We investigate picosecond laser ablation of metals using a hybrid simulation scheme. Laser energy input into the electron system and heat conduction within it are modeled using a finite-difference scheme for solving the heat conduction equation. Atom motion in the near-surface part (72 nm) of the sample is modeled using molecular dynamics. Energy transfer between the electronic and atomic subsystems due to electron-phonon coupling is taken into account. For the special case of 0.5 ps UV laser irradiation of copper, we investigate the fluence dependence of the ablation yield, the temperature and pressure evolution in the target, and the ablation mechanism.

Using molecular-dynamics simulation we simulate nanoindentation into the three principal surfaces—the (100), (110), and (111) surface—of Cu and Al. In the elastic regime, the simulation data agree fairly well with the linear elastic theory of indentation into an elastically anisotropic substrate. With increasing indentation depth, the effect of pressure hardening becomes visible. When the critical stress for dislocation nucleation is reached, even the elastically isotropic Al shows a strong dependence of the force-displacement curves on the surface orientation. After the load drop, when plasticity has set in, the influence of the surface orientation is lost, and the contact pressure (hardness) becomes independent of the surface orientation.

Iron undergoes a bcc to close-packed structural phase transition under pressure, at around 13 GPa, as shown by diamond anvil and shock experiments. Atomistic simulations have been able to provide insights into the transition, but without any plasticity occurring before the phase change, in single crystals, defective single crystals, or polycrystals. However, experiments in polycrystals do show clear evidence for plasticity. Here we study homogeneous uniaxial compression of polycrystalline Fe using several interatomic potentials: three embedded-atom-model potentials and one modified embedded-atom-model potential. We analyze grain-boundary rotation and dislocation activity, and find that the amount of dislocation activity as a function of strain depends greatly on the potential used. This variation can be explained in terms of the dislocation properties, calculated in this work for each of these potentials.

Nanograined metals have the merit of high strength, but usually suffer from low work hardening capacity and poor thermal stability, causing premature failure and limiting their practical utilities. Here we report a “nanodispersion-in-nanograins” strategy to simultaneously strengthen and stabilize nanocrystalline metals such as copper and nickel. Our strategy relies on a uniform dispersion of extremely fine sized carbon nanoparticles (2.6 ± 1.2 nm) inside nanograins. The intragranular dispersion of nanoparticles not only elevates the strength of already-strong nanograins by 35%, but also activates multiple hardening mechanisms via dislocation-nanoparticle interactions, leading to improved work hardening and large tensile ductility. In addition, these finely dispersed nanoparticles result in substantially enhanced thermal stability and electrical conductivity in metal nanocomposites. Our results demonstrate the concurrent improvement of several mutually exclusive properties in metals including strength-ductility, strength-thermal stability, and strength-electrical conductivity, and thus represent a promising route to engineering high-performance nanostructured materials.

Ultrafast laser irradiation of solids may ablate material off the surface. We study this process for thin films using molecular-dynamics simulation and thermodynamic analysis. Both metals and Lennard-Jones (LJ) materials are studied. We find that despite the large difference in thermodynamical properties between these two classes of materials—e.g., for aluminum versus LJ the ratio $T_c/T_{\text{tr}}$ of critical to triple-point temperature differs by more than a factor of 4—the values of the ablation threshold energy $E_{\text{abl}}$ normalized to the cohesion energy, ${ϵ}_{\text{abl}}=E_{\text{abl}}/E_{\text{coh}}$, are surprisingly universal: all are near 0.3 with $\pm 30\%$ scattering. The difference in the ratio $T_c/T_{\text{tr}}$ means that for metals the melting threshold ${ϵ}_m$ is low, ${ϵ}_m<{ϵ}_{\text{abl}}$, while for LJ it is high, ${ϵ}_m>{ϵ}_{\text{abl}}$. This thermodynamical consideration gives a simple explanation for the difference between metals and LJ. It explains why despite the universality in ${ϵ}_{\text{abl}}$, metals thermomechanically ablate always from the liquid state. This is opposite to LJ materials, which (near threshold) ablate from the solid state. Furthermore, we find that immediately below the ablation threshold, the formation of large voids (cavitation) in the irradiated material leads to a strong temporary expansion on a very slow time scale. This feature is easily distinguished from the acoustic oscillations governing the material response at smaller intensities, on the one hand, and the ablation occurring at larger intensities, on the other hand. This finding allows us to explain the puzzle of huge surface excursions found in experiments at near-threshold laser irradiation.