Electron Microscopy of Pulsed Laser Induced Rapid Solidification and Transient Solid State Phenomena in Nano-Scale Metal and Alloy Thin Films

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TECHNICAL SUMMARY: This project is an investigation of rapid solidification processes in pure metal and Al-rich Al(Cu) thin films. The main technique is in-situ transmission electron microscopy (TEM) using the dynamic TEM (DTEM) at Lawrence Livermore National Laboratory (LLNL) for ultrafast electron diffraction (UED) and imaging. There are no other measurement techniques currently capable of observing rapid solidification processes with interfacial velocities ranging from about 0.1 to 100 m/s in metal thin films with the required nano-scale spatial and temporal resolution offered by the DTEM. The research will reveal quantitative dynamic details of the extremely rapid liquid-solid transformation and other transient phenomena (e.g. solid-solid transitions) associated with Al and Al-rich Al(Cu) thin films and other FCC, BCC and HCP metals after single-shot pulsed laser melting. Robust modeling software codes, validated by direct comparison with quantitative measurements from in-situ DTEM experiments on pure metals, will be developed for the pulsed-laser melting. Post-mortem and in-situ microstructural characterization will be used to investigate microstructural evolution and defect formation in the solid state following solidification of pure metals with different crystal structures. The study of Al-rich Al(Cu) thin films is expected to help identify the conditions of morphological destabilization of the growth interface as function of pulsed laser power, alloy composition and heat-extraction parameters for hypo-eutectic, eutectic and hypereutectic compositions between Al and Al2Cu. The data obtained on parameters that determine the stability or instability of the transformation front will enable validation of theoretical solidification models. Apart from delivering basic scientific knowledge on metals under the extreme conditions of laser-induced rapid solidification, state-of-the-art experimental techniques for in-situ TEM will be developed for the purpose of probing the behavior of material volumes at time scales close to those accessible by computational modeling.NON-TECHNICAL SUMMARY: Solidification is a ubiquitous and fundamental process in materials fabrication, especially for metals, which are critical to energy generation and transmission, transportation and information technologies. Under extreme conditions of laser processing the growth dynamics determine the final microstructure and consequently performance-related properties in engineered components and devices. Understanding such phenomena is scientifically interesting and technologically important. The results of this research will be disseminated by peer-reviewed publications and presentations. The project involves outreach to high school students, via the Pennsylvania Junior Academy of Science and the development of semester-long research experiences for students enrolled in the new Pittsburgh Science & Technology Academy (grades 6-12). Students will receive training in vacuum and laser science, physical metallurgy, thin film science, micro-fabrication methods, scattering and diffraction physics, transmission and scanning electron microscopy, crystallography, thermodynamics, transport phenomena and numerical methods for materials modeling. Visits to, and continuous interaction with, LLNL will provide professional preparation outside the academic environment and access to state-of-the-art instrumentation.