Materials Week '97: Monday 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 Monday afternoon, September 15.

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

Session Chair: Robert C. Pond, The University of Liverpool, Liverpool, UK

INTERACTION OF DISLOCATIONS WITH INTERFACES IN A SOAP FROTH: M.E. Rosa, M.A. Fortes, Departmento de Engenharia de Materiais, Instituto Superior Tcnico, Lisboa, Portugal

The interaction of dislocations with interfaces was observed in tensile tests of mondispersed 2D foam samples between two parallel walls. A special device was used to prepare the samples and deform them. Dislocations are found to be reflected by the walls into the foam. When this happens, another dislocation is formed (Burgers vector conservation) that moves in the foam along the wall. Necking of the foam sample was observed in tension. Similar observations (i.e. reflection of dislocations) were made on bicrystalline foam samples (bubbles of two different diameters), but in this case there is some transfer of cells from one side to the other of the interface which eventually leads to an amorphous foam structure.

2:30 pm INVITED

STRUCTURE AND PROPERTIES OF DISLOCATIONS AND GRAIN BOUNDARIES IN SILICON: Matthew F. Chisholm, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN

With a scanning transmission electron microscope capable of forming a probe of atomic dimensions, a new approach to defect structure determination has become possible. Images formed using high-angle, elastically scattered electrons show strong atomic number (Z) contrast and reveal atomic column locations without the need for preconceived structure models. This experimental technique has been combined with simulations to study extended defects in silicon. This synergistic combination of experiment and theory has been used to determined the atomic and electronic structures of these important atomic configurations and has provided a remarkably self-consistent, atomic-scale picture of the segregation of As to silicon GBs.

3:00 pm INVITED

A THEORETICAL AND EXPERIMENTAL STUDY OF NON-PERFECT GRAIN BOUNDARY DISLOCATIONS: L. Sagalowicz, W.A.T. Clark, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210

Non-perfect grain boundary dislocations have been shown to appear in grain boundaries in a variety of materials. Recent theoretical approaches enable non-perfect grain boundary dislocations and grain boundary structures to be described, and utilize the power of group theory. Grain boundary dislocations may be divided into three classes: 1) perfect grain boundary dislocations, 2) imperfect grain boundary dislocations, and 3) partial grain boundary dislocations. Experimental transmission electron microscope evidence will be presented for boundaries in the diamond cubic structure, and it will be shown that imperfect and partial grain boundary dislocations play an important role in this system. The implications of these experimental observations for the description of grain boundary properties in terms of non-perfect dislocations will be discussed.

3:30 pm BREAK

3:40 pm INVITED

DISCONNECTIONS AS TRANSFORMATION DISLOCATIONS IN MARTENSITE TRANSFORMATIONS: R.C. Pond, Department of Materials Science and Engineering, The University of Liverpool, Liverpool L69 3BX, UK; J.P. Hirth, School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99614-2920

For phase transformations with well defined terrace planes, interface motion can occur by the motion of disconnections (defects with both step and dislocation character). Recent attention has focused on the role of such defects in diffusional transformations. The present work treats the role of disconnections in martensitic transformations. These defects are discussed in terms of symmetry theory. The symmetry imposes constraints that can influence the motion of the defects and hence the rate of the transformation.

4:10 pm INVITED

ANALYSIS OF MISORIENTATIONS AT THIN FILM INTERFACES BY THE PHENOMENOLOGICAL THEORY OF MARTENSITE CRYSTALLOGRAPHY: James M. Howe, Department of Materials Science & Engineering, University of Virginia, Charlottesville, VA 22903-2442; David A. Smith, Lehigh University, Bethlehem, PA 18015

It is quite common to find slight misorientations (tilts or rotations) associated with overgrowths on substrates in semiconductor multilayers and metal-ceramic systems. Several investigators have explained this phenomena using simple geometric analysis based on misfit accommodation at the interface. The purpose of this talk is two-fold: 1) to show that the misorientations at these interfaces can be readily explained by application of the phenomenological theory of martensite crystallography (PTMC) to the shape deformation between the overgrowths and substrates, and 2) to demonstrate that the simple geometric analyses that have been used to explain such phenomena are identical to the PTMC. Having made this comparison, it is apparent that the tilts and defects commonly observed in epitaxial layers may provide a number of examples with which to test the predictions of the PTMC in systems other than those having a martensite transformation. This work was performed during a visit with David Smith at IBM in May, 1992.

4:40 pm

HREM OBSERVATIONS OF MECHANICAL TWINNING IN Cu-Ti ALLOYS: STRUCTURE OF INCOHERENT AND COHERENT TWIN INTERFACES: T. Radetic, W.A. Soffa, V. Radmilovic , Department of Materials Science & Engineering, University of Pittsburgh, Pittsburgh, PA 15261; Department of Physical Metallurgy, University of Belgrade, 11001 Belgrade, Yugoslavia

The flow of Cu-Ti alloys hardened by coherent b' precipitates (Cu₄Ti, D1_a superstructure) is particularly interesting since these alloys show profuse twinning on multiple systems after small amounts of plastic flow by slip. In this paper, studies of the fine-scale structure of the mechanical twins which form in particle hardened Cu-Ti alloys are reported. The growth of deformation twins occurs via the motion of G-ledges at twin/matrix interfaces and via a coalescence of fine twins. The atomic structure of the coherent twin/matrix interfaces and associated single and multiple layer ledges are characterized. Incoherent interfaces at the tip of deformation twins generated by 90° and 30° twinning dislocations have been investigated. The twin/twin and slip/twin interactions and effect of grain boundaries on deformation twinning are also studied. Work supported in part by the MRC at the University of Pittsburgh sponsored by AFOSR and by the National Science Foundation (DMR) and by a Fellowship at the NCEM, Lawrence Berkeley National Laboratory, under Contract No. DE-AC-03-76SF00098.

5:00 pm

THE ROLE OF INTERFACES IN DEFORMATION TWINNING OF LAMELLAR TiAl CRYSTALS: L.M. Hsiung, T.G. Nieh, Lawrence Livermore National Laboratory, PO Box 808, L-370, Livermore, CA 94551-9900

Deformation twinning (DT) is known to play an important role in enhancing both room temperature ductility and high temperature creep resistance of lamellar TiAl alloys. The formation mechanism of DT in lamellar TiAl has recently been investigated using transmission electron microscopy. The (TiAl/Ti₃Al) interfaces in lamellar TiAl crystals provide additional nucleation sites for deformation twins, and promote the formation processes of DT in TiAl lamellae. It is suggested that DT in lamellar TiAl can be viewed as a stress-relief process for the pile-up of interfacial dislocations gliding along the lamellar interfaces during deformation. The deformation twins within TiAl lamellae are accordingly formed by a dislocation reaction based upon a stair-rod cross-slip mechanism.