Materials Week '97: Wednesday 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 Wednesday afternoon, September 17.

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

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

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

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

2:35 pm

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

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

3:05 pm INVITED

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

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

3:35 pm

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

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

4:05 pm

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

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

4:35 pm

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

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