In-situ and ex-situ characterization of ion-irradiation effects on third generation SiC fibers
Résumé
The objective of the present work is to study the irradiation effects on third generation SiC fibers. There are two SiC fibers which fulfill the minimum requisites for nuclear applications Hi-Nicalon type S manufactured by Nippon Carbon (Japan), hereafter HNS, and Tyranno SA3 manufactured by Ube Industry (Japan), hereafter TSA3. With this purpose, several irradiations with different ions have been carried out. In order to simulate the neutron induced damage, samples have been irradiated with 4 MeV Au3+, which interacts with SiC by creating large cascades of displaced atoms. Also, in order to simulate the interaction with fission products with different energy loss domains, samples have been irradiated with 12 MeV C4+ and 92 MeV Xe23+. These ions are characterized by a projected range in SiC greater than the fibers diameters. Thus, their energy loss will be mainly due to inelastic interactions. Two crucial aspects in their use as reinforcement for ceramic composites for nuclear applications are investigated their microstructural stability and their mechanical behavior under irradiation. First, the ex-situ characterization of the microstructural evolution of HNS and TSA3 under ion-irradiation to simulate the neutron damaging process is devoted to the study of the ion-amorphization of these fibers in terms of micro-Raman spectroscopy and transmission electron microscopy (TEM). It is reported that there is no-significant difference in the ion-amorphization threshold conditionsdose and temperatureof these fibers as compared to the model material, i.e. 6H-SiC single crystals. Moreover, in-situ and ex-situ scanning and transmission electron microscopy have been used to characterize the thermal annealing effects on the ion-amorphized samples. It is reported that thermal annealing at high temperatures not only induces the recrystallization of the ion-amorphized samples thus recovering its physic-chemical properties but also causes unrecoverable mechanical failurecracking and exfoliation. Regarding the mechanical irradiation behavior, an innovative approach has been used to characterize the creep behavior of TSA3 fibers under in-core-like conditions. This subject is of special concern for composite design for nuclear applications as excessive fiber creep would be an in-pile lifetime limiting condition. It is reported that ion-irradiation in dominant electronic energy loss regime induces a time-dependent strain under loads where thermal creep is negligible. This phenomenon, so called irradiation enhanced creep, is characterized by a linear correlation between the ion flux and the strain rate and square root dependence with the applied load.Finally, the above commented work has opened new questions about the ion irradiation behavior of SiC fibers which need to be answered with open perspectives and future work.