Program Organizers: Dr.Robert A.Schiffman, R. S. Research Inc., Barton, VT 05822. Professor Carlo Patuelli, Universita di Bologna, I-40126 Bologna, Italy
Monday, PM Room: Orange County 2
February 5, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: Edward A. Sloot, Canadian Space Agency, 6767 rue de l'Aeroport, St-Hubert, Quebec, Canada J3Y 8Y9
INTRODUCTION AND WELCOME BY THE ORGANIZERS
THE INFLUENCE OF CONVECTION AND THERMAL BOUNDARY LAYER THICKNESS ON NUCLEATION OF SOLID PHASE IN INFILTRATED METAL MATRIX COMPOSITES: C. Patuelli, S. Sprio, R. Tognato, Dipartimento di Fisica, Universita di Bologna, Via Irnerio 46, 40126 Bologna, Italy
In a recent work, infiltrated Al metal matrix composites (MMCs) with different volume fractions of ceramic preforms (10,19% Al2O3 and 23% Sic) were melted and solidified up to 720[[ring]]C and the crystallites size distribution were measured by X-ray diffraction analysis. It was found, in particular, that convection and thermal boundary layer (t.b.l.) thickness influence the nucleation rate of the solid phase. The phenomenon was more evident in the sample with the ceramic preform of 10% Al2O3 which has the highest aspect ratio, ie. much higher convection. The aim of this study is to investigate more deeply such phenomenon by melting and solidifying the same MMCs at higher temperatures (greater than 720[[ring]]C.). The results are discussed taking into account the effects of gravity or reduced gravity on t.b.l. thickness and viscosity, ie. on nucleation and growth of the Al matrix solid phase.
X-RAY TRANSMISSION MICROSCOPY STUDY OF THE DYNAMICS OF SOLID/LIQUID INTERFACIAL BREAKDOWN DURING METAL ALLOY SOLIDIFICATION: Peter A. Curreri, Marshall Space Flight Center, National Aeronautics and Space Administration, MSFC, AL 35812; William F. Kaukler, Center for Microgravity and Materials Research, The University of Alabama in Huntsville, Huntsville, AL 35899
In order to study the detailed dynamics of the solidification process, we are developing an apparatus to perform real-time x-ray microscopy of solidifying metallic systems. Only recently has technology become available to achieve this with the necessary resolution, speed, and contrast. We have applied the technique to Al-Cu and Al-Ag single phase alloys to study the dynamics of morphological instability. Due to the x-ray absorbing solute layer, Al-Ag alloys provide better contrast. With a thermal gradient of 45 C/cm, Al-10Ag alloy retains a planar interface up to a growth rate of 1 micron per second. The initial stages of cellular breakdown occur at a rate between 1.5 and 2 microns per second and steady state cellular growth is attained at 2 microns per second. Curvature of the cellular interface was recorded as the interface approached and engulfed voids. For both alloys we could clearly image the interfacial morphology in real time and determine the growth rate dependence on feature size and spacing.
CONVECTION IN HYPERMONOTECTIC ALLOYS AND THE INFLUENCE OF RESULTING COMPOSITIONAL VARIATIONS ON MICROSTRUCTURE: L.J. Hayes, J.B. Andrews, Department of Materials Science and Engineering, University of Alabama at Birmingham, Birmingham, AL 35294
This investigation focuses on the role of convection during vertically upwards directional solidification of hypermonotectic alloys. It should be possible under interfacially stable processing conditions (sufficiently high thermal gradient to growth rate ratio) to form a composite structure in these alloys consisting of aligned rods of the minority phase in a majority phase matrix if convection is minimized. Convective instability, however, is inherent in hypermonotectic alloys and results from adverse density gradients due to solute concentration variations in the liquid adjacent to the solidification front. Results from this work indicate that convective flow at low flow velocities (one-g processing) causes a composition variation along the length of the sample but does not impede the coupled growth process. Vigorous convective flow such as that experienced during portions of parabolic flight appears to completely disrupt the formation of the composite structure. Composition profiles and morphologies of both ground processed and KC-135 processed samples will be discussed.
CRYSTALLIZATION KINETICS OF THE UNDERCOOLED Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 ALLOY DURING CONTAINERLESS ELECTROSTATIC LEVITATION PROCESSING: Y.J. Kim, R. Bush, W.L. Johnson, MS 138-78, California Institute of Technology, Pasadena, CA 91125; S.K. Chung, W.K. Rhim, MS 183-401, Jet Propulsion Laboratory, Pasadena, CA 91109; A.J. Rulison, Microgravity Research, Space Systems/Loral, MS G03, Palo Alto, CA 94303
A glass forming alloy Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 was used to investigate the crystallization kinetics at different undercooling levels by constructing the time-temperature-transformation (TTT) diagram experimentally. The experiments were performed using the high temperature high vacuum electrostatic levitator at JPL. The measurements indicate that there are two distinct crystallization events at different undercooling levels. At low undercooling, there is a primary crystallization from the homogeneous liquid, while, at large undercooling, a secondary crystallization appears due to the phase separation in the supercooled liquid state. Based on the measured TTT diagram, the critical cooling rate required to bypass the crystallization was obtained to be 1.8 K/s.
3:35 pm BREAK
X-RAY TRANSMISSION MICROSCOPY OF Al-Pb MONOTECTIC ALLOYS DURING SOLIDIFICATION: W.F. Kaukler, Center for Microgravity and Materials Research, The University of Alabama in Huntsville, Huntsville, AL 35899-0001; P.A. Curreri, Marshall Space Flight Center, National Aeronautics and Space Administration, MSFC, AL 35812
In-situ observations of Al-Pb alloys both during and after unidirectional solidification will be discussed. X-ray microscopy permits viewing the features developing on either side of the s/l interfacial isotherm in real time. Our X-ray Transmission Microscope results reveal a diversity of phenomena not previously observed. Recent improvements to our techniques applied to this alloy have produced post-solidification images of fiber and aligned particulates with diameters of order of 10 micrometers. Previously reported features like solute layers in the melt and droplet formation can be observed with higher definition during solidification. Delicate, periodic striations parallel to the interface, presently only observed via x-ray microscopy, have been detected in these alloys with growth rates down to 1 micrometer/sec. It was previously thought there was a lower critical velocity for striation formation, but this new evidence suggests the possibility that this morphological feature simply becomes impractical to detect as growth rate is reduced.
MICROGRAVITY SOLIDIFICATION MICROSTRUCTURES AS ILLUSTRATED BY NICKEL-IRON AND STONY-IRON METEORITES: P.Z. Budka, 2135 Morrow Avenue, Schenectady, NY 12309; J.R.M. Viertl, 1403 Clifton Park Road, Schenectady, NY 12309; S.V. Thamboo, 2444 Brookshire Dr., Schenectady, NY 12309
The macro/microstructural features of nickel-iron and stony-iron meteorites provide visual, logical evidence that these materials solidified as castings in a microgravity environment. These features are remarkable because they exhibit regular patterns of phase formation over a wide range of masses and compositions. Such patterns, visible at low magnification, provide information on general conditions experienced during solidification: microgravity, undercooling and a shallow thermal gradient. This paper will present microstructural evidence to support this scenario. It will also illustrate classic solidification reactions such as eutectic, peritectic and isomorphous series using typical meteoritic microstructures. Understandings derived from these meteoritic structures can provide insights into fundamental segregation behavior and the solidification of large iron-nickel superalloy ingots.
MICRO- AND MACRO-SEGREGATION IN ALLOYS SOLIDIFYING WITH EQUIAXED MORPHOLOGY: D.M. Stefanescu, B.K. Dhindaw, J. Leon, Solidification Laboratory, The University of Alabama, Tuscaloosa, AL 35487; S. Sen, USRA, MSFC, Huntsville, AL 35812; P.A. Curreri, NASA, MSFC, Huntsville, AL 35812
A model has been developed to describe micro-segregation based on the 'closed system' assumption, i.e. no net mass transfer enters of leaves the domain during solidification. Further, this model has also been extended to include the impact of the 'open system' assumption (mass transport in and out of the system) and of the 'expanding system' assumption (coarsening and coalescence) on micro-segregation. Under purely diffusive conditions the 'closed system' assumptions can be considered to be valid. Solidification experiments during aircraft parabolic maneuvers and solute concentration measurements in Al-2%Cu and Al-5%Cu samples are being performed to test the 'closed system' model predictions. High and low g experimental data will be used to test the validity of the 'open system' model. Initial results and data analysis for these experiments will be discussed.
THE INFLUENCE OF CONVECTION ON BANDING IN PERITECTIC SYSTEMS: A COMPARISON OF Sn-Cd AND Pb-Bi: K.L. Zeisler-Mashl, T.A. Lograsso, Inst. for Physical Research and Tech., Iowa State University, Ames, IA 50011; R.K. Trivedi, Dept. of Matls. Sci. and Eng. and Ames Laboratory, US-DOE, Iowa State University, Ames, IA 50011
During directional solidification in peritectic systems, the primary phase and
peritectic phase can form at the solid-liquid interface as alternating layers
perpendicular to the growth direction for certain combinations of alloy
composition, growth velocity and temperature gradient in the liquid. This
banded microstructure is related to repeated solute accumulation and depletion
in the liquid ahead of the interface. For a system such as Sn-Cd, in which
convective mixing in the liquid can be minimized during growth in a terrestrial
environment, banding repeats over significant solidification distances. For a
system such as Pb-Bi, convective mixing in the liquid is significant during
growth in a terrestrial environment at the slow rates required for band
formation. As a consequence, the liquid composition progresses through the
regime in which banding occurs. The resulting microstructure exhibits a narrow
region containing a small number of bands that change in length, with the
length of the primary bands decreasing and the length of the peritectic bands
increasing as the fraction solidified increases. Current ground-based
directional solidification experiments using Sn-Cd alloys will be compared with
corresponding results from the Pb-Bi system.
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