Materials Week '97: Thursday 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 Thursday morning, September 18.

BOUNDARIES AND INTERFACES IN MATERIALS: THE DAVID A. SMITH SYMPOSIUM: Session VII

Session Chair: David B. Williams, Lehigh University, Bethlehem, PA 18015

GRAIN BOUNDARIES IN THE HIGH TEMPERATURE CUPRATE SUPERCONDUCTORS: P. Chaudhari, IBM, Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598

The electromagnetic properties of grain boundaries in the cuprate materials have greatly influenced the rate and the direction of progress in this field. This talk will review the properties of grain boundaries: both their deleterious and useful aspects. Their impact on critical currents, on processing of materials, superconducting devices such as SQUIDS, and on understanding the mechanism of superconductivity are presented.

9:00 am INVITED

GRAIN BOUNDARIES AND HETERO-INTERFACES IN Tl-BASED HIGH TEMPERATURE SUPERCONDUCTORS: Chris Grovenor, Annette Bramley, John O'Connor¹, Chris Eastell, Jo Moore, Department of Materials and ¹Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PH, UK

Many kinds of internal interfaces can influence the properties of both bulk and thin film samples of high temperature superconductors. We have studied the growth modes of Tl-2212, 2223 and 1223 thin films on different substrates and buffer layers by cross sectional TEM, and correlated the superconducting microwave performance with interfacial reaction characteristics and lattice defect concentrations. In addition, we have investigated the microstructure of Tl-1223 tapes by TEM, making observations on grain boundary structure, preferential alignment effects between grains and at 1223/silver interfaces and intercalation defects; again linking these observations to the macroscopic superconducting properties.

9:30 am INVITED

GRAIN BOUNDARIES AND THEIR ROLE IN THE DEVELOPMENT OF HIGH T_c SUPERCONDUCTING MATERIALS FOR LARGE-SCALE APPLICATIONS: Susan E. Babcock, Materials Science and Engineering and Applied Superconductivity Center, University of Wisconsin-Madison, 1500 Engineering Drive, Madison, WI 53706

Elucidation of structure-property relationships for individual grain boundaries, exploration of percolative current paths through textured materials, and engineering of the grain boundary microstructure and texture all remain critical issues in the development of the known high T_c superconductors. Professor Smith's research has influenced all of these topics both directly, through his work on superconducting materials, and indirectly through his insights into the general behavior of grain boundaries. His contributions to the study of high T_c superconductors will be reviewed in the context of current research and understanding of these materials.

10:00 am BREAK

10:10 am INVITED

EFFECT OF / INTERFACES ON CREEP BEHAVIOR OF SINGLE COLONY TITANIUM ALLOYS AT ROOM TEMPERATURE: S. Suri, G.B. Viswanathan, T. Neeraj, D-H. Hou¹, J.M. Scott² and M.J. Mills, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210; ¹Micron Technology Inc., Boise, ID 83706; ²Wright Laboratories, MLLM Materials Directorate, Wright Patterson AFB, OH 45433-6533

An important limitation in the application of titanium alloys for structural applications is their tendency to creep significantly at low homologous temperatures and at stress levels well below the yield strength. Several studies in the literature have shown that in two-phase alloys with a Widmanstatten structure, creep strain is affected by colony size and by sliding at / interfaces. In this investigation, the structure of / interfaces for a near- alloy, Ti-5Al-2.5Sn-0.4Fe, has been determined for the first time using diffraction contrast and high resolution TEM. In addition, the processes of slip transmission from to phases has been studied in detail via TEM investigation of single colony crystals which have been oriented and deformed to activate particular slip systems in the a matrix. The strong anisotropy of the constant strain rate and creep properties of these colony crystals will be discussed in terms of the observed mechanisms of slip transmission and interfacial sliding. Research supported by the Air Force Office of Scientific Research under grant #F49620-95-1-0153.

10:40 am INVITED

FINITE ELEMENT ANALYSIS OF PLASTIC YIELDING IN Fe-3%Si BICRYSTALS: R.H. Wagoner, Z.C. Yao¹ and Q. Wu², Dept. of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210; ¹National Steel Corp., Technical Research Center, 1745 Fritz Drive, Trenton, MI 48183; ²Department of Welding Engineering, Beijing Aeronautical Manufacturing Technology Research Institute, P.O. Box 863, Beijing, China

Over twenty years ago, R.E. Hook and J.P. Hirth observed the active slip systems for a range of Fe-3%Si bicrystals subjected to small compressive strain. These configurations have been stress-analyzed using anisotropic elastic finite element modeling (FEM) based on the commercial program ABAQUS. Nine bicrystals orientation pairs were investigated, all with the compression axis parallel to the grain boundary plane. The simulated stress patterns, when interpreted in terms of a critical resolved shear stress (CRSS) criterion, predicted the slip systems identified as "primary", "secondary", and "elastic incompatibility" types. Not only were the observed slip systems found to have the highest RSS at the boundary, but the front and back surfaces of the crystals were consistently differentiated by the FEM and the experiments. Only the slip systems labeled "second order" by Hook and Hirth had low RSS's. Elastic-plastic transition in bicrystals was also simulated to reveal the propagation of plastic deformation for these cases.

11:10 am

AN ATTEMPT TO CONTROL THE INTERFACIAL STRENGTH IN FeAl/TiB₂ COMPOSITES: J.H. Schneibel, R. Subramanian, Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6115

Composites consisting of a B2 iron aluminide matrix and 40vol.% of TiB₂ particles were processed by liquid phase sintering. Thermodynamic calculations suggest that by adding B or Ti, respectively, the iron aluminide melt may be enriched in B at the expense of Ti, or in Ti at the expense of B. This in turn may lead to segregation of either element at the FeAl/TiB₂ interfaces. Room temperature tests show slight increases in the flexure strengths (up to 1400 MPa) and the fracture toughness for composites microalloyed with B. Interfacial segregation of B may have contributed to this result. This research was sponsored by the Division of Materials Sciences, U.S. Department of Energy under contract number DE-AC05-96OR22464 with Lockheed Martin Energy Research Corp.

11:20 am

DEVELOPMENT OF A PREFERRED GRAIN BOUNDARY STRUCTURE IN AlCu INTERCONNECTS: David P. Field, TexSEM Laboratories, 226W 2230N, Provo, UT 84604

Analysis of AlCu interconnect lines indicates development of a preferred grain boundary structure in very narrow lines (<0.75 µm). This structure is observed to evolve because of grain boundary migration and preferential growth of given orientations during the post-patterning anneal. Experimental evidence is presented showing the evolution of interconnect microstructure subsequent to patterning of non-passivated lines. It is proposed that the structure develops because of surface energy minimization of both the top and sides of the lines, in addition to grain boundary energy considerations. Implications for the reliability of interconnect lines are discussed.

11:40 am INVITED

THE RELATION BETWEEN MECHANICAL AND ELECTRICAL PROPERTIES OF NANO-SCALE METALLIC CONTACTS: D.B. Williams, Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015-3195

Nano-scale metallic contacts have been made by several groups and their electrical and mechanical properties have been measured. This paper will first review these fascinating new results in the science of interfaces and the controversies that have arisen. It will then describe dynamical simulations of these contacts which model simultaneously their mechanical evolution and their electronic conductance. It will be shown that the jumps in the electronic conductance that are observed experimentally are almost always a consequence of a structural rearrangement inside the contact arising from a mechanical instability. If time permits we will also mention our most recent work on modeling electro-migration in these contacts.