Microscopic evidence of the connection between liquid-liquid transition and dynamical crossover in an ultraviscous metallic glass former

Liquid-liquid transitions are interesting to many researchers since they occur in systems as diverse as monoatomic liquids, multicomponent oxides, and metallic glass formers. In some cases, the crossover is accompanied by changes in the dynamical properties. By combining state-of-the-art synchrotron techniques, we followed the structure and atomic motion during quasistatic cooling of the Au49Cu26.9Si16.3Ag5.5Pd2.3 metallic glass former from the low-temperature supercooled liquid. With this thermal protocol, we were able to lower the glass transition temperature far enough to reveal a liquid-liquid crossover between two amorphous structures corresponding to two ultraviscous liquids with different kinetic behavior. This transition is in competition with vitrification, which occurs at conventional cooling rates, and is accompanied by structural changes not affecting the average density. Our results provide a direct connection between polyamorphism and dynamical crossover, and an alternative case to add to the highly debated topic on the low-temperature divergence of the dynamics in supercooled liquids.Peer ReviewedPostprint (published version

VarFish: comprehensive DNA variant analysis for diagnostics and research

VarFish is a user-friendly web application for the quality control, filtering, prioritization, analysis, and user-based annotation of DNA variant data with a focus on rare disease genetics. It is capable of processing variant call files with single or multiple samples. The variants are automatically annotated with population frequencies, molecular impact, and presence in databases such as ClinVar. Further, it provides support for pathogenicity scores including CADD, MutationTaster, and phenotypic similarity scores. Users can filter variants based on these annotations and presumed inheritance pattern and sort the results by these scores. Variants passing the filter are listed with their annotations and many useful link-outs to genome browsers, other gene/variant data portals, and external tools for variant assessment. VarFish allows users to create their own annotations including support for variant assessment following ACMG-AMP guidelines. In close collaboration with medical practitioners, VarFish was designed for variant analysis and prioritization in diagnostic and research settings as described in the software's extensive manual. The user interface has been optimized for supporting these protocols. Users can install VarFish on their own in-house servers where it provides additional lab notebook features for collaborative analysis and allows re-analysis of cases, e.g. after update of genotype or phenotype databases

Linking structure to fragility in bulk metallic glass-forming liquids

Using in-situ synchrotron X-ray scattering, we show that the structural evolution of various bulk metallic glass-forming liquids can be quantitatively connected to their viscosity behavior in the supercooled liquid near T[subscript g]. The structural signature of fragility is identified as the temperature dependence of local dilatation on distinct key atomic length scales. A more fragile behavior results from a more pronounced thermally induced dilatation of the structure on a length scale of about 3 to 4 atomic diameters, coupled with shallower temperature dependence of structural changes in the nearest neighbor environment. These findings shed light on the structural origin of viscous slowdown during undercooling of bulk metallic glass-forming liquids and demonstrate the promise of predicting the properties of bulk metallic glasses from the atomic scale structure.Article Copyright 2015 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The published article can be found at: http://scitation.aip.org/content/aip/journal/ap

On the bulk glass formation in the ternary Pd-Ni-S system

We report on the formation of bulk metallic glasses in the ternary Pd-Ni-S system. In a large compositional range, glass formation is observed by copper mold casting with a glass forming ability of up to 2 mm in diameter for the composition $Pd_{37}Ni_{37}S_{26}$. The best compromise of thermal stability upon heating from the as-cast state and glass forming ability was found for $Pd_{31}Ni_{42}S_{27}$, having a critical diameter of 1.5 mm and an extension of the supercooled liquid region of 27.2 K $(ΔTx = Tx – Tg)$. Differential scanning calorimetry and X-ray diffraction experiments were conducted in order to study the influence of the composition on the glass forming ability and thermal stability. The primary precipitating crystalline phases $Ni_{3}S_{2}$ and $Pd_{4}S$ are identified by in-situ high energy synchrotron X-ray scattering experiments upon heating from the glassy state as well as upon cooling from the equilibrium liquid. Finally, the origin of the bulk glass formation in this novel system is discussed regarding thermodynamics and kinetics and compared to current models for the prediction of the glass forming ability. Furthermore, the mechanical properties are investigated and discussed with respect to the rather fragile kinetic behavior. All in all, we gain new insights into the process of glass formation in this novel alloying system and give conclusions about the determining contributions for the glass forming ability and glass forming range

The kinetic fragility of Pt-P- and Ni-P-based bulk glass-forming liquids and its thermodynamic and structural signature

The thermodynamic and kinetic properties of Pt-P- and Ni-P-based bulk glass-forming liquids are investigated using differential scanning calorimetry and three-point beam bending. The kinetic fragility of the alloys is determined by measuring relaxation times and equilibrium viscosities. In the Pt-P-based alloy family a Pt-rich alloy is one of the most fragile bulk glass-forming liquids in the vicinity of the glass transition reported so far. The fragility parameter D$^∗$ increases significantly with decreasing Pt-content, which is due to a more pronounced loss of excess entropy upon undercooling for the fragile liquids. Relying on previous observations made for Pd-P-based alloys, it is argued that a bifurcation of the P environment into Pt-Ni-P and Pt-Cu-P structural units stabilizes the deeply undercooled liquid due to formation of medium range order (MRO). This is supported by synchrotron X-ray scattering experiments, which reveal the formation of a pre-peak in the total structure factor with increasing fragility. Adapting a previously reported empirical correlation, we show that the more fragile liquid alloys are characterized by a more pronounced dilatation of atomic pair correlations on the length scale of 1 nm, corroborating the idea that the observed fragility behavior of Pt-P-based bulk glass-forming liquids is inherently related to structural changes on the length scale of MRO

Structural evolution on medium-range-order during the fragile-strong transition in Ge 15 Te 85

Using synchrotron X-ray scattering, we investigate liquid Ge$_{15}$Te$_{85}$ spanning a wide temperature range from near Tg to the melt, and demonstrate that the density anomaly and fragile-strong transition are not only related to short-range-order (SRO) structural change (e.g. Peierls-like distortion), but also accompanied by a remarkable development of medium-range-order (MRO). The latter manifests as an emerging pre-peak in total structure factor S(Q) and atomic pair correlations on the length scale of ∼8 Å in the real space G(r) function. The results highlight the role of medium-range structural ordering in the evolution of the configurational entropy which, according to the Adam-Gibbs theory, can be linked to the fragile-strong transition (FS-transition). Based on the relation between structure and liquid dynamics, the FS-transitions at high pressures are examined in terms of experimental data and the Ehrenfest relation. This work identifies the length scale for the atomic correlations in MRO structural evolutions and presents a structural approach to exploring liquid dynamics, which may be useful for investigating relevant phase-change alloys

On the high glass-forming ability of Pt-Cu-Ni/Co-P-based liquids

The continuous and isothermal crystallization diagrams of the Pt$_{42.5}$Cu$_{27}$Ni$_{9.5}$P$_{21}$ and the Pt$_{60}$Cu$_{16}$Co$_2$P$_{22}$ bulk glass forming compositions are determined using calorimetric experiments. In the case of the Pt$_{42.5}$Cu$_{27}$Ni$_{9.5}$P$_{21}$ bulk metallic glass, the formation of the primary crystalline phase can be prevented by rapid cooling in a conventional DSC. In contrast, for similar cooling conditions, the formation of the primary precipitating compound in Pt$_{60}$Cu$_{16}$Co$_2$P$_{22}$ cannot be prevented in a conventional DSC as also observed in in-situ synchrotron X-ray scattering experiments. This is attributed to a critical overheating, above which remaining structures dissolve, resulting in a drastic increase of the degree of undercooling, similar to what is observed in Zr-based BMGs. Using the classical nucleation theory, the combined thermodynamic and kinetic data are used to model the isothermal crystallization data for Pt$_{42.5}$Cu$_{27}$Ni$_{9.5}$P$_{21}$, yielding an interfacial energy value of 0.11 J/m$^2$ between the primary nucleating crystal and the liquid. This value is three times higher than the value for good Zr-based glass-formers, suggesting that the interfacial energy plays a pivotal role in the exceptionally high glass-forming ability of Pt-P-based systems and compensates for the fragile liquid behavior and the large driving force for crystallization

Linking Structure to Fragility in Bulk Metallic Glass-Forming Liquids

Using in-situ synchrotron X-ray scattering, we show that the structural evolution of various bulk me- tallic glass-forming liquids can be quantitatively connected to their viscosity behavior in the super- cooled liquid near Tg. The structural signature of fragility is identified as the temperature dependence of local dilatation on distinct key atomic length scales. A more fragile behavior results from a more pronounced thermally induced dilatation of the structure on a length scale of about 3 to 4 atomic diameters, coupled with shallower temperature dependence of structural changes in the nearest neigh- bor environment. These findings shed light on the structural origin of viscous slowdown during undercooling of bulk metallic glass-forming liquids and demonstrate the promise of predicting the properties of bulk metallic glasses from the atomic scale structure

Connection Between Structure and Fragility of Metallic Glass-forming Liquids

We investigate the structural evolution of various metallic glass-forming liquids and quantitatively connect the structural changes to their viscosity behavior in the supercooled liquid region near Tg. Using in-situ synchrotron X-ray scattering, we find a common signature of thermally-induced structural changes on the short and medium range order length scale, which are consistently correlated with liquid’s fragility. These experimental findings suggest that the fragility of metallic glass-forming liquids is strongly affected by the changes in nearest neighbour atomic environment and the temperature dependence of atomic spatial correlations on a length scale of about 1 nm. The structural changes associated with fragility is consequently linked to temperature dependence of configurational entropy as well as mechanical properties of the glassy state. The present work provides new insight into the relation between liquid’s structure and their kinetic properties

Microscopic evidence of the connection between liquid-liquid transition and dynamical crossover in an ultraviscous metallic glass former

Liquid-liquid transitions are interesting to many researchers since they occur in systems as diverse as monoatomic liquids, multicomponent oxides, and metallic glass formers. In some cases, the crossover is accompanied by changes in the dynamical properties. By combining state-of-the-art synchrotron techniques, we followed the structure and atomic motion during quasistatic cooling of the Au49Cu26.9Si16.3Ag5.5Pd2.3 metallic glass former from the low-temperature supercooled liquid. With this thermal protocol, we were able to lower the glass transition temperature far enough to reveal a liquid-liquid crossover between two amorphous structures corresponding to two ultraviscous liquids with different kinetic behavior. This transition is in competition with vitrification, which occurs at conventional cooling rates, and is accompanied by structural changes not affecting the average density. Our results provide a direct connection between polyamorphism and dynamical crossover, and an alternative case to add to the highly debated topic on the low-temperature divergence of the dynamics in supercooled liquids.Peer Reviewe