On the nanomechanical characteristics of thermally-treated alloy 690: Grain boundaries versus grain interior

Alloy 690 has been widely used as steam generator tubing material in nuclear power plants owing to its excellent properties such as stress corrosion cracking resistance. In the present study, a series of nanoindentation experiments were conducted to explore the nanomechanical characteristics of the grain boundaries (GBs) and grain interior (GI) in the alloy 690 that were isothermally aged for four different times (for simulating the 20-80 years operation under power plant condition). While the accelerated aging induced no pronounced degradation, the GB and GI show remarkable and reproducible differences in nanohardness (plastic flow), pop-in stress (yielding), and strain-rate sensitivity. The results are discussed in terms of dislocation activities in the various mechanisms of deformation behavior.