Isabella Gallino

Kinetic and thermodynamic studies of the fragility of bulk metallic glass forming liquids

The thermodynamic functions of the bulk metallic glass (BMG) forming Pd43Ni10Cu27P20 alloy are determined calorimetrically as a function of temperature. Along with eight other BMG forming alloys, the available experimental thermodynamic and viscosity data are reassessed. For each alloy, consistent Vogel–Fulcher–Tammann (VFT) fits of the viscosity measurements are established, and the temperature dependence of the configurational entropy is calculated from thermodynamic data. Together with the VFT fits, fits to the Adam–Gibbs equation are performed using this configurational entropy change. We find remarkable agreement between the Adam–Gibbs and VFT fits. Moreover, the temperature T0 is obtained from the VFT fits at which the viscous flow diverges. This T0 matches very well the temperature where the configurational entropy vanishes in the corresponding Adam–Gibbs fits.

The effect of cooling rates on the apparent fragility of Zr-based bulk metallic glasses

We investigate the behavior of the kinetic fragility parameter D∗ as different cooling rates are applied to samples of the Zr57Cu15.4Ni12.6Al10Nb5 and Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 bulk metallic glass formers. The glassy samples are heated into the supercooled liquid region using differential scanning calorimetry (DSC) and cooled back into the glassy state with sets of different cooling rates, qC. The shifts in the glass transition are measured by determining the onset glass transition temperature, Tgonset, as well as the limiting fictive temperature, Tf′, upon reheating with a set of heating rates, qH. We then model the data by assuming a Vogel–Fulcher–Tammann-type behavior in the structural relaxation time, τ, and observe an apparent increase in the kinetic fragility parameter for slower cooling rates. These results show that fragilities calculated from DSC scans, where qH=qC are in good agreement with those from equilibrium viscosity data recently obtained by three-point beam-bending.

Glass transition with decreasing correlation length during cooling of Fe 50 Co 50 superlattice and strong liquids

A study of a non-liquid glass former reveals a correlation length that decreases as the transition temperature is approached from above, which is the opposite of what is expected. It suggests that ‘strong’ and ‘fragile’ liquids exist on opposite sides of an order–disorder phase transition.

X-Ray Photon Correlation Spectroscopy Reveals Intermittent Aging Dynamics in a Metallic Glass.

We use coherent x rays to probe the aging dynamics of a metallic glass directly on the atomic level. Contrary to the common assumption of a steady slowing down of the dynamics usually observed in macroscopic studies, we show that the structural relaxation processes underlying aging in this metallic glass are intermittent and highly heterogeneous at the atomic scale. Moreover, physical aging is triggered by cooperative atomic rearrangements, driven by the relaxation of internal stresses. The rich diversity of this behavior reflects a complex energy landscape, giving rise to a unique type of glassy-state dynamics.

Linking structure to fragility in bulk metallic glass-forming liquids

Using in-situ synchrotron X-ray scattering, we show that the structural evolution of various bulk metallic glass-forming liquids can be quantitatively connected to their viscosity behavior in the supercooled liquid near Tg. The structural signature of fragility is identified as the temperature dependence of local dilatation on distinct key atomic length scales. A more fragile behavior results from a more pronounced thermally induced dilatation of the structure on a length scale of about 3 to 4 atomic diameters, coupled with shallower temperature dependence of structural changes in the nearest neighbor environment. These findings shed light on the structural origin of viscous slowdown during undercooling of bulk metallic glass-forming liquids and demonstrate the promise of predicting the properties of bulk metallic glasses from the atomic scale structure.

On the Fragility of Bulk Metallic Glass Forming Liquids

In contrast to pure metals and most non-glass forming alloys, metallic glass-formers are moderately strong liquids in terms of fragility. The notion of fragility of an undercooling liquid reflects the sensitivity of the viscosity of the liquid to temperature changes and describes the degree of departure of the liquid kinetics from the Arrhenius equation. In general, the fragility of metallic glass-formers increases with the complexity of the alloy with differences between the alloy families, e.g., Pd-based alloys being more fragile than Zr-based alloys, which are more fragile than Mg-based alloys. Here, experimental data are assessed for 15 bulk metallic glasses-formers including the novel and technologically important systems based on Ni-Cr-Nb-P-B, Fe-Mo-Ni-Cr-P-C-B, and Au-Ag-Pd-Cu-Si. The data for the equilibrium viscosity are analyzed using the Vogel–Fulcher–Tammann (VFT) equation, the Mauro–Yue–Ellison–Gupta–Allan (MYEGA) equation, and the Adam–Gibbs approach based on specific heat capacity data. An overall larger trend of the excess specific heat for the more fragile supercooled liquids is experimentally observed than for the stronger liquids. Moreover, the stronger the glass, the higher the free enthalpy barrier to cooperative rearrangements is, suggesting the same microscopic origin and rigorously connecting the kinetic and thermodynamic aspects of fragility.

Atomic scale analysis of phase formation and diffusion kinetics in Ag/Al multilayer thin films

Thin films generally exhibit unusual kinetics leading to chemical reactions far from equilibrium conditions. Binary metallic multilayer thin films with miscible elements show some similar behaviors with respect to interdiffusion and phase formation mechanisms. Interfacial density, lattice defects, internal stresses, layer morphologies and deposition conditions strongly control the mass transport between the individual layers. In the present work, Ag/Al multilayer thin films are used as a simple model system, in which the effects of the sputtering power and the bilayer period thickness on the interdiffusion and film reactions are investigated. Multilayers deposited by DC magnetron sputtering undergo calorimetric and microstructural analyses. In particular, atom probe tomography is extensively used to provide quantitative information on concentration gradients, grain boundary segregations, and reaction mechanisms. The magnitude of interdiffusion was found to be inversely proportional to the period thickness...

Metallurgy Beyond Iron

Metallurgy is one of the oldest sciences. Its history can be traced back to 6000 BCE with the discovery of Gold, and each new discovery — Copper, Silver, Lead, Tin, Iron and Mercury — marked the beginning of a new era of civilization. Currently there are 86 known metals, but until the end of the 17th century, only 12 of these were known. Steel (Fe–C alloy) was discovered in the 11th century BCE; however, it took until 1709 CE before we mastered the smelting of pig-iron by using coke instead of charcoal and started the industrial revolution. The metallurgy of nowadays is mainly about discovering better materials with superior properties to fulfil the increasing demand of the global market. Promising are the Glassy Metals or Bulk Metallic Glasses (BMGs) — discovered at first in the late 50s at the California Institute of Technology — which are several times stronger than the best industrial steels and 10-times springier. The unusual structure that lacks crystalline grains makes BMGs so promising. They have a liquid-like structure that means they melt at lower temperatures, can be moulded nearly as easily as plastics, and can be shaped into features just 10 nm across. The best BMG formers are based on Zr, Pd, Pt, Ca, Au and, recently discovered, also Fe. They have typically three to five components with large atomic size mismatch and a composition close to a deep eutectic. Packing in such liquids is very dense, with a low content of free volume, resulting in viscosities that are several orders of magnitude higher than in pure metal melts.