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Liu, Chaoyang

Publications and source records attributed to Liu, Chaoyang.

5 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Stress breaks universal aging behavior in a metallic glass

Numerous disordered materials display a monotonous slowing down in their internal dynamics with age. In the case of metallic glasses, this general behavior across different temperatures and alloys has been used to establish an empirical universal superposition principle of time, waiting time, and temperature. Here we demonstrate that the application of a mechanical stress within the elastic regime breaks this universality. Using in-situ x-ray photon correlation spectroscopy (XPCS) experiments, we show that strong fluctuations between slow and fast structural dynamics exist, and that these generally exhibit larger relaxation times than in the unstressed case. On average, relaxation times increase with stress magnitude, and even preloading times of several days do not exhaust the structural dynamics under load. A model Lennard-Jones glass under shear deformation replicates many of the features revealed with XPCS, indicating that local and heterogeneous microplastic events can cause the strongly non-monotonous spectrum of relaxation times.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Energy Storage in Metallic Glasses via Flash Annealing

Abstract When the temperature rises in a kinetically frozen and out‐of‐equilibrium system, such as a structural glass, thermal activation usually drives a relaxation to a lower enthalpy state. This is the essence of the ubiquitous thermal ageing of glassy materials below the glass transition temperature. Here, it is shown that an ultrafast temperature rise without subsequent quenching can elevate the system to a higher enthalpy state, thereby erasing the relaxation history. The key of this temperature‐driven rejuvenation is a fast‐heating rate (flash annealing) that brings the glass into a temperature regime above the glass transition within less than a millisecond. Here, the structure is entropically driven towards the liquid state, lowering its free energy, but raising its enthalpy. Subsequent cooling freezes the increased enthalpy before the liquid state is reached, thereby leading to a global energy storage. This ultrafast rejuvenation process is demonstrated for a Zr‐based metallic glass, combining calorimetry, fast in‐situ X‐ray diffraction, and atomistic simulations, showing a tenfold increase in stored enthalpy via flash annealing.

Küchemann, Stefan↗
DOE OSTI2018

Shear banding leads to accelerated aging dynamics in a metallic glass

Traditionally, strain localization in metallic glasses is related to the thickness of the shear defect, which is confined to the nanometer scale. In this study, using site-specific x-ray photon correlation spectroscopy (XPCS), we reveal significantly accelerated relaxation dynamics around a shear band in a metallic glass at a length scale that is orders of magnitude larger than the defect itself. The relaxation time in the shear-band vicinity is up to ten-times smaller compared to the as-cast matrix, and the relaxation dynamics occurs in a characteristic three-stage aging response that manifests itself in the temperature-dependent shape parameter known from classical stretched exponential relaxation dynamics of disordered materials. We demonstrate that the time-dependent correlation functions describing the aging at different temperatures can be captured and collapsed using simple scaling functions. Finally, these insights highlight how an ubiquitous nano-scale strain-localization mechanism in metallic glasses leads to a fundamental change of the relaxation dynamics at the mesoscale.

36 MATERIALS SCIENCE↗
DOE OSTI2015

A peripheral component interconnect express-based scalable and highly integrated pulsed spectrometer for solution state dynamic nuclear polarization

High sensitivity, high data rates, fast pulses, and accurate synchronization all represent challenges for modern nuclear magnetic resonance spectrometers, which make any expansion or adaptation of these devices to new techniques and experiments difficult. Here, we present a Peripheral Component Interconnect Express (PCIe)-based highly integrated distributed digital architecture pulsed spectrometer that is implemented with electron and nucleus double resonances and is scalable specifically for broad dynamic nuclear polarization (DNP) enhancement applications, including DNP-magnetic resonance spectroscopy/imaging (DNP-MRS/MRI). The distributed modularized architecture can implement more transceiver channels flexibly to meet a variety of MRS/MRI instrumentation needs. The proposed PCIe bus with high data rates can significantly improve data transmission efficiency and communication reliability and allow precise control of pulse sequences. An external high speed double data rate memory chip is used to store acquired data and pulse sequence elements, which greatly accelerates the execution of the pulse sequence, reduces the TR (time of repetition) interval, and improves the accuracy of TR in imaging sequences. Using clock phase-shift technology, we can produce digital pulses accurately with high timing resolution of 1 ns and narrow widths of 4 ns to control the microwave pulses required by pulsed DNP and ensure overall system synchronization. The proposed spectrometer is proved to be both feasible and reliable by observation of a maximum signal enhancement factor of approximately −170 for {sup 1}H, and a high quality water image was successfully obtained by DNP-enhanced spin-echo {sup 1}H MRI at 0.35 T.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗