Materials Week '97: Wednesday 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 Wednesday morning, September 17.

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

Session Chair: Christopher R.M. Grovenor, the University of Oxford, Oxford, UK

STRUCTURE AND PROPERTIES OF TRIPLE JUNCTIONS IN ANISOTROPIC SYSTEMS: A.H. King, Department of Materials Science and Engineering, State University of New York, Stony Brook, NY 11794-2275

We have studies the morphology of triple junctions in systems that exhibit anisotropic grain boundary energy. It is shown that the grain boundaries adopt symmetrical configurations adjacent to the triple junctions in a very large fraction of all cases. Up to 70% of the triple junctions in gold thin films, for example, comprise three symmetric-tilt grain boundaries. We consider the energy balance that leads to such configurations and show that there is a very common set of cases in which the Herring equation (relating the morphology to the interfacial energies) appears to fail. It is then demonstrated that the triple junctions are stabilized in a number of different ways in these cases, usually involving the inclusion of additional defects at the junction itself, or along one of the grain boundaries. Research supported by the National Science Foundation, grant number DMR 9530314.

9:00 am INVITED

THE ROLE OF INTERFACIAL STRUCTURE IN DIFFUSIONAL CREEP: J.B. Bilde-Sorensen, Materials Research Department, Riso National Laboratory, PO Box 49, DK-4000 Roskilde, Denmark

The absorption and emission of vacancies at grain boundaries during diffusional creep take place at grain boundary dislocations (GBDs). The GBDs are confined to the boundary plane and in the general case their movement will therefore involve a combination of glide and climb. The coupling of vacancy absorption and emission to the movement of GBDs manifests itself on a larger structural scale in negative grain boundary sliding and negative grain boundary climb at certain grain boundaries as has been observed in materials deformed in diffusional creep. It also explains why denuded zones often are formed on only one side of the boundary in particle-containing materials.

9:30 am INVITED

SIMULATION STUDIES OF INTERFACE DIFFUSION AND PHASE FORMATION: J.M. Rickman, Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015

Two kinetic processes associated with grain boundaries are discussed. In the first study, we examine quantitatively the impact of heterogeneous nucleation on the temporal evolution of a phase transformation with particular emphasis on the correlation of nucleation site distribution and product phase microstructure. This is accomplished by investigating spatial correlations in the transforming system via the calculation of nonequilibrium correlation functions and by characterizing product grain sizes and shapes. Computer simulations of transformations are employed in order to validate our theoretical description and to relate microstructural features of the evolving phase to relevant length and time scales in the problem. In the second study, we investigate the kinetics of grain boundary diffusion using a spatially inhomegeneous lattice gasmodel. It is found that atomic transport can be accurately described by a series of approximate rate equations and that one can ascribe a bias, in a certain sense, to tagged atoms.

10:00 am BREAK

10:10 am INVITED

REVIEW OF INTERFACIAL SEGREGATION STUDIES BY ANALYTICAL ELECTRON MICROSCOPY: A.P. Sutton, T.N. Todorov, Department of Materials, Oxford University, OX1 3PH, UK

Solute segregation to grain boundaries and interphase interfaces can have significant effect on the properties of those interfaces, particularly if they are moving, e.g. during discontinuous precipitation, diffusion-induced grain boundary migration or electromigration. Study of interface migration was an abiding interest of David Smith throughout his career and he made seminal contributions to the literature in this field. The technique of analytical electron microscopy, is the principal tool for the measurement of the composition profiles that develop during interfacial segregation and this paper will review the advantages and limitations of X-ray energy-dispersive spectrometry and electron energy-loss spectrometry studies of interfacial segregation using the David Smith's work as examples where possible.

10:40 am INVITED

GRAIN BOUNDARY DIFFUSION AND THE GROWTH OF PRECIPITATES: G.J. Shiflet, Dept. of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903

The growth kinetics of grain boundary precipitates are measured as a function of grain misorientation, isothermal temperature and reaction time. The data are analyzed taking into account the short circuit diffusion path for substitutional elements provided by the grain boundary. Growth kinetics are found to increase by about an order of magnitude as the misorientation varies from 20 to 50 degrees for a given heat treatment period. The kinetics are analyzed using various models including Brailsford and Aaron. Solute segregation measurements are obtained using a FEG-TEM.

11:10 am

MEASUREMENT OF Cu DISTRIBUTION IN AN Au-4wt.% Cu THIN FILM BY AEM: D.T. Carpenter, M. Watanabe, D.B. Williams, K. Barmak, D.A. Smith, Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA

The addition of small amounts of copper to aluminum interconnects used in microelectronics may increase their lifetimes by several orders of magnitude. While the exact role of copper is not well understood, there is some qualitative evidence that enhanced lifetimes are related to increased copper segregation at the grain boundaries. The object of the current work is to quantitatively describe the distribution of Cu at grain boundaries in a typical interconnect material. A 300 kV FEG-STEM will be used to measure the distribution of Cu in a thin (100 nm) film of Al-4wt.% Cu using spatially resolved EDS. Analytical results from reduced raster scans and line profiles across a large number of grain boundaries to give information about both the amount and the distribution of Cu at each boundary will be presented.

11:30 am

INTERACTIONS BETWEEN GRAIN BOUNDARY SLIDING AND SOLUTE SEGREGATION: J.S. Vetrano, C.H. Henager, E.P. Simonen, S.M. Bruemmer, Pacific Northwest National Laboratory, Richland, WA 99352

The presence of solute atoms along sliding interfaces can have a profound effect on the climb and glide processes of extrinsic grain boundary dislocations. We have studied these effects by selected solute additions to aluminum and measurements of grain boundary dislocation stability and grain boundary sliding (GBS). Scanning Auger Microprobe measurements of thermally treated Sn- containing alloys showed a strong segregation of Sn to grain boundaries. Measurements of grain boundary sliding in these alloys also indicated that Sn altered the thermal stability of extrinsic grain boundary dislocations and increased the ability of the boundaries to slide. Also studied were solid solution Al-Mg alloys. When these alloys undergo GBS, the point defect and grain boundary dislocation motion combine to redistribute the Mg atoms heterogeneously along the boundary. This also indicates a strong interaction between solute atoms and the mechanisms of GBS. Implications for superplastically forming these alloys will be discussed. Work supported by the Materials Division, Office of Basic Energy Sciences, U.S. Department of Energy under Contract DE-AC06-76RLO 1830.

12:00 pm INVITED

INTERFACIAL DISLOCATIONS, INTERFACIAL STEPS, AND THE PEARLITE REACTION: Paul R. Howell, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802

For several decades, pearlite growth was considered to occur through the random attachment of atoms across the advancing interface. More recently, however, a ledge mechanism for pearlite growth has been proposed. Various linear defects in the pearlite.austenite and ferrite/cementite interfaces have been identified as either "growth steps/ledges" and/or "direction steps/ledges". The present paper will review data concerning the nature of the interfacial defects in pearlite with particular regard to the following (still controversial) questions: 1) How does a pearlite colony develop into the abutting austenite? 2) What is the importance (if any!) of crystallography in the pearlite reaction? Results from both ferrous and non-ferrous systems will be reported.