Effects of microstructure evolution on deformation and damage mechanisms during creep-fatigue testing of Alloy 709, The

Abstract

The improved economics and safety for next generation nuclear reactors depend, in part, on improved performance of advanced structural materials. Power plant structural components, for both fossil and nuclear fuels, are subject to thermal low cycle fatigue (LCF) during transient start-up and shut-down operations, and to creep during steady-state operation. Hence, the interaction of fatigue and creep degradation mechanisms (i.e. creep-fatigue) has been identified as a potential failure mode under such conditions. Alloy 709, a 20Cr-25Ni-1.5Mo-Nb-N solid solution and precipitation strengthened austenitic stainless steel, is a candidate material for structural components in Gen IV sodium cooled fast spectrum reactors (FSRs) due to its improved creep resistance over current qualified structural alloys. However, there are limited data on the creep-fatigue performance of this alloy. The objective of this research is to understand the deformation and damage mechanisms responsible for failure under creep-fatigue conditions relevant to nuclear service conditions. Laboratory creep and creep-fatigue tests are typically performed under conditions where damage is accumulated in an accelerated manner; i.e. at stresses and temperatures higher than expected during service. Initial strain-controlled LCF and creep-fatigue tests were conducted at the expected service temperature of 550 °C, and at a higher accelerated test temperature of 650 °C. The results indicated that the deformation mechanisms and damage were significantly different between the two test temperatures, which resulted in particularly poor creep-fatigue performance at 550 °C. The reduction in number of cycles to failure in creep-fatigue relative to LCF was significantly greater at 550 °C, compared to 650 °C, despite a higher creep resistance at the lower temperature. Additionally, the microstructural evolution of solution annealed (SA) material, namely precipitation of carbides and nitrides during testing, was shown to be significantly different at 550 and 650 °C. The effect of microstructural evolution on deformation and damage mechanisms in creep-fatigue was further investigated with SA material across a wider range of test temperatures (500 to 700 °C) and tensile hold times (0 to 30 min). Under conditions where dynamic precipitation is significant, a transition in slip behavior from planar to wavy leads to dynamic recovery at grain boundaries and a reduction of intergranular damage propagation as plastic deformation is enhanced at crack tips. Easier cross-slip is attributed to a consumption of solute atoms due to precipitation. Cyclic plastic deformation is also enhanced as precipitates coarsenduring testing. The greatest number of cycles to failure occurred where the accumulation of grain boundary creep damage (i.e. voids) was minimized and/or balanced with a large magnitude of cyclic plastic deformation. To further investigate the effect of microstructure evolution on creep-fatigue performance, a static aging treatment was used to produce a significant volume fraction of carbides and nitrides prior to creep-fatigue testing at 550 and 650 °C. Aging the alloy resulted in a three-fold increase in creep-fatigue life over the SA material at 550 °C, which was accompanied by more dynamic recovery and approximately the same amount of creep deformation per cycle compared to the SA condition. The significant improvement in creep-fatigue life at the expected FSR service temperature (550 °C) in an aged condition is encouraging for the applicability of Alloy 709 as a structural alloy. However, the results of this study indicate that the creep-fatigue performance during accelerated testing from a SA condition is not representative of long-term service performance due to the effects of microstructure on creep-fatigue deformation and damage mechanisms.