Relaxation spectrum of glass-forming alloys: Mechanical and microscopic insights

A dynamic map of metallic glasses

Recent decades have witnessed substantial progress in understanding relaxation dynamics of metallic glasses (MGs), marked by the discovery of multiple intrinsic dynamic modes and their correlation with properties. These advances have deepened microscopic insight into the nature of glasses and opened new avenues for addressing challenges in MGs design and property modulation. In this MRS Bulletin contribution, we summarize the dynamic modes in MGs, including structural α relaxation, slow β relaxation, nearly constant loss, fast β′ relaxation, γ relaxation, liquid-like fast dynamic mode, surface dynamic mode, vibrational excitations (boson peak and phonon-like modes), and ultraslow dynamic modes. We discuss their microscopic origins, with emphasis on the cooperative atomic motions that give rise to an emergent dynamic spatiotemporal ordering. We further highlight how these dynamic modes govern mechanical behavior, functional properties, stability and processing of MGs, demonstrating that the reliable dynamics–property relationship can help develop new MGs with tailored properties, analogous to structure–property relationships in crystalline materials. We also outline open questions regarding potential unified frameworks for these modes and the prospects for deliberately controlling dynamic modes to overcome property tradeoffs.

Current and future directions in probing structural dynamics and transport of metallic glasses

In this article, we discuss the challenges in assessing dynamics and structural changes of metallic glasses as amorphous out-of-equilibrium materials by means of coherent x-ray scattering. We focus on the fundamental understanding of the x-ray photon correlation spectroscopy (XPCS) technique and on how such an experimental probe may facilitate a deeper quantitative understanding of the underlying structural fluctuations occurring within the amorphous solid. Furthermore, we present how atomistic simulations and simulated x-ray photon correlation spectroscopy experiments can guide the interpretation of experimentally measured data. We conclude with a broader perspective on how contemporary advances in modeling, detector technology, and high flux x‑ray sources are transforming the study of glassy dynamics, enabling access to wider time and length scales and thus offering new avenues to probe the broad relaxation spectrum and complex nonequilibrium relaxation processes characteristic of amorphous solids. The X-ray photon correlation spectroscopy method uses coherent X-ray to probe structural relaxation processes in bulk metallic glasses by studying the temporal correlations in the obtained speckle patterns The X-ray photon correlation spectroscopy method uses coherent X-ray to probe structural relaxation processes in bulk metallic glasses by studying the temporal correlations in the obtained speckle patterns

Crystallization of Glasses: A Theoretical Analysis of the Role of Fragility and Polymorphism in Crystals.

Here we develop an elasticity-based theory of crystallization in glasses that incorporates structural heterogeneity, fictive temperature, and polymorph-mediated pathways. In a glass, structural degrees of freedom are effectively frozen, so that the fictive temperature Tf remains higher than the ambient temperature T, rendering the system intrinsically out of equilibrium. A central result is that the crystal-glass interfacial penalty is renormalized in fragile systems by soft, liquid-like regions, leading to a subquadratic mismatch energy scaling as ΣR3/2 rather than the classical R2 form. Applying this framework to ethanol, we show that nucleation proceeds preferentially via a two-step route through a plastic crystalline polymorph. The associated barriers are dramatically reduced: the glass-to-plastic step exhibits barriers of only  ∼5 kBT, compared to  ∼102 kBT for the direct glass-to-crystal transition. This large separation explains the dominance of the Ostwald pathway and the emergence of a pronounced time-temperature-transformation (TTT) nose. In contrast, silica retains the classical R2 scaling due to its rigid network, leading to very large barriers and suppressed bulk nucleation.

Metallic glasses: Deciphering their atomic choreography

Did you know that the strongest known engineering alloy is a metallic glass? This and many other world records have emerged along the success story of amorphous metals. With their unique structural and functional properties, and arising from a vast structural design space, metallic glasses are the amorphous counterpart to the high-entropy alloys paradigm. Sixty years after their discovery, metallic glasses have matured to industrial applications, but our structural understanding remains far behind. Presently, the advent of novel structural and dynamical probing methods, combined with new theoretical and modeling approaches, allow for the first time a one-to-one mapping of structure and structural dynamics. These developments emphasize medium-range order, extended network formation, and distinct structural partitioning in long-sought depth and detail for atomically disordered solids. Such a new view on metallic-glass microstructure and dynamics bears the promise of unifying numerous macroscopically observed phenomena, including negative creep, enthalpy recovery, stress-driven rejuvenation, and many more. This issue of MRS Bulletin aims at highlighting these advances, their promise to predictively understand macroscopic behavior, and to formulate reliable descriptors for structure and dynamics and for structure–property relationships of glassy materials. Metallic glasses are outstanding metallic alloys that combine the best properties from glassy solids and crystalline engineering alloys. This article introduces the current state-of-the-art and projects the future of structure and dynamics research of metallic glasses. As an overview article, it sets the stage for a series of subsequent focused contributions in this special issue Metallic glasses are outstanding metallic alloys that combine the best properties from glassy solids and crystalline engineering alloys. This article introduces the current state-of-the-art and projects the future of structure and dynamics research of metallic glasses. As an overview article, it sets the stage for a series of subsequent focused contributions in this special issue

Effect of molecular constraints on vibrational and quasilocalized excitations in glasses

Recent years have seen substantial progress in elucidating the statistical physics of the vibrational properties of structural glasses. Although many real-world glasses relevant for science and technology are molecular, the majority of computational studies concerning the mechanical and vibrational properties of structural glasses employ simple atomistic glass-forming models. Thus, the effects of stiff molecular constraints on mechanical and vibrational glass physics remain largely unexplored. In this work, we directly compare the properties of a molecular computer glass with those of an atomistic model featuring the same inter-molecular interaction potential, and created using the same formation protocol. We find that the molecular glass features a higher degree of mechanical disorder, with larger mesoscopic correlation lengths, while at the same time it hosts a lower number of soft, quasilocalized vibrations per atom -- compared to the atomistic glass. We rationalize these differences by accounting for the reduction in the effective number of degrees of freedom induced by the stiff molecular constraints. We additionally find that nonlinear plastic modes --- that carry plastic deformation in driven glassy solids --- couple much more strongly to volumetric strains in the molecular glass. Future research directions are discussed.

Exploring dynamic heterogeneity of metallic glass via anomalous creep