We report nano-scale mechanical heterogeneity of a metallic glass characterized by dynamic force microscopy. Apparent energy dissipation with the variation of ~12%, originating from non-uniform distribution of local viscoelasticity, was characterized.The correlation length of heterogeneous viscoelasticity was measured to be ~2.5±0.3 nm, which is well consistent with the dimension of shear transformation zones for plastic flow. This study provides the first experimental observation on the nano-scale mechanical heterogeneity in a metallic glass, and may fill the gap between atomic models and the macroscopic properties of metallic glasses.
Johari-Goldstein or b relaxation, persisting down to glassy state from a supercooled liquid, is a universal phenomenon of glassy dynamics. Nevertheless, the underlying micromechanisms leading to the relaxation are still in debate despite great efforts devoted to this problem for decades. Here we report experimental evidence on the structural origins of Johari-Goldstein relaxation in an ultra-quenched metallic glass. The measured activation energy of the relaxation (B26 times of the product of gas constant and glass transition temperature) is consistent with the dynamic characteristics of Johari-Goldstein relaxation. Synchrotron X-ray investigations demonstrate that the relaxation originates from short-range collective rearrangements of large solvent atoms, which can be realized by local cooperative bonding switch. Our observations provide experimental insights into the atomic mechanisms of Johari-Goldstein relaxation and will be helpful in understanding the low-temperature dynamics and properties of metallic glasses.
We report a hybrid atomic packing scheme comprised of a covalent-bond-mediated "stereochemical" structure and a densely packed icosahedron in a bulk metallic glass Pd40Ni40P20. The coexistence of two atomic packing models can simultaneously satisfy the criteria for both the charge saturation of the metalloid element and the densest atomic packing of the metallic elements. The hybrid packing scheme uncovers the structural origins of the excellent glass forming ability of Pd40Ni40P20 and has important implications in understanding the bulk metallic glass formation of metal-metalloid alloys.
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