Materials Week '97: Tuesday PM 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 afternoon, September 16.

MECHANICAL BEHAVIOR OF BULK NANO-MATERIALS: Session III

Session Chair: Shankar M. Sastry, Washington University, Campus Box 1185, One Brookings Drive, St. Louis, MO 63130

MECHANICAL BEHAVIOR OF BULK NANOSTRUCTURED METALS: W.W. Milligan, A.N. Fisher, J.E. Carsley, E.C. Aifantis, Michigan Technological University, Houghton, MI 49931-1295

Mechanical behavior of iron-copper alloys and tungsten alloys processed by attritor milling and consolidated by HIP or rapid forging will be discussed. Fully dense metals with grain sizes varying between 40 nm and 1700 nm were produced and tested in tension and compression. Behavior similar to that of metallic and polymeric glasses, including a strong tension/compression asymmetry, shear banding, and an apparent pressure dependence of the yield criterion, were all observed. Experiments at 77K, elevated temperature, and at various strain rates will be presented. The support of the National Science Foundation and the Air Force Office of Scientific Research are gratefully acknowledged.

2:30 pm

MECHANICAL ALLOYING, CONSOLIDATION, AND MECHANICAL PROPERTIES OF INTERSTITIAL AND SUBSTITUTIONAL ELEMENT ADDITION TO IRON ALLOYS: J. Rawers, D. Smith, D.Cook1, U.S. Dept. of Energy, Albany (Oregon) Research Center, 1450 Queen Ave. SW, Albany, OR 97321; 1Old Dominion University, Norfolk, VA

Alloying iron with interstitial and substitutional alloying elements affects mechanical properties differently. In this study interstitial (carbon and nitrogen) and substitutional (Al, Cr, Nb, and Ti) was mechanically alloyed into iron powder. The milled powder was subsequently compacted (hot-pressed) and several mechanical properties evaluated (hardness, strength). Powder characterization based on grain size, X-ray diffraction, and Mossbauer analysis, showed that a finer nanostructure results from alloying with interstitials. Characterization of compacts showed is little effect on the compacts due to alloy additions. Tensile and compression strengths were also found to be relatively independent of alloy addition, but characteristic of compacted ceramics.

2:55 pm

EFFECT OF B ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MECHANICALLY ALLOYED TiAl ALLOYS: H.H. Chung, N.J. Kim, Center for Advanced Aerospace Materials, Pohand University of Science and Technology, San 31, Hyojadong, Pohang, 790-784, Korea

TiAl alloys have been produced by mechanical alloying in an attritor mill using pre-alloyed powders. The mechanically alloyed (MA) powders were consolidated by vacuum hot pressing (VHP) at 1000°C for 2h. To prevent the coarsening of grain size during consolidation, B was added to the alloys. The hot pressed billets were subjected to various heat treatments and mechanical properties were measured by compression testing at room temperature. Yield strengths of MA alloys ranged from 736 MPa to 2720 MPa depending on the heat treatment. Heat treated materials showed considerable compressive ductility without any microcracking detected. Effects of microstructure on the mechanical properties of the MA materials are discussed.

3:20 pm BREAK

3:35 pm INVITED

HIGHEST STRENGTH PILE-UP PREDICTIONS FOR NANOMATERIALS: R.W. Armstrong, Department of Mechanical Engineering, University of Maryland, College Park, MD 20742-3035

The dislocation pile-up basis originally proposed for the Hall-Petch linear relation of flow strength versus the reciprocal square root of average grain diameter is pushed to the theoretical limit of nanometer grain sizes, say, as was proposed for bcc and fcc materials (R.W. Armstrong, "Strength Properties of Ultrafine Grain Metals", in ULTRAFINE GRAIN METALS, Syracuse University Press, 1970, 1-25). For the smallest pile-ups, the dislocation line tension is an important consideration that leads to a fall-off from the H-P extrapolation for bcc metals but increase in strength for fcc materials. Stress jumps needed for yielding at smaller pile-up lengths are smeared out, apparently, for real material grain size distributions and orientations. Intrinsic obstacle strengths, plastic instability, and inclusion effects are added complexities to consider.

4:05 pm

MECHANICAL CHARACTERIZATION OF RAPIDLY STRAIN HARDENING ALLOYS MP35N AND 70/30 BRASS ìINFLUENCE OF COLD WORK AND STRESS STATE ON THE DEFORMATION RESPONSE: E.A. El-Danaf, Ebrahim M. Shaji, S.R. Kalidindi, R.D. Doherty, Department of Materials Engineering, Drexel University, Philadelphia, PA 19104

MP35N (35% Co, 35% Ni. 10% Mo, 20% Cr) exhibits a remarkable combination of ultrahigh strength, high ductility, high fracture toughness. The alloy was found to show a 50% reduction in area before failure in tension tests of cold drawn and aged samples. 70/30 Brass was shown to exhibit similar strain hardening behavior. Although the materials start with grain sizes of the order of microns, during deformation, twinning reduces the slip length to the nanoscale. Our study includes determining: (i) strain hardening rate versus stress (or strain) for different stress states in simple compression plane strain compression and shear of annealed MP35N and 70/30 Brass. (ii) magnitude of secondary hardening as a function of prior cold-work level for different stress states for MP35N. (iii) influence of cold-work and aging on tensile ductility of cold drawn MP35N. (iv) propensity of alloy to exhibit extensive macro-scale shear banding in deformed plus aged condition.

4:30 pm

MECHANICAL PROPERTIES OF NANOCRYSTALLINE Ni: B.R. Elliot, J.R. Weertman, Northwestern University, Evanston, IL 60208

The influence of improved processing on the internal structure of nanocrystalline Ni has led to improved mechanical properties. Correlations between the internal structure (including grain size, pore size distribution, and impurities) and results of a variety of mechanical property measurements will be presented (including microhardness, tensile, and compression tests). Some comparison will also be made between samples produced by traditional inert gas condensation (IGC) and the newer jet blown arc IGC. Possible deformation mechanisms will be discussed in light of the structure and property measurements. *Research supported by Department of Energy Grant # DE-FG-02-86ER45229.