Under extreme conditions (e.g. changes in temperatures and pressures) phase transformations can proceed along various different transformation routes that are not accessible under equilibrium conditions, involving non-equilibrium processes, and can therefore result in various unusual and novel microstructures. The investigation of the transformation path for specific thermodynamic and kinetic conditions and understanding of the resulting microstructural evolution is both scientifically intriguing and technologically important, as many materials properties are strongly dependent on the scale and morphology of the microstructure of a material. Pulsed-laser-irradiation of thin films is a relatively novel processing route that induces melting and rapid re-solidification under extreme conditions of rapid heating-cooling cycles in thin films on amorphous substrates. To date most studies on laser processing of thin films have focused on rapid solidification of Si and Cu thin films. This thesis reports on the application of pulsed-laser processing and rapid solidification of technologically important Al thin films. It is demonstrated that pulsed-laser melting and rapid re-solidification is feasible for Al thin films in various configurations. Rapid solidification after pulsed-laser melting has been accomplished for Al films on bulk Si supported amorphous underlayers, with and without a thin synthetic amorphous capping layer, and also on free-standing and electron-transparent (no bulk Si support) Al films on thin amorphous underlayers, with and without thin synthetic amorphous capping layers. This dissertation reports only on a comparison of the microstructures resulting after laser processing of free-standing and electron-transparent (no bulk Si support) Al films on thin amorphous underlayer without thin synthetic amorphous capping layers. Excimer-laser-induced melting and rapid solidification of such Al thin films have been carried out in air. The resulting microstructure has been investigated post-mortem using transmission electron microscopy (TEM). The thin film microstructure resulting from re-solidification after pulsed-laser melting consisted predominantly of directionally solidified columnar grains, approximately 80nm thick (full film thickness), 0.5 micrometer wide and up to 5 micrometer long. Additionally, laser induced melting and re-solidification experiments were carried out using the dynamic transmission electron microscope (DTEM) at Lawrence Livermore National Laboratory, which enabled in-situ observations of the transient phenomena associated with the liquid-solid transformation with its unprecedented 15ns temporal and nano-meter (nm) spatial resolution. The in-situ DTEM experiments were used to study details of the morphology and dynamics at the liquid-solid interface, revealing a planar liquid-solid interface that moves at an average velocity of ~ 3m/s. Ex-situ post-mortem TEM investigations of samples that were laser-processed during the in-situ DTEM experiments showed that microstructures were similar in morphology for the in-situ and the ex-situ rapid solidification experiments
