High-power, short-duration, laser-driven, shock compression and recovery experiments on [001] silicon unveiled remarkable structural changes above a pressure threshold. Two distinct amorphous regions were identified: (a) a bulk amorphous layer close to the surface and (b) amorphous bands initially aligned with {111} slip planes. Further increase of the laser energy leads to the re-crystallization of amorphous silicon into nanocrystals with high concentration of nano-twins. This amorphization is produced by the combined effect of high magnitude hydrostatic and shear stresses under dynamic shock compression. Shock-induced defects play a very important role in the onset of amorphization. Calculations of the free energy changes with pressure and shear, using the Patel-Cohen methodology, are in agreement with the experimental results. Molecular dynamics simulation corroborates the amorphization, showing that it is initiated by the nucleation and propagation of partial dislocations. The nucleation of amorphization is analyzed qualitatively by classical nucleation theory.

Grain size has a profound effect on the mechanical response of metals. Molecular dynamics continues to expand its range from a handful of atoms to grain sizes up to 50 nm, albeit commonly at strain rates generally upwards of 10 6 s -1 . In this review we examine the most important theories of grain size dependent mechanical behavior pertaining to the nanocrystalline regime. For the sake of clarity, grain sizes d are commonly divided into three regimes: d > 1μm, 1 μm < d < 100 nm; and d < 100 nm. These different regimes are dominated by different mechanisms of plastic flow initiation. We focus here in the region d < 100 nm, aptly named the nanocrystalline region. An interesting and representative phenomenon at this reduced spatial scale is the inverse Hall-Petch effect observed experimentally and in MD simulations in FCC, BCC, and HCP metals. Significantly, we compare the results of molecular dynamics simulations with analytical models and mechanisms based on the contributions of Conrad and Narayan and Argon and Yip, who attribute the inverse Hall-Petch relationship to the increased contribution of grain-boundary shear as the grain size is reduced. The occurrence of twinning, more prevalent at the high strain rates enabled by shock compression, is evaluated.2

Strain rate, temperature, and microstructure play a significant role in the mechanical 11 response of materials. Using non-equilibrium molecular dynamics simulations, we characterize 12 the ductile tensile failure of a model body-centered cubic metal, tantalum, over six orders of 13 magnitude in strain rate. Molecular dynamics calculations combined with reported experimental 14 measurements show power-law kinetic relationships that vary as a function of dominant defect 15 mechanism and grain size. The maximum sustained tensile stress, or spall strength, increases with 16 increasing strain rate, before ultimately saturating at ultra-high strain rates, i.e. those approaching 17 or exceeding the Debye frequency. The upper limit of tensile strength can be well estimated by the 18 cohesive energy, or the energy required to separate atoms from one another. At strain rates below 19 the Debye frequency, the spall strength of nanocrystalline Ta is less than single crystalline 20 tantalum. This occurs in part due to the decreased flow stress of the grain boundaries; stress 21 concentrations at grain boundaries that arise due to compatibility requirements; and the growing 22 fraction of grain-boundary atoms as grain size is decreased into the nanocrystalline regime. In the 23 present cases, voids nucleate at defect structures present in the microstructure. The exact makeup 24 and distribution of defects is controlled by the initial microstructure and the plastic deformation 25 during both compression and expansion, where grain boundaries and grain orientation play critical 26 roles. 27 28 Keywords: tensile strength, spall, non-equilibrium molecular dynamics, tantalum 29 30 35 uniform. Concomitantly, the competition between void nucleation, void growth, and wave 36 propagation effects increases the complexity of the process.37

Understanding the effect of grain boundaries (GBs) on the deformation and spall behavior is critical to designing materials with tailored failure responses under dynamic loading. This understanding is hampered by the lack of in situ imaging capability with the optimum spatial and temporal resolution during dynamic experiments, as well as by the scarcity of a systematic data set that correlates boundary structure to failure, especially in BCC metals. To fill in this gap in the current understanding, molecular dynamics simulations are performed on a set of 74 bi-crystals in Ta with a [110] symmetric tilt axis. Our results show a correlation between GB misorientation angle and spall strength and also highlight the importance of GB structure itself in determining the spall strength. Specifically, we find a direct correlation between the ability of the GB to plasticity deform through slip/twinning and its spall strength. Additionally, a change in the deformation mechanism from dislocation-meditated to twinning-dominated plasticity is observed as a function of misorientation angles, which results in lowered spall strengths for high-angle GBs.