Sponsored by: MDMD Powder Metallurgy Committee and FEMS (Federation of European Materials Societies)
Program Organizers: Dr. David L. Bourell, The University of Texas at Austin, Materials Science & Engineering, MC C2201, Austin TX 78712; Dr. Liisa Kuhn-Spearing, Laboratory for the Study of Skeletal Disorders and Rehabilitation, Harvard Medical School, Children's Hospital, 300 Longwood Avenue, Boston MA 02115; Professor Dr. Herbert Gleiter, Karlsruhe Research Center, PO Box 3640, D-76021 Karlsruhe, Federal Republic of Germany
Wednesday, PM Room: Grand G
February 7, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: E.V. Barrera, Rice University, Mechanical Engineering 101, PO Box 1892, Houston TX 77251
GRAIN GROWTH OF NANOCRYSTALLINE MATERIALS - A REVIEW: T.R. Malow, C.C. Koch, Department of Materials Science and Engineering, North Carolina State University, PO Box 7907, Raleigh NC 27695-7907
Quantitative studies of grain growth in nanocrystalline materials (nc) will be reviewed. Methods for experimentally measuring nc grain size and their limitations are presented. The grain growth exponents n from Dl/n - Dol/n = kt for nc and conventional polycrystalline (D > 1 um) materials are compared. Grain growth data on nc materials from the literature and the authors' laboratory are analyzed by the above equation and one which takes grain boundary pinning forces into account. Activation energies for grain growth are also determined by both methods where applicable. Grain growth in nc Fe is emphasized in work from the literature and the authors' laboratory. Grain growth kinetics, in terms of the grain growth exponent, n, and the activation energy for grain growth, Q, are compared for nc Fe and conventional grain size (>=30 um) zone-refined Fe. An activation energy for grain growth in nc Fe is found to compare closely with that for grain boundary self diffusion in Fe.
DESIGN OF NANOCRYSTALS WITH STABILITY AGAINST GRAIN GROWTH: B. Fultz, L.B. Hong, Z.-Q. Gao, Mail 138-78, California Institute of Technology, Pasadena CA 91125
Nanocrystalline alloy powders were prepared by high energy ball milling, and their stabilities against grain growth during low temperature annealings were tested. Grain growth was measured by transmission electron microscopy dark field imaging, in combination x- ray diffraction lineshape analysis. Our nanocrystalline alloys were solid solutions in the as- milled state. Mössbauer spectrometry and x- ray diffractometry were used to measure the chemical ordering or chemical segregation that occurred during annealing. The grain growth in alloys of Fe3Si and Fe3(Al,Ge) slowed considerably after short- time annealings, which was coincident with the development of D03 chemical order. The addition of Nb to these alloys suppressed further their grain growth, evidently because the Nb segregated to grain boundaries. We argue that grain boundary chemical segregation can be useful for suppressing grain growth, as is the development of chemical order. An exception occurs for bcc Fe- Cu, however, in which the segregation of Cu atoms to grain boundaries has little effect on the suppression of grain growth. This work was supported by the NSF under contract DMR- 9213447.
MEASURING THE GRAIN SIZE DISTRIBUTION IN NANOCRYSTALLINE MATERIALS BY INDIRECT DECONVOLUTION OF THE BRAGG REFLECTION PROFILES: J. Weissmüller, National Institute of Standards and Technology, Bldg 223 Rm A153, Gaithersburg MD 20899; J. Löffler, Paul Scherrer-Institut, Villigen, Switzerland; C.E. Krill, R. Birringer, Fachbereich Physik, Universität des Saarlandes, D-66 Saarbrücken, Germany; H. Gleiter, Forschungszentrum Karlsruhe, Karlsruhe, Germany
It is often desirable to characterize the grain size distribution of nanocrystalline solids, in addition to the average values for the grain size. The grain size distribution can be obtained from scattering data by solving the linear set of equations relating the experimental Bragg reflection profile of a sample to the profiles of the particles from different size classes. The influence of the grain size on the atomic distribution and interference functions can be described by the intra- grain correlation function. The Bragg reflection profile is the cosine transform of this function. Each profile is also broadened due to instrumental effects and to the mean square lattice strain. The reflection profile of a nanocrystalline solid with a distribution of grain sizes is a weighted sum of the profiles of the individual grains. The determination of the size distribution involves no assumption on the functional form of the size distribution. The grain size distribution of nanocrystalline Palladium obtained in this way from X- ray diffraction data is compared.
DIFFERENTIAL MOBILITY ANALYZER FOR NANO PARTICLE SIZING: H. Fissan, Gerhard-Mercator-Universität Duisburg, Prozeß- und Aerosol-meßtechnik, Bismarckstr. 81, D-47057 Duisburg Germany; D. Pui, Particle Technology Laboratory, Mechanical Engineering Department, University of Minnesota, 111 Church St. SE, Minneapolis MN 55455
Nano-particles are commonly synthesized from the gas phase. For certain applications it is of great interest to be able to produce monodisperse nano-particles. However, nucleation, condensation and coagulation processes in the gas phase lead to polydisperse particles. Differential Mobility Analyzers (DMAs), based on the electrostatic classification principle, have been used with great success to select monodisperse fractions from the polydisperse particles and to perform accurate sizing of nanometer particles. The transfer function of the DMA provides a full description of the instrument performance and is used in the DMA data analysis. Up to now it has been assumed that the transfer function is only dependent on the flow and electrostatic effects and that there are no particle losses on the flow path within the instrument. While the assumption is valid for large particles, it is not applicable for particles below 100 nm diameter. It is important to take into account the diffusional effect of the nanometer particles, which causes the transfer function to broaden and significant particle losses in the instrument. Four commercially available DMAs of different designs have been evaluated for the present study. They are challenged with defined nano-particles and their size resolution and losses have been determined. In all cases the diffusional effect is too large to be negligible. Based on these results, we will discuss the requirements for an instrument especially designed for nano-particles and how an ideal instrument can be achieved.
3:30 pm BREAK
DENSIFICATION VERSUS GRAIN GROWTH IN NANOCRYSTALLINE ZrO2-3 MOL%Y2O3: HOW TO WIN: M.J. Mayo, Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802
The kinetics of densification and grain growth during the pressureless sintering of nanocrystalline (15 nm) ZrO2-3mol% Y2O3 were studied for samples with green densities from 30-58%, over temperatures ranging from 700-1300deg.C, times from 0-64 hours, and heating rates from 2-200deg.C/min. The results show that a rapid rate sintering approach, which uses fast heating rates to bypass a thermodynamically unfavorable, low temperature surface diffusion regime, cannot be applied to nanocrystalline zirconia due to thermal gradient problems unique to that material. With more ordinary heating rates, the densification kinetics show a strong dependence, not only on grain size and temperature, as expected, but also on pore size. Conversely, grain growth shows no dependence on pore size. By decreasing the pore size, then, it is possible to accelerate the densification kinetics relative to those of grain growth and achieve 99.9% dense zirconia with 80-90 nm grain diameters. Successful methods of decreasing the pore size include increasing the dry compaction pressure or, alternatively, using wet consolidation techniques. Simple manipulations of the temperature/time protocol during sintering have no effect on pore size evolution and thus no effect on the final density/grain size combinations that can be reached.
SOL-GEL SYNTHESIZED AND HOT PRESSED ALUMINA AND ALUMINA-ZIRCONIA NANOCOMPOSITES: J-F. Kuo, D.L. Bourell, Center for Materials Science and Engineering, University of Texas at Austin, MC C2200, Austin, TX 78712
Alumina and a co-precipitated nanocomposite of Yttria Stabilized Tetragonal Zirconia (YSTZ, 3 mol% Yttria) were synthesized using a sol-gel synthesis technique using chloride precursors. The effect of calcination on both phase characteristics and particle size was assessed. After crystallization of the amorphous gel, YSTZ remained tetragonal after calcination/sintering to 1250[[ring]]C, but the alumina underwent a series of phase transformations involving the phases, bayerite, boehmite, gamma, eta, theta and alpha. Particle size for ceramics and composites after calcination was as fine as 10 to 15 nm with agglomerates of the size 0.5 mm. Powders were cold and hot pressed. Sintering parameters were established to minimize coarsening and maximize density.
THERMAL STABILITY OF DYNAMICALLY CONSOLIDATED BALL-MILLED NANOCRYSTALLINE FE-2C POWDER: G.E. Korth, T.M. Lillo, Lockheed Martin Idaho Technologies, P.O. Box 1625, Idaho Falls ID 83415-2218; J.C. Rawers, U.S. Bureau of Mines, 1450 Queen Avenue SW, Albany, OR 97321-2198
Nanocrystalline material was synthesized by ball milling Fe-2C powder in an argon environment attritor which resulted in micron size powder particles with a substructure (grains) of 8-15 nm. TEM analysis was performed on dynamically consolidated material after 1 hour heat treatments up to 700deg.C. Results showed the 10-40 nm post-consolidated nanograins to be stable up to approximately 550deg.C, but after 1 hour at 700deg.C the grains had grown over an order of magnitude to approximately 500 nm. The grain size stability up to 550deg.C has real significance since preliminary property results indicate possible onset of superplastic behavior at approximately 500deg.C. Work supported by the U.S. Bureau of Mines under Subcontract No. J01345035 with the Department of Energy Idaho Operations.
EFFECT OF IN-SITU FORMATION OF NANOSCALE y-AL2O3 AND AlN DURING CRYOMILLING ON THE THERMAL STABILITY OF NANOCRYSTALLINE Fl AT ELEVATED TEMPERATURES: Robert J. Perez, Benlih Huang, E.J. Lavernia, Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92717-2575
The cryogenically attritor milled (cryomilled) Fe- 10 wt.%Al powders in liquid nitrogen have been found to retain nanocrystalline structures following consolidation at 550deg.C for 30 minutes using hot pressing and subsequent annealing at elevated temperatures for 60 minutes. Thermal stability of these heat treated powders was studied using dark field imaging of TEM. The crystallite size of the as- consolidated material was 11+/-5 nm. Subsequent annealing of the consolidated powders at 800deg.C and 950deg.C resulted in the crystallite size of 13+/-6 nm and 16+/-7 nm, respectively. Selected area diffraction (SAD) indicated the formation of nanoscale [[gamma]]- Al203 and AlN, which accounted for at least 2 volume percent in the cryomilled powders. Extraction replicas of the materials are being studied to determine the distribution and interparticle spacings of the [[gamma]]- Al203 and AlN. Zener's model is applied to provide insight into the interaction between the particles and the grain growth of the nanocrystalline materials. The authors would like to acknowledge the financial support provided by the Office of Naval Research under grants N00014- 93- 1072 and N00014- 94- 0017.
COMPUTER SIMULATIONS OF NANOPARTICLE INTERACTIONS: H.L. Heinisch, Pacific Northwest Laboratory, Richland, WA 99352
Interactions of idealized individual metal nanoparticles are simulated at the
atomic scale using molecular dynamics with an embedded atom potential for
nickel. The sintering of two identical, crystalline particles in free space,
each initially at 900 K, is simulated for an interaction time of 250 ps. The
particles reorient themselves to achieve registry of their crystal planes,
while necking occurs by surface diffusion. Surface layering, surface diffusion
coefficients and particle melting temperature compare favorably with measured
and calculated information. For simulations of more than two particles
interacting simultaneously, particle-particle interfaces are examined, as well
as the impact of the additional constraints on relative nanoparticle motion.
Pacific Northwest Laboratory is operated for the U.S. Department of Energy by
Battelle Memorial Institute under contract DE-AC06- 76RLO 1830.
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