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
Metallic glasses challenge the classical structure–property doctrine because the lack of long-range periodicity makes their structures complex and inherently multiscale. Although short-range order (SRO) within the first-neighbor cage has provided a useful vocabulary (e.g., icosahedral versus liquid-like motifs), growing evidence shows that SRO alone is not a reliable state variable for properties, such as stability and mechanical performance. In particular, the same nominal SRO motif can correspond to distinct energetic states depending on its embedding at more extended length scale, and local propensity indicators do not uniquely predict whether local plastic activity remains isolated or evolves into avalanches and extensive plastic deformation. We argue that rethinking glass structure requires shifting from per-atom SRO coordinates toward collective variables, defined over the range of relevant cooperativity. Within this framework, we clarify two kinds of medium-range order (MRO) that are frequently conflated in the literature at the intermediate scales (around second nearest- neighbor shell, 0.5–1 nm). One is the topological MRO, manifested as SRO motif connectivity/networking in atomistic models or as crystal-like nanodomains, probed by fluctuation electron microscopy and 4D-STEM. Another is defined by the persistent oscillations of pair distribution functions (i.e., the density wave fluctuations). Their coherence length is also about the same scale, and provides an alternative, more general view of the MRO. This motivates an energy landscape-based viewpoint in which density-fluctuation fields do not map to single basins, but condition the statistical accessibility of basins and activation barriers, thereby shaping the rigidity network that steers cascade propagation. We conclude by outlining quantitative opportunities to connect these scale-dependent collective descriptors, response-based diffraction, and energy landscape sampling into predictive, experimentally anchored structure–property relations. Data-driven atomistic analysis of metallic-glass structures using smooth overlap of atomic positions (SOAP) descriptors and transparent machine learning to identify radius of informative structural environments (RISE) Data-driven atomistic analysis of metallic-glass structures using smooth overlap of atomic positions (SOAP) descriptors and transparent machine learning to identify radius of informative structural environments (RISE)
Ultrastable glasses, amorphous solids with exceptionally low-energy states and enhanced kinetic, thermodynamic, and mechanical stability, have long been a subject of intense experimental interest. Over the past decade, their computational realization has emerged as a major goal in condensed matter physics, as numerical methods can exploit unphysical moves to access deeply supercooled and nonequilibrium glassy states far beyond the reach of conventional cooling protocols, thereby providing key insights into the nature of the glass transition and amorphous states and enabling the design of mechanically robust glassy materials. In this review, we outline the key steps underlying the most effective algorithms developed across the field. For each approach, we discuss its efficiency, limitations, and physical interpretation. We finally present a comparative analysis of the stability achieved across these methods, with the aim of equipping both newcomers and experts with an intuitive and comprehensive understanding of the field's current state and the opportunities it presents.
Fabio Leoni, M. Ozawa, John Russo et al.· Journal of Chemical Physics· 3 citations
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.
Rui Zhao, P. Luo, Hai-Yang Bai et al.· MRS bulletin· 0 citations
This work reviews recent findings on the dynamics of metallic glasses, focusing on the relationship between mechanical relaxation and microscopic dynamics, the latter explored by means of coherent x-ray scattering experiments. We first summarize the fundamental features of the mechanical relaxation spectrum of metallic glasses, encompassing α relaxation as well as secondary processes (such as β and γ). We examine the specificities and limitations of various characterization methods, and we survey what we currently know about how the relaxation spectrum evolves during cooling—from the supercooled liquid through the glass transition—and during physical aging of the glass. Subsequently, we contrast these macroscopic results with the microscopic dynamics. While macroscopic and microscopic observations appear consistent in the SCL, a notable divergence emerges in the glass state, with the emergence of compressed nonexponential relaxation functions. We discuss the possible origins of these differences in view of recent microscopic observations. Stress relaxation curves of a metallic glass at different temperatures. Overall relaxation map of metallic glasses showing the time scales expected for the primary and secondary relaxation processes. SCL, supercooled liquid Stress relaxation curves of a metallic glass at different temperatures. Overall relaxation map of metallic glasses showing the time scales expected for the primary and secondary relaxation processes. SCL, supercooled liquid
Peptide glasses are an emerging class of biofunctional amorphous materials, but their atomic-level structure has remained elusive. Here, we resolve the three-dimensional (3D) organization of peptide glasses using a cyclic dipeptide model, combining molecular dynamics simulations with 2D solid-state NMR fingerprinting. Our analysis quantifies three defining hallmarks of the glassy state: (i) pronounced conformational heterogeneity that distinguishes it from the crystalline state; (ii) reorganization of diverse H-bonding types producing annealing-rate-dependent spectral fingerprints; and (iii) dominance of non-hydrogen-bonded contacts that generate annealing - temperature-dependent spectroscopic signatures. These features give rise to molecular clusters with a branched H-bonding topology that quantitatively reproduce bulk spectroscopic properties, establishing a representative structural unit analogous to the crystalline unit cell. This framework enables the identification of structural organization in amorphous peptide glasses with different thermal histories, paving the way for rational design of functional small-molecule glassy materials.
Medium-range structure at a length scale between ~0.5 and 3 nm strongly influences the properties and phase transformations of metallic glass alloys. Electron diffraction methods based on four-dimensional scanning transmission electron microscopy are well suited for characterizing such structure using nanodiffraction with beams matched in size to the length scale of interest. Structural characterization using techniques including fluctuation electron microscopy, Ångström-beam electron diffraction, and angular correlation mapping have revealed widespread competition between crystalline and noncrystalline order, structural motifs responsible for crystallization and glass formation, and length scales controlling plasticity. Time-resolved, in situ electron correlation microscopy has demonstrated spatially heterogeneous dynamics in supercooled liquids and glasses and has hinted at connections between structure and atomic motions. Continued advances in techniques, sources, and detectors offer prospects for more discoveries to come. Electron nanodiffraction probes only a few hundred atoms at a time, offering unique views of the atomic structure and dynamics of metallic glasses Electron nanodiffraction probes only a few hundred atoms at a time, offering unique views of the atomic structure and dynamics of metallic glasses
P. Voyles, Harald Rösner, Gerhard Wilde· MRS bulletin· 0 citations