Large-scale atomistic simulations of the phase transformations and surface modification in short pulse laser processing of metals

Leonid Zhigilei, University of Virginia

Goergen 101, Sloan Auditorium

Abstract:
The microscopic mechanisms responsible for the material ejection and surface modification in short-pulse laser processing of metal targets are investigated in large-scale massively parallel atomistic simulations. The simulations are performed with a computational model that combines the classical molecular dynamic method with a continuum description of the laser excitation of conduction band electrons, electron-phonon coupling and electron heat conduction. The results of the simulations reveal a complex picture of highly non-equilibrium processes responsible for material modification and/or ejection in response to the fast laser energy deposition. The extreme heating and cooling rates realized in short pulse laser processing are defining the kinetics of the melting and resolidification processes and are responsible for the generation of unusual microstructure of the surface region. The resolidification, in particular, is controlled by the competition between the epitaxial regrowth of the substrate and nucleation of crystallites within the undercooled melted region, leading to the formation of nanocrystalline surface structure with a high density of stacking faults, twins, dislocations and point defects.  At higher laser fluences, in the ablation irradiation regime, the simulations reveal the microscopic mechanisms responsible for the material ejection and enable detailed analysis of the ablation plume (cluster size distributions, velocities and temperatures of different plume components) generated as a result of explosive boiling of the superheated surface region of the irradiated target. The peculiarities of laser ablation of thin films are also investigated for 5 – 20 nm metal films deposited on a silica substrate. Several distinct regimes of the film ablation are revealed in the simulations and connected to the corresponding nanoparticle size distributions. The results of the simulations are related to experimental data and the implications of the computational predictions for practical applications are discussed.

Bio:
Leonid V. Zhigilei was born in Vilnius, Lithuania, studied materials science and metallurgy at the Leningrad Polytechnic Institute (now St. Petersburg State Technical University), Russia, and did his Ph.D. dissertation work on the structure of metallic glasses at Tomsk State University and St. Petersburg State University, Russia (Ph.D. degree 1991). After several years of industrial work in Russia and Lithuania and a postdoctoral work in the Department of Chemistry at the Pennsylvania State University, in 2000 he joined the Department of Materials Science and Engineering at the University of Virginia. His research interests are in the general area of computational materials science, with focus on investigation of dynamic non-equilibrium processes in materials undergoing processing by short laser pulses, microscopic mechanisms of phase transformations, properties of nanostructured and non-crystalline materials. Zhigilei authored more than 100 journal papers that have been cited more than 5000 times (his current Google Scholar h-index is 41). He received the American Society for Mass Spectrometry Research Award in 2002 and the National Science Foundation CAREER award in 2004.