Chemical and topological disordering in M23C6 due to irradiation: An atomic-scale observation

Abstract

To clarify the stability of M23C6 precipitate in steels under irradiation, the chemical and topological disordering behaviors in M23C6 subjected to ion irradiation at elevated temperatures were investigated with the combined application of high-resolution electron microscopy (HREM) and electron energy-loss spectroscopy (EELS). Results show that the duplex structured M23C6 which is the core-crystalline and shell-amorphous was observed in the specimen irradiated at 623 K, nevertheless, no amorphous phase was observed in the specimen irradiated at 673 K. HREM observation confirmed the occurrence of the chemical disordering in irradiated M23C6, and the disordering of W atoms in the M23C6 unit cell was clarified by the peak intensity profile of the atomic column from the HREM image. EELS analyses indicate that no obvious chemical shift was observed by the core-loss spectrum, however, the change in the electron mean free path (λ) due to irradiation was confirmed by the low-loss spectrum, which is related to the formation of both irradiation defects and amorphous phase. It is proposed that the decreased material density upon irradiation is ascribed to the formation of W-vacancy-dominated defect clusters. An obvious change in interatomic distance was not observed in the irradiated M23C6 based on the radical distribution function analysis by extended energy-loss fine structure spectrum. However, the peak intensity decreased by irradiation, confirming the occurrence of chemical and topological disordering, regardless of the carbide being crystalline or amorphous.