The slow β relaxation is understood to be a universal feature of glassy dynamics. Its presence in bulk metallic glasses (BMGs) is evidence of a broad relaxation time spectrum that extends to deep within the glassy state. Despite the breadth of research devoted to this phenomenon, its microscopic origin is still not fully understood. The low-temperature aging behavior and atomic structural rearrangements of a Au 49 Cu 26.9 Si 16.3 Ag 5.5 Pd 2.3 BMG are investigated in the regime of the slow β relaxation by employing an ensemble of experimental techniques such as high-intensity synchrotron x-ray scattering, modulated differential scanning calorimetry (MDSC), dynamic mechanical analysis (DMA), impulse excitation, and dilatometry. Evidence of a distinct slow β-relaxation regime is seen in the form of (1) an excess wing of the DMA loss modulus beginning at ∼50• C, (2) a crossover effect of elastic modulus with isothermal aging at 50• C, and (3) a broad, nonreversing and largely irreversible sub-T g endotherm in the MDSC results. Atomic rearrangements occurring at the onset of the measured slow β-relaxation temperature regime were found to be confined mainly to the short-range order length scale while no significant atomic rearrangements occur on the length scale of the medium-range order. Furthermore, evidence is presented that suggests the crossover effect in Young's modulus is due to the evolution of chemical short-range order. These results support the emergent picture of a dynamically heterogeneous glassy structure, in which low-temperature relaxation occurs through atomic rearrangements confined mostly to the short-range order length scale.

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The correlations among the fragility of supercooled liquids, the fragility of superheated melts, and the glassforming ability for marginal metallic glasses Thermophysical and elastic properties of Cu 50 Zr 50 and ( Cu 50 Zr 50 ) 95 Al 5 bulk-metallic-glass-forming alloys Appl.

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Liquid-liquid transitions are interesting to many researchers since they occur in systems 19 as diverse as monoatomic liquids, multicomponent oxides and metallic glass-formers. In 20 some cases, the crossover is accompanied by changes in the dynamical properties, 21 although a direct microscopic evidence of this connection has not been reported so far. 22 By combining state-of-the-art synchrotron techniques, we followed the structure and 23 atomic motion during quasi-static cooling the Au49Cu26.9Si16.3Ag5.5Pd2.3 metallic glass-24 former from the low-temperature supercooled liquid. With this thermal protocol, we were 25 able to lower the glass transition temperature far enough to reveal a liquid-liquid 26 crossover between two amorphous structures corresponding to two ultra-viscous liquids 27 with different kinetic behavior. This transition is in competition with vitrification, which 28 occurs at conventional cooling rates, and is accompanied by structural changes not 29 affecting the average density. Our results provide a direct connection between 30 polyamorphism and dynamical crossover, and an alternative case to add in the highly-31 debated topic on the low-temperature divergence of the dynamics in supercooled liquids. 32 33 2Many systems, ranging from monoatomic liquids [1-3] to amorphous alloys [4][5][6][7], exhibit 34 transitions between liquid phases of different local structure. In glass-formers, liquid-liquid 35 transitions (LLTs) have been reported either in high temperature melts [3,[8][9][10][11][12][13] or in 36 supercooled liquids [4][5][6]14,15], suggesting the existence of intrinsic connections between the 37 kinetic properties of the system and the transition temperature [16]. LLTs are sometime 38 associated to a dynamical crossover with changes in the kinetic fragility [6,10,17]. When 39 observed upon cooling, the liquid evolves from a high temperature fragile phase with a steep 40 temperature dependence of viscosity and structural relaxation time, to a strong phase less 41 affected by temperature changes. For fragile liquids, like molecular and some metallic glass-42 formers, the LLT is expected at such a low temperature that the system may arrest beforehand 43 in the glass [16]. Still, no direct microscopic evidence of this and more in general of a direct 44 connection between LLT and dynamical crossover have been reported so far. This is also due 45 to the extreme difficulty of obtaining experimental measurements of the atomic collective 46 motion in glass-formers. 47 By taking advantage of state-of-the-art synchrotron techniques like x-ray photon correlation 48 spectroscopy (XPCS) and high energy x-ray diffraction (XRD), we present the first 49 experimental measurements of the microscopic dynamics of an ultra-viscous glass former with 50 relaxation times which were not accessible until now, neither experimentally nor with 51 numerical simulations. Our data show the occurrence of a LLT upon quasi-static cooling the 52 Au49Cu26.9Si16.3Ag5.5Pd2.3 metallic glass-former, which can be direct...