Alan Lindsay Greer

Poisson's ratio and modern materials

In comparing a materials resistance to distort under mechanical load rather than to alter in volume, Poissons ratio offers the fundamental metric by which to compare the performance of any material when strained elastically. The numerical limits are set by ½ and -1, between which all stable isotropic materials are found. With new experiments, computational methods and routes to materials synthesis, we assess what Poissons ratio means in the contemporary understanding of the mechanical characteristics of modern materials. Central to these recent advances, we emphasize the significance of relationships outside the elastic limit between Poissons ratio and densification, connectivity, ductility and the toughness of solids; and their association with the dynamic properties of the liquids from which they were condensed and into which they melt.

Characterization of supercooled liquid Ge2Sb2Te5 and its crystallization by ultrafast-heating calorimetry

Differential scanning calorimetry (DSC) is widely used to study the stability of amorphous solids, characterizing the kinetics of crystallization close to the glass-transition temperature T(g). We apply ultrafast DSC to the phase-change material Ge(2)Sb(2)Te(5) (GST) and show that if the range of heating rates is extended to more than 10(4) K s(-1), the analysis can cover a wider temperature range, up to the point where the crystal growth rate approaches its maximum. The growth rates that can be characterized are some four orders of magnitude higher than in conventional DSC, reaching values relevant for the application of GST as a data-storage medium. The kinetic coefficient for crystal growth has a strongly non-Arrhenius temperature dependence, revealing that supercooled liquid GST has a high fragility. Near T(g) there is evidence for decoupling of the crystal-growth kinetics from viscous flow, matching the behaviour for a fragile liquid suggested by studies on oxide and organic systems.

Transient nucleation effects in glass formation

Abstract We extend to the non-isothermal case a numerical technique that was developed to treat transient homogeneous nucleation in a one-component system by modeling directly the reaction by which clusters are produced. Calculations are presented for the nucleation frequency during the quench and for the number of nuclei produced and the volume fraction transformed at the end of quench for different rates of cooling from the melt. Three model systems are considered: an alkali silicate which is a relatively good glass former, and two metallic glasses. These show a wide range of critical cooling rates for glass formation. In some systems transient effects are predicted to be critical for glass formation. A simple technique is presented for determining when transient effects are important based on a calculation using steady state nucleation frequencies and macroscopic growth velocities.

Enhancement of room-temperature plasticity in a bulk metallic glass by finely dispersed porosity

Melts of Pd42.5Cu30Ni7.5P20 (at.u2009%) held under pressurized hydrogen are cast into bulk metallic glass (BMG) rods with fine (20–30μm diameter) pores uniformly dispersed. The low overall porosities (<4%) lead to only small reductions in Young modulus and yield strength, but to dramatically enhanced plasticity in compression: Rupture energy as high as 295MJm−3, compared to 16MJm−3 for the pore-free BMG. The pores force the proliferation of shear bands below the overall failure stress, a process of interest for toughening BMGs, materials for which shear localization in deformation restricts structural applications.

A new method for tensile testing of thin films

A new method for tensile testing of thin films is presented. The strain is measured directly on the unsupported thin film from the displacement of laser spots diffracted from a thin grating applied to its surface by photolithography. The diffraction grating is two-dimensional, allowing strain measurement both along and transverse to the tensile direction. In principle, Youngs modulus, Poissons ratio, and the yield stress of a thin film can be determined. Cu, Ag, and Ni thin films with strong ⟨111⟩ texture were tested. The measured Young moduli agreed with those measured on bulk crystals, but the measured Poisson ratios were consistently low, most likely due to slight transverse folding of the film that developed during the test. The yield stresses of the evaporated Cu and Ag thin films agreed well with an extrapolation of the Hall-Petch relation found for bulk materials. Ni thin films are known to deviate from a bulk Ni Hall–Petch relation for submicron grain sizes, and sputtered Ni films show much higher yield stresses than electrodeposited or vapor-deposited films of similar grain size. Our sputtered Ni films had higher yield stresses than other sputtered films from the literature.

Mechanisms for nanocrystal formation in metallic glasses

Abstract Possible mechanisms for nanocrystal formation in metallic glasses are discussed, focusing primarily on the Al–transition metal–rare earth glasses. The presented transmission electron microscopy (TEM) data prove that nanoscale phase separation occurs prior to crystallization in some Al–RE–Ni glasses. TEM observations and modeling studies of the devitrification kinetic data for some of these glasses are presented, which demonstrate preferential nucleation of nanocrystal α-Al grains near the boundaries of the phase separated regions. Preliminary studies show no evidence for phase separation in Al–RE–Fe glasses, which also crystallize to a nanoscale microstructure. A new model for homogeneous nucleation, coupling the interfacial and the long-range diffusion fluxes, is advanced to explain this.

Numerical modelling of crystal nucleation in glasses

Abstract Nucleation is important in influencing microstructures obtained by solidification. The fundamentals of crystal nucleation are most readily studied in glasses. Some recent results on metallic glasses are reviewed. Numerical modelling of transient nucleation kinetics can provide fits to measured data and can test theory. New extensions of the modelling to heterogeneous nucleation and to two-component systems with differing species mobilities are described.

Large bulk soft magnetic [(Fe0.5Co0.5)0.75B0.20Si0.05]96Nb4 glassy alloy prepared by B2O3 flux melting and water quenching

The large bulk soft magnetic glassy [(Fe0.5Co0.5)0.75B0.20Si0.05]96Nb4 alloy specimens with the diameters up to 7.7mm have been prepared by water quenching the melt immersed in the molten flux of B2O3. The maximum diameter of the obtained specimens is approximately 1.5 times as large as the previous result for copper mold casting. The bulk specimen with 7.7mm in diameter exhibits the saturation magnetization of 1.13T, the coercivity lower than 20A∕m at room temperature, and the Curie temperature of 732K. This bulk specimen is the thickest of any soft magnetic glassy alloys formed until now.

Phase selection, growth, and interface kinetics in undercooled Fe-Ni melt droplets

An electromagnetic levitation facility is used to containerlessly process Fe-Ni droplets; undercoolings ΔT of up to 300 K were achieved. Thermal measurements during solidification showed two types of recalescence behaviour for alloys containing between 7.5 and 17 at% Ni: A single recalescence step for ΔT ΔT* (primary growth of the metastable bcc phase, and the subsequent transformation of the bcc phase and solidification of the stable ccp phase). The critical undercooling ΔT* strongly increases with the Ni-content. The growth velocities at which the primary dendrites propagate through the droplet have been measured for a number of Fe-Ni alloy compositions. The velocities reflect the phase selection, i.e. primary bcc phases grow markedly more slowly than primary ccp phases. Thermodynamic modelling (CALPHAD) and an analysis of the velocity data within current theories of dendrite growth is undertaken to describe nucleation and growth behaviour. The results suggest that the metastable phase is nucleated in preference at high undercoolings because of its lower solid-liquid interface energy and that the kinetics at the bcc-liquid interface is considerably more sluggish than the kinetics at the ccp-liquid interface.

Fast and slow crystal growth kinetics in glass-forming melts

Published values of crystal growth rates are compared for supercooled glass-forming liquids undergoing congruent freezing at a planar crystal-liquid interface. For the purposes of comparison pure metals are considered to be glass-forming systems, using data from molecular-dynamics simulations. For each system, the growth rate has a maximum value U(max) at a temperature T(max) that lies between the glass-transition temperature T(g) and the melting temperature T(m). A classification is suggested, based on the lability (specifically, the propensity for fast crystallization), of the liquid. High-lability systems show fast growth characterized by a high U(max), a low T(max)/T(m), and a very broad peak in U vs. T/T(m). In contrast, systems showing slow growth have a low U(max), a high T(max)/T(m), and a sharp peak in U vs. T/T(m). Despite the difference of more than 11 orders of magnitude in U(max) seen in pure metals and in silica, the range of glass-forming systems surveyed fit into a common pattern in which the lability increases with lower reduced glass-transition temperature (T(g)/T(m)) and higher fragility of the liquid. A single parameter, a linear combination of T(g)/T(m) and fragility, can show a good correlation with U(max). For all the systems, growth at U(max) is coupled to the atomic/molecular mobility in the liquid. It is found that, across the diversity of glass-forming systems, T(max)/T(g) = 1.48 ± 0.15.