Zach Evenson

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.

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.

Relaxation processes and physical aging in metallic glasses

Since their discovery in the 1960s, metallic glasses have continuously attracted much interest across the physics and materials science communities. In the forefront are their unique properties, which hold the alluring promise of broad application in fields as diverse as medicine, environmental science and engineering. However, a major obstacle to their wide-spread commercial use is their inherent temporal instability arising from underlying relaxation processes that can dramatically alter their physical properties. The result is a physical aging process which can bring about degradation of mechanical properties, namely through embrittlement and catastrophic mechanical failure. Understanding and controlling the effects of aging will play a decisive role in our on-going endeavor to advance the use of metallic glasses as structural materials, as well as in the more general comprehension of out-of-equilibrium dynamics in complex systems. This review presents an overview of the current state of the art in the experimental advances probing physical aging and relaxation processes in metallic glasses. Similarities and differences between other hard and soft matter glasses are highlighted. The topic is discussed in a multiscale approach, first presenting the key features obtained in macroscopic studies, then connecting them to recent novel microscopic investigations. Particular emphasis is put on the occurrence of distinct relaxation processes beyond the main structural process in viscous metallic melts and their fate upon entering the glassy state, trying to disentangle results and formalisms employed by the different groups of the glass-science community. A microscopic viewpoint is presented, in which physical aging manifests itself in irreversible atomic-scale processes such as avalanches and intermittent dynamics, ascribed to the existence of a plethora of metastable glassy states across a complex energy landscape. Future experimental challenges and the comparison with recent theoretical and numerical simulations are discussed as well.

Unifying interatomic potential, g (r), elasticity, viscosity, and fragility of metallic glasses: analytical model, simulations, and experiments

An analytical framework is proposed to describe the elasticity, viscosity and fragility of metallic glasses in relation to their atomic-level structure and the effective interatomic interaction. The bottom-up approach starts with forming an effective Ashcroft-Born-Mayer interatomic potential based on Boltzmann inversion of the radial distribution function g(r) and on fitting the short-range part of

Role of the doping level in localized proton motions in acceptor-doped barium zirconate proton conductors

g(r)

Self-diffusion in liquid Al-Ge investigated with quasi-elastic neutron scattering

by means of a simple power-law approximation. The power exponent

Equilibrium viscosity, enthalpy recovery and free volume relaxation in a Zr44Ti11Ni10Cu10Be25 bulk metallic glass

\lambda

High temperature melt viscosity and fragile to strong transition in Zr–Cu–Ni–Al–Nb(Ti) and Cu47Ti34Zr11Ni8 bulk metallic glasses

represents a global repulsion steepness parameter. A scaling relation between atomic connectivity and packing fraction

β relaxation and low-temperature aging in a Au-based bulk metallic glass: From elastic properties to atomic-scale structure

Z \sim \phi^{1+\lambda}

Equilibrium viscosity of Zr-Cu-Ni-Al-Nb bulk metallic glasses

is derived. This relation is then implemented in a lattice-dynamical model for the high-frequency shear modulus where the attractive anharmonic part of the effective interaction is taken into account through the thermal expansion coefficient which maps the