Contribution Oral Presentation
In situ X-ray imaging investigation of solidification of high melting temperature materials: Silicon and superalloys
All growth and casting processes, used for example in the superalloy and photovoltaic industrial sectors, face challenges linked to the grain structure and crystalline defects left during the solidification step when aiming at improving the final desired properties. The post-mortem studies of the solidified ingots provide limited information on the mechanisms occurring during solidification, on their symbiosis or competition and on their kinetics which makes it difficult to control. As a consequence, it is essential to improve the understanding of the mechanisms of the formation of the final crystalline grain structure, of segregation and of the density of structural defects, namely dislocations.
Within this context, our contribution consists in characterizing the fundamental growth mechanisms of high melting temperature (up to 1800°C) materials using in situ X-ray imaging in a unique device named GaTSBI (Growth at high Temperature observed by X-ray Synchrotron Beam Imaging). Two imaging characterisation techniques are combined during solidification using X-ray synchrotron radiation at the ESRF (European Synchrotron Radiation Facility, Grenoble, France): X-ray radiography and X-ray Bragg diffraction. On the one hand, the X-ray radiography method brings information on the morphology and kinetics of the solid/liquid (S/L) interface as well as on solute segregation. On the other hand, the X-ray Bragg diffraction gives additional information about the evolution of the grain shape and structure, misorientation, defect formation and the local level of crystal lattice distortion during growth.
During the presentation, results concerning the solidification of Ni-Al alloys and silicon for PV applications will be presented. These experiments demonstrate the wide range of fundamental growth mechanisms that can be addressed and better understood with X-ray imaging applied during solidification. In metallic alloys, the formation of a dendritic network is studied as well as its interaction with convective flow. As concerns silicon for PV applications, essential features of twinning, grain competition and the origin and interaction of dislocations with grain boundaries in crystalline silicon are characterized.