Crystals at High Deformation Rates Displaying Glassy Behavior

Abstract

When investigated as a function of deformation rate and time, model nano‐crystals exhibit a clear threshold in mechanical and relaxation behavior, this observation being valid in both the Hooke (elastic) and plastic regime. At low deformation rate (ϵ˙<truetrueϵ˙c≃5×105 normals−1), materials are able to equilibrate with ongoing deformation, whereas ultra‐fast deformations bring the system out of equilibrium on the same nanosecond timescale. The time evolution of stress is investigated and suggests features that are typical of glassy relaxation with a stretched exponential behavior for stress relaxation that is only encountered for large deformation rates. Related relaxation times furthermore reveal a rapid increase with trueϵ˙. These results actually link general aspects of plastic deformation, irreversibility and thermal equilibrium.

Understanding the strain rate sensitivity of nanocrystalline copper using molecular dynamics simulations

“…The Young's modulus depends on the applied strain rate. This indicates that the system is out of equilibrium during deformation especially at high strain rate [55]. The response of the macroscopic stress with respect to the increase of the applied strain depends strongly on the mobility of dislocations in the materials [56].…”

“…Schiøtz et al [29] have also concluded that to completely relax the systems, strain rates must be far below what is normally used in molecular dynamics ε < 10 7 s −1 ). The stress relaxation behavior of these models has also been investigated [55]. The results show that the stress holds constant (σ(t) = σ inf ) when a stress relaxation test is applied on configurations deformed at strain rate below εc .…”