Changes in short- and medium-range order in metallic liquids during undercooling
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troduction There has been considerable recent progress in the understanding of atomic-scale processes in the undercooling of metallic glass-forming liquids, and their subsequent vitrification as well as potential applications.1 This understanding has been advanced by the development of new and more powerful computational and experimental approaches that have provided unique insights into the time-correlated atomic-scale processes and their more tractable experimental measurements, such as pair-correlation functions. Despite these advances, the relationships between the atomic configurations in the undercooled liquid2–4 and the changes they undergo with the rate of cooling,5 and the various stable and metastable phases that form remain questions.6 This includes the relationship between the developing short- and medium-range order as a function of undercooling, the rate of undercooling, and the kinetic and thermodynamic stability of nuclei that are forming. The overwhelming majority of metallic glasses form near deep eutectics.7 Why is the nucleation of the nearby ground-state compounds suppressed in many of these systems? Atomistic models, while still far from simulating experimental cooling rates, suggest that the deeply undercooled liquid is heterogeneous with nanoscale regions with varying topology resulting in segmented regions of varying diffusivity.8–12 These simulations
suggest it is not the local packing alone, but how these various polytopes form into more extended structures that control both the vitrification and phase selection upon heating.13–16 In this article, we explore some recent developments in the field and also provide insights into possible future directions.
Implications of fragility on experiments and modeling Landmark observations in this field date back several decades, when it was shown that many metallic glasses arise from fragile liquids that show a large deviation from the Arrhenius behavior in their temperature dependence in viscosity or relaxation time (Figure 1).17–20 Changes in the heat capacity, viscosity,21 and more recently, in situ neutron and x-ray scattering22–24 have all revealed a non-Arrhenius dependence of the changes in the undercooled liquid with temperature. Subsequent atomistic models suggest that this fragility arises from the heterogeneous nature of the “polymerization” of the metallic liquids similar to that of water.25 The challenge with connecting the molecular dynamics (MD) simulations and the experimental observations is one of spatial and temporal scales. The critical cooling rate to form metallic glasses, the lowest cooling rate where a particular composition remains glassy, varies by nearly 10 orders of magnitude, with some alloys having as low as a few K/s for
M.J. Kramer, Materials Science and Engineering Division, US Department of Energy Ames Laboratory, USA; [email protected] Mo Li, Georgia Institute of Technology, USA; [email protected] doi:10.1557/mrs.2020.272 • available © The Author(s), 2020, published on behalf of Materials Research by Cambrid
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