Materials Week '97: Monday PM 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 afternoon, September 15.

MICROSTRUCTURE EVOLUTION, CHARACTERIZATION AND MODELING: Session II: Liquid Solid Systems II

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

MODELING OF THE GROWTH AND INTERACTIONS OF EQUIAXED DENDRITES ON A MESOSCOPIC SCALE: B. Kauerauf¹, I. Steinbach¹, C. Beckermann² and J. Guo¹, ¹ACCESS e.V., D-52056 Aachen, Germany; ²The University of Iowa, Iowa City, IA 52242-1527

The interactions between multiple equiaxed dendritic grains during diffusion-controlled growth into the undercooled melt of a pure substance are modeled using a novel mesoscopic simulation technique. The mesoscopic scale is of the order of the diameter of the dendrite envelopes, which is large compared to the interdendritic spacings. In the model, the calculation of the temperature field in the undercooled liquid is coupled with a modified stagnant film model for dendritic growth, and the evolution of the internal solid fraction inside the grain envelopes is predicted. Three-dimensional numerical results are presented for the transient growth of multiple grains in the presence of strong thermal interactions.

2:30 pm

MESOSCALE MODELING OF CONVECTIVE EFFECTS DURING SOLIDIFICATION: S.P. Marsh and S.G. Lambrakos, Code 6320, Materials Science and Technology Division, Naval Research Laboratory, Washington, DC 20375-5343

A stereology-based modeling approach is presented that combines macroscopic fluid flow with microscopic solidification mechanisms. This mesoscale method allows mass balances arising from buoyancy-driven flows to be coupled directly to local solidification phenomena. Simulation results describing the effect of fluid flow on cellular spacings will be presented and compared with experimental data. This work is being supported by NASA under Grant NAG8-1272.

3:00 pm

DEVELOPING MICRO-SEGREGATION MODELS FOR MULTI-COMPONENT ALLOYS: Vaughan R. Voller, Saint Anthony Falls Laboratory Mississippi River at 3rd Ave., SE University of Minnesota, Minneapolis, MN 55414

During the solidification of an alloy mass transport on the local scale of the secondary dendrite arm spaces is controlled by diffusion. This process is referred to as micro-segregation. The standard micro-segregation models focus on closed systems in binary alloys solidifying in a prescribed fashion. The term closed system implies that the mixture concentration within a given Representative Elementary Volume (REV) of the mushy region remains fixed during the solidification. In the context of a complete solidification model this feature is not realistic. In real solidification systems the mixture concentration in the REV will change with time--Macro-segregation. Furthermore, the alloy may not be binary and the solidification path may not be known a-priori. The object of this paper is to investigate models that can be used to model micro-segregation in multi-component alloys in open REV's (i.e., in a regime with time varying mixture concentrations). The focus will be on: (1) an investigation of the possible numerical schemes for modeling the micro-segregation (e.g., explicit vs. Implicit and fixed grid vs. deforming grid) and (2) a discussion on the coupling between micro and macro scale models, i.e., looking at the question -What REV values should be used to control the micro-segregation model?

3:30 pm

CRYSTALLOGRAPHIC EVOLUTION IN DIRECTIONALLY SOLIDIFIED MICROSTRUCTURES: Krishna Rajan, Jeffrey Trogolo, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590

In this presentation we describe the crystallographic characterization of directionally solidified nickel-based superalloys using electron diffraction techniques. It is shown that there appears to be some maximum level misorientation that is prevalent in these systems. Transmission electron microscopy of the low angle boundaries in the region of the microstructure near the higher levels of misorientations shows high levels of twisting of the grain boundary plane along the axis of solidification growth. The relationship between the evolution of grain boundary plane misorientation and the mechanism of misorientation evolution of the overall sample is discussed.

4:00 pm

MICROSTRUCTURE EVOLUTION DURING SOLIDIFICATION OF A VIGOROUSLY STIRRED MELT: J. Roplekar, J.A. Dantzig, University of Illinois, Dept. of Mech & Industrial Eng., 1206 W. Green St., Urbana, IL 61801

Production of metallic parts using semi-solid forming techniques has become a commercially viable process. The process uses feedstock which is characterized by rounded primary phase, surrounded by solute-rich regions. During reheating, these solute-rich regions melt at lower temperature, resulting in a mixture which is relatively easily deformed into complex shaped parts. In this work, we describe the MHD-DC casting process, wherein a rotating electromagnetic field is used to impart rotation to the melt during solidification. Studies are described to correlate the observed microstructures with experimental conditions.

4:30 pm

THE EFFECT OF IRON CONCENTRATION ON POROSITY FORMATION IN 319 CAST ALUMINUM ALLOY: J.W. Zindel, Ford Motor Company, MD 3182 SRL, P.O. Box 2053, Dearborn, MI 48121-2053

Iron is a ubiquitous element in aluminum alloys. Sources for iron include impurities in the bauxite ore, contamination in the recycling stream, intentional additions to reduce the propensity for die soldering in the die casting process, and poor molten metal handling practices. Iron has been attributed to increasing the propensity for microporosity formation in castings. This work studied the effect of increasing the iron content in a 319 type alloy on microporosity formation in a well fed casting. The casting was a wedge shape with a chill placed at the thin edge of the wedge to generate parallel isotherms progressing towards a large riser and a solidification times which ranged from 15 to 2150 seconds. Four iron concentrations were studied: 0.38, 0.59, 0.83, and 0.95. Density measurements were used to determine porosity levels. The calculated porosity is a strong function of the absolute density of the material and the various techniques used to determine the absolute density will be discussed. The absolute density of the material did increase with iron concentration but the porosity did not appear to be a function of iron concentration in this casting configuration.

5:00 pm

EFFECT OF GRAVITY ON THE MICROSTRUCTURAL EVOLUTION OF TUNGSTEN HEAVY ALLOYS: A. Tewari, A.M. Gokhale, School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta GA 30318

Tungsten heavy alloys are usually manufactured by liquid phase sintering of tungsten powder along with Ni and Fe powders. This results in a two phase material having grains of almost pure tungsten embedded in a matrix of W, Ni, Fe alloy. The evolution of microstructure during LPS also depends on gravity. To understand the role of gravity on the evolution of microstructure, LPS experiments were performed in normal gravity and under micro-gravity conditions of space shuttle. The microstructure of these sintered alloys were quantitatively characterized in detail using Digital Image Analysis to gauge the effect of gravity on the process of LPS. This article presents differences found in the evolution of microstructures of liquid phase sintering under gravity and micro-gravity environment.