Spallation Characteristics of Single Crystal Aluminum with Copper Nanoparticles Based on Atomistic Simulations.

In this study, the effects of Cu nanoparticle inclusion on the dynamic responses of single crystal Al during shockwave loading and subsequent spallation processes have been explored by molecular dynamics simulations. At specific impact velocities, the ideal single crystal Al will not produce dislocation and stacking fault structure during shock compression, while Cu inclusion in an Al-Cu nanocomposite will lead to the formation of a regular stacking fault structure. The significant difference of a shock-induced microstructure makes the spall strength of the Al-Cu nanocomposite lower than that of ideal single crystal Al at these specific impact velocities. The analysis of the damage evolution process shows that when piston velocity up ≤ 2.0 km/s, due to the dense defects and high potential energy at the interface between inclusions and matrix, voids will nucleate preferentially at the inclusion interface, and then grow along the interface at a rate of five times faster than other voids in the Al matrix. When up ≥ 2.5 km/s, the Al matrix will shock melt or unloading melt, and micro-spallation occurs; Cu inclusions have no effect on spallation strength, but when Cu inclusions and the Al matrix are not fully diffused, the voids tend to grow and coalescence along the inclusion interface to form a large void.

Resolving dynamic fragmentation of liquids at the nanoscale with ultrafast small-angle X-ray scattering.

High-brightness coherent ultrashort X-ray free-electron lasers (XFELs) are promising in resolving nanoscale structures at the highest temporal resolution (∼10 fs). The feasibility is explored of resolving ultrafast fragmentation of liquids at the nanoscale with single-shot small-angle X-ray scattering (SAXS) on the basis of large-scale molecular dynamics simulations. Fragmentation of liquid sheets under adiabatic expansion is investigated. From the simulated SAXS patterns, particle-volume size distributions are obtained with the regularization method and average particle sizes with the weighted Guinier method, at different expansion rates. The particle sizes obtained from simulated SAXS are in excellent agreement with direct cluster analysis. Pulse-width effects on SAXS measurements are examined. The results demonstrate the feasibility of resolving the nanoscale dynamics of fragmentation and similar processes with SAXS, and provide guidance for future XFEL experiments and data interpretation.

Hcp/fcc nucleation in bcc iron under different anisotropic compressions at high strain rate: Molecular dynamics study.

Previous researches have revealed the importance of shear and the orientation dependence in the structural transition of iron. In this work, we introduce a series of shear deformations by adjusting the strain ratio between the longitudinal ([001]) and transversal ([010] and [100]) directions, and then investigate this structural transition under different anisotropic compressions with molecular dynamics simulations. It is found that the shear deformation can lower the transition pressure notably, and even change the nucleation structure and morphology. Under 1D-dominated compression (along (001) direction), there only appears hcp nucleation with a few fcc stacking faults. For other cases, more equivalent planes will be activated and fcc structure begins to nucleate. Under 2D-dominated compression (along (010) and (001) directions), the fcc mass fraction is already over the hcp phase. At last, we compare the variations of shear stress and potential energy for different phases, and present the sliding mechanism under typical anisotropic compressions.

Effects of temperature and void on the dynamics and microstructure of structural transition in single crystal iron.

With classic molecular dynamics simulations, we investigate the effects of temperature and void on the bcc to hcp/fcc structural transition in single crystal iron driven by 1D ([0 0 1]) and 3D (uniform) compressions. The results show that the pressure threshold does not reduce monotonously with temperature. The pressure threshold firstly increases and then decreases in the range of 60-360 K under 1D compression, while the variation trend is just opposite under 3D compression. As expected, the initial defect may lower the pressure threshold via heterogenous nucleation. This effect is found to be more distinct at lower temperature, and the heterogenous nucleation mainly results in hcp structure. Under the condition of strain constraint, the products of structural transition will respectively form flaky hcp twin structure ((1 0 0) or (0 1 0)) and lamellar structure ({1 1 0}) of mixed phases under 1D and 3D compressions. During the structural transition, we find the shear stress (1D compression) of hcp phase is always lower than that of bcc phase. The cold energy calculations indicate that the hcp phase is the most stable under high pressure. However, we observe the evident metastable state of bcc phase, whose energy will be much higher than both hcp and fcc phases, and then provides the possibility for the occurrence of fcc nucleation.