Atomistic Simulation Study of Short Pulse Laser-Induced Generation of Crystal Defects in Metal Targets

Abdul Karim, Eaman, Engineering Physics - School of Engineering and Applied Science, University of Virginia
Zhigilei, Leonid, Department of Materials Science and Engineering, University of Virginia


Modification of surface structure of metals by short pulse laser irradiation is increasingly used for tailoring surface properties for the needs of practical applications. The rapid energy deposition and shallow heat affected zones in the irradiated metal targets minimize the residual damage and enable micro/nanostructuring of surfaces with good quality and reproducibility. The limited understanding of the underlying physics and interrelations between the processes responsible for laser-induced structural transformations, however, slows down the design of new laser driven techniques for advanced material processing. Under conditions when the fast rates of the laser-induced processes and the small size of laser-modified regions present a challenge for experimental characterization, computational modeling is playing an increasingly important role in the development of the theoretical understanding of the laser-materials interactions and the advancement of laser applications.
In this computational study, the microscopic mechanisms and kinetics of fast laser-induced phase transformations in metal targets irradiated by short laser pulses are investigated in large-scale atomistic computer simulations. The computational model used in the simulations combines the classical molecular dynamics method with a continuum description of the laser excitation of conduction band electrons, electron-phonon equilibration, and electron heat conduction in the irradiated metal targets. The characteristics of short pulse laser interaction with metal targets in vacuum and under conditions of spatial confinement by a solid transparent overlayer are investigated in a series of atomistic simulations. Three distinct regimes of the material response to the laser energy deposition are identified in the simulations, namely, the regime of laser-induced melting and solidification of the surface region of the target, the regime of photomechanical spallation, and the so-called “phase explosion regime” characterized by an explosive disintegration of a superheated surface region into a vapor and small liquid droplets. The confinement by a transparent overlayer has strong effect on the laser induced processes, including the decrease in the maximum depth of melting and suppression of the spallation and phase explosion.
The computational investigation of the laser-induced generation of crystal defects reveals the formation of a high density of point defects (mostly vacancies and some interstitials) in the surface regions of the metal targets. The generation of a high density of vacancies have important implications on the physical, chemical, and mechanical properties of the surface layers. Computational analysis reveals a strong correlation between the solidification front velocity and the concentration of vacancies, which in turn suggests that the vacancies are mainly generated at the rapidly advancing solidification front under conditions of strong undercooling below the melting temperature of the target material.
The laser-induced generation of dislocations is also observed in the simulations of bcc Cr targets and is found to have a strong dependence on the crystallographic orientation of the target surface. For (001) Cr targets, a transient appearance of small loops of unstable partial dislocations outlining islands of stacking faults takes place at the initial stage of the relaxation of laser-induced stresses. For (110) and (111) Cr targets, the emission of a high density of perfect dislocations from the melting front is observed a few picoseconds after the laser pulse and is related to the high values of the resolved shear stress generated in many of the 48 slip systems of the bcc structure.
The computational predictions from this study have implications for optimization of the experimental conditions in current and emerging applications based on short pulse laser-induced surface modification.

PHD (Doctor of Philosophy)
crystal defects, femtosecond laser ablation, laser melting, laser spallation, molecular dynamics, two temperature model
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