstructure appreciably. It is found in this study that 1225°C is the lowest solutionizing temperature with 4 hours holding time and cooling to room temperature by AAC (accelerated air cooling) or WQ (water quenching) to produce the single phase solid-solution. Aging treatments were performed subsequent to 1200 C/4h/AAC and 1250°C/24h/AAC solution treatments, and similar microstructures were obtained. A precipitate morphology change from spheroidals to cuboidals was observed at 1050°C. A very important result of this study is that no double aging is necessary to develop a unimodal-cuboidal precipitate microstructure as cited in the literature. A single aging treatment at 1050°C for 24 hours subsequent to the solution treatment at 1200°C/4h yields unimodal-cuboidal precipitates. IN738LC shows a unimodal grain size microstructure up to 1140°C beyond which a very distinct duplex precipitate grain size microstructure sets in. Below 1140°C, precipitate grain growth was through precipitate coalescence and solute absorption from the matrix. Above 1140°C, precipitate dissolution is more favorable. The activation energy calculations show that beyond 1140°C, the precipitates are in dissolution mode, not in growth.

4:00 pm

MICROSTRUCTURAL EVOLUTION OF DEFORMED Ni-BASE SINGLE CRYSTAL SUPERALLOYS PRIOR TO RECRYSTALLIZATION: A.M. Dalley, H. Dong, D. Zhao, W.L. Moore, J.J. Valencia, Concurrent Technologies Corporation, 1450 Scalp Avenue, Johnstown, PA 15904

Single crystal nickel-base superalloys CMSX-6 (1st generation) and CMSX-4 (2nd generation) have been examined for microstructural evolution of the gamma prime strengthening phase in the as-cast and thermo-mechanically processed (TMP) conditions. The goal of this work is to advance the ability to manufacture components from cast single crystals. After being solution heat treated, specimens were plastically deformed by compression, rolling, or bending. Combinations of processing temperatures, strain rates and total strain produced non-recrystallized and recrystallized microstructures, including some that exhibited a secondary transformation of the cellular precipitation type. Those that did not recrystallize were subsequently heat treated to evaluate the ability of the microstructure to resist recrystallization. Light optical, SEM and micro-orientation textural analyses were performed to document the gamma prime microstructural evolution. This work was conducted by the National Center for Excellence in Metalworking Technology, operated by Concurrent Technologies Corporation, under contract N00140-92-C-BC49 to the U.S. Navy as part of the U.S. Navy Manufacturing Science and Technology Program.

4:30 pm

STUDY OF THE PRECIPITATION KINETIC OF Al-Zn AND Al-Mg ALLOYS USING DILATOMETRIC TECHNIQUES: Ney José Luiggi, Mirna Betancourt1, Glenys Hernandez and Luis Acuña, GFM. Dpto. de Física. Escuela de Ciencias, Universidad de Oriente, Cumana 1I, UT Cumana, Apdo. Postal 299, Sucre, Venezuela

A dilatometric study was undertaken to characterize the precipitation process in Al-26\% wt. Zn and Al-12\% wt. Mg binary alloys. The samples were cut in 26x4x4 mm³ parallelepipeds and homogenized at 500°C for 15 hours. The measurements were taken in an automatic 402 E NETZSCH dilatometer with a sensibility of 0.2 m_m. A previous study at different heating rates permitted us to determine the interval of temperature where the dilatation process exhibits a slope change, which is associated with a structural transformation process. This temperature range lies between 142 and 310°C for the Al-Zn alloy and between 240 and 417°C for the Al-Mg alloy. Isothermal aging of the samples were carried out in those temperature ranges, three different behaviors being observed for the Al-Mg alloy: a) A moderate initial growth to a peak followed by a decrease of the volume of the samples in the temperature range between 242 and 280°C. b) A monotonous and steady growth in the neighborhood of 320°C and c) A violent initial growth and a subsequent waning in the volume of the samples. All these behaviors indicate different precipitation processes. The Al-Zn alloy manifest a violent volume growth, followed by a decrease associated with the dissolution of the phase previously developed. The thermal expansion curves, smoothed and normalized to the maximum of the volume change at a given temperature, were used to obtain the precipitated or dissolved fraction of the phases formed at each aging temperature. At the end of this aging process, these observances allowed us to determine the solubility curve of the different phases under study. A Jhonson-Melh-Avrami type model is proposed to analyze the kinetics and evaluate the typical kinetic parameters for each alloy. In the particular case of the Al-Zn alloy we detect two processes with different activation energy, the values of which are (50 ± 5) Kcal/mol for the lowest temperature and (30 ± 5) for temperatures between 270 and 310°C. A similar study was undertaken for the Al-Mg alloy, the results being in agreement with the experience. TTT curves are reported for each case.

MICROSTRUCTURE PROPERTY RELATIONSHIPS IN / TITANIUM ALLOYS: Session I: Developments in Ti-6-22-22 and Other Alloys

Session Chair: Patrick L. Martin, Rockwell Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360-2398

ALPHA/BETA TITANIUM ALLOY DEVELOPMENT FOR HIGH SPEED AIRCRAFT STRUCTURES: William D. Brewer, Terryl A. Wallace, R. Keith Bird, NASA Langley Research Center, Mail Stop 188A, Hampton, VA 23681-0001

Alpha/Beta titanium alloys are among those being considered for strength, stiffness and fracture critical applications on advanced high speed aircraft. However, to meet some projected mission requirements, alloy performance must be improved. This presentation will discuss on-going research on some conventional and emerging alloys, including Ti-6Al-2Mo-4Zr-2Sn-0.08Si (Ti-6242S), Ti-6Al-2Cr-2Mo-2Zr-2Sn (Ti-62222), Ti-4Al-4Mo-2Sn-0.5Si (IMI 550), and Ti-4.5Al-5.4Mo-2Cr-1Ni (Corona X), that show promise for application to high speed aircraft structures. Effects of heat treatment, cryo- and elevated-temperatures, and selected processing on alloy microstructure and mechanical properties will be discussed. The emphasis is on improving strength-toughness combinations in sheet product.

2:40 pm INVITED

THE EFFECTS OF MICROSTRUCTURE AND COMPOSITION ON THE PROPERTIES OF IMI-550: R.R. Boyer, Boeing Commercial Airplane Group, P.O. Box 3707, MS 6H-CJ, Seattle, WA 98124; R.J. Lederich, McDonnell Douglas Aerospace Co., P.O. Box 516, MC S111-1041, St. Louis, MO 63116: R.J. Tisler, McDonnell Douglas Aerospace Co., 2401 E. Wardlow Rd., MC C071-0034, Long Beach, CA 90807

The effects of heat treatment on the microstructure of IMI 550 (Ti-4Al-4Mo-2Sn-0.5Si) on the resultant properties will be discussed. Heat treatment parameters studied include solution treatment temperature, cooling rate from the solution treatment temperature, and aging temperature. The effects of these variables on tensile, fatigue, fracture toughness and crack-growth resistance will be discussed. Generally, as the solution treatment increases and the cooling rate decreases, fracture toughness and crack-growth resistance are improved - the trends for tensile and fatigue strengths are not as well defined. Chemistry modifications of this alloy, reduced O₂ and reduced O₂+Si were also investigated. These modifications provided a significant toughness increase with a moderate reduction in tensile strength. The fatigue properties of the reduced O₂+Si were somewhat better than the "standard" and low O₂ grades of the alloy, though the differences were not large. Rationale for the trends observed will be provided.

3:20 pm

KINETICS OF ORDERING IN Ti-6Al-2Mo-2Cr-2Sn-2Zr: X. Tang, H.J. Rack, Clemson University School of Chemical and Materials Engineering, 501 Rhodes Hall, Clemson, SC 29634-0922

A significant loss in fracture toughness may be observed in Ti-6Al-2Mo-2Cr-2Sn-2Zr containing 0.1 to 0.3 wt.% Si following thermal exposure to temperatures between 1000 and 1200°F. Prior explanations for this loss have centered upon either silicide formation and/or ordering of the alpha matrix. This investigation has considered the kinetics of the latter ordering reaction as determined by in-situ electrical resistivity measurements. It will examine the effects of alpha phase composition and Si content on the ordering kinetics, the former controlled by prior alpha-beta solution temperature, the latter by overall alloy composition. Finally, analysis and separation of these contributions to the fracture toughness loss in Ti-6Al-2Mo-2Cr-2Sn-2Zr will be presented.

3:40 pm

MICROSTRUCTURE-PROPERTY RELATIONSHIPS IN Ti-6-22-22: X.D. Zhang, H. Fraser, Department of Materials Science and Engineering, P. Bonniwell, W. Baeslack, Welding Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210; D.J. Evans, ML/Wright Laboratories, WPAFB, OH; T. Ginter, T. Bayha, Lockheed-Martin Co., Marietta, GA

A wide range of microstructures has been effected through heat-treatment of the alloy Ti-6Al-2Mo-2Cr-2Sn-2Zr with and without additions of 0.2Si, and these have been characterized in detail using optical metallography, scanning electron microscopy and high resolution and analytical transmission electron microscopy. It is shown that in conditions of heat-treatment which resemble those adopted commercially, the w phase and also (2 are found to be present. Silicides are also observed to form, but only after relatively high temperature anneals. The compositional variations that result from and / heat-treatments and following aging in the temperature range 480 to 600°C have been determined using EDS in the TEM. The tensile strengths and ductilities, and fracture toughnesses of samples in these various heat-treated conditions have been measured, and the relationship between these properties and the microstructures has been determined. This research has been supported by the U.S. Air Force.

4:00 pm

THE MICROSTRUCTURAL BASIS OF TOUGHNESS LOSS IN AGED Ti-6-22-22: Z.M. Wang, R. Crooks, Analytical Services & Materials, Inc., 107 Research Drive, Hampton, VA 23666

The aging temperature has a significant effect on the mechanical properties of the / titanium alloy Ti-6222. Two product forms of alloy Ti-62222, 2 in. (50 mm) plate and 0.060 in. (1.5 mm) sheet, have been studied in an attempt to explain a drop in toughness associated with aging above 1000°F. Samples of plate products aged 8 hours at 1000 and 1100°F, and sheet product aged 8 hours at 1000, 1100, and 1200°F were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Microstructures and fracture behavior were studied in both product forms, and the alpha phase deformation mode was examined in the sheet material. A monolithic, FCC interface phase was found in ion-milled foils of low-toughness plate, and the volume fraction of a ₂ (Ti₃Al) precipitates increased with higher aging temperatures. Dislocation analyses of deformed sheet samples revealed a slip mode transition and a decrease in the number of active slip systems after aging at higher temperatures. The changes in dislocation behavior are apparently related to the precipitation of ₂.

4:20 pm

PHASE TRANSFORMATIONS IN Corona-X (Ti-4.5Al-5Mo-2Cr-1Ni): P. Martin, Rockwell Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360-2398; H.J. Rack, Clemson University School of Chemical and Materials Engineering, 208 Rhodes Hall, Clemson, SC 29634-0922

This presentation will examine the phase transformations observed in - Corona-X titanium following rapid cooling from solution treatment at temperatures between 750 and 910°C. The types and extent of these transformations were found to be functions of the amount/compositions of the a and b phases, the latter having been determined by quantitative x-ray analysis. BSE, X-ray, and TEM have shown that as the phase stability increases the morphology of the resulting microstructures progressively changes from martensitic platelets to retained equiaxed . TEM analysis of retained further indicates that athermal w may also form on cooling from the solution treatment temperature. These results will be discussed by considering the observed transformations as competitive processes involving the formation of the martensitic and athermal w phases.

4:40 pm

THE EFFECT OF SOLUTION TREATMENTS ON THE MICROSTRUCTURE AND PROPERTIES OF THE + TITANIUM ALLOY Ti-4Al-4Mo-2Sn-05Si (wt %): L.J. Hunter, M. Strangwood, The University of Birmingham, School of Metallurgy and Materials, Elms Rd., Edgbaston, Birmingham, B15 2TT, UK

The room temperature tensile strength and fracture toughness have been determined for Ti-4Al-4Mo-2Sn-0.5Si (wt %) for solution treatment in the and fully phase fields followed by cooling at three rates prior to a standard aging treatment. The resulting microstructures have been characterized by transmission electron microscopy and related to the mechanical property behavior determined. Failure in the + solution treated specimens was controlled by interfacial regions between primary a and transformed ( and within the transformed , whilst ductile tearing also occurred in the fully solution treated material.

5:00 pm

QUANTIFYING THE AGEING RESPONSE OF MARTENSITE IN THE Ti-6Al-2Sn-4Zr-6Mo (wt.%) ALLOY: M. Strangwood, D.J. Meadows, The University of Birmingham, School of Metallurgy and Materials, Elms Road, Edgbaston, Birmingham, B15 2TT, UK

Samples of Ti-6246 have been quenched to a fully ('') microstructure prior to aging for up to 4 hours at temperatures of 650, 700, and 750(C. The resulting material was characterized microstructurally using transmission electron microscopy and by microhardness. In addition, ultrasonic methods were used to follow variations in Young's modulus and internal friction for in-situ aging over the same time and temperature regimes. The aging treatments produced microstructures consisting of mixtures of '', w and , which have been related to the variation in physical and mechanical properties recorded.

NON-LINEAR EFFECTS IN MATERIALS SCIENCE: Session II: Phase Transformations

Session Chair: L. Brush, Dept. of Materials Science and Engineering, University of Washington, Seattle, WA 98195

MODELS OF BOUNDARY MOTION DURING PHASE TRANSFORMATIONS: Robert F. Sekerka, University Professor, Physics and Mathematics, Carnegie Mellon University, Pittsburgh, PA 15213

Modeling of the motion of the free boundaries that separate phases during a phase transformation is an intrinsically nonlinear problem. For a few simple shapes, namely quadric surfaces, analytical solutions are possible by the method of Ivantsov or equivalent. For shapes that depart only slightly from these simple shapes, one can obtain approximate solutions by means of perturbation theory, as is done in the linear theory of morphological stability. Some weakly nonlinear results can also be obtained by carrying out perturbation theory to second and third order. This leads to information about the nature of the bifurcation at the onset of instability, as will be illustrated for perturbations of spheres and circular cylinders. For more complicated growth forms, one must resort to numerical methods or computer simulations. Examples such as the boundary integral method and the phase field model will be discussed, along with typical results. Strengths, limitations and prognoses for future improvements of these methods will be discussed. This work is supported by the National Science Foundation under grant DMR 9634056.

2:30 pm INVITED

NON-LINEAR INFLUENCE OF CRYSTAL-MELT CAPILLARITY ON DENDRITE SHAPE AND GROWTH SPEED: M.E. Glicksman, M.B. Koss, Afina Lupulescu, J.C. LaCombe, L.T. Bushnell, Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180-3590

Dendritic patterns depend on the transport of heat and solute from the crystal-melt interface, and on capillarity occurring at that interface. The planforms and 3D shapes of dendrites are affected non-linearly by the material and the growth direction of the prunary axis, both of which alter the interracial energy and its anisotropy. The kinetics of dendrite growth were studied under microgravity conditions in two space flight experiments, called Isothermal Dendrite Growth Experiment (IDGE), flown by NASA in March, 1994, and in March, 1996. Results from these space flight experiments will be discussed, including the relationship of tip shape and speed to the interfacial energy and its anisotropy.

3:00 pm INVITED

NUMERICAL SIMULATION OF DENDRITIC ALLOY SOLIDIFICATION USING A PHASE FIELD METHOD: W.J. Boettinger , J.A. Warren, Metallurgy Division, Materials Science and Engineering Laboratory, NIST, Gaithersburg, MD 20899

The phase field method provides a promising approach to the description of solidification phenomena. In binary alloys, realistic microsegregation patterns produced by dendritic growth have been obtained. The phase field method enjoys much of its success because it removes the difficult numerical problem of tracking the liquid-solid interface, by giving up the notion of a mathematically sharp interface. A benefit of the diffuse interface model is that it naturally allows for topology changes. Two such changes, the coalescence of dendritic sidebranches during growth and fragmentation of dendrites during melting, are examined here.

3:30 pm BREAK

3:40 pm INVITED

ANHARMONIC CONTRIBUTIONS TO THE VIBRATIONAL ENTROPY OF PHASES OF CO₃V: B.T. Fultz and Laura Nagel, California Institute of Technology, Pasadena, CA 91125; J.L. Robertson, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831

Inelastic neutron scattering was used to measure phonon density-of-states (DOS) curves of CO₃V with the L12, hP24, and fcc phases at temperatures from 625 to 1060 C. The phonon DOS curves showed a significant dependence on temperature owing to enharmonic lattice vibrations. At low temperatures, the difference in vibrational entropies of the L12 and hP24 phases is about (0.12±0.03) kB/atom, with the vibrational entropy of the L12 phase being larger. Phonon softening causes the entropy of the hP24 phase to increase by about 0.055 kB/atom per 100°C, however. Such large enharmonic effects are important in understanding the thermodynamics of phase transformations that occur at elevated temperatures. This work was supported by the U. S. Department of Energy under contract DE-FG0396ER45572.

4:10 pm INVITED

AN ANALYSIS OF THE MORPHOLOGY AND STRUCTURE OF G.P. ZONES: J.K. Lee, Dept. of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931; H.I. Aaronson, Dept. of Materials Science and Engineering, Carnegie Mellon, University, Pittsburgh, PA 15213

Using the Discrete Atom Method to examine the elastic strain energy associated with arbitrarily-shaped, elastically-inhomogeneous, coherent precipitates, the three-dimensional morphology and structure of Guinier-Preston zones in a model system similar to Al-Cu are investigated. As the anisotropy factor, A = 2C44/(C,C,2), increases, the shape of G. P. zones changes from globules (A = 0.4) to five-atomic-layer plates (A= 3.2), and then to three-layer plates (A = 10). The internal structure is found to depend on the Cu-A1 bond length; the smaller this length, the greater the volumetric misfit strain energy. At a given value of A, if the Cu-AI bond length is taken to be similar to that of Cu-Cu, the G. P. zones remain pure fcc Cu, whereas if this length is close to that of Al-AI, the zones form an ordered crystal with tetragonal symmetry, a structure reminiscent of the theta-double-prime phase in this alloy system.

4:40 pm

CRITICAL EVENTS IN EVOLVING MICROSTRUCTURES: John W. Cahn, W. Craig Carter, Jean E. Taylor, Mathematics Department, Rutgers University, Piscataway, NJ 08540; Materials Science and Engineering Laboratory, NIST, Gaithersburg, MD 20899

Symmetry considerations are important in a Landau-type of bifurcation theory for determining some characteristic features of higher order phase transitions and spinodals in multicomponent alloys, not only for phase diagrams, but also for critical parameters that determine major differences in how the microstructures evolve. Similar symmetry considerations affect interfacial phase transitions and their effect on microstructure evolution. Phase separation in ternary alloys will be compared with the onset of edge and corner formation and faceting in evolving crystal shapes in three dimensions. Ordering of fcc to L1₂ and L1₀ will be compared to ordering of bcc to B₂ and DO₃, not only with regard to major differences in phase diagram features, but also with respect to critical aspects of their respective interfaces and two-phase and domain microstructures.

5:00 pm INVITED

ORDER PARAMETER COUPLING AND KINETICS OF PHASE TRANSFORMATIONS: L.Q. Chen, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802

The formation and sequence of various morphological patterns during phase transformations, as a result of nonlinear coupling between different order parameters, were investigated. Cases to be discussed include coupling between (I) a composition and a long-range order parameter, (II) a composition and the transformation strain, (m) a long-range order parameter and the transformation strain, and (V) the transformation strain and an applied stress. Examples to be presented include the kinetics of precipitation of ordered intermetallic precipitates, transformation-strain-induced morphological evolution, and microstructural evolution under an external applied stress. Where possible, the morphological patterns and their evolution predicted from our computer simulations will be compared with experimental observations in practical alloy systems.

5:30 pm

UNSTEADY CONVECTION AD LAYERED STRUCTUR FORMATION IN PERITECTIC SYSTEMS: P. Mazumder, R.K. Trivedi, Ames Laboratory U. S. Department of Energy and Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011

The solidification of two phase microstructures in peritectic systems has recently received quantitative examination. Both the diffusive and boundary layer models have been developed to predict the conditions under which a layered growth can occur. Experimental studies, however, have shown that neither of these models are operative in the experiments carried out in the Pb-Bi and Sn-Cd systems. The layered structure, in fact, is an oscillatory structure that is interconnected in three dimensions. We shall show that these interconnected structures evolve due to unsteady convection in the melt. A theoretical model, which includes oscillatory convection, has been developed that shows that the oscillatory behavior of the solute profile in the liquid can give rise to the simultaneous growth of the primary and peritectic phases in which the primary phase forms as a macroscopic Christmas tree. This unique structure forms in a nonlinear regime and the results of numerical computation based on the mass, momentum, and heat transport equations will be presented. The importance of oscillatory convection will be demonstrated through experimental results in the Pb-Bi and Sn-Cd system in which the oscillatory structures disappear when the alloy is solidified in finer tubes in which convection effects are minimized.

PROCESS MODELING IN THE HIGH TEMPERATURE MATERIALS INDUSTRY: Session II

Session Chair: Carlos Ruiz, Allied Signal Aerospace, 1130 W. Warner Road, Tempe, AZ 85284

APPLICATION OF PROCESS MODELING TO AIRCRAFT ENGINE FORGINGS: Shesh K. Srivatsa, GE Aircraft Engines, Mail Drop M87, 1 Neumann Way, Cincinnati, OH 45215

This paper will describe the application of process modeling to the forging of critical aircraft engine rotating components. With parts getting larger in size and processing windows for new materials getting tighter, process modeling is a key tool in making new parts in a cost and time effective manner with improved quality and satisfactory properties. Today process modeling for forged components is a production tool and an up-front requirement and a key factor in "making right the first time" parts of size/shape/materials for which there is no prior experience. Generation of modeling inputs (i.e., material property data and boundary conditions) and validation of outputs will also be discussed. Without realistic inputs and validated outputs, the full potential of modeling cannot be realized.

2:30 pm

HOT WORKABILITY OF ALLOY C-276: G.T. Velarde¹, C.J. Van Tyne², Y-W Cheng³, F.S. Suarez⁴, ¹Kaiser Aluminum, Spokane, WA, ²Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401, ³NIST, Boulder, CO, ⁴INCO Alloys International, Huntington, WV

Alloy C-276, a nickel-based superalloy used primarily in flue-gas desulfurization scrubbers, has at times exhibited diminished ductility and surface cracking during hot rolling. Hot workability of Alloy C-276 was investigated by hot-compression testing on a Gleeble 1500. The as-received material exhibited extremely high workability at temperatures above 870°C. Below this temperature, recovery and recrystallization were sluggish, causing lower workability. In order to simulate more accurately the conditions of the commercial hot rolling operation, heat treatments in an oxidizing atmosphere were also used to control both surface condition and grain size prior to compression testing. The resulting ductility was lower than in the as-received material for all strain rates and temperatures tested. Surface oxides were observed to promote cracking by providing a brittle initiation site. Large grain size as well as segregation of residual elements to grain boundaries probably also enhanced cracking. A ductility trough was detected in the test temperature range from 925°C to 1040°C. A possible linkage between this trough and the precipitation of P-phase carbides is postulated. This work was conducted at the Advanced Steel Processing and Products Research Center, an NSF sponsored industry-university cooperative research center, at Colorado School of Mines.

3:00 pm

APPLICATIONS OF MODELING TO HOT DEFORMATION PROCESSES AT ALLVAC: Ramesh S. Minisandram, Laurence A. Jackman, Robin M. Forbes Jones, Chris OíBrien, Allvac-An Allegheny Teledyne Company, P.O. Box 5030, Monroe, NC 28110-0530

Process modeling at Allvac encompasses melting and solidification processes (VAR, ESR, PCHM) and hot deformation processes such as open die press forging, radial forging and rolling. Following a brief overview of these activities, specific applications of radial forging and rolling simulations will be provided. Improvements in computer technology have greatly extended modeling capabilities; however, limitations still prevail because of insufficient material property information and the inability to predict metallurgical features such as segregation during solidification and microstructures developed during hot deformation. Cooperative efforts with various research organizations are under way to address these issues.

3:30 pm

PROCESS MODLES: TOOLS FOR KNOWLEDGEABLE PROCESSES ENGINEERS AND METALLURGISTS: R.A. Jaramillo, D. Lambert, S.J. Patel, Inco Alloys International, 3200 Riverside Drive, Huntington, WV 25705-1771

A philosophy for developing and implementing relevant process models is presented along with current endeavors in process modelling. The recent advances in computational ability brought about by developing computer technologies is making process modelling a viable method for enhancing process control in industry. However, the investment in time and resources required to procure and develop accurate process models is significant. Issues regarding material properties, boundary conditions, numerical methods, hardware, etc. expand the expertise required to develop process models. Identifying the needs of process engineers and metallurgists as a foremost priority, a methodology for efficiently employing process models evolves. Also, current efforts in development and use of process models at Inco Alloys International is presented.

4:00 pm

PROCESS MODELING OF SHAPE CHANGE DURING HOT ISOSTATIC PRESSING: J.J. (Sean) Conway, F.J. Wulczynski, Crucible Compaction Metals, 1001 Robb Hill Road, Oakdale, PA 15071

Thousands of powder metallurgy (P/M) parts made via hot isostatic pressing (HIP) have been produced commercially over the last twenty years. Several of these parts are near net shapes with complex two-dimensional or three-dimensional geometries. Since HIP P/M parts undergo volumetric shrinkage while consolidating to full density, the starting steel can which contains the powder must be designed appropriately to produce the desired shape. Unfortunately, the dimensional shrinkage is not uniform and this anisotropy can be difficult to predict. Through the years, HIP manufacturers have employed engineering intuition and previous experience to develop the starting can design. Recently, other approaches have been evaluated, namely empirical and continuum mechanics/finite element modeling. The results of these process models are discussed

4:30 pm

FEEDBACK CONTROL OF MICROSTRUCTURE DEVELOPMENT DURING HOT FORGING: W. Garth Frazier, W.M. Mullins, R. Dennis Irwin, Enrique A. Medina, James C. Malas, Materials Process Design, Materials Directorate, WL/MLIM, Building 653, Wright-Patterson Air Force Base, OH 45433-7746

The advantages of using real-time feedback for the control of applied stress, strain and strain rate during a hot forging operation are investigated. At present, state of the art design for most commercial hot metal forming processes is limited to the use of open loop control. Limitations in model fidelity, however, always affect the performance these open loop control systems. By assuming that reliable sensors are available, this work evaluates the advantages of using real time feedback in a forging process. A high fidelity simulation of a system that includes dynamics of the forging press, workpiece loading, and microstructure evolution is developed. By using real-time measurements of applied stress, strain and strain rate, a closed loop control system for forging processes can modify parameters such as ram velocity and temperature in order to achieve desired deformation paths. Results of closed loop simulations are compared to those of existing open loop design approaches and recommendations are made.

5:00 pm

FINITE ELEMENT ANALYSIS OF RESIDUAL STRESSES IN ELECTROSLAG BUTT WELDS: Leilei Zhang and David Atteridge, Oregon Graduate Institute of Science and Technology, Department of Materials Science and Engineering, P.O. Box 91000, Portland, OR 97291-1000

Electroslag welding is a potentially attractive process for joining thick plates such as those used in constructing ships, storage tanks, pressure vessels, bridges, buildings and other heavy structures. However, concerns about low fracture toughness of the resulting weld have limited the use of the process for some thick-plate applications. Fracture mechanics analyses have generally assumed that the weld-induced tensile stress field is of yield strength magnitude. This assumption greatly reduces the allowable flaw size, and frequently results in allowable flaw sizes smaller than those which can be reliably detected by standard non-destructive testing techniques. In this paper, a computational model is developed to more accurately calculate the magnitude and distribution of residual stresses for electroslag welding of plates. The model consists of two parts, a temperature analysis model and thermal stress model. Elastic-plastic temperature dependent mechanical properties are included in this model. Based on these calculations, critical flaw sizes are reevaluated.