Molecular dynamics studies of the roles of microstructure and thermal effects in spallation of aluminum

• Two stages plasticity is observed in shock front structure of the nanocrystalline Al sample.
• The thermal dissipation rate in void nucleation stage is higher than other stages.
• A mutual promotion mechanism between melting and cavitation process is discovered.
• Competition effect of strain rate hardening and temeprature softening on spall strangth is found.

Spallation behaviors of nanocrystalline aluminum under shock loading are studied by nonequilibrium molecular dynamics simulations. Simulations on single-crystal aluminum are conducted for comparison. Both classical spallation and micro-spallation are studied. Our simulations show that the shock front structure of the nanocrystalline aluminum sample apparently displays two stages plasticity –– the grain boundary mediated stage and the dislocation mediated stage. The spallation mechanism is dominated by cavitation, i.e., nucleation, growth, and coalescence of voids. It is found that void nucleation mechanism is different in single-crystal aluminum and nanocrystalline aluminum. Void nucleation is induced by dislocation activities in single-crystal aluminum, while is induced by mechanical separation and sliding of grain boundaries in nanocrystalline aluminum. Thermal dissipation during cavitation is studied, and the mutual promotion between cavitation and melting is discovered. Thermal dissipation during cavitation leads to temperature arising in the vicinity of voids and promotes melting around voids. On the other hand, melting of materials leads to dropping of spall strength and thus facilitates the cavitation process. Some quantitative discussions on spall strength and thermal dissipation rate are proposed. The spall strength of single-crystal aluminum first increases and then decreases as shock intensity increases, which is attributed to the competition mechanism between strain rate hardening and temperature softening. Grain boundary effect on spall strength is more remarkable in cases of lower shock intensity. The thermal dissipation rate in void nucleation stage is much larger than in void growth–coalescence stage.

Nucleation, growth, and coalescence of voids are accompanied by thermal dissipation. Temperature arises in the vicinity of voids and promotes melting around voids.In single-crystal Al, the relation between spall strength and shock intensity is found to be determined by the competition mechanism between strain rate hardening and temperature softening. The spall strength of nanocrystalline Al is less sensitive to the shock intensity than that of single-crystalline Al in classical spallation.Figure optionsDownload as PowerPoint slide