Materials Week '97: Tuesday 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 Tuesday morning, September 16.

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

Session Chair: Kevin M. Knowles, The University of Cambridge, Cambridge, UK

DISLOCATIONS IN STRAINED LAYER INTERFACES: Peter J. Goodhew, Department of Materials Science & Engineering, The University of Liverpool, Liverpool L69 3BX, UK

Two dislocation configurations are particularly important in semiconductors. Threading dislocation (TDs) intersect the interfaces between layers and can have any Burgers vector. Misfit dislocations (MDs) have a component of their Burgers vector which acts to relieve the strain in the system, and they often lie in or parallel to interfaces between layers. In this paper we consider the origin of both types of dislocation, and their scope for multiplication. The device and growth requirements of strained layers and fully-relaxed layers with predictable lattice parameters but long term stability are different and sometimes conflict. We will report experimental work on III-V systems, particularly InGaAs grown on GaAs. We have studied the distribution of spacings of MDs and the implications these have for the dislocation sources which must be operating, their effect on the surface topography of a growing layer, the effect of vicinal plane substrates on TD formation, and the use of etching techniques to reveal TD densities even in thin heterolayers.

9:00 am INVITED

INTERFACE PROPERTIES AND PHASE STABILITIES IN METALLIC MULTILAYERS: Hamish L. Fraser, Rajarshi Banerjee, Suliman Dregia, Mark Asta^*, Andrew Quong^*, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH; ^*Sandia National Laboratory, Livermore, CA

In a recent study of phase stability in thin multilayered samples of Al and Ti, it has been noted that as the scale of the microstructure is decreased, the Ti layers undergo a transition from hcp to fcc and then revert back to hcp in even thinner layers, whereas the Al layers undergo a transition from fcc to hcp. We have developed a thermodynamic model to account for these observations, which is based on the influence of the relative energies of the interfaces between the various stable and metastable configurations. Because of the apparent importance of these various interfaces on phase stabilities in these new types of materials, their structures have been characterized using high resolution and analytical electron microscopy, and the degree of compositional intermixing is being evaluated by high resolution SIMS. The information gained from these experiments is being used in theoretical computations (using a density functional approach) of bulk and interfacial energies so that a detailed basis to the thermodynamic model may be developed.

9:30 am INVITED

SHEAR BOUNDARIES IN LAMELLAR TiAl: P.M. Hazzledine, Materials Directorate, Wright Laboratory, WL/MLLM, Wright-Patterson AFB, OH 45433; UES Inc, 4401 Dayton-Xenia Road, Dayton, OH 45432

In lamellar TiAl six orientation variants of the tetragonal phase form a multilayer in which the lamellar boundaries are {111} planes. The tetragonality of the unit cell causes in-plane misfits in the lattice parameters of the order of 1%. In very thin lamellae the misfits are taken up elastically whereas in thicker lamellae some of the misfit is taken up by van der Merwe dislocations. The elastic strains are shears in the plane of the lamellae and the dislocations are either single sets of screw dislocations or cross grids of screw dislocations. Both the elastic coherency stresses and the mismatch dislocations affect the strength of the multilayer; the coherency stresses by direct interaction with glissile dislocations and the mismatch dislocations by acting as barriers to glissile dislocations crossing the lamellar boundaries. Neither has any effect on dislocations which are glissile in the plane of the lamellae. In this paper the strength of TiAl is discussed with reference to the structure of the lamellar boundaries.

10:00 am BREAK

10:10 am INVITED

GRAIN BOUNDARIES IN ORDERED ALLOYS: Ian Baker and Easo George, Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, NH 03755

Many strongly-ordered intermetallic compounds fail through brittle intergranular fracture. In this paper research on the structure and chemistry of grain boundaries in ordered alloys is reviewed. The observations are discussed in terms of the technique used for grain boundary examination with emphasis on the limitations of each technique. Most work on grain boundary structure/chemistry in ordered alloys has been on L1₂ compounds, in particular Ni₃Al. In this material, several researchers have noted that the grain boundaries in Ni₃Al can be enriched in nickel. This is usually observed when the material is nickel-rich and doped with boron. Grain boundaries in Ni₃Si seem to be similarly enriched in nickel. There have been few studies of grain boundaries in B2 compounds. However, grain boundary compositions in the B2 compounds NiAl and FeAl were usually found to be the same as the nearby matrix. The implications of these observations are discussed in terms of the mechanisms which have been presented for the brittle nature of the grain boundaries in these materials. Research supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences, contract #DE-FG02-87ER45311.

10:40 am

GRAIN BOUNDARY STRUCTURE AND SEGREGATION IN NiAl: David E. Luzzi, Richard W. Fonda¹, Min Yan², Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA; ¹Naval Research Laboratory,Washington, DC; ²Los Alamos National Laboratory, Los Alamos, NM

A combined experimental and theoretical modeling approach utilizing HREM and atomistic structure calculations has been applied to the structure of a =5 (310) [001] grain boundary in Ni-rich NiAl. The resultant structure model, containing nickel antisite defects adjacent to the boundary plane, is consistent with HREM data, is the lowest energy structure at 0K via molecular statics calculations, and is stable at higher temperatures as determined from Monte Carlo calculations. The calculations were then extended to the study of stoichiometric and Al-rich grain boundaries. Segregation of point defects to the boundary is favored in all off stoichiometric NiAl. In Al-rich NiAl, either Ni vacancies or Al antisite defects may be found at the grain boundary, whereas only Ni constitutional vacancies are favored within the bulk.

11:00 am

EFFECTS OF INTERLAYERS ON THE ADHESION OF THIN COPPER FILMS: Michael D. Kreise, Neville R. Moody^*, William W. Gerberich, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN; ^*Sandia National Laboratories, Livermore, CA

Nanoindentation techniques are used to evaluate the effect of very thin "adhesion promotion" layers of refractory elements such as chromium and tungsten on the adhesion of thin copper films of 100nm to 1000nm thickness on SiO₂ substrates. Such interlayers produce changes in the work of adhesion, W_ad, which has previously been identified as leveraging all other contributions to the adhesion toughness, G_l. The present study examines in particular the effect of interlayers on the plastic dissipation contribution. Other variables of interest are film thickness, residual stress, and the effect of thick refractory overlayers.

11:20 am

STRUCTURE AND MECHANICAL PROPERTIES OF Cu/Nb AND Cu/Ni NANOSTRUCTURED MULTILAYERS: T.E. Mitchell, Y.C. Lu, J.D. Embury, M. Nastasi, H. Kung, Center for Materials Science, Mail Stop K765, Los Alamos National Laboratory, Los Alamos, NM 87545

Cu/Nb multilayers prepared by sputtering onto Si substrates with layer thicknesses ranging from 11Å to 5000Å have been characterized by transmission electron microscopy and nanoindentation. The films are strongly textured with fcc and bcc close-packed planes and directions tending to be parallel. For the 11Å layers the Cu is found to grow pseudomorphically on Nb in the bcc structure. It is thought that, for thicker layers, the bcc Cu loses coherency and transforms martensitically to the fcc phase, resulting in the observed Kurdjumov-Sachs orientation relationship. The hardness follows a Hall-Petch relationship for the larger thicknesses, but is controlled by Orowan dislocation bowing at the smallest thicknesses. In addition, Cu/Ni multilayers have been grown epitaxially on NaCl and Cu single crystal substrates. The multilayers are single crystal but contain a high density of dislocations. They are being deformed in order to understand slip propagation across the interfaces.

11:40 am INVITED

COMBINED ATOMISTIC AND HREM STUDIES OF INTERFACES IN AB TYPE INTERMETALLICS: NiAl vs TiAl: V. Vitek, Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104

The atomic structure of grain boundaries and other interfaces in intermetallic compounds have been studied very extensively since they are often paths of brittle fracture or affect critically the plastic properties. Most of the atomic level calculations have been performed using the central-force potentials of the embedded-atom type to describe atomic interactions. In the case of NiAl such calculations revealed structures that are in an excellent agreement with HREM observations while in TiAl calculations employing such central force potentials exhibit some significant discrepancies. On the other hand ab initio electronic structure calculations lead to a very good agreement with HREM observations. It will be shown that the reason for this difference between NiAl and TiAl must be sought in a notably different nature of bonding in these two alloys. In particular, the bonding is practically metallic in NiAl while it has significantly covalent character in TiAl. Hence, combined HREM observations and atomistic calculations of interfacial structures can contribute significantly to our understanding of the nature of bonding in compounds and alloys.