Chemical and structural analysis of grain boundaries in Inconel 690 for corrosion resistance

摘要

Stress Corrosion Cracking (SCC) is a failure mechanism that results from the combination of tensile stress, corrosive environment and material susceptibility; it is frequently an intergranular attack. Material-environment combinations for SCC readily exist in nuclear power plants, and are critical for to the longevity of the reactor components. Inconel 690 (alloy 690 UNS N06690) is an alloy that has been put into service in the nuclear industry over the past 20 years due to its relatively good resistance to SCC. A new generation of nuclear plants is likely to be built in the US and the life of existing and new nuclear plants are expected to extend to 60-80 years. The study of alloy 690, as well as other structural metals, is important in order to understand, predict, and avert costly and dangerous failures that could occur due to SCC later in the life of the plants. The microstructure of an alloy has an important effect on its corrosion and SCC behavior. In particular, high energy grain boundary structures in austenitic Ni-base alloys and stainless steels have been shown to have greater SCC susceptibility. This thesis studies the fundamental structural and chemical properties of grain boundaries in alloy 690, to better understand the SCC resistances and susceptibilities of different grain boundary structures. In order to investigate the grain boundaries based on their structure, an integrated approach was developed to allow for site-specific chemical and mechanical characterization.