A Study of Heavy Ion Irradiation‐Induced Segregation at Grain Boundaries in Alloy 800H

Abstract

Environmentally‐Assisted Cracking (EAC) of structural components in a nuclear reactor core of light water reactors (LWRs) and water‐cooling systems in fusion reactors is of concern in that it can directly influence on the safety of a nuclear reactor. Previous studies showed that one of the many factors that governing the EAC of a structural component is the changes of the chemical composition of the grain boundaries of the material used after exposed to neutron radiation [1]. This phenomenon is also known as irradiation‐assisted stress corrosion cracking (IASCC). The changes of the chemistry of grain boundaries induced by irradiation, i.e. radiation‐induced segregation (RIS), and the increase in irradiation‐induced defects can promote hardening of the material, thereby affecting the resultant mechanical and environment‐sensitive behaviour. With long‐term (~80 years) reactor operation under consideration for power generation, Alloy 800H is one of the candidate alloy systems that may be used in the next generation advance light water nuclear energy system [2]. One of the reasons for this choice is that Alloy 800H is code‐certified for high temperature (up to 760 °C) use in nuclear power systems [3]. Also, the high Ni and Cr concentrations in the alloy provide good resistance to void swelling and corrosion in light water environments [4]. In this study, RIS in ion‐irradiated Alloy 800H was investigated. A commercial grade Alloy 800H was subjected to simultaneous Fe ++ and He ++ ion‐beam irradiation to simulate the effect of neutron irradiation‐induced segregation. The alloy was irradiated to a dose of 16.6 dpa at temperature of 440°C using the facility at University of Michigan Ion Beam Laboratory, Ann Arbour, Michigan. The microstructural characterisation was performed using a spherical aberration‐corrected FEI Titan G2 80‐200 with Super X EDX (ChemiSTEM™) operated at 200kV and equipped with a GIF Quantum 965 EELS to provide independent compositional analyses of the intergranular segregation induced during ion irradiation. Scanning transmission electron microscopy (STEM) – Electron Energy Loss Spectroscopy (EELS) analyses show that the grain boundaries in Alloy 800H are enriched with Ni, Si and Ti after irradiation, as shown in Figure 1. Fe and Cr depletion was also detected at the same grain boundary, as shown in Figures 2 and 3. Apart from that, a denuded zone with no chemical‐related irradiation‐induced defects decorating along the grain boundary was observed. However, diffraction contrast imaging shows that a high number density of the irradiation‐induced dislocation loops was observed along the grain boundary. This observation and its implications for the alloy will be further discussed.