Microstructure evolution of alloy 709 during static-aging and creep-fatigue testing

Alloy 709 (20Cr–25Ni-1.5Mo–Nb steel), a candidate structural alloy for Gen IV nuclear reactors, is expected to undergo microstructural evolution during long-term service at elevated temperatures, which in turn affects the mechanical behavior through changing contributions from precipitate and solute strengthening. For safe reactor design, it is important to understand how microstructural evolution and mechanical behavior differ between long-term service and accelerated laboratory tests, such as low cycle fatigue (LCF) and creep-fatigue. In this study, a combined electron microscopy and small angle neutron scattering (SANS) analysis has been performed to quantify and compare the precipitation evolution in the alloy after a range of static aging treatments (at 550 to 750 °C for 1–2500 h) and laboratory LCF and creep-fatigue tests (at 550 and 650 °C). The experimental precipitation quantification results from the static-aged material are found to be deviated from the equilibrium thermodynamic predictions. In addition, dynamic precipitation that occurs during fatigue testing leads to finer and more uniform distribution of carbide precipitates with a higher volume fraction compared to static aging under similar time and temperature conditions. Thus, deformation during accelerated testing results in a significantly different microstructure compared to that expected during service conditions where the accumulation of deformation occurs over a much longer period.