Pressure-induced non-monotonic crossover of steady relaxation dynamics in a metallic glass

Relaxation dynamics, as a key to understand glass formation and glassy properties, remains an elusive and challenging issue in condensed matter physics. In this work, in situ high-pressure synchrotron high-energy x-ray photon correlation spectroscopy has been developed to probe the atomic-scale relaxation dynamics of a cerium-based metallic glass during compression. Although the sample density continuously increases, the collective atomic motion initially slows down as generally expected and then counter-intuitively accelerates with further compression (density increase), showing an unusual non-monotonic pressure-induced steady relaxation dynamics crossover at ~3 GPa. Furthermore, by combining in situ high-pressure synchrotron x-ray diffraction, the relaxation dynamics anomaly is evidenced to closely correlate with the dramatic changes in local atomic structures during compression, rather than monotonically scaling with either sample density or overall stress level. These findings could provide new insight into relaxation dynamics and their relationship with local atomic structures of glasses.Comment: 21 pages, 4 figure

Fractal atomic-level percolation in metallic glasses

Metallic glasses are metallic alloys that exhibit exotic material properties. They may have fractal structures at the atomic level, but a physical mechanism for their organization without ordering has not been identified. We demonstrated a crossover between fractal short-range (<2 atomic diameters) and homogeneous long-range structures using in situ x-ray diffraction, tomography, and molecular dynamics simulations. A specific class of fractal, the percolation cluster, explains the structural details for several metallic-glass compositions. We postulate that atoms percolate in the liquid phase and that the percolating cluster becomes rigid at the glass transition temperature

Pressure-induced tuning of lattice distortion in a high-entropy oxide

As a new class of multi-principal component oxides with high chemical disorder, high-entropy oxides (HEOs) have attracted much attention. The stability and tunability of their structure and properties are of great interest and importance, but remain unclear. By using in situ synchrotron radiation X-ray diffraction, Raman spectroscopy, ultraviolet–visible absorption spectroscopy, and ex situ high-resolution transmission electron microscopy, here we show the existence of lattice distortion in the crystalline (Ce$_{0.2}$La$_{0.2}$Pr$_{0.2}$Sm$_{0.2}$Y$_{0.2}$)O$_{2−δ }$ HEO according to the deviation of bond angles from the ideal values, and discover a pressureinduced continuous tuning of lattice distortion (bond angles) and band gap. As continuous bending of bond angles, pressure eventually induces breakdown of the long-range connectivity of lattice and causes amorphization. The amorphous state can be partially recovered upon decompression, forming glass–nanoceramic composite HEO. These results reveal the unexpected flexibility of the structure and properties of HEOs, which could promote the fundamental understanding and applications of HEOs

High-Pressure Induced Phase Transitions in High-Entropy Alloys: A Review

High-entropy alloys (HEAs) as a new class of alloy have been at the cutting edge of advanced metallic materials research in the last decade. With unique chemical and topological structures at the atomic level, HEAs own a combination of extraordinary properties and show potential in widespread applications. However, their phase stability/transition, which is of great scientific and technical importance for materials, has been mainly explored by varying temperature. Recently, pressure as another fundamental and powerful parameter has been introduced to the experimental study of HEAs. Many interesting reversible/irreversible phase transitions that were not expected or otherwise invisible before have been observed by applying high pressure. These recent findings bring new insight into the stability of HEAs, deepens our understanding of HEAs, and open up new avenues towards developing new HEAs. In this paper, we review recent results in various HEAs obtained using in situ static high-pressure synchrotron radiation x-ray techniques and provide some perspectives for future research

Spatial Resolution Limit for Nanoindentation Mapping on Metallic Glasses

Spatial heterogeneity, as a crucial structural feature, has been intensively studied in metallic glasses (MGs) using various techniques, including two-dimensional nanoindentation mapping. However, the limiting spatial resolution of nanoindentation mapping on MGs remains unexplored. In this study, a comprehensive study on four representative MGs using nanoindentation mapping with a Berkovich indenter was carried out by considering the influence of a normalized indentation spacing d/h (indentation spacing/maximum indentation depth). It appeared to have no significant correlation with the measured hardness and elastic modulus when d/h &gt; 10. The hardness and elastic modulus started to increase slightly (up to ~5%) when d/h &lt; 10 and further started to decrease obviously when d/h &lt; 5. The mechanism behind these phenomena was discussed based on a morphology analysis of residual indents using scanning electron microscopy and atomic force microscopy. It was found that the highest spatial resolution of ~200 nm could be achieved with d/h = 10 using a typical Berkovich indenter for nanoindentation mapping on MGs, which was roughly ten times the curvature radius of the Berkovich indenter tip (not an ideal triangular pyramid) used in this study. These results help to promote the heterogeneity studies of MGs using nanoindentation that are capable of covering a wide range of length scales with reliable and consistent results

Pressure-Induced Structural Evolution and Band Gap Shifts of Organometal Halide Perovskite-Based Methylammonium Lead Chloride

Organometal halide perovskites are promising materials for optoelectronic devices. Further development of these devices requires a deep understanding of their fundamental structure–property relationships. The effect of pressure on the structural evolution and band gap shifts of methylammonium lead chloride (MAPbCl₃) was investigated systematically. Synchrotron X-ray diffraction and Raman experiments provided structural information on the shrinkage, tilting distortion, and amorphization of the primitive cubic unit cell. In situ high pressure optical absorption and photoluminescence spectra manifested that the band gap of MAPbCl₃ could be fine-tuned to the ultraviolet region by pressure. The optical changes are correlated with pressure-induced structural evolution of MAPbCl₃, as evidenced by band gap shifts. Comparisons between Pb-hybrid perovskites and inorganic octahedra provided insights on the effects of halogens on pressure-induced transition sequences of these compounds. Our results improve the understanding of the structural and optical properties of organometal halide perovskites

Role of rare earth elements addition in enhancing glass-forming ability and magnetic softness of a Co75B25 metallic glass: Theoretical prediction and experimental verification

The effects of light/heavy rare earth (L/HRE) elements addition on local atomic structures, glass-forming ability (GFA), and soft magnetic properties of a Co75B25 metallic glass (MG) were comprehensively investigated using theoretical prediction and experimental verification methods. Ab initio molecular dynamics (AIMD) simulations based on density functional theory (DFT) calculations revealed that the Co-RE-B (RE = La, Sm, Gd, and Y) MGs exhibit unique structural heterogeneities, with B-centered prism units dominating the local atomic structures. Compared to the Co-LRE-B (LRE = La and Sm) MGs, the Co-HRE-B (HRE = Gd and Y) MGs possess stronger bond strengths and higher fractions of icosahedral-like (ico-like) units, leading to remarkable enhancements in structural stability, fivefold symmetry, atomic packing density, and sluggish diffusion, ultimately resulting in enhanced GFA. Furthermore, the lower magnetic anisotropy energy (MAE) and range of the local loosely packed regions (LLPRs) in the HRE-containing MGs can significantly promote magnetic softness. The theoretical analysis predicts that the MGs containing the HRE possess higher GFA and magnetic softness with the order of Co75B25 < Co71.5La3.5B25 < Co71.5Sm3.5B25 < Co71.5Gd3.5B25 < Co71.5Y3.5B25. These predictions have been successfully verified by the experiments in both local atomic structures and properties, through the in situ high-energy synchrotron X-ray diffraction (HEXRD) characterization, as well as evaluations of thermal and magnetic properties