Materials Week '97: Tuesday 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 Tuesday morning, September 16.

MICROSTRUCTURE EVOLUTION, CHARACTERIZATION, AND MODELING: Session III: Novel Systems & Methods

Session Chair: S.P. Marsh, Naval Research Laboratory, Code 6325, 4555 Overlook Ave. SW., Washington, DC 20375-5343

GRAIN AREA DISTRIBUTIONS IN EVOLVING THIN FILMS: M.A. Palmer, M.E. Glicksman, K. Rajan. Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, NY

The properties of a thin film for example, mechanical strength, electrical conductivity, and creep resistance depend upon the size of the individual grains which make up the thin film. The distribution of grain sizes as well as the average grain size will therefore be important. In this paper the grain area distribution in an evolving thin film is examined. It is found that two dimensional theories accurately describe the behavior of larger grains. However these models do not take into account the finite thickness or the finite width of the thin film. It has been found that these characteristics cause deviations from what would be predicted by the two dimensional models. Most notably the grain size distribution appears to broaden with time. Several tabulated distributions, as well as those predicted by numerous computer simulations, will be compared with the data, and the implications of the broadening will be discussed.

9:00 am

SIMULATION OF MICROSTRUCTURAL EVOLUTION IN ELECTRONIC GLASS-CERAMICS: Indrajit Sinha, Rajiv Kumar Mandal, School of Materials Science and Technology, Institute of Technology, Banaras Hindu University, Varanasi-221 005, India

It has been recognized that the presence of a connected network of the crystalline phase improves the performance of electronic glass-ceramics. Keeping this in view the theoretical simulation of microstructural evolution of the ceramic phase in a glassy matrix using the static Monte Carlo technique has been done in two and three dimensions. The problem has been approached from the perspective of the percolation theory. A normal distribution of probable sites of the ceramic phase in question has been done and the condition in commensurate with the physical realization that nucleation sites have a low probability of occurrence has been used to identify them.

9:30 am

MICROSTRUCTURE EVOLUTION DURING SPRAY FORMING OF LIQUID IMMISCIBLE ALLOYS: Rajiv Kumar Mandal, S.N. Ohja, School of Materials Science and Technology, Institute of Technology, Banaras Hindu University, Varanasi-221 005, India

Liquid immiscible alloys based on Al-Pb and Cu-Pb Systems were spray deposited using different processing conditions. The scanning electron microscopy of spray deposits invariably revealed uniform dispersion of submicron size lead particles in fine equiaxed grains of the matrix phase. In contrast, the atomized and overspray powder particles of these alloys indicated a bimodal size distribution of lead particles in the intercellular or interdendritic regions. The X-ray diffraction study and EPMA of the preform and atomized powders indicated presence of a new metastable phase in Cu-Pb alloys. The microstructural evolution during spray forming of liquid immiscible alloys will be discussed in light of the heat flow at the gas-droplet interface and consequent rapid solidification of droplets as well as that of the preform during spray deposition process.

10:00 am

CHARACTERIZATION OF THE BETA TO AMORPHOUS PHASE TRANSFORMATION IN A BULK TITANIUM-BASED ALLOY: K.J. Doherty¹, D.J. Li², G.J. Shiflet¹, S.J. Poon² , ¹University of Virginia, Dept. of Materials Science and Eng., Charlottesville, VA 22903; ²University of Virginia, Dept. of Physics, Charlottesville, VA 22901

Partial amorphization of bulk titanium-based alloys was obtained via annealing of a metastable crystalline material. A BCC solid solution (b) was formed in the Ti-Cr-TM (TM = one or more transition metals) system after arc-melting or air-cooling of a solutionized ingot. Initial examinations using x-ray diffraction patterns show a broadening of the major crystalline peaks along with an emergence of an amorphous halo with increasing aging time. Further evaluation using electron microscopy reveals complex phase transformations which vary significantly by modifying the aging temperature and time. Compositional fluctuations also change the amorphization characteristics. The premise for amorphization is that the free energy difference between an amorphous phase and the metastable crystalline phase is lowered by the appropriate additions of the proper transition metals to the binary Ti-Cr system.

10:30 am

ATOMIC-SCALE MEASUREMENT OF COMPOSITION PROFILES NEAR GROWING PRECIPITATES IN THE Cu-Co AND Ni-Al SYSTEMS: Ian Rozdilsky, A. Cerezo, G.D.W. Smith, Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, U.K.

The three-dimensional atom probe can reconstruct the positions of the majority of atoms within a small volume of material to sub-nanometer resolution. This technique has been used to measure atomic scale composition profiles in the immediate vicinity of nanometer scale precipitates during diffusional growth/coarsening in the Cu-Co and Ni-Al systems. Compositional data obtained from the interface of individual precipitates is used to examine the atomic attachment/detachment process.

11:00 am

A MACROSCOPIC MODEL FOR MULTIPHASE PRECIPITATION AS BASED ON THE CLASSIC NUCLEATION AND GROWTH THEORY: Juan C. Márquez, Ney Luiggi A., GFCES, Dpto. de F'sica, Escuela de Ciencias, Universidad de Oriente. Cumaná. Apdo. Postal 299, Sucre, Venezuela

The study of the multiphase precipitation process of a binary alloy system, with the nucleation and growth theory as its starting point, has led us to determine how the concentration of clusters of different sizes varies in a system where the decomposition of the solid solution is simultaneously shifted into a metastable phase and a stable phase. Both the metastable and the stable clusters evolve independently until the onset of the process of dissolution of the metastable phase. This premise leads us to a system of differential equations with as many equations as clusters participate in the process. As all the metastable clusters can be integrated into a single metastable phase and all the stable clusters can be integrated into one single stable phase, this system may well reduced to a macroscopic scheme for each phase, with one single differential equation defining all metastable cluster and one single differential equation defining all stable clusters. The insertion or non-insertion of the nucleation stage in this model depends on how a critical sizes of the stable and metastable clusters are defined and fixed. If the starting clusters are dimmers only the growth stage is characterized, three differential equations being sufficient to typify the process. The solutions obtained are mathematically different from the empirical Johnson-Melh-Avrami model. Our model is applied to the Pb-Ca binary and commercial aluminum alloy, our results being in qualitative agreement with the experiment. The not quantitative agreement is related to how the model is assumed relative to the reaction constant for each type of solute in the system examined.

11:30 am

ON THE TIME FOR COMPLETE RECRYSTALLIZATION: C.H. W[sinvcircumflex]rner, Instituto de F'sica, Universidad Cat--lica de Valpara'so, Casilla 4059, Valpara'so 02, Chile

Usually, Johnson-Mehl-Avrami-Kolmorogov (JMAK) kinetics is employed to follow the recrystallization process. In this paper, by using simple models of nucleation and growth, it is shown that the time for completion is finite (as opposed to the JMAK theory). It was found that in one, two, and three-dimensional systems, with instantaneous nucleation conditions, this time scales as 1/vn, 1/vn^1/2, and 1/vn^1/3, respectively; n being the nucleus density per unit length (area, volume) and v the growing interface isotropic speed.

12:00 noon

CONTAINERLESS DIRECTIONAL SOLIDIFICATION OF TEXTURALLY ALIGNED LAMELLAR TiAl Bimal Kad, Mike Scott,¹ and Dennis Dimiduk,² AIMES-0411, University of California San DIego, LaJolla, CA 92093, ¹UES Inc., 4401 Dayton-Xenia Road, Dayton, Ohio, ²WL/MLLM Materials Laboratory, WP-AFB, Ohio 45443.

The plastically anisotropic response of PolySysnthetically Twinned (PST) colonies of lamellar TiAl is such that the best combination of strength and ductility is obtained when the laminates are aligned parallel to the loading axis. This characteristic mechanical response has led to intensive efforts to texturally align lamellar TiAl microstructures by solidification, or deformation, processing based methodologies. Solidification schemes in the (Ti-48-50at%Al) composition range of interest are particularly complicated on account of closely spaced L+ and L+ peritectic phase fields, where the texture of the primary solidification product dictates the final laminate texture, based on epitaxial solid state transformation schemes. Additionally, the details of the L-> S portion of the phase diagram, in this composition range, are sketchy at best, and any phase boundary variabilities introduced by interstitial impurities or minor ternary additions are unknown. This presentation will report on our progress in producing aligned are unknown. This presentation will report on our progress in producing aligned Lamellar TiAl, within the framework of above mentioned phase diagram uncertainties. In particular efforts are directed at skirting the L+ phase field and extending the L+ phase field, with a view to sustaining a desirable texture in the primary solid.