152 research outputs found
Thermal-activation model for freezing and the elastic robustness of bulk metallic glasses
Despite significant atomic-scale heterogeneity, bulk metallic glasses well below their glass transition temperature exhibit a surprisingly robust elastic regime and a sharp elastic-to-plastic transition. Here it is shown that, when the number of available structural transformations scales exponentially with system size, a simple thermal-activation model is able to describe these features, where yield corresponds to a change from a barrier energy dominated to a barrier entropy dominated regime of shear transformation activity, allowing the system to macroscopically exit its frozen state. A yield criterion is then developed, which describes well the existing experimental data and motivates future dedicated deformation experiments to validate the model
Local structural excitations in model glasses
Structural excitations of model Lennard-Jones glass systems are investigated
using the Activation-Relaxation-Technique (ART), which explores the potential
energy landscape of a local minimum energy configuration by converging to a
nearby saddle-point configuration. Performing ART results in a distribution of
barrier energies that is single-peaked for well relaxed samples. The present
work characterises such atomic scale excitations in terms of their local
structure and environment. It is found that, at zero applied stress, many of
the identified events consist of chain-like excitations that can either be
extended or ring-like in their geometry. The location and activation energy of
these saddle-point structures are found to correlate with the type of atom
involved, and with spatial regions that have low shear moduli and are close to
the excess free volume within the configuration. Such correlations are however
weak and more generally the identified local structural excitations are seen to
exist throughout the model glass sample. The work concludes with a discussion
within the framework of and relaxation processes that are
known to occur in the under-cooled liquid regime.Comment: 34 Pages, 13 Figure
Searching for activated transitions in complex magnetic systems
The process of finding activated transitions in localized spin systems with
continuous degrees of freedom is developed based on a magnetic variant of the
Activation-Relaxation Technique (mART). In addition to the description of the
method and the relevant local properties of the magnetic energy landscape, a
criterion to efficiently recognize failed attempts and an expression for the
step magnitude to control the convergence are proposed irrespective of the
physical system under study. The present implementation is validated on two
translational symmetric systems with isotropic exchange interactions. Then, in
one example, diffusion processes of a skyrmion vacancy and a skyrmion
interstitial are revealed for a skyrmion system on a square spin lattice. In
another example, the set of activation events about a metastable state of a 2D
dipolar spin glass is investigated and the corresponding energy barrier
distribution is found. Detailed inspection of the transition states reveals the
participation of nearest neighbour pairs affording a simplified analytical
understanding
Shear-band arrest and stress overshoots during inhomogeneous flow in a metallic glass
At the transition from a static to a dynamic deformation regime of a shear band in bulk metallic glasses, stress transients in terms of overshoots are observed. We interpret this phenomenon with a repeated shear-melting transition and are able to access a characteristic time for a liquidlike to solidlike transition in the shear band as a function of temperature, enabling us to understand why shear bands arrest during inhomogenous serrated flow in bulk metallic glasses
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