A. L. Greer

Intrinsic plasticity or brittleness of metallic glasses

The intrinsic plasticity or brittleness of crystalline metals correlates with the ratio of the elastic shear modulus μ to the bulk modulus B; when the ratio μ/B exceeds a critical value, the metal is brittle. Sufficient data on elastic moduli and toughness are now available to permit an assessment for metallic glasses. We find a similar correlation, with the critical value of μ/B for metallic glasses (0.41–0.43) more sharply defined than for crystalline metals. This critical value applies also for annealing-induced embrittlement of metallic glasses. The clear correlation between mechanical behaviour (plasticity or brittleness) and μ/B assists in understanding flow and fracture mechanisms, and in guiding alloy design to alleviate brittleness of metallic glasses.

Formation of bulk metallic glass by fluxing

Bulk specimens (0.4–4 g mass) of the alloy Pd40Ni40P20 have been undercooled consistently to the glass state, with no detectable superficial crystallinity, in a molten flux of dehydrated boron oxide. The minimum dimension of the most massive glass specimen, so formed, was 1.0 cm. The absence of crystallinity in the specimens was confirmed by x‐ray diffraction, scanning electron microscopy, and calorimetry.

Solidification Microstructures: Recent Developments, Future Directions

The status of solidification science is critically evaluated and future directions of research in this technologically important area are proposed. The most important advances in solidification science and technology of the last decade are discussed: interface dynamics, phase selection, microstructure selection, peritectic growth, convection effects, multicomponent alloys, and numerical techniques. It is shown how the advent of new mathematical techniques (especially phase-field and cellular automata models) coupled with powerful computers now allows the following: modeling of complicated interface morphologies, taking into account not only steady state but also non-steady state phenomena; considering real alloys consisting of many elements through on-line use of large thermodynamic data banks; and taking into account natural and forced convection effects. A series of open questions and future prospects are also given. It is hoped that the reader is encouraged to explore this important and highly interesting field and to add her/his contributions to an ever better understanding and modeling of microstructure development.

Thickness of shear bands in metallic glasses

A review of measurements and atomistic modeling shows that shear bands in metallic glasses have a characteristic thickness of ∼10nm. Such extreme localization of plastic deformation, within a thicker liquidlike layer implied by fracture-surface morphology, cannot have a thermal origin. By analogy with granular materials, the thickness is linked to the local structural rearrangements required to generate dilatation. This analysis suggests that first-coordination-shell clusters may be significant structural units in metallic glasses.

Bulk formation of a metallic glass: Pd40Ni40P20

Molten spheroids of Pd40 Ni40 P20, of up to 0.53‐cm minor diameter, were slowly cooled (1.4 K/s) on a fused silica surface under 10−6 Torr vacuum to a form which was entirely glassy except for some superficial crystallinity comprising less than 0.5% of the volume. The occurrence of crystallization was eliminated by subjecting the specimens to surface etching followed by a succession of heating and cooling cycles. The absence of crystallization in the bulk was confirmed by x‐ray diffraction, transmission electron microscopy, and calorimetry. Using the last technique, the heat of crystallization of the glass was measured to be 5.3±0.3 kJ/g atom.

Atomic transport and structural relaxation in metallic glasses

Abstract Structural relaxation effects in metal-metalloid glasses are reviewed. Data on viscous flow and atomic diffusion are presented, and possible mechanisms of structural relaxation are discussed.

Grain refinement of alloys by inoculation of melts

Recent progress in understanding the inoculation of aluminium melts is reviewed. Transmission electron microscopy of inoculant particles in a metallic glass reveals details of the mechanism of nucleation of aluminium grains. While such studies define some of the conditions under which inoculation is effective or not, they do not permit a prediction of grain size. Unusually for a nucleation–related phenomenon, quantitative prediction is possible. For potent inoculation such as is practised in aluminium alloys, grain initiation is limited by inoculant particle size, occurring first on the largest particles. Simple thermal models can then describe the dependence of grain refinement on alloy content and processing conditions, and enable consideration of inoculant design.

Electromigration effects on compound growth at interfaces

Interfacial reactions are important in microelectronic devices and can be accelerated or decelerated by imposing a direct electric current normal to the interface. These effects are analyzed by including electromigration-driven interchange of atomic species in a conventional analysis of reaction layer thickening in a binary system controlled by interdiffusion in the layer and by an interfacial reaction barrier. New types of behavior are predicted. When the electromigration augments interdiffusion, layer growth can accelerate as the layer thickens, in contrast to the usual deceleration. When the electromigration opposes interdiffusion, there is a limiting layer thickness, inversely proportional to the applied current.

Interdiffusion studies in metallic glasses using compositionally modulated thin films

Abstract Interdiffusivities are measured in compositionally modulated amorphous thin films as a function of modulation wavelength. The sensitivity of this method is demonstrated by the measurement of very low interdiffusivities, and permits the observation of structural relaxation effects. From these results we are able to determine the general thermodynamic behavior of the alloy system, and to derive the bulk interdiffusivity. Our data suggest that, contrary to earlier reports, the Stokes-Einstein equation (relating diffusivity and viscosity) is not valid in metallic glasses.

Thermodynamic evidence for a poisoning mechanism in the Al-Si-Ti system

Abstract The poisoning of grain refinement in aluminium alloys owing to the presence of silicon is investigated using CALPHAD based thermodynamic modelling software. The existence of primary phases other than α-aluminium, namely TiSi2 and TiSi, which are stable at typical melt holding temperatures, is consistent with microstructural observations of other researchers. The compositional and temperature range of these phases are calculated. The software is also used to determine the effect of silicon and titanium additions on the growth rates of α-aluminium grains via the growth restriction parameter Q. These results are used to explain poisoning in silicon containing foundry alloys via a thorough review and discussion of the relevant literature.