Atomic-Level Structure and Deformation in Metallic Glasses
Metallic glasses (MGs) are a relatively new class of materials discovered in 1960 and lauded for its high strengths and superior elastic properties. Three major obstacles prevent their widespread use as engineering materials for nanotechnology and industry: 1) their lack of plasticity mechanisms...
| Summary: | Metallic glasses (MGs) are a relatively new class of materials discovered in 1960
and lauded for its high strengths and superior elastic properties. Three major obstacles
prevent their widespread use as engineering materials for nanotechnology and
industry: 1) their lack of plasticity mechanisms for deformation beyond the elastic
limit, 2) their disordered atomic structure, which prevents effective study of their
structure-to-property relationships, and 3) their poor glass forming ability, which
limits bulk metallic glasses to sizes on the order of centimeters. We focused on
understanding the first two major challenges by observing the mechanical properties
of nanoscale metallic glasses in order to gain insight into its atomic-level structure
and deformation mechanisms. We found that anomalous stable plastic flow emerges
in room-temperature MGs at the nanoscale in wires as little as ~100 nanometers
wide regardless of fabrication route (ion-irradiated or not). To circumvent experimental
challenges in characterizing the atomic-level structure, extensive molecular
dynamics simulations were conducted using approximated (embedded atom
method) potentials to probe the underlying processes that give rise to plasticity in
nanowires. Simulated results showed that mechanisms of relaxation via the sample
free surfaces contribute to tensile ductility in these nanowires. Continuing with characterizing
nanoscale properties, we studied the fracture properties of nano-notched
MGnanowires and the compressive response of MG nanolattices at cryogenic (~130
K) temperatures. We learned from these experiments that nanowires are sensitive
to flaws when the (amorphous) microstructure does not contribute stress concentrations,
and that nano-architected structures with MG nanoribbons are brittle at low
temperatures except when elastic shell buckling mechanisms dominate at low ribbon
thicknesses (~20 nm), which instead gives rise to fully recoverable nanostructures regardless
of temperature. Finally, motivated by understanding structure-to-property
relationships in MGs, we studied the disordered atomic structure using a combination
of in-situ X-ray tomography and X-ray diffraction in a diamond anvil cell
and molecular dynamics simulations. Synchrotron X-ray experiments showed the
progression of the atomic-level structure (in momentum space) and macroscale volume
under increasing hydrostatic pressures. Corresponding simulations provided
information on the real space structure, and we found that the samples displayed
fractal scaling (r<sup>d</sup> ∝ V, d < 3) at short length scales (< ~8 Å), and exhibited a
crossover to a homogeneous scaling (d = 3) at long length scales. We examined
this underlying fractal structure of MGs with parallels to percolation clusters and
discuss the implications of this structural analogy to MG properties and the glass
transition phenomenon. |
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