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.

Ultrasonic vibration has been widely applied to modify the properties of glasses, yet its influence on glass relaxation remains elusive. Here, we systematically examine the effects of ultrasonic vibrations over broad frequency and amplitude ranges on an organic molecular glass, N,N-bis(3-methylphenyl)-diphenyl-benzidine, during isothermal annealing below the glass transition temperature. Calorimetric up-scan indicates that samples annealed with ultrasound exhibit a higher probability and extent of crystallization compared to those annealed without ultrasound, while their enthalpy recovery remains unchanged. These results indicate that ultrasonic vibration facilitates local structural ordering towards crystallization, through a distinct pathway without detectably affecting the relaxation process and potential-energy evolution.

The glass transition is marked by a rapid increase in the barriers for molecular rearrangement, and this leads to the vitrification of supercooled liquids. Recent theories suggested that at low temperature, molecular rearrangements generate elastic stresses that dissipate through the surrounding material and add a non-local contribution to relaxation barriers. However, it is difficult to experimentally measure this elastic contribution. Here we measure the elastic barriers by investigating the transformation of vapour-deposited stable glasses, influenced by distant boundaries of varying elasticity. Rigid boundaries preserve bulk super-Arrhenius dynamics in which both relaxation times and barriers increase on supercooling. By contrast, distant soft boundaries facilitate fast Arrhenius relaxations—even below the glass transition temperature—by allowing faraway stress dissipation. As such, the constant soft-substrate barrier is attributed to local interactions, whereas the differences between barriers on soft and rigid boundaries arise from non-local elastic barriers. Our results show that the rapid dynamical slowdown and vitrification of supercooled liquids are governed by the emergence and growth of elastic barriers, and provide a direct experimental basis for elasticity-based descriptions of the glass transition. Elastic stresses are believed to play a role in the vitrification process in glasses, but measuring them is challenging. Imposing elastic boundary conditions is now shown to provide a way to probe elastic effects within glassy samples.

Amorphous materials, owing to their cross-scale structural uniformity, circumvent the inherent sensitivity to defects as in traditional crystalline materials. As a result, they have irreplaceable and critical applications in numerous advanced technological fields. However, due to their thermodynamically non-equilibrium nature, amorphous materials undergo structural relaxation toward equilibrium, causing performance degradation or even failure during the period of service. Additionally, the complex and disordered structure of amorphous materials results in low-energy excitations, such as boson peak and tunneling two-level systems, which contribute to internal friction and thermal noise of the materials. These factors significantly limit their performance in advanced technological applications. Therefore, effectively enhancing the stability of amorphous materials and suppressing low-energy excitations are crucial steps toward surpassing their performance limits. Recent studies have demonstrated that atomic-level fabrication based on enhanced surface dynamics can successfully produce ultrastable amorphous materials, achieving an unprecedented degree of control over their microstructure, stability, and low-energy excitations, far beyond what conventional methods can attain. This article delves deeply into the underlying mechanisms of atomic-level fabrication for amorphous materials, focuses on the structural features and superior performances of ultrastable amorphous materials compared to conventional ones, and outlines future research directions and development trends of atomic-level fabrication in this field.

Metallic glasses (MGs) that mainly made up of metallic elements are a new member of the glassy materials family. This new kind of glass combines the characteristics of liquids and solids, glasses and metals, making it fascinating to both scientists and industrialists. With the discovery of more and more systems, MG is becoming one of the most active research field in metallic materials, and some concepts and technologies derived from MGs also facilitate the development of other materials from quasi-crystals to high entropy alloys. MGs have now been successfully used in aerospace, robotics, medicine, consumer electronics, etc. and the practical applications of MGs are still growing. On the other hand, the diverse properties and the unique structure of the MGs render them ideal models to study major open issues including the structural model of disordered materials, glass transition, collective motion and energy landscape. However, understanding the emerging properties and phenomena of MGs still poses enormous challenges, which has stimulated a wealth of efforts, including the development of new experimental approaches, the synthesis of systems with tailored properties, and the advancements in experimental techniques, theoretical models, and numerical simulations. In this Roadmap, we try to provide a broad overview of recent and potential future activities in the MG field, and present a roadmap for the development and applications of MGs by gathering contributions form scientists with diverse backgrounds, illustrating the major challenges and discussing the latest technology and strategy to tackle these challenges with experts covering various developments in general concepts, synthesis and characterisation, and theoretical and simulation methods.

Amorphous materials avoid the inherent sensitivity to defects in traditional crystalline materials due to their cross-scale structural uniformity.Therefore,they have irreplaceable and important applications in many advanced technical fields.However,due to their thermodynamically non-equilibrium nature,amorphous materials experience structural relaxation towards equilibrium,leading to performance degradation or even failure during use.Additionally,the complex and disordered structure of amorphous materials results in low-energy excitation,such as boson peaks and tunneling two-level systems,which can cause internal friction and thermal noise in the materials.These factors significantly limit their performance in advanced technical applications.Therefore,effectively improving the stability of amorphous materials and suppressing low-energy excitation are key steps towards breaking through their performance limits.Recent studies have shown that atomic-level fabrication based on enhanced surface dynamics can successfully produce ultrastable amorphous materials,achieving unprecedented control over their microstructure,stability,and low-energy excitation,far exceeding the level achievable by traditional methods.The exceptional advantages of ultrastable amorphous materials endow them with significant application potential in advanced domains such as gravitational wave detection.This article delves into the underlying mechanisms of atomic-level fabrication for amorphous materials,highlighting their structural features and superior performances compared with traditional amorphous materials,and it also outlines future research directions and development trends of atomic-level fabrication in this field.

Fast relaxation is the precursor to α-relaxation, yet its microscopic origin and role in the emergence of glassy dynamics remain elusive. Using inelastic neutron scattering on the fragile glass-former Ca0.4K0.6(NO3)1.4, we show that the fast relaxation dynamics are strongly modulated by local structure and exhibit a distinct temperature dependence that correlates with shear phonon propagation and the Arrhenius-to-super-Arrhenius crossover of α-relaxation. These results indicate that fast relaxation arises from the interplay of shear phonon softening and local structural arrangements, serving as the intermediate dynamical process that connects the elastic interactions to kinetic facilitation underlying the emergence of dynamic heterogeneity in supercooled liquids.

Ultrastable glasses can be produced via vapor deposition, exhibiting superior thermodynamic and kinetic stability compared to their liquid-cooled counterparts. Although often considered equivalent to liquid-cooled glasses aged for 106 of years, their structural evolution with increasing stability remains poorly understood. Here, using Cs-corrected scanning transmission electron microscopy, we directly observe the nanoscale spatial heterogeneity of vapor-deposited and liquid-cooled Zr-Cu-Al metallic glasses as a function of stability or energy. We find that the correlation length of spatial heterogeneity reaches a minimum in the most stable vapor-deposited metallic glasses with lowest energy, whereas larger heterogeneity is observed in the more stable liquid-cooled glasses. Complementary simulations reveal that this minimization of heterogeneity arises from the reduction of distorted icosahedral structures with medium-range ordering. Our study provides compelling evidence that vapor-deposited metallic glasses undergo a distinct evolution of spatial heterogeneity compared to their liquid-cooled counterparts as stability increases.

Enabled by surface-mediated equilibration, physical vapour deposition can create high-density stable glasses comparable with liquid-quenched glasses aged for millions of years. Deposition is often performed at various rates and temperatures on rigid substrates to control the glass properties. Here we demonstrate that on soft, rubbery substrates, surface-mediated equilibration is enhanced up to 170 nm away from the interface, forming stable glasses with densities up to 2.5% higher than liquid-quenched glasses within 2.5 h of deposition. Gaining similar properties on rigid substrates would require 10 million times slower deposition, taking ~3,000 years. Controlling the modulus of the rubbery substrate provides control over the glass structure and density at constant deposition conditions. These results underscore the significance of substrate elasticity in manipulating the properties of the mobile surface layer and thus the glass structure and properties, allowing access to deeper states of the energy landscape without prohibitively slow deposition rates.

Studying the flow behavior is critical to understand the deformation mechanism of amorphous solids. However, detecting the basic flow events in amorphous solids is challenging. Here, by simultaneous SAXS/WAXS, elementary flow carriers in wound metallic glasses are identified from flow induced structural heterogeneities with a radius of gyration of 2.5 similar to 3.5 nm. Their size increases and morphology changes from sphere-like to rod-like under flow. Moreover, the atomic structure exhibits an unusual change to a more disordered state during winding/annealing at the temperature of similar to 0.8 Tg. This work provides an atomic-to-nanoscale description of the flow carriers of amorphous solids during deformation.

Surfaces mediate the formation of stable glasses (SGs) upon physical vapor deposition (PVD) for a wide range of glass formers. The thermodynamic and kinetic stability of SGs and their anisotropic packing structures are controlled through the deposition parameters (deposition temperature and rate) as well as the chemical structure and composition of the glass former. The resulting PVD glass properties can therefore be related to the structure and dynamics of the glass surface, which can have oriented packing, enhanced surface diffusion, and a lower glass transition temperature, and can facilitate an enhanced aging rate of the interfacial region. We review our current understanding of the details of this surface-mediated SG formation process and discuss key gaps in our knowledge of glass surface dynamics and their effect on this process.

Revealing the microscopic structural and dynamic pictures of glasses is a long-standing challenge for scientists 1 , 2 . Extensive studies on the structure and relaxation dynamics of glasses have constructed the current classical picture 3 – 5 : glasses consist of some ‘soft zones’ of loosely bound atoms embedded in a tightly bound atomic matrix. Recent experiments have found an additional fast process in the relaxation spectra 6 – 9 , but the underlying physics of this process remains unclear. Here, combining extensive dynamic experiments and computer simulations, we reveal that this fast relaxation is associated with string-like diffusion of liquid-like atoms, which are inherited from the high-temperature liquids. Even at room temperature, some atoms in dense-packed metallic glasses can diffuse just as easily as they would in liquid states, with an experimentally determined viscosity as low as 10 7 Pa·s. This finding extends our current microscopic picture of glass solids and might help establish the dynamics–property relationship of glasses 4 .

Comprehending and controlling the stability of glasses is one of the most challenging issues in glass science. Here we explore the microscopic origin of the ultrastability of a Cu-Zr-Al metallic glass (MG). It is revealed that the ultrastable window (0.7-0.8 T-g) of MGs correlates with the enhanced degree of nanoscale-to-mesoscale structural/mechanical heterogeneity and the connection of stabilityfavored clusters. On one side, the increased fraction of stabilityfavored clusters promotes the formation of a stable percolating network through a critical percolation transition, which is essential to form ultrastable MG. On the other side, the enhanced heterogeneity arising from an increased distribution in local clusters may promote synergistically a more efficient and frustrated packing of amorphous structure, contributing to the ultrastability. The present work sheds new light on the stability of MGs and provides a step toward nextgeneration MGs with superior stability and performances.

The dynamics of molecular associates in a methanol/water mixture was investigated using quasielastic neutron scattering. By measuring the signal from four methanol/water samples differing only by their isotopic composition, the relative motion of the water to methanol molecules, i.e. their mutual dynamics, was determined at the nanoscale. The thus obtained nanoscopic mutual diffusion coefficient signals a significantly slower process than the single particle diffusion of either methanol or water in the system as well as their macroscopic mutual diffusion. The data do not provide any indication of microsegregation in this preeminent alcohol/water mixture; however, they do indicate the existence of long lived but dynamic molecular associates of water and methanol molecules. Analysis of the structural relaxation shows that the lifetime of molecular association through hydrogen bonding determines the fact that viscosity of the mixtures at intermediate concentrations is higher than that of both pure components.

Employing wide-angle neutron spin echo spectroscopy, we measured the Q-dependent coherent intermediate scattering function of the prototypical ionic glass former Ca0.4K0.6(NO3)1.4, in the equilibrium and supercooled liquid states beyond the hydrodynamic regime. The data reveal a clear two-step relaxation: an exponential fast process, and a stretched exponential slow alpha process. de Gennes narrowing is observed in all characteristic variables of the alpha process: the relaxation time, amplitude, and stretching exponent. At all length scales probed, the relative amplitude of the alpha-relaxation decreases with increasing temperature and levels off in the normal liquid state. The temperature dependence of the stretching exponent and the relaxation time at different Q's indicate that modifications of the relaxation mechanisms at the local length scales, manifested as temperature independent dynamic heterogeneity and smaller deviations from Arrhenius behavior, have occurred even above the alpha-beta (Johari-Goldstein) bifurcation temperature.

By employing quasielastic neutron scattering, we studied the atomic-scale relaxation dynamics and transport mechanism of La50Ni15Al35 and Ce70Cu19Al11 metallic glass melts in the temperature range of >200K above their liquidus temperatures. The results show that both liquids exhibit stretched exponential relaxation and Arrhenius-type temperature dependence of the effective diffusion coefficient. The La50Ni15Al35 melt exhibits an activation energy of 0.545 +/- 0.008 eV and a stretching exponent similar to 0.77 to 0.86 in the studied temperature range; no change of activation energy, as suggested in previous reports, associated with liquid-liquid phase transition was observed. In contrast, the Ce70Cu19Al11 melt exhibits larger diffusivity with a much smaller activation energy of 0.201 +/- 0.003 eV and a smaller stretching exponent similar to 0.51 to 0.60, suggestive of more heterogeneous dynamics.

2-Propanol was investigated, in both the liquid and supercooled states, as a model system to study how hydrogen bonds affect the structural relaxation and the dynamics of mesoscale structures, of approximately several Ångstroms, employing static and quasi-elastic neutron scattering and molecular dynamics simulation. Dynamic neutron scattering measurements were performed over an exchanged wave-vector range encompassing the pre-peak, indicative of the presence of H-bonding associates, and the main peak. The dynamics observed at the pre-peak is associated with the formation and disaggregation of the H-bonded associates and is measured to be at least one order of magnitude slower than the dynamics at the main peak, which is identified as the structural relaxation. The measurements indicate that the macroscopic shear viscosity has a similar temperature dependence as the dynamics of the H-bonded associates, which highlights the important role played by these structures, together with the structural relaxation, in defining the macroscopic rheological properties of the system. Importantly, the characteristic relaxation time at the pre-peak follows an Arrhenius temperature dependence whereas at the main peak it exhibits a non-Arrhenius behavior on approaching the supercooled state. The origin of this differing behavior is attributed to an increased structuring of the hydrophobic domains of 2-propanol accommodating a more and more encompassing H-bond network, and a consequent set in of dynamic cooperativity.

The surface of a glassy material exhibits enhanced mobility compared to the bulk counterpart, however the underlying mechanism for this remains elusive. Herein, we present studies of the dynamical properties of a prototypical glass-forming metallic liquid ${\mathrm{Zr}}_{50}{\mathrm{Cu}}_{50}$ as a function of the distance from both the free surface and pinned surface using molecular dynamics simulations. We found that the surface mobility increases gradually on approaching the free surface, with a concomitant increase of the non-Gaussianity. The phonon density of states at the free surface exhibits lower characteristic frequencies than in the bulk and pinned surface. These results suggest phonon softening caused by anisotropic fluctuations at free surfaces as an alternative physical mechanism leading to the enhanced dynamics at free glassy surfaces from the perspective of collective excitations.

By decreasing the rate of physical vapor deposition, ZrCuAl metallic glasses with improved stability and mechanical performances can be formed, while the microscopic structural mechanisms remain unclear. Here, with scanning transmission electron microscopy and high-energy synchrotron X-ray diffraction, we found that the metallic glass deposited at a higher rate exhibits a heterogeneous structure with compositional fluctuations at a distance of a few nanometers, which gradually disappear on decreasing the deposition rate; eventually, a homogeneous structure is developed approaching ultrastability. This microscopic structural evolution suggests the existence of the following two dynamical processes during ultrastable metallic glass formation: a faster diffusion process driven by the kinetic energy of the depositing atoms, which results in nanoscale compositional fluctuations, and a slower collective relaxation process that eliminates the compositional and structural heterogeneity, equilibrates the deposited atoms, and strengthens the local atomic connectivity.

Amorphous pure metals are important for technological applications and provide a simple system for the study of fundamental science. Through detailed analysis of the microscopic structure of vapor-deposited Ta film, we reveal the presence of various micro-regions with distinct packings of different atomic clusters . These results suggest that high-throughput production of atomic cluster packing can be generated during the packing of vaporized atoms on a large depositing substrate, allowing sampling the energy landscape of the system in an extended range. The proper packing of atomic clusters favoring glass formation and corresponding to stable amorphous states located in the deep valley of energy landscape can be arrested, finally giving rise to the formation of amorphous Ta states that can hardly be obtained by traditional liquid-quenching methods.