Materials Week '97: Monday AM Session

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 session will be held Monday morning, September 15.

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

Session Chair: J.A. Dantzig, University of Illinois, Dept. of Mech & Industrial Eng., 1206 W. Green St., Urbana, IL 61801

THE MICROSTRUCTURE OF HIGH VOLUME FRACTION SOLID-LIQUID MIXTURES: T.L. Wolfsdorf, P.W. Voorhees, Dept. Materials Science and Engineering, Northwestern University, Evanston, IL 60208

The success of a variety of industrial materials production techniques, including liquid state sintering and semi-solid processing, is strongly influenced by the skeletal structures that form at a high volume fraction of solid in solid-liquid mixtures. In order to gain a fundamental understanding of the relation between processing, structure, and properties in these materials, we investigate the morphology and formation of the solid skeleton which forms in high volume fraction solid-liquid mixtures. Using microstructural tomography, a novel technique for imaging 3-D microstructures, we characterize the details of the skeletal topology and connectedness. Electron back-scattered diffraction analysis of the solid particles in the skeleton yields quantitative evidence for the mechanisms that are operative during skeletal formation. Our data suggests a model for skeletal formation and the origin of the skeletal stability. Based on this model, we recommend two specific methods to engineer the properties of these materials. Work supported by the Microgravity Sciences Div. of NASA.

9:05 am

DIFFUSION LIMITED COARSENING OF PRECIPITATES: Martin E. Glicksman, H. Mandyam, S.P. Marsh*, Materials Science and Engineering Department, Chemical Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180-3590; *Naval Research Laboratory, Washington, DC 20375-5343

Interactions among precipitate particles undergoing late-stage diffusion-limited phase coarsening were modeled with several theoretical approaches: 1) time-dependent multiparticle simulations; 2) snap-shot simulations and perturbation methods; 3) nearest-neighbor distribution functions; 4) direct screening, with random field cells. These techniques will be described briefly, and their kinetic predictions for the evolution of a two-phase microstructure compared. Consistency is found at low volume fractions (V_V < 0.1) of the dispersed phase with those methods that permit, or are consistent with, diffusional Debye screening. Leading-order corrections to the coarsening rate constants always start as V_v^1/2 with contributions from dipolar interactions occurring next. At somewhat higher volume fractions (0.10 < V_V < 0.3) correlation effects become more significant, and direct (geometric) screening seems to work well. This work supported by the National Science Foundation, Division of Materials Research, under grant DMR-9633346.

9:40 am

TEM STUDY OF THE TWO-PHASE PARTITIONLESS MICROSTRUCTURE EVOLUTION IN RAPID SOLIDIFIED CO-18.5Al ALLOYS: H. Sieber, D.R. Allen, J. Perepezko, Dept. of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706

Rapid solidification is known to produce solute trapping but in the limit of full partitionless solidification, only one phase typically forms for a given cooling rate or alloy composition. In rapidly quenched cobalt-aluminum (Co-Al) foils for 17.22 at.%Al XRD investigations show two partitionless phases are formed, a face-centered cubic (fcc) and an ordered body-centered cubic (B2) phase. The microstructure of the alloy changes with decreasing cooling rate from the edge towards the middle of the quenched foils. The different microstructure regions in Co-Al splat quenched (SQ) foils have been analyzed in detail by TEM investigations in plan view and cross section geometry. A comparison of the anti-phase domains size to the grain size indicates that the B2 phase solidifies as disordered bcc and orders during cooling in the solid state. The results are compared with partitionless phase evolution observed in other systems and consider in terms of the kinetic constraints imposed by this novel microstructure. The authors gratefully acknowledge the support of NASA (NAGW-2841 and NAG8-1278) and a NASA Graduate Student Researchers Program fellowship (for DRA).

10:15 am BREAK

10:30 am

PHASE FIELD SIMULATIONS OF COALESCENCE AND FRAGMENTATION IN A BINARY ALLOY: James A. Warren, William J. Boettinger, NIST, Metallurgy Division, Bldg. 223, B164, Gaithersburg, MD 20899

In the last few years, many advancements have been made in the use of the phase-field method for solidification modeling. This approach has allowed researchers to predict realistic microsegregation patterns produced by dendrite growth in binary alloys. One of the advantages of this approach, besides its computational simplicity, is that topological changes such as the coalescence of dendritic sidebranches during growth and fragmentation of dendrites during melting are handled without any modification of the model. Recent efforts into three dimensional alloy simulations, as well as directional dendritic solidification will be discussed.

11:05 am INVITED

SIMULATION OF MICROSTRUCTURAL EVOLUTION IN TWO-PHASE SYSTEMS: Veena Tikare, Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87110

Modeling of microstructural evolution presents unique challenges because of the large number thermodynamic, mechanistic and spatial variables which must be considered simultaneously. Microstructural evolution in two-phase systems is further complicated by characteristics of the two individual phases plus their interactive characteristics. These are thermodynamic characteristics such as mutual solubility of each in the other, wetting or non-wetting of each by the others, etc. and mechanistic characteristic such as transport mechanism of each of the two phases. The spatial arrangement of the two phases must also be considered. In this paper, the incorporation of characteristics of two separate phases into the Potts and the phase-field model will be presented. The advantages and disadvantages of each of the models will be discussed. The appropriate application of each of the models will also be discussed. This work was performed at Sandia National Laboratories, supported by the U.S. Department of Energy under contract number DE-AC04-95AL85000.

11:40 am

PHASE FIELD COMPUTATIONS USING AN ADAPTIVE GRID TECHNIQUE: N. Provatas, N. Goldenfeld, J. Dantzig, University of Illinois, Dept. of Mech & Industrial Eng., 1206 W. Green St., Urbana, IL 61801

In recent years, the phase field technique has been developed to model evolution of solidification microstructures. In the method, the liquid-solid interface is modeled as a diffuse region whose thickness is characterized by an order parameter, known as the phase field. One of the difficulties in applying the phase field method is the conflicting requirements of high resolution needed to successfully capture the physical phenomena at the interface, and the simultaneous need to fully resolve the diffusion field ahead of the advancing front. This had led to the use of very dense grids, with concomitantly large computation times, and also to the study of high growth regimes, where the diffusion field is relatively small. In this work, we describe an adaptive gridding procedure for solving the phase field equations, where high resolution is available near the interface, with more appropriate grid dimensions to resolve the diffusion field.