1997 TMS Annual Meeting: Monday Session

CHEMISTRY AND PHYSICS OF NANOSTRUCTURES AND RELATED NONEQUILIBRIUM MATERIALS: Session I: Diffusion and Phase Transformations

Session Chairperson: Brent Fultz, 138-78, California Institute of Technology, Pasadena, CA 91125

8:30 am INVITED

KINETICS OF ATOMIC TRANSPORT IN THE FORMATION OF NONEQUILIBRIUM MATERIALS: Michael J. Aziz, Division of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge MA 02138

Rapid solidification can result in the production of nonequilibrium materials either primarily due to nucleation or primarily due to growth. Rapid growth can trap in solute or point defect supersaturations or chemical disorder, or can cause the formation of nonequilibrium microstructures. Additionally, large driving forces can permit the nucleation and growth of other metastable phases. The phenomenology and mechanisms for these processes will be discussed. Models and experiments will be reviewed.

9:00 am INVITED

DEVIATIONS FROM LINEAR DIFFUSION DURING SOLID-STATE AMORPHIZATION IN Ni-Hf DIFFUSION COUPLES: M. Atzmon, W.S.L. Boyer, The University of Michigan, Department of Nuclear Engineering and Radiological Sciences, Materials Science and Engineering, Ann Arbor, MI 48109-2104

When analyzing diffusion measurements, the diffusion coefficient is often assumed to be independent of composition and time. In this paper, studies of possible deviations from this simple picture will be reported. During amorphous alloy growth in an elemental diffusion couple, the amorphous phase has a wide homogeneity range, and the interdiffusion coefficient is unlikely to be uniform. Experimental results in Ni-Hf thin-film couples will be described, which indicate that the interdiffusion coefficient varies significantly with composition. This variation leads to significant systematic errors in determining the interfacial compositions from measured composition profiles. A more reliable measurement method will be presented. A number of authors have suggested that evolution of macrostress during amorphization by interdiffusion affects the rate of the latter. Using a combination of substrate curvature and lattice parameter measurements, we have determined the evolution of stress during interdiffusion in Ni-Hf thin-film couples. The amorphous phase is observed to form under large tensile stress, which relaxes by creep during subsequent growth. Neither this stress, nor that induced by ion irradiation, affects the interdiffusion coefficient.

9:30 am INVITED

SINTERING OF SMALL ASSEMBLIES: R.S. Averback, Huilong Zhu, M. Yeadon, J.M. Gibson, Materials Science and Engineering, University of Illinois, 1304 W. Green St., Urbana, IL 61801

The sintering of small assemblies of nanoparticles has been investigated by a combination of molecular dynamics computer simulations and in situ observations in a UHV transmission electron microscope. Because the radius of curvature of nanoparticles is so small, enormously large interfacial stresses are developed when nanoparticles come in contact with other nanoparticles or substrates, and this leads to sintering by plastic deformation of the particles on the time scale of some picoseconds. Surprisingly, these particles rotate and form twins and other low energy configurations. Several examples of such sintering will be illustrated, including nanoparticle assemblies of Cu, amorphous alloys and intermetallic compounds. Interactions between nanoparticles and substrates will also be discussed.

10:00 am INVITED

EFFECT OF FREE SURFACES ON THE KINETICS AND MORPHOLOGY OF SPINODAL DECOMPOSITION: Long-Qing Chen, Materials Science and Engineering, Penn State University, University Park, PA 16802

The kinetics of spinodal decomposition and morphological evolution near a crystalline surface were investigated by microscopic master equations in the point and pair approximations and a second-neighbor interaction model. Both two-dimensional (2-D) and 3-D model systems were considered. It is shown that, in the presence of a surface, spinodal decomposition initially involves surface segregation, followed by anisotropic decomposition in the near-surface region, and then followed by isotropic decomposition in the bulk. It is demonstrated that, due to segregation, a surface spinodal decomposition may take place for alloys whose overall average compositions are outside the bulk spinodal. It is found that the presence of a surface results in a dominant concentration wave which produces interesting transient morphological patterns such as distorted hexagonal precipitate lattices for relatively low-volume fractions and straight stripes at high volume fractions in the near-surface region. The relevance of these results to the mechanisms of diffusional phase transformations, such as spinodal ordering and decomposition, in nanoscale crystalline materials will be discussed.

10:30 am BREAK

10:45 am INVITED

SELF-PROPAGATING REACTIONS IN NANOSCALE MULTILAYER MATERIALS: Tim Weihs, Materials Science & Engineering, The Johns Hopkins University, Baltimore, MD 21218

This presentation describes self-propagating, exothermic reactions in nanoscale, multilayer materials. The talk begins with a review of self-propagating reactions in powder-based materials and then focuses on similar exothermic reactions in multilayer foils. The multilayer foils consist of alternate layers of materials that have high heats of mixing, such as Al and Ni. The individual layers (of Al or Ni) are only nanometers thick, but hundreds of these layers are sputter deposited to form one foil with a total thickness between 10 and 100 microns. The layered foils arc easily removed from their substrates, and their exothermic mixing can be started at room temperature with a small spark. Te exothermic reactions propagate al speeds greater than 10m/s and they reach final temperatures as high us 1600°C The reactions can propagate in air, vacuum, or liquids. The velocities, the heats, and the temperatures of the reactions will be presented for at least two different multilayer systems, and the chemical and structural parameters that control these properties will be addressed using an analytical model. The final segment of the talk will describe how reactive multilayer foils can be used as local heat sources to joint structural components.

11:15 am INVITED

NANOSTRUCTURED PALLADIUM ALLOY MEMBRANE MATERIALS: Kenneth J. Bryden, Jackie Y. Ying, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139-4307

Membranes are often fabricated from palladium since it has a very high hydrogen permselectivity. Diffusion through the metal is often the rate-limiting step in hydrogen transport through palladium-based membranes. The flux through these membranes can be increased by tailoring a microstructure that allows for higher hydrogen diffusivity. Nanostructured palladium has a much higher hydrogen diffusivity than conventional palladium due to its large volume fraction of grain boundaries. Thus, nanostructured metal membranes would provide higher hydrogen fluxes and better performance. By doping palladium with another element, enhanced stability against grain growth, stability against the alpha to beta phase transition which causes cracking, and enhanced poisoning resistance can be achieved. In this study nanostructured pure palladium and palladium alloy membranes were synthesized by pulsed electrodeposition. The hydrogen permselectivity through these materials was determined in a membrane reactor and the effect of common poisons (e.g. carbon monoxide and hydrogen sulfide) on hydrogen diffusivity was also measured to relate the hydrogen permeation properties of these materials to their microstructure and elemental composition.