Materials Week '97: Tuesday AM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Tuesday morning, September 16.

MECHANICAL BEHAVIOR OF BULK NANO-MATERIALS: Session II

Session Chair: Fernand Marquis, Dept. of Metallurgical Engineering, South Dakota School of Mines & Technology, Rapid City, SD 57701

SINTERING OF NANOPARTICLES: R.S. Averback, Huilong Zhu, M. Ghaly, M. Yeadon, J.M. Gibson, Department of Materials Science and Engineering, University of Illinois, 1304 W. Green St., Urbana, IL 61801

The sintering of nanoparticles has been investigated by a combination of molecular dynamics computer simulations and in situ observations in a UHV TEM. Because their small radii of curvature, nanoparticles begin to sinter when they contact by plastic deformation. Different mechanisms have been identified in metals: dislocation glide in pure metals, viscous flow along grain boundaries of intermetallics and bulk viscous flow in amorphous alloys. Similar mechanisms occur when nanoparticles contact substrates. The time scale for sintering is some tens of picoseconds. Direct observations of particle-particle sintering and particle substrate sintering by TEM will also be reported.

9:00 am

NANO-DUPLEX STRUCTURED (Mo,Ti)-DISILICIDES REACTIVELY SINTERED FROM PRETREATED POWDERS: T. Aizawa, M. Suzuki, B.K. Yen, Univ. of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan

Spex-type milling was used to yield nano-grained powder mixture both in Mo-Si and Ti-Si systems. These pretreated powders were mixed with the specific chemical composition, and reactively densified by using the hot-pressing, resulting in nearly-full dense, nano-grained (Mo-Ti)- disilicide duplex structure. Variation of hardening at room temperature and flexure strength at elevated temperatures with increasing the mole content of TiSi₂ into MoSi₂ was investigated to correlate the nano-structured duplex microstructure with the mechanical properties.

9:25 am

MICROSTRUCTURE AND PROPERTIES OF NANOCOMPOSITES OBTAINED THROUGH SPD-CONSOLIDATION OF COMPOSITE POWDERS: Y.T. Zhu¹, I.V. Alexandrov ², V.A. Shundalov ², R.Z. Valiev², T.C. Lowe¹, ¹Mail Stop G755, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545; ²Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, Ufa 450000, Russia

Nanocomposites have great potential for structural applications because they exhibit not only excellent mechanical properties such as high yield strength and hardness, but also much better thermal stability than single-phase nanocrystalline materials. In this study, fully dense Cu-SiO₂ and Al-A1₂O₃ nanocomposites were obtained through consolidation of nanocrystalline composite powders using Severe Plastic Deformation (SPD). The microstructure and texture were analyzed to understand how microstructure development during deformation and subsequent annealing correlates with microhardness. Theoretical analysis using a dislocation model is employed to interpret the results.

9:50 am

INFLUENCE OF DYNAMIC DENSIFICATION ON NANOCRYSTALLINE STRUCTURE IN Ti-Si ALLOY: P. Counihan, N.N. Thadhani, School of Materials Science and Engineering, Georgia Tech, Atlanta, GA 30332-0245

Dynamic densification was used for consolidation of mechanically alloyed, amorphous Ti-Si alloy powders, employing a 3-capsule, plate-impact, gas-gun loading assembly. The powders densified under two-dimensional shock loading conditions at impact velocities of 300 and 500 m/s were observed to retain the amorphous structure as evidenced by XRD and TEM analysis. The densified material was subsequently annealed at various temperatures (600-1200°C) and times to produce a crystallized (Ti₅Si₃) nanoscale microstructure (>40 nm). TEM and XRD analysis along with microhardness measurements were used to characterize the microstructure and properties of the resulting material. In this presentation characteristics of the compacts and the influence of dynamic loading on the nanocrystalline structure in the crystallized compacts will be discussed. Funded by ARO and Georgia Tech.

10:15 am BREAK

10:30 am INVITED

SUPERPLASTICITY IN NANOCRYSTALLINE MATERIALS: R.S. Mishra, A.K. Mukherjee, Dept. of Chemical Eng. and Matls. Sci., Univ. of California Davis, CA 95616

Superplasticity is a grain size dependent phenomenon, because of which, it is expected that nanocrystalline materials would exhibit the effect at lower temperatures or higher strain rates. Both these features are attractive for technological applications. Lower forming temperature for ceramics can result in significant cost saving on power and tooling. The optimum strain rate for conventional superplasticity is in the range of 10^-5-10^-3 s^-1, which is quite slow for large scale forming of components. A shift in optimum superplastic strain rates to 10^-1-1 s^-1 is desirable. Our initial results on tensile deformation in a number of metallic and intermetallic materials show that the fundamental mechanism of superplasticity might be different. The results are analyzed to provide a mechanistic insight on superplasticity in nanocrystalline materials. The role of grain size on diffusional and dislocation accommodation during superplasticity is discussed.

11:00 am

LOW-CYCLE FATIGUE AND LOW TEMPERATURE CREEP BEHAVIOR OF ULTRAFINE-GRAIN (UFG) COPPER: S.R. Agnew¹, J.R. Weertman¹, R.Z. Valiev ², ¹Northwestern University, Evanston, IL 60208; ²Institute for Metals Superplasticity Problems, Ufa, Russia

Studies of low-cycle fatigue and low temperature (1/3T_m) creep behavior of UFG Cu made by severe plastic deformation have been designed to reveal information about the dominant deformation mechanisms, in addition to determining the microstructure's stability. During fully reversed low-cycle fatigue a considerable degree of cyclic softening has been observed, e.g., at _p =1.5% the stress amplitude decreased from 380 to 230 MPa (40%) prior to saturation. Changes in the microstructure resulting from the deformation were determined via TEM analysis before and after testing. A marked change in creep behavior has been observed at 160°C, where a long transient is followed by a reduced creep rate. TEM shows that the grain size has increased from 0.25 to 0.5 µm, with a high density of twins observed in the post-creep case. Research supported by Los Alamos National Laboratory Contract 7764Q0016-35.

11:25 am

MICROSTRUCTURAL CHARACTERISTICS OF ULTRAFINE-GRAINED Al AND Al ALLOYS PRODUCED USING EQUAL-CHANNEL ANGULAR PRESSING: Y. Iwahashi¹, Z. Horita¹, M. Furukawa², M. Nemoto¹, T.G. Langdon³, ¹Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 812-81, Japan; ²Department of Technology, Fukuoka University of Education, Munakata, Fukuoka 811-41 Japan; ³University of Southern California, Los Angeles, CA 90089-1453

Ultrafine-grained structures of pure Al (99.9%) and A1-Mg solid solution alloys were produced using equal-channel angular extrusion whereby an intense plastic strain is introduced into the material. The structural evolution with respect to the imposed strain was examined using transmission electron microscopy. It is demonstrated that homogeneous equiaxed grain structures, with grain sizes of the order of ~1 µm and ~0.5 µm, may be attained in A1 and an Al-1% Mg alloy by subjecting the materials to equivalent strains of ~400% and ~600%, respectively. The results confirm that the addition of 1% Mg to the A1 matrix is effective in promoting grain refinement.