ICYS Annual Report 2023

20/50

ICYS Annual Report 2023The glass transition is a phenomenon in which disordered motions of atoms and molecules at high temperatures are quickly cooled down and frozen without crystallization, leaving the disordered structure of atoms and molecules. The freezing of the motion progresses rapidly within a temperature range of a few Kelvins near the glass transition temperature, and the viscosity increases by more than ten orders of magnitude. Simulations revealed that there are heterogeneous regions in the speed of atomic motion near the glass transition temperature. This heterogeneity of atomic motion is called dynamic heterogeneity. Since the correlation length of the dynamical heterogeneity the glass shows critical phenomenon-like behavior near transition temperature, it is predicted that the dynamical heterogeneity plays an important role in glass transition. It is also indicated by simulation that there is a relationship between dynamic heterogeneity and the heterogeneity in atomic structure (structural heterogeneity). However, the relationship has not been confirmed experimentally. This is because it is difficult to observe structural and dynamic heterogeneities during the transition from the liquid state to the glassy state at nano-level resolution. In this study, I focus on convergent beam electron diffraction (CBED) which has a nano-level spatial resolution. Because CBED can analyze a local atomic structure. Thus, spatial series of CBED can visualize the structural heterogeneity. In addition, the temporal series of CBED can measure an intensity of a motion of the local atomic structure. Thus, the spatiotemporal series of CBED can visualize the dynamic heterogeneity. The spatiotemporal series of CBED can measure both heterogeneities, simultaneously. We developed this method to obtain the spatiotemporal distribution of diffraction patterns and call the method as 5D-STEM1. We applied 5D-STEM and succeeded to measure both heterogeneities simultaneously. As a result, it was revealed that ordered structures tend to move slowly2. Then, we tried to investigate the relaxation process in more detail.We measured relaxation time by correlating diffraction patterns using two-time correlation function (TTCF), while X-ray-based studies often rely one-time correlation function (OTCF)3. The two approaches yield notably different relaxation times. Since OTCF is measured around an average, a long observation time is required to capture full relaxation. In contrast, TTCF calculates relaxation time from direct image-to-image comparisons, making it less sensitive to total observation time and capable of measuring relaxation even if it is not fully complete. Thus, TTCF method has the potential to measure a slower relaxation process observed below glass transition temperature (Tg).In glassy materials, multiple relaxation processes occur below Tg, including fast β-relaxation and slow α-relaxation. Previously, Research Digest 1. Outline of Research2. Research ActivitiesKatsuaki NAKAZAWAFig. 1. Spatial distributions of τ1, τ2, γ1, γ2, and the ratio in PNCP at 473K.References 1) K. Nakazawa et al., Microscopy, 72, 5, 446-449 (2023) 2) K. Nakazawa et al., NPG Asia Materials., 57, 16 (2024) 3) A. Madsen et al., New Journal of Physics, 12 (2010)determining their individual contributions was difficult. Here, using 5D-STEM from 473–573 K, we quantified both the ratio and timescales of β and α-relaxations in Pd42.5Ni7.5Cu30P20(PNCP). Fitting a two-step KWW function,revealed heterogeneous distributions in τ1, τ2, γ1, γ2, and the ratio. We correlated local structural features—such as diffraction symmetry and statistical moments—with these parameters. Notably, regions exhibiting greater local structural order tended toward higher α-relaxation fractions, consistent with mode-coupling theory.Upon increasing temperature, α-relaxation times drop sharply near Tg (~560 K). The fraction of β-relaxation also increases at rapid early-stage higher relaxation. Meanwhile, local structural correlations weaken with rising temperature, suggesting accelerated relaxation reduces spatial coherence. Overall, these findings demonstrate the advantages of TTCF, offering deeper insights into the complex multi-stage relaxation in metallic glasses and revealing how structural regularity influences the balance between α- and β-relaxations.temperatures, indicating more 18Measurement of Dynamic Heterogeneity Near the Glass Transition Temperature Using Advanced Transmission Electron Microscopy