Microstructures and properties of materials under repeated laser irradiation

This research program has explored the stability of alloys under pulsed laser irradiation. Two primary directions were investigated: (i) phase transitions during a single laser pulse, and (ii) phase stability under repeated laser irradiation. The first theme was primarily concerned with both the crystalline to amorphous phase transition and the transition of liquids and glasses to crystalline matter. The second project examined the phase evolution during laser pulsing in situations where plastic deformation was prevalent (high-energy laser pulses). Both computer simulation and experimental programs were undertaken. Our work using computer simulations had several notable successes. For the work connected with multiple pulsing, we used molecular dynamics (MD) to simulate the behavior of alloys under severe plastic deformation. We found that during high strain-rate deformation atomic mixing of chemical species is random, independent of the detailed thermochemical properties of the system. This result contrasts with recent reports. In this work, we also developed two new methods of analyzing atomic mixing, one is based on relative mean square displacements (RMSD) of atoms and the other, Burgers vector analysis (BVA), on nearest neighbor displacements. The RMSD analysis is valuable in that it specifies the length scales over which deformation processes take place, and we applied it to understand deformation in nanocrystalline, amorphous and large-grained systems. The BVA analysis, on the other hand, reveals if the damage is homogeneous. Finally we showed that at elevated temperatures, the phase stability is not determined from a simple competition between shearing events and vacancy diffusion, which has long been assumed, but rather atomic mixing in the shearing events is temperature dependent. This work is significant in that it elucidates the fundamental mechanisms that underlie high strain rate deformation, and it provides computational tools for other researchers to perform related work. Our work on MD simulation also examined shock-induced spall. We showed that as voids develop in the early stage of spall, an amorphous region forms around them and that voids coalescence via mass transfer through this amorphous medium. Finally the MD work began examining solidification at deep undercoolings; we found the surprising result that solidification was limited by defects in the liquid and that these defects have virtually identical properties as interstitialcy atoms in the crystalline state. This work is being continued in the renewal project since it offers new insight into the melting and solidification processes under extreme conditions. The experimental work began looking at plastic deformation and liquid-state diffusion using nanosecond laser pulses, but taking advantage of an NSF equipment award, we switched to study solidification at deep undercooling using a femtosecond laser system. We have developed in this program the first application of non-linear reflectivity using third harmonic generation (THG) of light to monitor the solid-liquid/liquid-solid (or any phase transition with 10 fs time resolution). THG decreases by three orders of magnitude on melting whereas the change in linear reflectivity in metals is only 5%. We have also showed that THG can be used as an ultrafast (sub picosecond) thermometer for systems such as Si