High temperature creep-fatigue behavior of alloy 617 and alloy 230

The Very High Temperature Reactor (VHTR) is one of the Gen-IV reactor design concepts which embody a common goal of providing safe, longer lasting, proliferation-resistant, and economically viable nuclear energy. One of the biggest challenges in the research and development of Gen-IV reactor systems is the performance and reliability issues involving structural materials for both in-core and out-of-core applications.;Currently, Ni-based superalloy alloy 617 (Inconel 617) and alloy 230 (Haynes 230) are considered as the leading metallic candidate materials for applications in the VHTR because of their favorable mechanical properties at elevated temperatures. The major damage mechanism for materials used in the VHTR is predicted to be creep-fatigue damage, which arises due to startup and shutdown or power transients during normal operation. The mechanism of synergic interaction between creep and fatigue is not well understood and there are no effective modeling methods for predicting the creep-fatigue life of both alloys at present. To better understand and address these issues, Low Cycle Fatigue (LCF) and creep-fatigue tests of alloy 617 and alloy 230 were conducted in this study. Creep-Fatigue life prediction methods, such as linear damage summation and frequency-modified tensile hysteresis energy modeling, were evaluated. In addition, various microanalysis techniques were used to identify the underlying damage mechanism of alloy 617 and alloy 230 under LCF and creep-fatigue tests. The experiment and analysis results lead to the following conclusions: 1. Under all LCF test conditions, alloy 230 performed slightly better than alloy 617. 2. Compared to the LCF results, the cycles to failure for both materials were reduced under creep-fatigue test conditions. Again, alloy 230 exhibited longer creep-fatigue life than alloy 617. It was also found that longer hold time at the maximum tensile strain would cause a further decrease in both materials creep-fatigue life. 3. The linear damage summation failure criterion was able to predict the creep-fatigue life of alloy 617 for limited test conditions, but considerably underestimated the creep-fatigue life of alloy 230. In contrast, frequency-modified tensile hysteresis energy modeling showed promising results. 4. Creep-fatigue test conditions tended to transform the transgranular cracking mode observed in LCF tests of both materials to intergranular cracking mode especially for the creep-fatigue tests with longer dwell time at peak tensile strain. Moreover, alloy 230 demonstrated better resistances to intergranular cracking than alloy 617 under the creep-fatigue deformation. For both materials, additional creep damage in creep-fatigue testing was also manifested by material interior intergranular cracking. 5. During the LCF tests, the deformation tended to concentrate in the original grain boundary (GB) region especially for tests done at higher total strain range. The localized deformation near original GB region was enhanced for both materials during creep-fatigue tests due to the additional creep deformation and grain boundary sliding process incurred at the strain hold period. 6. Similar to the case of Stress Corrosion Cracking (SCC), oxidation-assisted crack propagation along GB promoted intergranular cracking of alloy 617 and resulted in faster crack growth rate in the creep-fatigue test of alloy 617 at small total strain range. 7. The higher volume fraction of carbide precipitates and better oxidation resistance of alloy 230 contributed to its dominance in LCF and creep-fatigue life over alloy 617. 8. GB cellular precipitation formed by discontinuous precipitation reaction in alloy 230 might have a deleterious effect on the LCF and creep-fatigue properties of alloy 230 and can be avoided by appropriate heat treatment and chemistry control to further improve the mechanical properties of alloy 230.