Abstract:Inelastic deformation of metallic glasses occurs via slip events with avalanche dynamics similar to those of earthquakes. For the first time in these materials, measurements have been obtained with sufficiently high temporal resolution to extract both the exponents and the scaling functions that describe the nature, statistics and dynamics of the slips according to a simple mean-field model. These slips originate from localized deformation in shear bands. The mean-field model describes the slip process as an avalanche of rearrangements of atoms in shear transformation zones (STZs). Small slips show the predicted power-law scaling and correspond to limited propagation of a shear front, while large slips are associated with uniform shear on unconstrained shear bands. The agreement between the model and data across multiple independent measures of slip statistics and dynamics provides compelling evidence for slip avalanches of STZs as the elementary mechanism of inhomogeneous deformation in metallic glasses. 2 One Sentence Summary:We show that bulk metallic glasses deform via slip avalanches of "weak spots", by demonstrating agreement of new high temporal resolution measurements of the slip-statistics and dynamics with the predictions of a simple mean field model for plastic deformation. Main Text:We show here that slowly sheared metallic glasses deform plastically via slip avalanches of weak spots. The weak spots are shear transformation zones (STZs), which are collective rearrangements of 10-100 atoms [1].During high temperature deformation of metallic glasses (close to the glass transition), STZs operate independently and the material flows homogeneously, in agreement with STZ theory predictions over several orders of magnitude of stress and strain rate [1,2]. At lower temperatures metallic glasses deform inhomogenously via intermittent slips on narrow shear bands [3]. At low strain rates, these slip events are manifested as sudden stress drops, called serrated flow. Analytical [4,5] and computational investigations [6,7,8] suggest STZ operation, but experimental support has been challenging because slip events are both fast (with millisecond durations) and highly localized (with thicknesses <1 µm) [3]. Here we report experimental results on the stress drop dynamics and statistics, finding excellent agreement with analytic model predictions for the slip avalanche statistics of weak spots or STZs.Many other materials-including crystals and densely packed granular solids-exhibit sudden slips during inelastic deformation. Although the mechanisms of deformation differ, the statistics and dynamics of the slip events are described by the same simple mean-field model of plastic deformation [9,10]. The model assumes that weak spots slip and then restick whenever the local shear stress exceeds a local slip threshold. Weak spots in crystals are dislocations, while in a metallic glass they are STZs. Through elastic interactions a slipping weak spot can trigger others to slip creating a slip avalanche. In crystal...

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Aliphatic urethane polymers have been synthesized and characterized, using monomers with high molecular symmetry, in order to form amorphous networks with very uniform supermolecular structures which can be used as photo-thermally actuable shape memory polymers (SMPs). The monomers used include hexamethylene diisocyanate (HDI), trimethylhexamethylenediamine (TMHDI), N,N,N',N'-tetrakis(hydroxypropyl)ethylenediamine (HPED), triethanolamine (TEA), and 1,3-butanediol (BD). The new polymers were characterized by solvent extraction, NMR, XPS, UV/VIS, DSC, DMTA, and tensile testing. The resulting polymers were found to be single phase amorphous networks with very high gel fraction, excellent optical clarity, and extremely sharp single glass transitions in the range of 34 to 153°C. Thermomechanical testing of these materials confirms their excellent shape memory behavior, high recovery force, and low mechanical hysteresis (especially on multiple cycles), effectively behaving as ideal elastomers above T g . We believe these materials represent a new and potentially important class of SMPs, and should be especially useful in applications such as biomedical microdevices.

Slowly-compressed single crystals, bulk metallic glasses (BMGs), rocks, granular materials, and the earth all deform via intermittent slips or “quakes”. We find that although these systems span 12 decades in length scale, they all show the same scaling behavior for their slip size distributions and other statistical properties. Remarkably, the size distributions follow the same power law multiplied with the same exponential cutoff. The cutoff grows with applied force for materials spanning length scales from nanometers to kilometers. The tuneability of the cutoff with stress reflects “tuned critical” behavior, rather than self-organized criticality (SOC), which would imply stress-independence. A simple mean field model for avalanches of slipping weak spots explains the agreement across scales. It predicts the observed slip-size distributions and the observed stress-dependent cutoff function. The results enable extrapolations from one scale to another, and from one force to another, across different materials and structures, from nanocrystals to earthquakes.

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We have investigated the possibility of void nucleation from the coalescence of excess free volume generated in shear bands during deformation of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass. Excess free volume in a shear band results in excess free energy relative to a relaxed glass with less free volume. To calculate the free energy of the material in a shear band with excess free volume, we model it as having the same structure as a glass solidified at an elevated glass transition temperature, which we call the fictive temperature. This excess free energy can be correlated with a free volume chemical potential that provides a driving force for void nucleation. The results of this modeling indicate that any free volume generated in the shear band during deformation is unstable, with the consequence that voids are predicted to form spontaneously from the coalescence of free volume. These voids are then expected to coarsen. Under tension, void growth and linkage would be facilitated by a tensile stress state, perhaps leading to premature fracture, whereas a compressive stress state would hinder void growth. This may explain the common observation that failure under uniaxial tension occurs as the result of the propagation of a single shear band, whereas multiple shear bands can form under uniaxial compression without causing failure.

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