In situ TEM and Continuum Modeling of Laser-induced Rapid Solidification of Aluminum and Aluminum Copper Alloys
In situ characterization of rapid solidification processes has proven too challenging for conventional characterization techniques as they fail to meet the spatio-temporal resolution requirements for observing the rapid transient processes. Recent advances in ultra-fast time-resolved in situ transmission electron microscopy enabled characterization of pulsed laser induced melting and rapid solidification processes in Al and Al – Cu alloys with unprecedented spatio-temporal resolution using the unique Dynamic Transmission Electron Microscope (DTEM). The DTEM achieves nano-scale spatio-temporal resolution by modifying a conventional TEM with two laser systems – the image formation laser system and process initiation laser system, for observing rapid solidification process in a thin film geometry. 
 
In this study, in-situ DTEM experimentation has been utilized to document rapid solidification processes in Al and Al – Cu alloy thin films, enabling accurate determination of average solidification velocity evolution and associated crystal growth mode changes during rapid solidification process in pure aluminum and hypo-eutectic and hyper-eutectic Al – Cu alloys. Enthalpy transport based computer modeling has been performed and benchmarked by experimental metrics obtained from in situ DTEM experimentation to calculate the spatio-temporal thermal field evolution during the rapid solidification process in Al. This demonstrated the unique capability of in situ DTEM experimentation to deliver quantitative metrics from direct observation with nano-scale spatio-temporal resolution for the validation of computer modeling. Post-mortem characterization provided detailed insights on microstructural evolution during rapid solidification process by establishing the correlation between solidification conditions and resultant microstructural constitution. The respective influence of heat transfer, crystallography and Cu concentration on the details of the dynamics of the rapid solidification process in hypo-eutectic and hyper-eutectic Al – Cu alloys were examined and quantified. The investigation showed that rapidly solidified microstructures in pulsed laser irradiated TEM transparent Al – Cu thin films exhibit equivalent microstructural features developed in bulk alloy samples after laser surface melting. DTEM experimentation uniquely allowed direct observation of rapid solidification processes in Al and Al-Cu alloys, and facilitated high precision determination of process metrics such as critical velocities for crystal growth mode changes, which are important to improved understanding of alloy microstructure evolution under the driven, far-from-equilibrium conditions pertaining to rapid solidification.