Nickel-based superalloys are commonly used in many high-temperature applications in various industries, including nuclear, chemical, aerospace, and defense. This project will utilize a novel stress-strain microprobe (SSM) to characterize the condition of welds in one such superalloy, alloy 718, following various thermal treatments which result in the deterioration of their mechanical and fracture properties. Recently, the automated ball indentation (ABI) technique of the SSM system was proven to yield a new parameter of indentation energy to fracture (IEF). The IEF values were successfully correlated to the conventionally-measured fracture toughness of several nuclear pressure vessel steels. Since ABI is essentially non-destructive, the technique could be used periodically to evaluate and monitor the condition of such structures in service to assure integrity and safety, as well as to avoid premature decommissioning. For materials such as alloy 718, which degrade due to high temperature service conditions, correlations will be established in this project between their microstructure and ABI measurements in order to apply the technique for damage characterization and in remaining life prediction. Phase I will establish these correlations by a detailed characterization of the microstructure of as-fabricated and aged alloy 718 welds and their ABI parameters. Phase II will address extensions of these correlations to characterization of the property changes of alloy 718 welds in-service and the development of a miniature SSM testing device for field and remote testing of metallic structures in various power plants.
Commercial Applications and Other Benefits as described by the awardee:The in-situ nondestructive SSM technology will be used to determine key mechanical properties and to monitor aging and assess the integrity of welded structural components over their design service life and in lifetime extension evaluations in the nuclear, chemical, aerospace and defense industries. This will ensure safe operation and avoid premature decommissioning of power plants and other expensive structures.
