Analysis of the effect of aging on distributed relaxations, hardness, and embrittlement in Cu59Zr41 and Fe80B20 glasses

Distribution of thermally activated plastic events in a flowing glass

The potential energy landscape of a flowing metallic glass is revealed using the activation-relaxation technique. For a two-dimensional Lennard-Jones system initially deformed into a steady-state condition through quasistatic shear, the distribution of activation energies is shown to contain a large fraction of low-energy barriers, consistent with a highly nonequilibrium flow state. The distribution of plastic strains has a fundamentally different shape than that obtained during quasistatic simulations, exhibiting a peak at finite strain and, after elastic unloading, a nonzero mean plastic strain that evidences a polarization of the flow state. No significant correlation is found between the activation energy of a plastic event and its associated plastic strain.

Identification and characterization of potential shear transformation zones in metallic glasses

The stability of local atomic configurations in a Ni-Zr metallic glass is studied by molecular dynamics. It is shown that individual atom displacements induce irreversible atomic rearrangements under different glass relaxation, temperature, and strain conditions. The number of regions with an unstable topology depends on the glass relaxation degree. Their time evolution is governed by thermal activation, the activation energy decreasing with elastic strain. It is also shown that unstable regions are located in correspondence of shear transformation zones operating under plastic deformation.

Small isotope effect of diffusion in disordered structures

The isotope effect E of a single jump vacancy diffusion mechanism in statically disordered lattices is investigated by Monte Carlo simulation. It is found that E decreases significantly with increasing disorder. This effect is attributed to percolation processes and ensuing reduction of the effective dimension of space for the diffusing particle.

Simulation of plastic deformation in a two-dimensional atomic glass by molecular dynamics IV

Plastic deformation in a structurally well-relaxed two-dimensional atomic glass was simulated by a computer molecular dynamics approach. The simulation, which was carried through yielding and to substantial plastic strains, demonstrated that the principal mechanism of plastic strain production is by local partly dilatant shear transformations nucleated preferentially in the boundaries of liquid-like material separating the small quasi-ordered domains that form when the glass is well relaxed. Under imposed forward shear-strain increments, local shear transformations in atomic clusters were found to be mostly in the same direction as the applied stress. There were, however, substantial levels of shear transformations in other random directions, including many opposed to the applied stress. In all instances, however, nucleaton of shear transformations reduced the Gibbs free energy monotonically, which is governed largely by the locked-in excess enthalpies of the glassy state. At shear strains above 15%, localization of shear into bands was observed to begin. This steadily intensified and formed well-defined sharp shear bands into which all the shear strain became concentrated by the end of the simulation at a strain of 27%. A strong correlation was found between the tendency for shear localization and retained shear-induced dilatation.

Topological features of structural relaxations in a two-dimensional model atomic glass II

The topological features of atom motions in a high-temperature melt, a sub-cooled melt above T g , and a glass below T g , were analysed in detail by means of a two-dimensional molecular dynamics simulation. A striking analogy was observed between the structure and properties of the liquid-like material separating quasi-ordered domains of atom clusters, and high-angle grain boundaries. The main feature of the structural relaxation below the melting point, both above and below T g was the gradual dissolution and disappearance of the liquid-like material, permitting increasing order in the previously quasi-ordered domains and a growth in their sizes. In these processes, many sequences reminiscent of cancellation of dislocation pairs, or mutual reactions to give more stable sets, were observed.