F. Spaepen

A microscopic mechanism for steady state inhomogeneous flow in metallic glasses

Abstract An empirical deformation map for metallic glasses is introduced and the two modes of deformation, homogeneous and inhomogeneous flow are reviewed. The microscopic mechanism for steady state inhomogeneous flow is based on a dynamic equilibrium between stress-driven creation and diffusional annihilation of structural disorder. The formalism is developed using the free volume as the order parameter. The boundary line between the homogeneous and inhomogeneous flow regions on the deformation map is calculated. The stress-strain relation in inhomogeneous flow approaches ideally plastic behavior.

Research opportunities on clusters and cluster-assembled materials —A Department of Energy, Council on Materials Science Panel Report

The Panel was charged with assessing the present scientific understanding of the size-dependent physical and chemical properties of clusters, the methods of synthesis of macroscopic amounts of size-selected clusters with desired properties, and most importantly, the possibility of their controlled assembly into new materials with novel properties. The Panel was composed of both academic and industrial scientists from the physics, chemistry, and materials science communities, and met in January 1988. In materials (insulators, semiconductors, and metals) with strong chemical bonding, there is extensive spatial delocalization of valence electrons, and therefore the bulk physical properties which depend upon these electrons develop only gradually with cluster size. Recent research using supersonic-jet, gas-aggregation, colloidal, and chemical-synthetic methods indeed clearly establishes that intermediate size clusters have novel and hybrid properties, between the molecular and bulk solid-state limits. A scientific understanding of these transitions in properties has only been partially achieved, and the Panel believes that this interdisciplinary area of science is at the very heart of the basic nature of materials. In Sec. V (Future Challenges and Opportunities), a series of basic questions for future research are detailed. Each question has an obvious impact on our potential ability to create new materials. Present methods for the synthesis of useful amounts of size-selected clusters, with surface chemical properties purposefully controlled and/or modified, are almost nonexistent, and these fundamentally limit our ability to explore the assembly of clusters into potentially novel materials. While elegant spectroscopic and chemisorption studies of size-selected clusters have been carried out using molecular-beam technologies, there are no demonstrated methods for recovery and accumulation of such samples. Within the past year, the first reports of the chemical synthesis of clusters with surfaces chemically modified have been reported for limited classes of materials. Apparatus for the accumulation and consolidation of nanophase materials have been developed, and the first promising studies of their physical properties are appearing. In both the chemical and nanophase synthesis areas, clusters with a distribution of sizes and shapes are being studied. Progress on macroscopic synthetic methods for size-selected clusters of controlled surface properties is the most important immediate goal recognized by the Panel. Simultaneous improvement in physical characterization will be necessary to guide synthesis research. Assuming such progress will occur, the Panel suggests that self-assembly of clusters into new elemental polymorphs and new types of nanoscale heterogeneous materials offers an area of intriguing technological promise. The electrical and optical properties of such heterogeneous materials could be tailored in very specific ways. Such ideas are quite speculative at this time; their implementation critically depends upon controlled modification of cluster surfaces, and upon development of characterization and theoretical tools to guide experiments. The Panel concluded that a number of genuinely novel ideas had been enunciated, and that in its opinion some would surely lead to exciting new science and important new materials.

Strain localization in amorphous metals

Abstract Inhomogeneous flow in metallic glasses is studied in this paper within the context of continuum mechanics. Motivated by similar work for elastic-plastic solids, the possibility of strain localization into a shear band is investigated for a metallic glass which is modelled as a nonlinear viscoelastic solid. The essential features of the localization problem are brought out through an analysis of the constitutive law which reveals a catastrophic softening via free volume creation. Analytic expressions for the stress at catastrophic softening agree very closely with the stress at strain localization calculated from the numerical solution of the full set of shear band equations.

Interfaces and stresses in thin films

A review of the current understanding of the effect of interfaces on the intrinsic stresses in polycrystalline thin films is given. Special attention is paid to the measurement, modeling and application of surface and interface stresses. Mechanisms for generating the compressive and tensile components of the intrinsic stress are assessed. Prospects for future research are presented.

A structural model for the solid-liquid interface in monatomic systems

Abstract A structural model for the solid-liquid interface has been developed by applying the construction rules for the liquid to the boundary condition of a crystal plane. In particular, an interface has been created between a dense random packing of hard spheres and a close packed crystal plane by preferentially forming tetrahedral holes (typical for the liquid) and disallowing octahedral holes (necessary for the crystal). The resulting interface has zero density deficit in agreement with conclusions from positron lifetime studies. The surface tension, of entropic origin, is of the same magnitude as the experimental values. The model can account qualitatively for the ease of step nucleation during crystal growth.

Calorimetric studies of crystallization and relaxation of amorphous Si and Ge prepared by ion implantation

Amorphous Si and Ge layers, produced by noble gas (Ar or Xe) implantation of single crystal substrates, have been crystallized in a differential scanning calorimeter (DSC). The MeV implantation energies resulted in amorphous layers of micron thickness whose areal densities were determined using the Rutherford backscattering and channeling of 1‐MeV protons. These techniques allow determination of the amorphous‐crystal interface velocity (which is proportional to the rate of heat evolution ΔHac) and the total enthalpy of crystallization ΔHac. Amorphous Ge was found to relax continuously to an amorphous state of lower free energy, with a total enthalpy of relaxation of 6.0 kJ/mol before the onset of rapid crystallization. The interface velocity for crystallization on (100) substrates, was found to have an Arrhenius form with an activation energy of 2.17 eV. The value of ΔHac was found to be 11.6±0.7 kJ/mol, the same as for samples prepared by deposition. For Si, ΔHac was determined to be 11.9±0.7 kJ/mol wit...

The kinetics of structural relaxation of a metallic glass

Abstract The change in viscosity of a Pd 82 Si 18 glass during isothermal annealing has been measured in the temperature range of 424–537 K for times up to 325 h. It was observed that the viscosity increased linearly with time and that the rate of increase exhibited an Arrhenius-type temperature dependence with an activation energy of 31 ± 2 kJ/mole. The isoconfigurational viscosity exhibited an activation energy of 192 ± 17 kJ/mole for all states measured in this temperature range. The kinetics of the viscosity changes are explained quantitatively by a new model for structural relaxation, based on an extension of the free volume model for flow.

On the approximation of the free energy change on crystallization

Abstract Approximations of the Gibbs free energy change for crystallization of an undercooled liquid. ΔG, are discussed and compared. When liquid heat capacity data are available, ΔG can be approximated to various degrees of accuracy depending on the completeness of the data. In the absence of these data, it is necessary to make further approximations. It is shown that Turnbulls [4] simple linear approximation for ΔG is generally applicable to pure metals. A new expression is proposed for use with easy glass forming alloys such as Au81.4Si18.6. An approximation due to Hoffman [1] is not appropriate for use with metals and alloys but is adequate for use with organic substances, such as ortho-terphenyl.

Effect of sample size on deformation in amorphous metals

Uniaxial compression tests were performed on micron-sized columns of amorphous PdSi to investigate the effect of sample size on deformation behavior. Cylindrical columns with diameters between 8μm and 140nm were fabricated from sputtered amorphous Pd77Si23 films on Si substrates by focused ion beam machining and compression tests were performed with a nanoindenter outfitted with a flat diamond punch. The columns exhibited elastic behavior until they yielded by either shear band formation on a plane at 50° to the loading axis or by homogenous deformation. Shear band formation occurred only in columns with diameters larger than 400nm. The change in deformation mechanism from shear band formation to homogeneous deformation with decreasing column size is attributed to a required critical strained volume for shear band formation.

Structural rearrangements that govern flow in colloidal glasses

Structural rearrangements are an essential property of atomic and molecular glasses; they are critical in controlling resistance to flow and are central to the evolution of many properties of glasses, such as their heat capacity and dielectric constant. Despite their importance, these rearrangements cannot directly be visualized in atomic glasses. We used a colloidal glass to obtain direct three-dimensional images of thermally induced structural rearrangements in the presence of an applied shear. We identified localized irreversible shear transformation zones and determined their formation energy and topology. A transformation favored successive ones in its vicinity. Using continuum models, we elucidated the interplay between applied strain and thermal fluctuations that governs the formation of these zones in both colloidal and molecular glasses.