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Metallic glasses: Elastically stiff yet flowing at any stress
Summary
Researchers demonstrated that metallic glass, an amorphous solid with high yield stress, lacks a true microscopic elastic limit. Using coherent X-ray scattering, they found that even extremely small stresses accelerate atomic-scale transport within the material. The findings reveal fundamental differences in how amorphous and crystalline solids respond to mechanical stress at the atomic level.
Crystalline solids have a minimum stress needed to displace atoms or to move defects. This stress defines the true elastic limit and is generally a sizeable share of the macroscopic yield stress. Here we demonstrate that a metallic glass, an amorphous solid with a yield stress in the giga-pascal regime, lacks such a true microscopic elastic limit. Leveraging in-situ coherent x-ray scattering, we uncover a strongly accelerated atomic-scale transport upon the application of a stress as small as 0.005 times the yield stress. With increasing stress levels, the distribution of structural relaxation times changes from compressed exponential to simple exponential form, revealing a stress–temperature equivalence in the time-scale domain. These findings strongly promote a microstructurally heterogeneous picture of metallic glasses, in which a part of the amorphous microstructure controls macroscopic yielding whereas another part admits microplastic flow at any stress.
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