This study uses non-equilibrium molecular dynamics simulations to explore the dynamic failures and deformation mechanisms of a cylindrical shell composed of nanocrystalline nickel-titanium alloy under implosion loading. We discover that some individual spall planes are sequentially generated in the material along radial stress wave propagating, indicative of the formation of multiple spallation. For the first spallation, the void-nucleation behavior at larger grain size exhibits an intergranular/transgranular coexisting pattern, while voids tend to nucleate along the grain boundaries with decreasing the grain size. For the secondary spallation, localized shearing zones and grain boundaries at larger grain size play a role on offering potential void-nucleated sites. The formation of shear deformation bands promotes grain refinement, contributing to a deterioration of dislocation-induced strengthening effect. Consequently, a lower spall strength is produced, as opposed to that for the first spallation. Lastly, a notable penetration behavior between two spall planes is observed in the sample with large grain size, which is attributed to the fact that voids nucleate on linking grain boundaries with temperature beyond the melting point.
 

Metallic shells; Multiple Spallation and its penetration; Shear localization; Cylinder implosion loading; Atomistic modeling.

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