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Title: A microscopic continuum model for defect dynamics in metallic glasses

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Journal Article: Publisher's Accepted Manuscript
Journal Name:
Journal of the Mechanics and Physics of Solids
Additional Journal Information:
Journal Volume: 104; Journal Issue: C; Related Information: CHORUS Timestamp: 2017-12-15 02:48:05; Journal ID: ISSN 0022-5096
Country of Publication:
United Kingdom

Citation Formats

Acharya, Amit, and Widom, Michael. A microscopic continuum model for defect dynamics in metallic glasses. United Kingdom: N. p., 2017. Web. doi:10.1016/j.jmps.2017.03.014.
Acharya, Amit, & Widom, Michael. A microscopic continuum model for defect dynamics in metallic glasses. United Kingdom. doi:10.1016/j.jmps.2017.03.014.
Acharya, Amit, and Widom, Michael. 2017. "A microscopic continuum model for defect dynamics in metallic glasses". United Kingdom. doi:10.1016/j.jmps.2017.03.014.
title = {A microscopic continuum model for defect dynamics in metallic glasses},
author = {Acharya, Amit and Widom, Michael},
abstractNote = {},
doi = {10.1016/j.jmps.2017.03.014},
journal = {Journal of the Mechanics and Physics of Solids},
number = C,
volume = 104,
place = {United Kingdom},
year = 2017,
month = 7

Journal Article:
Free Publicly Available Full Text
This content will become publicly available on April 7, 2018
Publisher's Accepted Manuscript

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  • In the paper K. L. Ngai et al., [J. Chem. 140, 044511 (2014)], the empirical correlation of ductility with the Poisson's ratio, ν{sub Poisson}, found in metallic glasses was theoretically explained by microscopic dynamic processes which link on the one hand ductility, and on the other hand the Poisson's ratio. Specifically, the dynamic processes are the primitive relaxation in the Coupling Model which is the precursor of the Johari–Goldstein β-relaxation, and the caged atoms dynamics characterized by the effective Debye–Waller factor f{sub 0} or equivalently the nearly constant loss (NCL) in susceptibility. All these processes and the parameters characterizing themmore » are accessible experimentally except f{sub 0} or the NCL of caged atoms; thus, so far, the experimental verification of the explanation of the correlation between ductility and Poisson's ratio is incomplete. In the experimental part of this paper, we report dynamic mechanical measurement of the NCL of the metallic glass La{sub 60}Ni{sub 15}Al{sub 25} as-cast, and the changes by annealing at temperature below T{sub g}. The observed monotonic decrease of the NCL with aging time, reflecting the corresponding increase of f{sub 0}, correlates with the decrease of ν{sub Poisson}. This is important observation because such measurements, not made before, provide the missing link in confirming by experiment the explanation of the correlation of ductility with ν{sub Poisson}. On aging the metallic glass, also observed in the isochronal loss spectra is the shift of the β-relaxation to higher temperatures and reduction of the relaxation strength. These concomitant changes of the β-relaxation and NCL are the root cause of embrittlement by aging the metallic glass. The NCL of caged atoms is terminated by the onset of the primitive relaxation in the Coupling Model, which is generally supported by experiments. From this relation, the monotonic decrease of the NCL with aging time is caused by the slowing down of the primitive relaxation and β-relaxation on annealing, and vice versa.« less
  • Understanding of the structure and dynamics of liquids and glasses at an atomistic level lags well behind that of crystalline materials, even though they are important in many fields. Metallic liquids and glasses provide an opportunity to make significant advances because of its relative simplicity. We propose a microscopic model based on the concept of topological fluctuations in the bonding network. The predicted glass transition temperature, Tg, shows excellent agreement with experimental observations in metallic glasses. To our knowledge this is the first model to predict the glass transition temperature quantitatively from measurable macroscopic variables.
  • In molecular-dynamics simulations of BeF/sub 2/ glass, it is observed that a few percent of the ions are capable of displacements of the order of 1 A in 10 ps at temperatures lower than 300 K. It is shown that these ions are located at well-defined defect sites in the glass. The activation energy for such motion is between 0.03 and 0.2 eV. Ions making large displacements often return to their starting points after a few picoseconds (periodic motion). It is suggested that such mobile ions are responsible for the low-temperature anomalous properties of glass.
  • The electronic and transport properties of three metallic glass systems, /ital a/-Cu/sub 60/Zn/sub 40/, /ital a/-Mg/sub 75/Zn/sub 25/, and /ital a/-Ni, are studied by means of realistic microscopic real-space calculations. At low temperature, the transport properties are contrilled by the magnitude and the shape of the conductivity function sigma/sub /ital E// near the Fermi energy. It is shown that for a stable metallic glass the Fermi energy is quite close to a local minimum in sigma/sub /ital E// and this causes the negative temperature coefficient of resistivity which is purely due to the elastic scattering of the conduction electrons frommore » the disordered atoms.« less
  • To establish a relationship between microstructure and mechanical properties, we systematically annealed a Zr-based bulk metallic glass (BMG) at 100 ~ 300°C and measured their mechanical and thermal properties. The as-cast BMG exhibits some ductility, while the increase of annealing temperature and time leads to the transition to a brittle behavior that can reach nearly-zero fracture energy. The differential scanning calorimetry did not find any significant changes in crystallization temperature and enthalpy, indicating that the materials still remained fully amorphous. Elastic constants measured by ultrasonic technique vary only slightly with respect to annealing temperature and time, which does obey themore » empirical relationship between Poisson’s ratio and fracture behavior. Nanoindentation pop-in tests were conducted, from which the pop-in strength mapping provides a “mechanical probe” of the microscopic structural heterogeneities in these metallic glasses. Based on stochastically statistic defect model, we found that the defect density decreases with increasing annealing temperature and annealing time and is exponentially related to the fracture energy. A ductile-versus-brittle behavior (DBB) model based on the structural heterogeneity is developed to identify the physical origins of the embrittlement behavior through the interactions between these defects and crack tip.« less