INVESTIGATION INTO THE MICROSTRUCTURAL CHANGES IN ALLOY X-750 SPACER MATERIAL DURING Kr~(2+)/He~+ DUAL BEAM IRRADIATION USING IN-SITU TEM OBSERVATION

摘要

Alloy X-750, an aged hardened nickel based super alloy, is used in the cores of water cooled reactors. In CANada Deuterium Uranium (CANDU) reactors X-750 is used for fuel channel spacers that maintain a gap between fuel channel pressure tubes and the surrounding calandria tubes. Ex-service CANDU spacers have been found to be embrittled. The change in properties is attributed to the neutron irradiation which leads to lattice defects and cavities in the structure of the alloy X-750. This study deals with the simulation of in-service, irradiation-induced microstructural changes in alloy X-750 employing the dual beam irradiation method with 1 MeV Kr~(2+) ions and 10 KeV He~+ ions, in the temperature range of (200-500 °C). The results of the simulation study indicate that various types of defects are created in the microstructure of X-750 spacer material, including cavities, stacking fault tetrahedra and Frank loops. Additionally, the simultaneous He implantation leads to the formation of cavities at all irradiation temperatures. Moreover, increases in doses level at constant irradiation temperature results in significant growth of cavities. In addition, cavity density decreases, and the cavities size increases, as the irradiation temperature increases from 200 to 500 °C. Weak-beam TEM micrographs showed very small (2-5 nm) SFTs as well as large (12-20 nm) Frank loops generated during irradiation. Furthermore, the γ'-Ni_3(Al,Ti) phase becomes disordered at irradiation temperatures below 400 °C and at irradiation doses above 1.3 dpa but remains ordered at higher temperatures and at lower dpa. The results of current study can be extremely helpful to understand and predict the mechanical properties of X-750 spacer material as a function of irradiation temperature and dose.