Program Organizers: Professor Carl C. Koch, Materials Science and Engineering Department, North Carolina State University, Box 7907, Raleigh, NC 27695; Dr. Robert D. Shull, NIST, Bldg. 223 B152, Gaithersburg, MD 20899
Monday, PM Room: Orange County 1
February 5, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: Dr. Robert D. Shull, NIST, Bldg. 223 B152, Gaithersburg, MD 20899
2:00 pm Invited
RADIATION-INDUCED CRYSTAL-TO-GLASS TRANSFORMATION IN INTERMETALLIC COMPOUNDS:
P.R. Okamoto, N.Q. Lam, Materials Science Division, Argonne National Laboratory, Argonne, IL 60439
Molecular dynamic simulations of defect- induced amorphization of intermetallic compounds suggests that static atomic displacements can be used as a general order parameter for both homogeneous and inhomogeneous defect structures. The results indicate that a unified approach to defect- induced amorphization can be based on a generalized version of the Lindemann melting criterion, which in its simplest form assumes that melting of a defective crystal occurs when the sum of the static and dynamic mean square displacement exceeds a critical value identical to that for melting of the perfect crystal. A direct consequence of this hypothesis is that the effective Debye temperature, average shear modulus and melting temperature of the defective crystal, all must decrease with increasing static atomic disorder. Experimental observations and measurements in support of the generalized Lindemann melting criterion will be discussed.
2:30 pm Invited
NON-EQUILIBRIUM PROCESSING OF MATERIALS BY ENERGETIC ION IRRADIATION AT HIGH AND LOW TEMPERATURE: R.S. Averback, P. Partyka, Mai Ghaly, Department of Materials Science and Engineering, University of Illinios at Urbana-Champaign, Urbana, IL; Y. Lee, C.P. Flynn, Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL; P. Bellon, SRMP, Saclay, France
Irradiation with energetic particles is a highly controlled and convenient means to synthesize metastable materials. At low temperatures, irradiation drives systems away from equilibrium by mixing atoms in energetic collision cascades, while at high temperatures, radiation-induced defects restore systems to their equilibrium state. Although the concepts are clear, the details of these processes are complex and have been controversial for many years. Recently, molecular dynamics computer simulations have begun to provide a detailed understanding of the mixing process in cascades, and the results of these simulations will be reviewed here. The high temperature phenomena are more difficult to understand because of a competition between disordering processes in cascades, and ordering processes from mobile defects. This competition will be illustrated by radiation-induced disordering in order-disorder alloys and interfacial roughening in immiscible systems. Results from both experiment and computer simulation will be presented.
3:00 pm
SYNTHESIS, STRUCTURAL STABILITY AND PHASE SEPARATION OF AMORPHOUS MoSi2NX: H. Kung, T.R. Jervis, T.E. Mitchell, M. Nastasi, Los Alamos National Laboratory, Los Alamos, NM; J-P. Hirvonen, Technical Research Centre of Finland, Espoo, Finland
The structural stability and phase separation of amorphous MoSi2Nx films, with nitrogen content x varying between 0 and 4, have been studied. The MoSi2Nx films were synthesized using reactive magnetron sputtering technique. Cross-sectional transmission electron microscopy was used to examine the structural evolution of as-deposited and annealed (up to 1000[[ring]]C in vacuum) films. Electron energy loss spectroscopy was utilized to investigate the local bonding structure. Nanoindentation was employed to measure the corresponding mechanical properties. An amorphous structure is observed in the as-sputtered MoSi2Nx. For large nitrogen content (x>2.5) films, the amorphous structure is preserved even after 1000[[ring]]C annealing but undergoes a phase separation (spinodal decomposition) into two phases. At lower nitrogen concentration, the MoSi2Nx films crystallize to form the C40 structure, which is a metastable structure of MoSi2. The hardness and modulus is observed to vary with the nitrogen content and annealing conditions. The role of nitrogen in influencing the structural stability, phase separation and mechanical properties will be discussed and presented.
3:20 pm BREAK
3:40 pm Invited
SOFT AND HARD MAGNETIC PROPERTIES OF Fe-RICH NANOCRYSTALLINE ALLOYS CONTAINING INTERGRANULAR AMORPHOUS PHASE: Akihisa Inoue, Institute for Materials Research, Tohoku University, Sendai 980 77, Japan
The partial crystallization of amorphous Fe rich Fe-M-B (M=Zr, Hf, Nb or Nd) alloys with 90 at% Fe concentration was found to cause finely mixed structures of nanoscale bcc-Fe particles surrounded by the remaining amorphous phase in the Zr-, Hf- and Nb-containing alloys and of nanoscale bcc-Fe and bct-Fe14Nd2B particles embedded in the remaining amorphous phase. The particle size is about 10 to 15 nm for the bcc-Fe phase and 20 to 30 nm for the bcc-Fe and Fe14Nd2B phases. Furthermore, the phase transition of bcc-Fe + amorphous to bcc-Fe + Fe14Nd2B + amorphous in a Fe90(Zr-Nd)7B3 system occurs around 4 at% Nd. The mixed phase alloys exhibit good magnetic properties combined with high magnetization, i.e., permeability above l.5x104 at 1 kHz and saturation magnetization above 1.5 T for the former phase alloys, and coercivity of 250 kA/m , remanence of 1.3 T and maximum energy product of 145 kJ/m3 for the latter phase alloys. The appearancc of the soft and hard magnetic properties in coexistence with the remaining amorphous phase for the Fe-rich alloys is important for future development of new nanocrystalline magnetic alloys.
4:10 pm
STRUCTURAL EVOLUTION OF Nd2Fe14B DURING MECHANICAL MILLING: W.F. Miao, J. Ding, R. Street, P.G. McCormick, Research Centre for Advanced Mineral and Materials Processing, University of Western Australia, Nedlands, W.A. 6907
The development of metastable structures during the mechanical milling of Nd2Fe14B has been investigated. Changes in structure and composition accompanying milling were followed with x-ray diffraction, Mossbauer spectroscopy and differential scanning calorimetry measurements. Milling was found to initially result in disordering and amorphization of the Nd2Fe14B phase. Continued milling caused disproportionation of the amorphous phase to a two phase [[alpha]]-Fe / amorphous mixture at longer milling times. Structural and compositional changes ocurring during heat treatment of the as-milled material will also be reported.
4:30 pm
THE EFFECT OF NITROGEN AND OXYGEN ON THE THERMAL STABILITY OF NANOCRYSTALLINE Fe SYNTHESIZED BY CRYOMILLING: Benlih Huang, Robert J. Perez, Eurique J. Laverina, Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92717-2575
Recently, cryomilling of aluminum and NiAl has been found to result in the in-situ formation of nanoscale g-Al2O3 and AlN, which impede the grain growth of the nanocrystalline structures during heat treatment. The present study has been conducted to synthesize Fe/ g-Al2O3, AlN nanocomposites by cryomilling of Fe-10 wt.%Al in liquid nitrogen. The as-milled powders following 25 hours of cryomilling were consolidated at 550[[ring]]C at 350 MPa for 30 minutes, and subsequently annealed at elevated temperatures for 60 minutes. TEM dark field imaging indicated that nanocrystalline structure with crystallite size of 10-20 nm was maintained for the consolidated powders annealed at 950deg.C. Selected area diffraction (SAD) indicated the formation of g-Al2O3 and AlN. In contrast, cryomilling of Fe-10 wt.%Al in liquid argon, and cryomilling of pure Fe in liquid nitrogen have not been able to produce nanocrystalline structures with thermal stability at 650deg.C. Therefore, incorporation of nitrogen, oxygen, and aluminum to Fe during mechanical alloying is shown to be imperative to the achievement of thermal stability of nanocrystalline structures. 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.
4:50 pm
VIBRATIONAL ENTROPY OF Ni3V: Laura Nagel, Brent Fultz, Department of Materials Science, California Institute of Technology, Pasadena, CA 91125; Lee Robertson, Steve Spooner, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831
We have measured the difference in vibrational entropy for two different ordered states of Ni3V: as a chemically ordered cubic crystal and as the equilibrium tetragonal DO22 structure. From these measurements, we have determined the Debye temperatures for Ni3V in two states of order. Data were obtained from low-temperature calorimetry (80K to 360K). From these data we estimate differences in vibrational entropy between the two states to be on the order of 0.1 kB per atom. About half of this result can be explained by harmonic contributions, and the other half by anharmonic effects. In order to interpret these results, we have performed thermal expansion measurements and inelastic neutron scattering experiments. This work was supported by the U.S. Department of Energy under Contract No. DE-FG03-86ER45270.
5:10 pm
MECHANICAL CRYSTALLIZATION BY CRYOGENIC BALL MILLING: Benlih Huang, Robert J. Perez, Paula J. Crawford, Enrique J. Lavernia, Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92717-2575; Steven R. Nutt, Department of Materials Science and Engineering, University of Southern California, Los Angeles, CA 90089-0241
Cryogenic ball milling of Metglas Fe78B13Si9 in an attritor in liquid nitrogen
has resulted in the synthesis of nanocrystals of [[alpha]]-Fe(Si) and Fe2B with
crystallite size of 2-6 nm in diameter. The mechanical crystallization during
cryogenic attritor milling consists of two stages: the initial stage involves
the crystallization through the formation of shear bands, and the latter stage
involves the crystallization through a wear-like mechanism. To further provide
insight into the mechanisms of mechanical crystallization, thermal analysis
indicated that the formation of [[alpha]]-Fe(Si) consisted of both thermal and
athermal mechanisms, while that of the Fe2B consisted of only thermal
mechanism. To understand the effect of composition on the stability of the
glassy phase, elements of Co and Ni were incorporated to the Metglas Fe78B13Si9
using both cryogenic attritor milling and Spex milling. TEM study and thermal
analysis have been used to investigate the mechanisms which control the
stability of Metglas Fe78B13Si9 during mechanical crystallization. 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.
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