Ultrahigh X-ray imaging and diffraction operando studies of ultrasound processing of Al alloys and graphites

This thesis presents a comprehensive investigation into the ultrasound-assisted processing of Al alloys and graphite materials using ultrafast synchrotron X-ray imaging and megahertz (MHz) X-ray free electron laser (XFEL) microscopy techniques, with emphasis on understanding the multiscale and multiphysics mechanisms driving multiphase evolution of Al alloy in solidification and the layer exfoliation of graphite materials. The unifying theme is the exploration of how ultrasonically generated physical phenomena, i.e., cavitation, shockwaves, and acoustic streaming, govern microstructural evolution at multiple scales. This comparative approach provides a comprehensive understanding of ultrasound-matter interactions, revealing universal principles that can be applied to optimize processing parameters for a wide range of materials. The key findings are summarized as follows:(1) In-situ synchrotron X-ray tomography and diffraction revealed, for the first time, the nucleation and co-growth dynamics of multiple Fe-rich intermetallics and their dynamic interactions with the Al dendrites of the recycled Al-5Cu-1.5Fe-1Si alloy in solidification under ultrasound melting processing (USMP). USMP significantly refined the α-Al dendrites, modified the morphologies of α-Fe and β-Fe phases, and altered their spatial distributions, thereby enhancing structural homogeneity and potentially improving mechanical properties. (2) By taking the full advantage of MHz XFEL microscopy, we have imaged and quantified, at ns time scale and μm length scale simultaneously, the local shockwaves produced by the implosion of a single cavitation bubble, multiple bubbles and bubble clouds, as well as the layer exfoliation dynamics under such shockwave impacts. The results confirmed that exfoliation is governed not by a single implosion event, but by repeated cyclic forces induced by shockwave impact, with exfoliation behaviour strongly dependent on local defects and graphite structure. (3) Using quasi-simultaneous synchrotron X-ray tomography and diffraction, we have studied in-situ the complicated peritectic reaction mechanisms involving Al4Mn and Al6Mn phases in an Al-8Mn alloy during the solidification process. The real-time evolution of the spatial and orientation relationships between these intermetallics was quantified for the first time, revealing the highly anisotropic faceted growth and intricate coalescence patterns that deviated from the equilibrium solidification models.