Synchrotron x-ray–based imaging techniques, such as radiography and tomography, have reached sufficient spatio-temporal resolution that they are now used almost routinely to investigate solidification in technologically important systems such as steel, Ni, Al, and other alloy systems. Studies of this type have been used to image equiaxed and columnar dendrites and to compare their behaviour with theories of crystal shape, coarsening, and morphological stability.
In this talk I will present three examples of how synchrotron X-ray radiography can be used to catch the dynamic of solidification and show why it has become an indispensable tool in the field. In the first study the fragmentation of columnar grains in Al-Cu alloys with and without an applied Pulsed Electro-Magnetic Field (PEMF) during directional solidification was investigated. The fragmentation rate and spatio-temporal distribution of the fragments in the mushy zone was measured under a wide range of solidification conditions and the data analysed and used to develop a comprehensive fragmentation qualitative model. In the second part I will present the results of a large study of equiaxed solidification in which solidifying grain refined Al-Cu foil samples were imaged by synchrotron X-ray radiography and the grain formation rate measured by automatically counting the number of grains appearing in the field of view in each frame using a bespoke machine learning algorithm. The study, which could have not been carried out without automatic analysis due to the very large amount of data, comprised more than 100 experiments covering a wide range of conditions. The resulting thousands of grain formation and growth rate measurements represent the largest data set of this type. Key insights from the results, such as effect of cooling rate on grain formation, as well as the machine learning approach, will be described. In the final example I will show the results of a series of experiments aimed at investigating fluid flow effects on mass transport and formation of defects within the mushy zone.