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Session Chairperson: Patrice E.A. Turchi, Chemistry and Materials Science Department (L-268), Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551
2:00 pm INVITED
MOLECULAR DYNAMICS SIMULATION OF MECHANICAL PROPERTIES OF AMORPHOUS ALLOYS: Tomoyasu Aihara Jr., Tsuyoshi Masumoto, and Yoshiyuki Kawazoe, Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-77, Japan
Macroscopic mechanical properties of materials arise from cooperative dynamics of atoms. However, it is difficult to detect the atomistic dynamical behavior by in situ experiment. We performed a large scale molecular dynamics simulation to study the deformation and fracture processes in Zr-Ni amorphous alloy. Finnis-Sinclair type pair functional potentials is used. Uni-axial strain is applied for a nano-size rod with constant strain rate at various temperatures. Tensile processes are traced by checking the internal energy and by the snapshots for atom configuration. The discrete atomic model reproduces the behavior of the continuum matter. The correlation between the inhomogeneous atom configuration and atomic level stress is analyzed.
2:40 pm INVITED
METASTABILITY AND PROPERTIES OF METALLIC BULK GLASS FORMING ALLOYS: Hans J. Fecht, Institute of Metallic Materials, Technical University Berlin, Hardenbergstrasse 36, PN 2-3, D-10623 Berlin, Germany
The absence of crystallization over a wide time/temperature window can be utilized to produce bulk metallic glass by relatively slow cooling of the melt. For a number of alloys including the multicomponent Pd-Ni-P, Au-Pb-Sb, Zr-Cu-Ni-Al, and Zr-Ti-Ni-Cu-Be alloys the relevant thermodynamic and thermomechanical properties of the metastable glassy and undercooled liquid states have been measured below and above the glass transition temperature. These measurements include specific heat, viscosity, density and elastic properties as functions of temperature. As a result it becomes obvious that the maximum undercooling for these alloys is given by an isentropic condition before an enthalpic or isochoric instability is reached. Alternatively, these glasses can also be produced by mechanical alloying, thus replacing the thermal disorder by static disorder and resulting in the same thermodynamic glass state. For the "weaker" glasses the isentropic instability also coincides with a divergence of the viscosity. During heating through the undercooled liquid a nanoscale phase separation occurs for most glasses as a precursor of crystallization. Further measurements of the mechanical and tribological wear properties point to the unique engineering applications of this new class of advanced materials.
3:20 pm BREAK
3:40 pm INVITED
ENVIRONMENTAL EFFECT IN BULK AMORPHOUS ALLOYS: C.T. Liu, L. Heatherly, D.S. Easton, Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6115; C.H. Chen, Health Science Research Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, Akihisa Inoue, Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980-77, Japan
Recent studies indicate that moisture-induced hydrogen embrittlement is a major cause of low ductility and brittle fracture of many intermetallic alloys in moist air at ambient temperature. This embrittlement involves the reaction of reactive elements in intermetallic alloys with the moisture in air and the generation of atomic hydrogen which penetrates into crack tip and causes loss of tensile ductility. In the current study, bulk amorphous alloys based on Zr-Al-Ti-Cu-Ni were tested in tension at room temperature in various environments. Preliminary results indicate that the tensile fracture strength of about 1400 MPa is not strongly affected by test environments. These results alone do not rule out the possibility that these amorphous alloys may react with moisture during tensile testing. Additional work involving laser desorption spectroscopy will be conducted in order to detect this reaction. Research sponsored by the Laboratory Directed Research and Development Program of the Oak Ridge National Laboratory, U.S. Department of Energy, under contract number DE-AC05-96OR22464 with Lockheed-Martin Energy Research Corporation.
4:20 pm INVITED
SEEBECK PHENOMENON ON AMORPHOUS-CRYSTALLINE INTERFACE AND AMORPHOUS-CRYSTALLINE THERMOCOUPLE: Mikhail V. Finkel, DAATH-Scientific Center, 9926 Haldeman Avenue #36A, Philadelphia, PA 19115; Jim S.-J. Chen, Mechanical Engineering Department, Temple University, 12th & Norris Street, Philadelphia, PA 19122
Thermo-electric Seebeck phenomenon on amorphous-crystalline interface in several alloys is studied. The thermocouples, consisting of amorphous and crystalline parts of the same alloy are proposed and investigated. The amorphous-crystalline transition zone formed by heating of amorphous alloys in heterogeneous temperature field serves as a hot junction. It is shown that this zone could be made as narrow as 10-100 mm. Thermo-Electric Moving Force (TEMF) for thermocouples made from Fe-B, Ni-Fe-Si-B, and Co-Fe-Si-B alloys was measured. TEMF of thermocouple made from Co-Fe-Si-B alloy in its as-cast condition can achieve 8.1 µV/K, and it is constant in the range 293K to 593K. It is shown that relaxation of the amorphous structure affects the TEMF of amorphous-crystalline thermocouple. (Patent is applied)
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