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.

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

Session Chair: Alexander H. King, SUNY at Stony Brook, Stony Brook, NY 11794

STUDIES OF BOUNDARIES AND INTERFACES ON THE ATOMIC SCALE BY ATOM PROBE FIELD ION MICROSCOPY: G.D.W. Smith, Department of Materials, University of Oxford, Oxford, UK

David Smith was the first person to interpret correctly the image contrast observed from grain boundaries in the field ion microscope (FIM). This paper will begin with a short review of the contributions which he made to the understanding of grain boundary dislocations, and to the FIM observation of the atomic scale topography of interfaces. The second part of the paper will consist of an overview of atom probe FIM studies of the fine-scale chemical composition of grain boundaries and interface phases. The contributions of this technique to the understanding of interfacial chemistry will be illustrated by examples from ferrous and non-ferrous metals, semiconductor materials, and oxide superconductors.

2:30 pm INVITED

ATOMISTIC STUDIES OF GRAIN BOUNDARIES AND HETEROPHASE INTERFACES: EXPERIMENTS AND SIMULATIONS: David N. Seidman, Northwestern University, Department of Materials Science and Engineering, Evanston, IL 60208-3108

This talk is a review of our research on both grain boundaries (GBs) in single-phase binary metal alloys and ceramic/metal (C/M) heterophase interfaces. The emphasis is on obtaining a detailed atomistic picture of solute-atom segregation as obtained from both experiments, theory, and simulations. Heavy use is made of atom-probe field-ion microscopy to determine directly solute-atom segregation at GBs, whose five macroscopic degrees of freedom are measured by transmission electron microscopy; thereby systematically exploring the eight-dimensional GB phase space. Our experimental work is both complemented and supplemented by Monte Carlo simulations of solute-atom segregation for a wide range of twist and tilt boundaries. The chemistry of pristine C/M interfaces, as well as ones at which a segregant is present, are studied by atom-probe and Z-contrast microscopies. Local density functional theory (LDFT) and molecular dynamics studies are presented and compared with the experimental observations. This research is supported by the Department of Energy/Basic Energy Sciences and the National Science Foundation/Division of Materials Research.

3:00 pm INVITED

TRANSMISSION ELECTRON MICROSCOPY OF INTERFACES IN ENGINEERING CERAMICS: K.M. Knowles, Department of Materials Science and Engineering, The University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK

Interphase and grain boundaries in ceramic materials can be much more complex chemically than in metallic materials. For example, covalently bonded materials such as silicon nitride tend to contain thin (Å 1nm wide) amorphous grain boundary phases left behind after liquid phase sintering at high temperature. Fibre-reinforced glass ceramics, of interest as light, potentially damage tolerant materials, tend to possess Å0.1µm wide interphase regions between the matrix and the nanocrystalline fibres. In these interphase regions there are a variety of different possible chemical species, the precise details of which will be a sensitive function of heat treatment for a given combination of matrix and fibre. Electronic ceramics such as zinc oxide and strontium titanate internal barrier layer capacitors are further examples where interfacial phenomena dictate device properties. In this talk, we will use examples from a number of different engineering ceramics to illustrate the variety of ceramic interfaces that can arise and show how a combination of transmission electron microscopy techniques can be used to gain insight into the structure, chemistry and materials properties of the ceramic interfaces at near-atomic level.

3:30 pm BREAK

3:40 pm INVITED

THE EFFECT OF INTERFACES IN SOLID-STATE REACTIONS BETWEEN OXIDES: Matthew T. Johnson, Paul G. Kotula, C. Barry Carter, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455

The structure of an interface is an important factor in determining its response to different applied forces. This basic premise, which underlies much of the work of the late David A. Smith, has led to continuing studies on internal interfaces in polycrystalline materials. In ceramic oxides, the bonding is mixed covalent and ionic, which means that local changes in density can occur; this factor has important consequences for the movement of internal interfaces in these materials. Solid-state reactions occur by the movement of heterophase boundaries. This movement can be driven by either chemical or electrochemical driving forces. Through the use of pulsed-laser deposition (PLD), thin-films of NiO, In₂O₃ and Fe₂O₃ have been deposited onto monocrystalline bulk substrates of a-Al₂O₃ and MgO to produce initially planar phase boundaries. This geometry of a thin film on a bulk substrate has proved to be ideal for studying some of the fundamental processes occurring in solid-state reactions as they relate to the interfaces. In the case of the NiO/Al₂O₃ system, the effect of interfaces (or substrate orientation) on the reaction kinetics has been studied. The substrate orientation controls the overlayers and therefore the structure of the interfaces. In the case of the In₂O₃/MgO and Fe₂O₃/MgO systems, interfacial stability has been studied when the systems were reacted both without and under the influence of an electric field. The electric field provides a driving force for mass transport that affects both the reaction and the interfacial stability. Following the classical approach used by David Smith, the systems were all characterized using transmission electron microscopy (TEM); the TEM images were complemented by those obtained using a field-emission scanning electron microscope (FESEM).

4:10 pm INVITED

INTERFACE STRUCTURE-PROPERTY RELATIONSHIPS IN DIRECTIONALLY SOLIDIFIED EUTECTICS (DSEs) OF OXIDES: Elizabeth C. Dickey*, Vinayak P. Dravid, M.R. Notis+, C.E. Lyman+, and Alexandre Revcolevschi++, Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208; +Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015; ++Université de Paris, France

When properly grown, DSEs exhibit aligned fibrous/lamellar microstructure which contain numerous crystallographically identical heterophase interfaces amenable to extensive analysis. Over the last decade we have investigated a large number of lamellar DSEs, both for their structure as well as properties as reflected in crack propagation or phase transformations. The presentation will cover the connection between all length scales of interface microstructure and its influence on residual stresses and crack propagation behavior in DSEs. This presentation covers a collaborative effort which spans a full decade, three generations of mentor-student relationships and the Atlantic ocean. One of the authors (VPD) in this long list was fortunate to have late Prof. David A. Smith as his PhD committee member. He was a clear beneficiary of David's advice and words of wisdom, which he carried with him to Northwestern.

4:40 pm

DISLOCATION BEHAVIOR AT OXIDE/METAL INTERFACES UNDER NANOSCALE CONTACTS: D.E. Kramer, W.W. Gerberich, Dept. of Chemical Engineering & Materials Science, 151 Amundson Hall, Washington Avenue SE, Minneapolis, MN 55455

Metal single crystals with native oxide films exhibit unusual loading behavior when probed by nanoindentation. Initially, loading is elastic, up to stresses that approach the theoretical shear strength of the metal. The initiation of plastic deformation manifests itself in the form of a yield excursion or "pop in" in the load/displacement curve. The presence of mechanical deformation near the oxide/metal interface and the oxide thickness are shown excursion behavior. These interactions have been investigated on electropolished Fe-3% Si single crystals. Thicker oxide films require greater loads to initiate plastic flow. A mechanical deformation layer at the oxide / metal interface results in plastic deformation prior to, or the elimination of, the excursion event. A model for dislocation nucleation at the oxide/metal interface is discussed in light of the results.

5:00 pm

EDS, EELS, AND AUGER STUDIES OF METAL-CERAMIC INTERFACES: R.Y. Hashimoto, E.S.K. Menon, M. Saunders, A.G. Fox, Center for Materials Science and Engineering, Department of Mechanical Engineering, Naval Postgraduate College, Monterey, CA 93943

Metal-ceramic interfaces are important in electronics packaging applications. We have studied the copper-alumina and aluminum-alumina systems using transmission electron microscopy (TEM), energy-dispersive x-ray spectroscopy (EDS), electron energy-loss spectroscopy (EELS) and Auger electron spectroscopy (AES). The interfaces were created under vacuum by diffusion bonding of 100mm metal foils pressed between polished alumina substrates (99.98% purity) for several hours at approximately 90% of the metal melting temperature. The presence and distribution of impurities is investigated by EDS, EELS, and AES. Of particular importance is the role of silicon (the major impurity in commercially available alumina) which previous studies have suggested should migrate to the interface during bonding thus influencing its thermomechanical behavior. The energy-loss near edge structure (ELNES) of the EELS spectra is also considered in an attempt to characterize the interfacial chemistry. The ultimate aim is to relate the mechanical properties of the interface to the chemical properties.