Merging of the alpha and beta relaxations and aging via the Johari–Goldstein modes in rapidly quenched metallic glasses

This paper provides evidence that the physical aging of deeply and rapidly quenched metallic glasses is promoted by the Johari–Goldstein slow beta relaxation, resulting in a significant irreversible increase in the mechanical modulus on initial heating. Dynamic mechanical analysis has been used to characterize relaxation phenomena of a strong and a fragile metallic glass. In addition, we can extrapolate the temperature dependence of beta- and alpha-relaxation peaks to higher temperatures and calculate the merging temperature for both types of glasses

Bimodal crystallization rate curves of a molecular liquid with fieldinduced polymorphism

In this study, we use dielectric spectroscopy to explore how frequency and amplitude of an applied strong electric field affect the overall crystallization kinetics over a range of temperatures, focusing on a molecular system with field-induced polymorphism: vinyl ethylene carbonate (VEC). The volume fraction of the field-induced polymorph can be controlled by the parameters of the high-electric field, i.e., frequency and amplitude. We find that the crystallization rate maximum of the field induced polymorph is located at lower temperatures relative to the that of the regular polymorph. The temperature of the highest crystallization rate for the regular polymorph was found to be unaffected by the electric field, but the overall rates increase with increasing field amplitude. The dimensionality of crystal growth is also analyzed via the Avrami parameter and is frequency invariant but affected by the field amplitude. Our results demonstrate that a detailed knowledge of the influence of high fields on crystallization facilitates control over the crystallization behavior and the final product outcome of molecular systems, providing new opportunities for material engineering and improving pharmaceuticals

Approximate square-root-time relaxation in glass-forming liquids

We present data for the dielectric relaxation of 43 glass-forming organic liquids, showing that the primary (alpha) relaxation is often close to square-root-time relaxation. The better an inverse power-law description of the high-frequency loss applies, the more accurately is square-root-time relaxation obeyed. These findings suggest that square-root-time relaxation is generic to the alpha process, once a common view, but since long believed to be incorrect. Only liquids with very large dielectric losses deviate from this picture by having consistently narrower loss peaks. As a further challenge to the prevailing opinion, we find that liquids with accurate square-root-time relaxation cover a wide range of fragilities

Fundamental Link between β Relaxation, Excess Wings, and Cage-Breaking in Metallic Glasses

In glassy materials, the Johari–Goldstein secondary (β) relaxation is crucial to many properties as it is directly related to local atomic motions. However, a long-standing puzzle remains elusive: why some glasses exhibit β relaxations as pronounced peaks while others present as unobvious excess wings? Using microsecond atomistic simulation of two model metallic glasses (MGs), we demonstrate that such a difference is associated with the number of string-like collective atomic jumps. Relative to that of excess wings, we find that MGs having pronounced β relaxations contain larger numbers of such jumps. Structurally, they are promoted by the higher tendency of cage-breaking events of their neighbors. Our results provide atomistic insights for different signatures of the β relaxation that could be helpful for understanding the low-temperature dynamics and properties of MGs

From Single-Particle to Collective Dynamics in Supercooled Liquids

It has been recognized recently that the considerable difference between photon-correlation (PCS) and dielectric (BDS) susceptibility spectra arises from their respective association with single-particle and collective dynamics. This work presents a model that captures the narrower width and shifted peak position of collective dynamics (BDS), given the single-particle susceptibility derived from PCS studies. Only one adjustable parameter is required to connect the spectra of collective and single-particle dynamics. This constant accounts for cross-correlations between molecular angular velocities and the ratio of the first-rank and second-rank single-particle relaxation times. The model is tested for three supercooled liquids, glycerol, propylene glycol, and tributyl phosphate, and is shown to provide a good account of the difference between BDS and PCS spectra. Since PCS spectra appear to be rather universal across a range of supercooled liquids, this model provides a first step toward rationalizing the more material specific dielectric loss profiles

Enhanced diffusivity in supercooled liquids

The enhancement of self and probe diffusivity over the Stokes Einstein values by orders of magnitude is observed in the viscous regime of polymers, molecular glass-formers and supercooled metallic systems. We investigate an alternative to identifying enhanced diffusivity with the fast components of the primary structural relaxation process: high translational mobility is being associated with cooperative dynamics in string-like topologies, which is responsible for fast diffusive particle motion and the slow secondary beta-relaxation observed below the dynamic crossover temperature in fragile glass-formers. This approach provides a unifying view of diffusivity across the different classes of glass-forming materials