Sponsored by: MDMD Powder Metallurgy Committee and FEMS (The 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, P.O. Box 3640, D-76021 Karlsruhe, Federal Republic of Germany
Wednesday, AM Room: Grand G
February 7, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: B.T. Fultz, Mail 138-78, California Institute of Technology, Pasadena CA 91125
8:30 am
SYNTHESIS, PROCESSING AND MECHANICAL PROPERTIES OF NANOCRYSTALLINE MATERIALS PRODUCED BY SOLUTION PHASE SYNTHESIS: Shankar M.L. Sastry, W.E. Buhro, Washington University, St. Louis MO 63130
Solution phase synthesis (SPS) is an extremely promising approach for producing nanometer-scale particles of metals, intermetallics and ceramics. In this method, solution-phase chemical reactions are conducted at low temperatures (< 200deg.C) in solution to form solid precursors which are subsequently annealed in the solid state at temperatures of 500 - 1000deg.C, to produce by nucleation and growth 10 - 100 nm crystallites. We have synthesized by SPS nanocrystallites of Cu, TiB2, ZrB2, TiC, TiN, MoSi2, Ni3Al, NiAl, TiAl, and TiAl3. The consolidation characteristics, room and elevated temperature strength, high temperature creep and room temperature fracture toughness of several of the above nanocrystalline materials will be discussed. This research was conducted under NSF Grant No. ECS-9119006 and AFOSR Grant No. F 49620-93-1-0131.
8:50 am
DENSIFICATION OF NANOCRYSTALLINE POWDER PRODUCED BY SOLUTION PHASE SYNTHESIS: THEORETICAL MODELING AND COMPARISON WITH EXPERIMENTS: R. Suryanarayanan, S.M.L. Sastry, Department of Mechanical Engineering. T.J. Trentler, B.E. Waller, W.E. Buhro, Department of Chemistry, Washington University, St. Louis MO 63130
A theoretical model for the densification of nanoparticles by hot pressing and/or hot isostatic pressing is developed. Based on the Helle- Easterling- Ashby model for hot isostatic pressing, the present model attempts to take into account the effect of agglomeration, surface impurities, and particle size distribution in nanoparticle systems. Precision density measurements are performed on nanocrystalline copper and nanocrystalline molybdenum disilicide, that have been compacted by a combination of hot pressing or hot isostatic pressing. Measurements are performed on compacts that have been produced with and without exposure to air during precompaction powder handling. The model results are compared with the experimental data. The model is used to predict the dominant densification mechanism and these results are compared with experiments by performing microstructural characterization of the nanocompacts. Preliminary results show that impurities on particle surfaces lead to dramatic decrease in densification rates, requiring greater temperatures and/or pressures to achieve full densification. The results of this model may partially explain why attempts to densify nanoparticle systems by several researchers have been unsuccessful, in-spite of expectations of high densification rates due to orders of magnitude increase in diffusivities.
9:15 am
HIGH PRESSURE CONSOLIDATION OF 'NANO-NANO' ALUMINA COMPOSITES: R.S. Mishra, Department of Chemical Engineering and Materials Science. C.E. Lesher, Department of Geology. A.K. Mukherjee, Department of Chemical Engineering and Materials Science, University of California, Davis CA 95616
Fully dense 'nano- nano' alumina composites have been obtained by sintering at a nominal pressure of 1 GPa in the temperature range of 1073- 1223 K. The temperature of sintering depends on the volume fraction and chemistry of the second phase. Results are presented on Al203- SiC and Al203- TiO2 composites. Application of high pressure makes it possible to achieve 'nano- nano' structure, where both phases are nanocrystalline (the reported literature results for Al203- SiC show large Al203 grains with intragranular nanocrystalline SiC). The 'nano- nano' alumina composites show high toughness and hardness (>26 GPa). The sintering kinetics agree with the theoretical prediction of densification by dislocation creep mechanism. The implication of the present results on the diffusional densification rate is discussed.
9:35 am
PROCESSING OF NANOCRYSTALLINE ZIRCONIA AND ZIRCONIA-ALUMINA COMPOSITE POWDER BY HOT PRESSING AND HIP: G. Prabhu, D.L. Bourell, Center for Materials Science and Engineering, University of Texas at Austin, MC C2200, Austin, TX 78712
Three mol% Yttria Stabilized Tetragonal Zirconia (YSTZ) and a YSTZ - alumina nanocomposite were synthesized using a co-precipitation technique using chloride precursors. The effect of calcination on both phase characteristics and particle size was assessed. Xray diffraction studies revealed that crystallization of YSTZ began at 550[[ring]]C. The tetragonal phase appears and is retained after sintering to 1225[[ring]]C. The nanocomposite had to be calcined at 1000[[ring]]C to observe any crystallinity. Alumina peaks were observed only after calcination at 1400[[ring]]C. As TGA showed that the attached water of hydration in these ceramics was lost at 550[[ring]]C, the powders were calcined at this temperature and used as starting material in the sintering studies. The powders used had particle sizes of approximately 10 to 15 nm. YSTZ sintered to greater than 99% theoretical density at temperatures higher than 1225[[ring]]C while the nanocomposite sintered to similar density at 1300[[ring]]C. Powders were also cold pressed and hot isostatically pressed (HIP) using argon as the pressurizing medium. Sintering parameters to minimize coarsening and maximize density were established.
10:00 am
A MICROSTRUCTURAL STUDY OF CONSOLIDATION OF FE AND CU NANOPOWDERS: O. Dominguez, Y. Champion, J. Bigot, CECM-CNRS, 15 rue Georges Urbain, 94407 Vitry sur Seine, France
Previous studies of compacted nanocrystalline metal powders suggest that for certain metals, high green density cannot be obtained even at high consolidation pressures due to their intrinsic high yield strength. This study concerns the different behaviour of nanometric powders as a function of their crystal structure and of the nature of the compaction process. Two initial nanometric powders of Fe and Cu with equivalent particle size distributions, were prepared by levitation melting in liquid nitrogen. TEM analysis of the as- prepared nanopowders shows, for Cu, planar defects, thought to be twins, in a substantial fraction of the particles, whilst the iron particles are totally free of any defect. A comparative analysis of the consolidation process of nanometric Fe and Cu powders, using die- compaction and isostatic pressing, shows a remarkable difference in density and calculated yield strength between the two bulk specimens. The evolution of the defects for both nanopowders was studied as a function of pressure. We discuss these changes in relation to the observed consolidation and mechanical behaviour.
10:25 am
SINTERING OF NANOCRYSTALLINE POWDERS: EXPERIMENTS AND COMPUTER SIMULATIONS: R. S. Averback, J.M. Gibson, D. Olynick, R. Tao, H. Zhu, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801
Sintering of nanocrystalline materials has been investigated at three levels of observations. On the atomistic scale, molecular dynamics computer simulations have been used to study the initial stages of sintering when two or more nano-particles first come into contact. They show that because of the high shear stress developed in the nano-particle contacts, the particles undergo extensive densification in matters of picoseconds by dislocation motion. In situ electron microscopy of similarly small assemblies of particles has enabled us to follow the sintering to longer times and at somewhat larger length scales and observe the role of surface diffusion and particle contamination. Finally, measurements of densification on bulk specimens undergoing shear deformation provide information on the macroscopic behavior of sintering involving large assemblies of nanocrystals. Attempts will be made to relate the macroscopic sintering behavior of nanocrystals to the microscopic processes elucidated by the simulation and TEM.
10:50 am
THE RELATIONSHIP BETWEEN MICROSTRUCTURE AND MECHANICAL PROPERTIES OF NANOCRYSTALLINE WC BASED HARD METALS: R. Porat, S. Berger, A. Rosen, Technion, Israel Institute of Technology, Haifa 32000, Israel
WC belongs to the family of cemented carbides, widely used in industry due to unique combination of mechanical, physical, and chemical properties. To improve properties, especially hardness - fracture toughness relationship, reduction of the grain size to the nano-scale is suggested. The research concentrates on investigating various synthesis processes of nanocrystalline WC and selecting the optimal process with respect to grain size, microstructure and composition. A novel approach is the investigation of sintering kinetics by means of dilatometry. The study consists of the characterization of the microstructure, the composition and the mechanical properties; the major goal is to understand the microstructure to the finest details, which in turn is expected to lead toward improvement of the hardness and plasticity of the final products.
11:15 am
GRAIN SIZE STABILIZATION PRODUCED BY DISPERSED FULLERENES: E.V. Barrera, J. Sims, D.L. Callahan, Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX 77251-1892
Recent research on processing copper, tin, aluminum, iron, and stainless steel with fullerenes has focused on several issues: dispersing fullerenes on a nanometer size molecular level both in and at grain boundaries in polycrystalline materials, maintaining their stability throughout the processing and subsequent anneals, and verifying their contribution to strengthening. Nanocrystalline copper and stainless steel have been processed with fullerene additives where well dispersed fullerenes contributed to the control of the starting grain size and to their stability. Fullerenes have been shown to effectively stabilize grain size and are currently being studied as a phase stabilizing agent as well. Films of co-deposited aluminum and fullerenes have been studied by transmission electron spectroscopy where nanometer size particles have been observed in grains and less frequently at grain boundaries where grain size has also been stabilized. Discussion will be given as to the role of the fullerenes in metals and to the advantages in using fullerenes for mechanical property enhancement. Support has been provided by the National Science Foundation, under grant number DMR- 9357505.
11:35 am
A MODEL FOR FULLERENE GROWTH MECHANISM: H. Yoshida, National Institute for Advanced Interdisciplinary Research, 1-1-4 Higashi, Tsukuba, Ibaraki 305, Japan
A model for fullerene growth is proposed that describes stochastical
formation of its five and six membered rings. For the model, the following is
assumed: (1) The intermediates consist of only pentagons and hexagons; (2) The
isolated pentagon rule (IPR) is applied; (3) Structure transformations are
forbidden; (4) All pentagons and hexagons adjacent to the intermediate form at
once. The expressions of creation probability of C60 and C70, P(C60) and
P(C70), with P are obtained; P is the mean probability of sticking of five
membered ring to intermediate. P(C60) is formed to be a monotonically
increasing function of P for 0P<=1. The limit as P goes to 1 of the quantity
P(C60) is equal to 0.5, while P(C60)=0 for P=0. P(C60) of P has a singularity
at P=1 that is caused by IPR. P(C70) has a peak at about P=0.9, and
P(C70)1->0 for P->0 and 1. P(C70) has no singularity at P=1. For
P(C70)/P(C60)=0.1; the present model gives P=0.7. This means that pentagon is
more easily created than hexagon except for the restriction on the creation of
pentagon owing to IPR.
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